Publications

In this study, we perform quantum calculations of the spin dynamics of a small spin system that includes nine coupled electrons and one nucleus placed in an external magnetic field and exposed to microwave irradiation. This is an extension of a previous work in which we have demonstrated on a system of 11 coupled electron spins the dynamics of the electron polarizations composing the electron paramagnetic resonance (EPR) line during static dynamic nuclear polarization (DNP) experiments. There we have shown that the electron polarizations are determined by a spectral diffusion process, induced by the dipolar interaction and cross-relaxation. Additionally, we showed that a distinction had to be made between strong and weak dipolar-coupled systems relative to the inhomogeneity of the EPR line with only the first behaving according to the thermal mixing DNP (with two electron spin temperatures) description. The EPR spectra in the weak and strong dipolar interaction cases show different types of spectral features. In the extended spin system, we again make a distinction between weak and strong electron electron interactions and show that the DNP spectra for the two cases are different in nature but that the DNP spectra can be derived in all cases from the EPR line shapes using the indirect cross effect.

A deeper understanding of parameters affecting Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP), an emerging nuclear magnetic resonance hyperpolarization method, is crucial for the development of new polarizing agents and the successful implementation of the technique at higher magnetic fields (>10 T). Such progress is currently impeded by computational limitation which prevents the simulation of large spin ensembles (electron as well as nuclear spins) and to accurately describe the interplay between all the multiple key parameters at play. In this work, we present an alternative approach to existing cross-effect and solid-effect MAS-DNP codes that yields fast and accurate simulations. More specifically we describe the model, the associated Liouville-based formalism (Bloch-type derivation and/or Landau-Zener approximations) and the linear time algorithm that allows computing MAS-DNP mechanisms with unprecedented time savings. As a result, one can easily scan through multiple parameters and disentangle their mutual influences. In addition, the simulation code is able to handle multiple electrons and protons, which allows probing the effect of (hyper) polarizing agents concentration, as well as fully revealing the interplay between the polarizing agent structure and the hyperfine couplings, nuclear dipolar couplings, nuclear relaxation times, both in terms of depolarization effect, but also of polarization gain and buildup times.

We present a bimodal Floquet analysis of the recently introduced refocused continuous wave (rCW) solid-state NMR heteronuclear dipolar decoupling method and compare it with the similar looking X-inverse X (XiX) scheme. The description is formulated in the rf interaction frame and is valid for both finite and ideal pi pulse rCW irradiation that forms the refocusing element in the rCW scheme. The effective heteronuclear dipolar coupling Hamiltonian up to first order is described. The analysis delineates the difference between the two sequences to different orders of their Hamiltonians for both diagonal and off-diagonal parts. All the resonance conditions observed in experiments and simulations have been characterised and their influence on residual line broadening is highlighted. The theoretical comparison substantiates the numerical simulations and experimental results to a large extent. (C) 2016 Elsevier Inc. All rights reserved.

Gd(iii) complexes have emerged as spin labels for distance determination in biomolecules through double-electron-electron resonance (DEER) measurements at high fields. For data analysis, the standard approach developed for a pair of weakly coupled spins with S = 1/2 was applied, ignoring the actual properties of Gd(iii) ions, i.e. S = 7/2 and ZFS (zero field splitting) ≠ 0. The present study reports on a careful investigation on the consequences of this approach, together with the range of distances accessible by DEER with Gd(iii) complexes as spin labels. The experiments were performed on a series of specifically designed and synthesized Gd-rulers (Gd-PyMTA-spacer-Gd-PyMTA) covering Gd-Gd distances of 2-8 nm. These were dissolved in D₂O-glycerol-d₈ (0.03-0.10 mM solutions) which is the solvent used for the corresponding experiments on biomolecules. Q- and W-band DEER measurements, followed by data analysis using the standard data analysis approach, used for S = 1/2 pairs gave the distance-distribution curves, of which the absolute maxima agreed very well with the expected distances. However, in the case of the short distances of 2.1 and 2.9 nm, the distance distributions revealed additional peaks. These are a consequence of neglecting the pseudo-secular term in the dipolar Hamiltonian during the data analysis, as is outlined in a theoretical treatment. At distances of 3.4 nm and above, disregarding the pseudo-secular term leads to a broadening of a maximum of 0.4 nm of the distance-distribution curves at half height. Overall, the distances of up to 8.3 nm were determined, and the long evolution time of 16 μs at 10 K indicates that a distance of up to 9.4 nm can be accessed. A large distribution of the ZFS parameter, D, as is found for most Gd(iii) complexes in a frozen solution, is crucial for the application of Gd(iii) complexes as spin labels for distance determination via Gd(iii)-Gd(iii) DEER, especially for short distances. The larger ZFS of Gd-PyMTA, in comparison to that of Gd-DOTA, makes Gd-PyMTA a better label for short distances.

Dynamic Nuclear Polarization (DNP) experiments on solid dielectrics can be described in terms of the Solid Effect (SE) and Cross Effect (CE) mechanisms. These mechanisms are best understood by following the spin dynamics in electron-nuclear and electron-electron-nuclear model systems, respectively. Recently it was shown that the frequency swept DNP enhancement profiles can be reconstructed by combining basic SE and CE DNP spectra. However, this analysis did not take into account the role of the electron spectral diffusion (eSD), which can result in a dramatic loss of electron polarization along the EPR line. In this paper we extend the analysis of DNP spectra by including the influence of the eSD process on the enhancement profiles. We show for an electron-electron-nuclear model system that the change in nuclear polarization can be caused by direct MW irradiation on the CE electron transitions, resulting in a direct CE (dCE) enhancement, or by the influence of the eSD process on the spin system, resulting in nuclear enhancements via a process we term the indirect CE (iCE). We next derive the dependence of the basic SE, dCE, and iCE DNP spectra on the electron polarization distribution along the EPR line and on the MW irradiation frequency. The electron polarization can be obtained from ELDOR experiments, using a recent model which describes its temporal evolution in real samples. Finally, DNP and ELDOR spectra, recorded for a 40 mM TEMPOL sample at 10-40 K, are analyzed. It is shown that the iCE is the major mechanism responsible for the bulk nuclear enhancement at all temperatures.

