Isotropic Linewidths Research Articles

Pure Isotropic Proton NMR Spectra in Solids using Deep Learning

AbstractThe resolution of proton solid‐state NMR spectra is usually limited by broadening arising from dipolar interactions between spins. Magic‐angle spinning alleviates this broadening by inducing coherent averaging. However, even the highest spinning rates experimentally accessible today are not able to completely remove dipolar interactions. Here, we introduce a deep learning approach to determine pure isotropic proton spectra from a two‐dimensional set of magic‐angle spinning spectra acquired at different spinning rates. Applying the model to 8 organic solids yields high‐resolution 1H solid‐state NMR spectra with isotropic linewidths in the 50–400 Hz range.

Pure Isotropic Proton NMR Spectra in Solids using Deep Learning.

The resolution of proton solid-state NMR spectra is usually limited by broadening arising from dipolar interactions between spins. Magic-angle spinning alleviates this broadening by inducing coherent averaging. However, even the highest spinning rates experimentally accessible today are not able to completely remove dipolar interactions. Here, we introduce a deep learning approach to determine pure isotropic proton spectra from a two-dimensional set of magic-angle spinning spectra acquired at different spinning rates. Applying the model to 8 organic solids yields high-resolution 1 H solid-state NMR spectra with isotropic linewidths in the 50-400 Hz range.

Spin relaxation in Si nanoclusters embedded in free-standing SiGe nanocolumns

Separated nanocolumns (NCs) with embedded Si nanoclusters were prepared using the top-down technique that combines a bio-template and the defect-free neutral beam etching of Si0.75Ge0.25/Si/Si0.75Ge0.25 double-quantum-well layers. The electron spin resonance (ESR) was studied in the dark and under illumination for the structures with different lateral sizes of NCs. For the structure with a NC diameter in the range of 20–25 nm, the ESR signal is characterized by the isotropic line width. The spatial separation of nanoclusters results in the suppression of the Dyakonov-Perel mechanism of spin relaxation. A decrease in the NC diameter down to 13–14 nm leads to electron localization under the bottom of NCs, making the orientation dependence of the ESR line width anisotropic. Illumination results in the increase in spin lifetimes in both the types of NC structures, relocating the electrons to the center of NCs in the narrow NC structure, and making electron localization stronger in the thick NCs.

Unraveling the role of microenvironment and hydrodynamic forces on the vibrational relaxation rates of pyridine–water complexes

Polarization dependent Raman spectra of pyridine dissolved in water have been measured in a wide concentration range from dense to extreme dilute solutions. The isotropic band frequencies and line widths of the ν12(Α1) mode have been analyzed by using various models of line broadening mechanisms. The vibrational dephasing and vibrational frequency modulation have been analyzed by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. Spectral features such as bandwidths, band frequencies and characteristic dephasing times have been estimated and interpreted in the context of concentration induced variations and effects due to solute–solvent interactions. The time-correlation functions of vibrational dephasing were obtained for the ν12(Α1) mode and it was found that there is a general agreement in the whole concentration range with the modeling proposed by the Rothschild approach, which applies to complex liquids as is the case of pyridine–water solutions. The evolution of several parameters, such as the characteristic times and the vibrational second moments is indicative of drastic variations at extreme dilution revealing changes in the vibrational relaxation of the pyridine complexes in the aqueous environment. The microenvironment prevailing in the neighborhood of the ν12(Α1) mode seems to be the key factor in obtaining information concerning vibrational dynamics. Significant correlation is observed between vibrational relaxation rate and solvent parameters namely viscosity, density, refractive index and molecular radius. Microviscosity, involving the size of solute and solvent molecules, is found to be crucial in determining the relaxation rate. The microenvironment appears to play an important role in the vibrational relaxation process.