Dynamic nuclear polarization is typically explained either using microscopic systems, such as in the solid effect and cross effect mechanisms, or using the macroscopic formalism of spin temperature which assumes that the state of the electrons can be described using temperature coefficients, giving rise to the thermal mixing mechanism. The distinction between these mechanisms is typically made by measuring the DNP spectrum-i.e. the nuclear enhancement profile as a function of irradiation frequency. In particular, we have previously used the solid effect and cross effect mechanisms to explain temperature dependent DNP spectra. Our past analysis has however neglected the effect of depolarization of the electrons resulting from the microwave (MW) irradiation. In this work we concentrate on this electron depolarization process and perform electron-electron double resonance (ELDOR) experiments on TEMPOL and trityl frozen solutions, using a 3.34 Tesla magnet and at 2.7-30 K, in order to measure the state of the electron polarization during DNP. The experiments indicate that a significant part of the EPR line is affected by the irradiation due to spectral diffusion. Using a theoretical framework based on rate equations for the polarizations of the different electron spin packets and for those of the nuclei we simulated the various ELDOR line-shapes and reproduced the MW frequency and irradiation time dependence. The obtained electron polarization distribution cannot be described using temperature coefficients as required by the classical thermal mixing mechanism, and therefore the DNP mechanism cannot be described by thermal mixing. Instead, the theoretical framework presented here for the analysis of the ELDOR data forms a basis for future interpretation of DNP spectra in combination with EPR measurements.

We report magic angle spinning, dynamic nuclear polarization (DNP) experiments at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrices. The H-1 DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d(21)-BDPA). In addition, and in contrast to other DNP mechanisms such as the solid effect or cross effect, the experimental data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a H-1 reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramolecular delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE saturates at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP experiments in chemical, biophysical, and physical systems at high magnetic fields. (C) 2014 AIP Publishing LLC.

To study the solid state ¹H-DNP mechanism of the biradical TOTAPOL under static conditions the frequency swept DNP enhancement spectra of samples containing 20 mM and 5 mM TOTAPOL were measured as a function of MW irradiation time and temperature. We observed that under static DNP conditions the biradical TOTAPOL behaves similar to the monoradical TEMPOL, in contrast to MAS DNP where TOTAPOL is considerably more effective. As previously done for TEMPOL, the TOTAPOL DNP spectra were analyzed taking a superposition of a basic SE-DNP lineshape and a basic CE-DNP lineshape with different amplitudes. The analysis of the steady state DNP spectra showed that the SE was dominant in the 6-10 K range and the CE was dominant above 10 K. DNP spectra obtained as a function of MW irradiation time allowed resolving the individual SE and CE buildup times. At low temperatures the SE buildup time was faster than the CE buildup time and at all temperatures the CE buildup time was close to the nuclear spin-lattice relaxation time, T_1n. Polarization calculations involving nuclear spin-diffusion for a model system of one electron and many nuclei suggested that the shortening of the T_1n for increasing temperatures is the reason why the SE contribution to the overall enhancement was reduced.

A Floquet description of a phase alternated homonuclear recoupling scheme for perdeuterated systems is presented. As a result, we demonstrate improvements in the recoupling efficiency of the DOuble Nucleus Enhanced Recoupling [DONER; J. Am. Chem. Soc. 131 (2009) 170541 technique by utilizing Phase Alternated Recoupling Irradiation Schemes [PARIS; Chem. Phys. Lett. 469 (2009) 342]. The effect of proton and deuterium radio frequency irradiation during recoupling has been systematically studied and theoretical observations have been verified experimentally using a deuterated model compound, L-Alanine, at 10 and 20 kHz magic angle spinning frequency. Experimental results are well in agreement with theoretical observations, thereby significantly increasing the recoupling efficiency of conventional DONER in perdeuterated systems. (C) 2013 Elsevier Inc. All rights reserved.

The ¹³C solid state Dynamic Nuclear Polarization (DNP) mechanism using trityl radicals (OX63) as polarizers was investigated in the temperature range of 10-60 K. The solutions used were 6 M ¹³C urea in DMSO/H ₂O (50% v/v) with 15 mM and 30 mM OX63. The measurements were carried out at ∼3.5 T, which corresponds to Larmor frequencies of 95 GHz and 36 MHz for the OX63 and the ¹³C nuclei, respectively. Measurements of the ¹³C signal intensity as a function of the microwave (MW) irradiation frequency yielded ¹³C DNP spectra with temperature dependent lineshapes for both samples. The maximum enhancement for the 30 mM sample was reached at 40 K, while that of the 15 mM sample at 20-30 K. Furthermore, the lineshapes observed showed that both the cross effect (CE) and the solid effect (SE) DNP mechanisms are active in this temperature range and that their relative contribution is temperature dependent. Simulations of the spectra with the relative contributions of the CE and SE mechanisms as a fit parameter revealed that for both samples the CE contribution decreases with decreasing temperature while the SE contribution increases. In addition, for the 15 mM sample the contributions of the two mechanisms are comparable from 20 K to 60 K while for the 30 mM the CE dominates in this range, as expected from the higher concentration. The steep decrease of the CE contribution towards low temperatures is however unexpected. The temperature dependence of the OX63 longitudinal relaxation, DNP buildup times and ¹³C spin lattice relaxation times did not reveal any obvious correlation with the DNP temperature dependence. A similar behavior of the CE and SE mechanism was observed for ¹H DNP with the nitroxide radical TEMPOL as a polarizer. This suggests that this effect is a general phenomenon involving a temperature dependent competition between the CE and SE mechanisms, the source of which is, however, still unknown.

Deuterium magic angle spinning (MAS) NMR is used to study the dynamics of an organic molecule, N-[triethoxysilylpropyl]acetamide-d₃, grafted at the inner surface of the mesoporous silica material, MCM-41. The grafted molecule has a deuterated methyl group at its free terminus to monitor its local mobility through changes in its dynamic ²H-MAS NMR spectrum. Different spectra were recorded as a function of temperature from two different water containing samples. Observation shows that a major part of the grafted molecule remains static, irrespective of the temperature and hydration state of the sample, whereas the rest shows spectral changes indicative of a two-site jump motion of the methyl groups. Experimental observations were substantiated using molecular dynamic (MD) simulations of the grafted molecule. Subsequently, the MD results corroborate a model for the grafted molecules experiencing an exchange between two conformations consistent with the analysis of the ²H-MAS NMR spectra.

Proton Dynamic Nuclear Polarization (DNP) experiments were conducted on a 3.4 T homebuilt hybrid pulsed-EPR-NMR spectrometer, on static samples containing 10 mM or 40 mM TEMPOL in frozen glassy solutions of DMSO/water. During DNP experiments proton-NMR signals are enhanced with the help of microwave (MW) irradiation on or close to the Electron Paramagnetic Resonance (EPR) spectrum of the free radicals in the sample, transferring polarization from the free electrons to the nuclei. In the solid state a distinction is made between three DNP enhancement mechanisms: the Solid Effect (SE), the Cross Effect (CE) and Thermal Mixing (TM). In an effort to determine the dominant DNP mechanisms responsible for the enhancement of the nuclear signals, electron and nuclear spin-lattice relaxation rates, enhancement buildup times and microwave (MW) swept DNP spectra were measured as a function of temperature and MW irradiation strength. We observed lineshape variations of the DNP spectra that indicated changes in the relative contributions of SE-DNP and CE-DNP with temperature and MW power. Using a theoretical model describing the SE-DNP and CE-DNP the DNP spectra could be analyzed without involving the TM-DNP mechanism and the relative SE-DNP and CE-DNP contributions to the nuclear enhancement could be determined. From this analysis it follows that lowering the temperature beyond 20 K increases the SE-DNP and decreases the CE-DNP contributions. Possible explanations for this behavior are suggested.

A spectrometer specifically designed for systematic studies of the spin dynamics underlying Dynamic Nuclear Polarization (DNP) in solids at low temperatures is described. The spectrometer functions as a fully operational NMR spectrometer (144 MHz) and pulse EPR spectrometer (95 GHz) with a microwave (MW) power of up to 300 mW at the sample position, generating a MW B₁ field as high as 800 KHz. The combined NMR/EPR probe comprises of an open-structure horn-reflector configuration that functions as a low Q EPR cavity and an RF coil that can accommodate a 30-50 μl sample tube. The performance of the spectrometer is demonstrated through some basic pulsed EPR experiments, such as echo-detected EPR, saturation recovery and nutation measurements, that enable quantification of the actual intensity of MW irradiation at the position of the sample. In addition, DNP enhanced NMR signals of samples containing TEMPO and trityl are followed as a function of the MW frequency. Buildup curves of the nuclear polarization are recorded as a function of the microwave irradiation time period at different temperatures and for different MW powers.

The application of Floquet theory in solid-state nuclear magnetic resonance is discussed. The Floquet approach has been used in the study of various effects related to quadrupolar nuclei varying from the basic magic-angle spinning (MAS) Hamiltonian, rotational resonance, and rotary-resonance conditions to the effect of quadrupole nuclei on spin 1/2 coupled to them. An extension of Floquet theory, multimode-multipole Floquet theory (MMFT), in which a multipole operator basis has been used is applied to the analysis of heteronuclear decoupling, the calculation of the R² condition width, and depolarization effects in double-quantum recoupling experiments. Cross polarization (CP) is one of the most important techniques for the detection of rare spins in solid-state nuclear magnetic resonance (NMR). MAS leads to improvement in the spectral resolution making the various sites distinguishable and improving the signal to noise ratio but it complicates the analysis of the spectra resulting from dynamic processes.

Dynamic deuterium magic angle spinning NMR has been applied to study the slow motion of small molecules close to a silica surface. In particular, alanine-d(3) molecules dissolved in an aqueous solution were loaded into the pores of the mesoporous materials SBA-15 and MCM-41. Deuterium spectra were measured as a function of the water content of these materials and the temperature. From the analysis of these spectra and the corresponding proton spectra, using a simple molecular exchange model, relatively slow desorption rates of the binding of alanine to the inner pore surface were obtained and were correlated with the low proton concentrations at the pore surfaces.

We present a rf scheme designed to excite triple quantum (TQ) coherences for proton solid state NMR. This recoupling scheme is based on the phase modulated Lee Goldburg sequence combined with echo pulses and applied nonsynchronous with the magic angle spinning period. Based on the effective bimodal Floquet Hamiltonian we optimize the conditions for TQ coherence excitation. Numerical simulations are used to further adjust the recoupling conditions as well as define the sequence limitations. Experimental TQ filtered one-dimensional spectra and two-dimensional correlations of TQ to single quantum coherences are presented for standard amino acids. These results are compared with the crystal structures showing that this scheme can aid in resonance assignments and in resolving local spin topologies.

The elimination by HF of the silica matrix from the composites obtained by the two-step reaction deposition of CsxH3-xPW12O40 (CSHPW) salt nanocrystals with a CS/W-12 ratio equal 2.5 on SBA-15 yields materials with substantially lower Cs/W-12 ratios of 1.7-2.0. The value of the CS/W-12 ratio in the nanocasts is determined by the Cs-precursor (Cs n-propoxide or Cs-acetate) used at the first stage of materials preparation. The surface area of the CsHPW nanocasts is 41-45 times higher than their co-precipitated analogs at the same Cs/W-12, ratios. We report here that implementation of the nanocasting preparation technique yields for the first time a bulk CsHPW material that combines a high concentration of acid sites (CS/W-12= 1.7-2.0) with a high surface area of 41-93 m(2) g(-1). Co-precipitated analogues at the same CS/W-12 ratios are nonporous and exhibit a surface area smaller than 5 m(2) g(-1). Our nanocasted CsHPW materials are stable against leaching and colloidization in polar solvents, and their catalytic performance exceeded that of bulk Cs2.5H0.5PW12O40, known as the most active among the acidic HPW salts. The catalytic activity of CsHPW nanocasts in MTBE synthesis and in the isopropanol dehydration reactions is shown to be higher by a factor of 2-3 than that of the standard Cs2.5H0.5PW12O40 material. (c) 2007 Elsevier B.V. All rights reserved.

Davies electron-nuclear double resonance spectra can exhibit strong asymmetries for long mixing times, short repetition times, and large thermal polarizations. These asymmetries can be used to determine nuclear relaxation rates in paramagnetic systems. Measurements of frozen solutions of copper(L-histidine)₂ reveal a strong field dependence of the relaxation rates of the protons in the histidine ligand, increasing from low (g_∥) to high (g_⊥) field. It is shown that this can be attributed to a concentration-dependent T_1e-driven relaxation process involving strongly mixed states of three spins: the histidine proton, the Cu(II) electron spin of the same complex, and another distant electron spin with a resonance frequency differing from the spectrometer frequency approximately by the proton Larmor frequency. The protons relax more efficiently in the g_⊥ region, since the number of distant electrons able to participate in this relaxation mechanism is higher than in the g _∥ region. Analytical expressions for the associated nuclear polarization decay rate T_een^-1 are developed and Monte Carlo simulations are carried out, reproducing both the field and the concentration dependences of the nuclear relaxation.

The effects of grafting the alumina species at the surface of silica support on the H3PW12O40 (HPW) immobilization capacity and performance of loaded HPW in acid-catalyzed reactions were studied with regular silica-gel and ordered mesostructured silica SBA-15. The states of alumina and HPW, in terms of the acidity/basicity, material texture, and structure of adsorbed HPW species, were characterized by XRD, XPS, NH3-TPD, UV-vis-, and FTIR-spectroscopy, as well as H-1 and P-31 MAS NMR, NH3- and CO2-TPD, and N-2 adsorption. It was demonstrated that grafting of silica with small alumina clusters at partial to full surface coverage produces isolated basic sites of the same strength as at the surface of pure alumina, but with a surface concentration that is an order of magnitude lower. These sites anchor the molecular species of HPW, retaining their polyanion structure and acidity-catalytic activity patterns in several acid-catalyzed reactions. The reason for the low activity of HPW moieties immobilized at the surface of pure alumina is a polydentate adsorption of polyanions yielding a high extent of HPW acidity neutralization. The use of mesostructured SBA-15 as a support yielded catalysts with 30-70% higher activity compared with that based on regular silica gel. This is a result of higher surface area and surface concentration of silanols in SBA-15, which makes it possible to immobilize more HPW as molecular species. © 2005 Elsevier Inc. All rights reserved.

We here report on a high-resolution pulse scheme for double-quantum (DQ) proton NMR spectroscopy in the solid-state. The pulse scheme employs a combination of multiple-pulses and magic-angle spinning (MAS) for both the excitation and conversion of DQ coherences and their evolution under homonuclear dipolar decoupling. This is made possible in this two-dimensional experiment by an effective combination of homonuclear dipolar decoupling method of phase modulated Lee-Goldburg and symmetry adapted sequence for homonuclear dipolar recoupling under MAS. DQ spectra of monoethyl fumaric acid, glycine, and histidine are presented to highlight the utility of the pulse scheme together with some of the existing drawbacks. (C) 2004 Published by Elsevier B.V.

The proton-carbon polarization exchange in Lee-Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly C-13-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon-proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear C-13-H-1 spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in (CH)-C-13-(CH)-C-13 and (CH2)-C-13-C-13 spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton-carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U-C-13] tyrosine.HCl and [U-C-13, N-15] histidine.HCl.H2O samples are in satisfactory agreement with simulations. (C) 2003 American Institute of Physics.

We demonstrate here a new windowed multiple pulse sequence for the detection of protons in solid state NMR. Acquisition windows are inserted in the phase modulated Lee-Goldburg scheme (PMLG). This enables one-dimensional acquisition as against two-dimensional in windowless schemes. The pulse scheme, theoretical justifications and experimental spectra of a few samples are presented. In addition two-dimensional proton-proton and carbon-proton correlation experiments are performed with windowed PMLG proton detection. (C) 2002 Published by Elsevier Science B.V.

We present a method for the synthesis of CdS and CdS_xSe_1-x nanocrystals by precipitation from a solution of cadmium carboxylate in dimethyl sulfoxide (DMSO) with or without elemental S (Se). The reaction in the presence of hydrazine has also been investigated. The CdS particle size, determined by X-ray diffraction, varied between 2 and 7 nm, depending on the reaction conditions. The CdS_xSe_1-x crystal size varied between 5 and 10 nm. UV spectra of the particles showed an increase of the optical band gap as the size of the crystals decreased. The surface structure of the CdS nanocrystals was characterized by IR and NMR spectroscopy. The spectra of CdS particles prepared with cadmium acetate and sulfur showed four types of binding sites at the crystallite surfaces: oxidized sulfur, acetate adsorbed to the surface cadmium in a bridging complex, oxygen-bound DMSO, and hydroxyl ions.

The backbone conformation of peptides and proteins is completely defined by the torsion angles (phi,psi,omega) of each amino acid residue along the polypeptide chain. We demonstrate a solid-state NMR method based on heteronuclear distance measurements for determining (phi,psi) angles. Simple and reliable deuterium phase modulated pulses (PM5) reintroduce dipolar couplings between H-2 and a spin-1/2 nucleus. Measuring the C-13(i-1){H-2(i)alpha} REDOR distance across a peptide bond results in the torsion angle phi (i) as a consequence of;he restricted geometry of the peptide backbone. The N-15(i+1){H-2(i)alpha} REDOR distance across a peptide bond defines the torsion angle psi (i). This approach is demonstrated for both the 3-spin X{H-2(2)}REDOR case of glycine and the 2-spin X{H-2}REDOR case, represented by L-alanine, using two different tripeptides. It is shown that the technique can handle multiple sample conformations. PMS-REDOR decay curves of the psi angle show distinctly different behaviors between alpha -helix and beta -sheet backbone conformations.

Dipolar recoupling techniques of homonuclear spin pairs are commonly used for distance or orientation measurements in solids. Accurate measurements are interfered with by broadening mechanisms. In this publication narrowband RF-driven dipolar recoupling magnetization exchange experiments are performed as a function of the spinning frequency to reduce the effect of zero-quantum T-2 relaxation. To enhance the exchange of magnetization between the coupled spins, a fixed number of rotor-synchronous pi-pulses are applied at spinning frequencies approaching the rotational resonance (R-2) conditions. The analysis of the powder averaged dipolar decay curves of the spin magnetizations as a function of the spinning frequency provides a quantitative measure of the dipolar coupling. An effective Hamiltonian for this experiment is derived, taking into account all chemical shift parameters of the spins. The length of the nbRFDR mixing time and the number of rotor cycles per pi-pulse are optimized by numerical simulations for sensitive probing of the dipolar coupling strength. The zero-quantum T-2 relaxation time can easily be taken into account in the data analysis, because the overall exchange time is almost constant in these experiments. Spinning-frequency-dependent nbRFDR experiments near the m = 1 and m = 2 R-2 condition are shown for doubly C-13-labeled hydroxybutyric acid. (C) 2000 Academic Press.

Phase modulated pulses for deuterium recoupling in H-2-C-13 REDOR NMR spectroscopy have been introduced to improve dephasing of the detected C-13 nuclei. The deuterium inversion properties of phase modulated recoupling pulses have been studied experimentally on L-alanine-2-d(1) and theoretically using average Hamiltonian theory and exact simulations of the equation of motion of the density matrix. The best C-13 dephasing was observed when XYXYX (PM5) deuterium recoupling pulses were applied. A comparison to the 900-180 degrees-90 degrees (CPL) composite pulse scheme revealed an improvement of recoupling on the order of 2.5. Simple CW recoupling pulses of the same length of PM5 and CPL pulses showed the weakest C-13 dephasing. Simulations have shown that the H-2 recoupling efficiency of PM5 REDOR experiments approach the very efficient REAPDOR results. However, in our case a REAPDOR study of L-alanine-2-d(1) resulted in a significant decrease of the C-13 signal intensity due to pulse imperfections of C-13 pi-pulses. The new PM5-REDOR technique has been employed to study the torsion angle between C1/2 and C5 in ethylmalonic acid-4-d(2). (C) 2000 Academic Press.

A new type of fast amplitude modulated pulse scheme is presented here that yields a significant sensitivity enhancement in the triple-quantum magic angle spinning NMR spectrum of a spin-5/2 nucleus. Enhancement is achieved by fast phase alternation of the triple to single-quantum conversion pulse, which transfers triple to single-quantum coherence in a direct, non-adiabatic manner. The success of this pulse relies on the difference between the quadrupolar frequency and the intensity of the rf irradiation field, during its short segments. Numerical optimizations and experimental results are presented showing the improved performance. (C) 2000 Elsevier Science B.V. All rights reserved.

CdS nanoparticles precipitated from aqueous solution were studied by ¹H NMR. The nanoparticles were deliberately not capped by any surface termination agent. The samples had porous structure. Proton high spinning speed magic angle spinning (MAS) NMR spectra revealed that there are three abundant proton species in nanoparticle samples prepared with an excess of Cd, having different chemical shifts: a relatively narrow peak due to hydroxyl groups and two broader lines resulting from adsorbed water molecules with different chemical environments. The relative intensities of the lines were temperature-dependent, reflecting a complicated chemical equilibration processes. A small fraction of water molecules (∼5%) experiences anisotropic dynamic motion as deduced from the analysis of the dipolar sideband patterns. The main part of the spectrum was due to rapidly exchanging protons. The exchange between protons of the same type occurs on a time scale not much longer than 1 μs. The exchange between different lines was studied by 2D exchange MAS spectroscopy. The analysis of the proton-proton correlation spectra as a function of the mixing time led us to the conclusion that the nanocrystalline surface is covered by clusters of water. The clusters are well separated from each other and are formed by protons occupying sites with the same chemical environment. The size of the pores was also estimated to be of the order of a few nanometers.

Multiple-quantum magic-angle-spinning (MQMAS NMR) spectroscopy has become a routine method to obtain high-resolution spectra of quadrupolar nuclei. One of the main problems in the performance of this experiment has been the poor efficiency of the radio-frequency pulses used in converting multiple-quantum coherences to the observable single-quantum signals. As the MQMAS experiment is basically an echo experiment this problem can be related to the efficiency with which continuous wave pulses can normally achieve the multiple- to single-quantum conversion for different crystallites in a spinning powdered sample. In this paper we investigate various aspects involved in this multiple-to-single quantum conversion, in the hope to facilitate the devise of new experimental schemes that can lead to significant MQMAS signal enhancements. We examine in particular a recently suggested experiment for MQMAS spectroscopy which employs amplitude-modulated radio-frequency pulses, and which can yield substantial signal and even resolution enhancements over the commonly used pulse schemes in MQMAS experiments. The mechanisms of operation of continuous-wave and of amplitude-modulated pulses as applied to the selective manipulation of spin-3/2 coherence elements are examined in detail, with the aid of the fictitious spin-1/2 formalism in combination with quadrupolar adiabaticity arguments. New insight into the nature of the MQMAS experiment is thus revealed, and the superior performance of suitable amplitude modulations toward the formation of MQMAS powder echoes is justified. Experimental results highlighting the utility of this scheme in samples possessing multiple quadrupolar sites with varying quadrupolar anisotropies and chemical shift offsets are demonstrated, as is the relative insensitivity of the new signal-enhancement technique to the actual level of rf irradiation. Further implications and uses of this new irradiation scheme are also briefly discussed.

Diluted magnetic semiconductor samples Cd0.99Fe0.01S and Cd0.994Co0.006S were investigated by Cd-113 magic angle spinning NMR spectroscopy in the temperature range of 180-400 K. These alloys were prepared by mixing Cd0.97M0.03S (M = Co, Fe) and the binary compound CdS in the appropriate molar ratios and maintaining the mixtures at about 1000 degrees C for seven-days. The macroscopic homogeneity of the samples was determined by energy dispersive spectroscopy. The microscopic homogeneity of the paramagnetic ion distribution in these samples can be studied by analyzing their Cd-113 NMR spectra. These spectra contain a set of Cd* bands (the * sign indicates the observed cadmium atoms) that are shifted by the transferred hyperfine (THF) interaction between the cadmium nuclei and their neighboring paramagnetic ions. Using the temperature dependence of the positions, the relaxation times, and anisotropies of these bands, we assigned each band to a well-defined next-nearest neighbor M(2)-S-Cd(1)-S-Cd* conformation, and correlated the THF interaction constants of all conformations to the M(2)-Cd* distances. All cadmium bands in the spectra comprise a set of lines that correspond to Cd* atoms in conformations of the type M(3)-S-Cd(2)-S-Cd(1)-Cd*-S-Cd(1)-S-M(2). The relative intensities of the lines can be calculated for given x values, when we assume a random magnetic ion impurity distribution. For the Cd0.994C0.006S sample, we found that the calculated line intensities were not consistent with the experimental intensities. To explain this deviation and to simulate a spectrum that fits the experimental data, we assumed that this sample consists of clusters with different concentrations of the impurity. Good agreement was obtained when about half of the cadmium atoms do not interact with the paramagnetic ions, and the other half interacts with ions that are randomly distributed in an alloy composed of Cd0.987Co0.013S. [S0163-1829(99)14335-2].

The application of short composite pulse schemes (90(x)degrees-90(x)degrees-90(x)degrees and 90(x)degrees-180(x)degrees-90(x)degrees) to the rotational echo double-resonance (REDOR) spectroscopy of X-H-2 (X: spin 1/2 observed) systems with large deuterium quadrupolar interactions has been studied experimentally and theoretically and compared with simple 180 degrees pulse schemes. The basic properties of the composite pulses on the deuterium nuclei have been elucidated, using average Hamiltonian theory, and exact simulations of the experiments have been achieved by stepwise integration of the equation of motion of the density matrix, REDOR experiments were performed on N-15-H-2 in doubly labeled acetanilide and on C-13-H-2 in singly H-2-labeled acetanilide. The most efficient REDOR dephasing was observed when 90(x)degrees-180(x)degrees-90(x)degrees composite pulses were used. It is found that the dephasing due to simple 180 degrees deuterium pulses is about a factor of 2 less efficient than the dephasing due to the composite pulse sequences and thus the range of couplings observable by X-H-2 REDOR is enlarged toward weaker couplings, i.e., larger distances. From these experiments the H-2-N-15 dipolar coupling between the amino deuteron and the amino nitrogen and the H-2-C-13 dipolar couplings between the amino deuteron and the alpha and beta carbons have been elucidated and the corresponding distances have been determined, The distance data from REDOR are in good agreement with data from X-ray and neutron diffraction, showing the power of the method. (C) 1999 Academic Press.

Proton magic angle spinning (MAS) spectra of a model spin system, consisting of six protons, were calculated for different values of the external magnetic field and the spinning frequencies. Floquet theory was used to evaluate these spectra. The reduction of the effective homonuclear dipolar interaction for increasing spinning frequency was investigated. The influence of an increase of the external magnetic field and the spinning frequencies on the linewidths of the centerband spectra is discussed. This Floquet description of the rotating proton spin system will assist us in our calculations of the CPMAS spin dynamics of a low abundant spin interacting with a set of coupled protons. (C) 1998 Academic Press.

CdS and CdS:Mn nanoparticles were studied by ¹¹³Cd and proton NMR. Nanoparticle samples were synthesized by precipitation of CdS from an aqueous solution of Na₂S and CdSO₄. The nanoparticles were not deliberately capped by any surface-termination agent. The NMR spectra of CdS nanoparticles prepared with an excess of Cd consist of three frequency bands: a bulk line corresponding to the inner Cd atoms with full sulfur coordination, a broad band due to surface Cd atoms, and a sharp line of CdSO₄ in solution. The assignment of these lines is accomplished by experiments on S-rich samples, by the study of the temperature dependence of the spectra, and by comparing the chemical shift values of the lines with tabulated chemical shift values of different cadmium compounds. A significant amount of water was found in the samples. This water is trapped in microscopic pores resulting in a lowering of the water-ice phase transition and a significant shortening of the proton T₁ and T₂ relaxation times. The NMR spectra of CdS:Mn do not show any additional spectral structure. Mn²⁺ ions remain in the included water and/or are located at the nanocrystallite surface and do not appear to be incorporated inside the nanoparticles.

We have investigated the reorientational mobility of n-hexane and n-pentanol guest molecules in Dianin's compound and in zeolite 5A by molecular mechanics/dynamics calculations (MM/MD) and temperature-dependent solid-state H-2 NMR. To choose the proper motional models for the different host-guest systems, MM calculations were first performed and minimum-energy structures for the compounds were established. The calculations were performed on a crystallographic unit cell, with periodic boundary conditions to minimize edge effects. MD methods were then applied to the minimized structures and provided an insight into the different dynamic modes experienced by the guest molecules in their host cages. Several structural parameters of the host-guest systems were monitored for a total simulation period of up to 100 ps both at low temperature (77 K) and high temperature (300 K). H-2 NMR spectra of perdeuterated and specifically deuterated guest molecules in their inclusion compounds were acquired as a function of temperature. After detection the observed Line shapes were analyzed in terms of motional models, taking into account experimental parameters and the results of the MD calculations. The solid-state H-2 NMR showed a very high degree of agreement with the results of the molecular modeling calculations.

Solid extractants for metal ions have been prepared by chemical bonding of jojoba wax to a polystyrene backbone, followed by phosphonation or sulfur-chlorination of the jojoba moiety. In this study, the intermediates and final solid products of the reactions were characterized by solid-state C-13 and P-31 nuclear magnetic resonance spectroscopy. The spectra showed the expected chemical shifts of the atoms involved in the chemical reactions, as well as other parts of the reacting molecules. Thus, the carbonyl carbon of the jojoba chain appears at 175 ppm, the methyl carbons at 15 ppm, tie polystyrene backbone at 40-42 ppm (aliphatic carbons) and 128, 137, 143-147 (aromatic carbons). Carbons adjacent to N, S, and P appear at 45-55, 60, and 48 ppm, respectively.

The p-xylene adduct of Dianin's compound was studied by solid state NMR and molecular modelling. Molecular potential calculations were performed considering a p-xylene guest molecule in a single host cage. These calculations confirmed the existence of six different possible orientations of the guest molecules in the cage. The orientations correspond to two types of guests, which are not related by any symmetry, each consisting of three C-3 symmetry related sites. The two types induce significant distortions to the local structure of the host crystal cage, and these are most pronounced at the bottle-neck of the cage. In the calculations no diffusion paths were found that transferred guest molecules of one type to the other. X-Ray studies confirmed that the dimensions of the p-xylene/Dianin's cages are essentially equal to those reported previously. In our case, the R (3) over bar space groups failed to describe the X-ray diffraction data accurately. Single crystal H-2-NMR of the p-xylene/Dianin's compound, specifically deuterated in the hydroxyl groups of the cage, provided detailed information on the distortions of the hexagons of hydroxyl groups interconnecting two cages. The hexagon distortions were correlated to the orientation of the guest molecules. C-13 CP-MAS NMR confirmed that the methyl groups of the host, in the middle of the cage, are influenced strongly by the inclusion of the p-xylene molecules. Based on the experimental observations, a model describing the distortions in the crystal is proposed.

The results of various N-15 solid-state NMR experiments performed on solid samples of doubly N-15-labeled 3,5-dimethylpyrazole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyrazole are reported, In the solid state, these compounds form various hydrogen-bonded complexes, The principal values of the N-15 chemical-shift tensors (CST) of amine and imine nitrogen atoms are derived by lineshape analysis of the N-15 NMR spectra of the static powders obtained under the conditions of H-1-N-15 cross polarization and H-1 decoupling, The orientations of the N-15 CST in the molecular principal axis system are obtained by taking into account the N-15-N-15 dipolar interactions and the N-15-D dipolar interaction after deuteration of the mobile proton sites, The relative orientations of the amine and imine CST in PMP are independently checked by one-dimensional off-magic axis sample spinning magnetization transfer experiments, The isotropic chemical shifts, the principal elements, and the orientations of the CST of both nitrogen atoms and the ND distances depend only slightly on the chemical structure and the associated hydrogen-bonded structure, The intramolecular structure of pyrazoles, therefore, does not vary substantially when different types of hydrogen-bonded complexes are formed. (C) 1996 Academic Press, Inc.

The sensitivity of one-dimensional dynamic magic-angle spinning (MAS) and off-MAS H-2 nuclear magnetic resonance spectra to changes in the parameters of jump-type molecular motions is studied. The Floquet theory approach is used to simulate spectra of spins with I = 1, which are involved in exchange processes in rotating solids. The solution of the Bloch-McConnell equations for rotating samples are derived and some simulated frequency spectra are shown. The dependence of the lineshapes of the center and sidebands of the MAS and off-MAS spectra on the exchange parameters are discussed. Experimental results of H-2 spectra of perdeuterated dimethyl sulfone, obtained in the temperature range 20-55 degrees C, are demonstrated. The methyl groups in this molecule undergo pi flips at rates that can be detected by MAS and off-MAS NMR. The shapes of the experimental sidebands are compared with simulated results.

Multinuclear solid-state nuclear-magnetic-resonance (NMR) experiments have been performed on a single-crystal sample of bulk composition Hg0.78Cd0.22Te. The NMR spectra of the three distinct lattice species (Hg, Cd, and Te) have been observed and assigned. Some double-resonance Hg-Te and Hg-Cd experiments clarify the assignment of Te resonance to chemical environment. These experimental results confirm our previous assignments of Te shifts to distinct chemical environments. They further suggest that in melt-grown, homogeneously distributed samples, cation distributions differ little from predictions of a statistical model.

The combination of transferred-echo double resonance (TEDOR) with rotational-echo double resonance (REDOR) has been used to measure an 8-angstrom fluorine-carbon internuclear distance in a nine-residue fragment of the peptide antibiotic emerimicin. The fragment is (FCH2CO-Phe-MeA-MeA-[1-C-13]MeA-[N-15]Val-Gly-Leu-MeA-MeA-OBzl)-F-19 (MeA = alpha-methylalanine or aminoisobutyric acid). The TEDOR part of this magic-angle-spinning, solid-state NMR experiment selects the C-13 label by its dipolar coupling to N-15 and suppresses the natural-abundance carbon background. The REDOR part of the experiment measures dipolar coupling of the selected carbon to F-19. The TEDOR-REDOR combined experiment works with a variety of spin 1/2 nuclei and can be used to characterize internuclear distances and geometry in macromolecular aggregates that do not crystallize.

The Floquet theory for the description of magic angle sample spinning (MASS) nuclear magnetic resonance (NMR) experiments is introduced. MASS NMR signals of single spin systems and homonuclear two-spin system in the presence of direct and indirect dipolar couplings are evaluated. The single spin system is utilized to develop our Floquet formalism for the MASS NMR experiments and the coupled spin system is discussed to demonstrate its methodology and its applicability. This theoretical approach enables the calculation of the positions, the amplitudes, and line shapes of MASS center and sideband. The diagonalization of the MASS Floquet matrices of the different spin systems provides eigenvalues and eigenvectors which in turn determine the frequencies and intensities of the various bands in the MASS spectra. Their frequency shifts can be explained in terms of anticrossing of Floquet states and their amplitudes can be calculated via the matrix elements of transition-amplitude Floquet operator. New Floquet operators are introduced and the commutation relations between them utilized to enable the evaluation of diagonalization matrices. The Floquet theory is applied to determine the rotational resonance conditions for homonuclear dipolar-coupled spin pairs. It is shown that significant band shifts are expected when degeneracies between Floquet states occur. These shifts endow the centerband and sidebands with power line shapes from polycrystalline samples. Simulations of experimental line shapes can provide direct and indirect dipolar coupling constants, including their relative signs. This approach is valid for all possible dipolar coupling strengths as well as for all sample spinning speeds and isotropic and anisotropic chemical shift parameters. The decay of zero-quantum coherences in the coupled spin-systems is also considered in terms of the Floquet approach. The sensitivity of the MASS line shapes close to the rotational resonance conditions with respect to the re

Solid-state NMR techniques were employed in order to study the structure and the dynamics of porphine. The changes observed in the line width of the ¹H NMR signal between 173 and 443 K suggest that the porphine macrocycles rotate in the crystals. This was confirmed by recording the ¹³C CPMAS NMR spectra at different temperatures which showed, in addition to the expected coalescence of signals due to the central hydrogens tautomerism, a broadening of the resonances due to the overall molecular rotation. These studies, coupled to measurements at different temperatures and fields of the relaxation times of the ¹H magnetization in the rotating frame, allowed us to obtain activation parameters for the motion which are, within experimental error, equal to those made available by CPMAS NMR for the tautomerism of the central hydrogens. These results suggest an explanation for what seems to be a contradiction between the structure of porphine observed by X-ray, according to which the central hydrogens are localized in opposite pairs of nitrogens, and the structure observed by CPMAS in which the hydrogens migrate between the four central nitrogens. If it is assumed that the migration of the central hydrogens is coupled to a 90° rotation of the molecules, the translational symmetry of the crystal will not be changed by the tautomerism, and an X-ray analysis would always detect a single tautomer.

Deuterium and carbon-13 NMR of specifically labeled benzenehexoyl hexa-n-hexanoate in the various solid-state phases are reported. The spectra exhibit dynamic line shapes which change discontinuously at the phase transitions. The results are interpreted in terms of sequential \u201cmelting\u201d of the side chains on going from the low-temperature solid phases IV, III, etc., toward the liquid. In phase IV the molecules are very nearly static, except for fast rotation of the methyl groups about their C₃axes. The results in phase III were quantitatively interpreted in terms of a two-site isomerization process involving simultaneous rotation by 95° about C!-C₂and transition from gtg to g'g't (or equivalently g^ftg^fto ggt) for the rest of the chain. The specific rate of this reaction at 0 °C is ~ 10⁵s^{-12 3 4 5 6 7 8 910}. In phase II additional chain isomerization processes set-in which were, however, not analyzed quantitatively. Further motional modes, involving reorientation of whole chains about their C^ar-0 bonds, appear on going to phase I. In all solid phases the benzene ring remains static.