MATPASS/CPMG: A sensitivity enhanced magic-angle spinning sideband separation experiment for disordered solids

A Carr–Purcell Meiboom–Gill (CPMG) sensitivity-enhanced spinning sideband separation experiment is presented. The experiment combines the idea of magic-angle turning and phase-adjusted sideband separation (MATPASS), allowing for isotropic/anisotropic chemical shift separation of disordered solids with line widths far greater than the magic-angle spinning frequency. The use of CPMG enhances the sensitivity of the wide-line spectra by an order of magnitude via multiple-echo acquisition. The MATPASS/CPMG protocol involves acquisition of time-domain data using a MAT/CPMG pulse sequence followed by f1 shearing during data processing to arrive at the PASS representation. Such a protocol has √2 higher sensitivity than the conventional PASS method because all CPMG echo signals are used for the final PASS spectrum. Application of this method is demonstrated using a GeSe4 glass sample with both 77Se isotropic line widths and chemical shift anisotropy that far exceed the spinning frequency. The sideband separation allows for the measurement of chemical shift anisotropy of the disordered solids.

Heteronuclear dipolar decoupling effects on multiple‐quantum and satellite‐transition magic‐angle spinning NMR spectra

We here report on the influence of heteronuclear dipolar decoupling on the (27)Al 3QMAS, 5QMAS, and the double-quantum filter-satellite-transition magic-angle spinning (DQF-STMAS) spectra of a strongly dipolar-coupled system, gibbsite. The requirements for heteronuclear dipolar decoupling increase with the order of coherence evolving in the indirect dimension of a two-dimensional (2D) experiment. The isotropic line width of the high-resolution 2D spectra, in samples like gibbsite, is composed of four parts: the distribution of isotropic shifts (delta(ISO), delta(QIS)), the homogeneous broadening related to the proton-proton flip-flop terms, the (27)Al-(27)Al homonulcear dipolar couplings, and the (1)H-(27)Al heteronuclear dipolar couplings. It is shown that, even in the case of gibbsite, where a strong proton-proton bath exists, the main resolution limiting factor in these experiments resides in the (1)H-(27)Al dipolar interaction.

Localized in vivo isotropic‐anisotropic correlation 1H NMR spectroscopy using ultraslow magic angle spinning

In a previous work (1), the susceptibility broadening in the (1)H NMR metabolite spectrum obtained in a live mouse was separated from the isotropic information, which significantly increased the spectral resolution. This was achieved using ultraslow magic angle spinning (MAS) of the animal combined with a modified phase-corrected magic angle turning (PHORMAT) pulse sequence. However, PHORMAT cannot be used for spatially selective spectroscopy. This article introduces a modified sequence called localized magic angle turning (LOCMAT) that makes this possible. Proton LOCMAT spectra were obtained from the liver and heart of a live mouse while the animal was spun at a speed of 4 Hz in a 2 Tesla field. It was found that even in this relatively low field, LOCMAT provided isotropic line widths that were a factor of 4-10 times smaller than those obtained in a stationary animal. Furthermore, the susceptibility broadening of the heart metabolites showed unusual features that are not observed in dead animals. The limitations of LOCMAT and possible ways to improve the technique are discussed. It is concluded that in vivo LOCMAT can significantly enhance the utility of NMR spectroscopy for biomedical research.

High resolution 31P nuclear magnetic resonance study of Cr3+-doping effects on KTiOPO4

P nuclear magnetic resonance was employed to investigate Cr 3þ -doping effects on KTiOPO4. The high resolution 31 P isotropic chemical shift and linewidth measurements sensitively revealed changes in the microscopic environments caused by the doping.

Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.

Vibrational dephasing in bromocyclohexane: how to separate contributions from different mechanisms

Raman and IR spectra of bromocyclohexane dissolved in methylcyclohexane, methanol, 1-propanol and 2-propanol have been recorded. The isotropic line widths of the CBr stretching mode for the axial (658 cm −1) and the equatorial (686 cm −1) conformers as well as of the ν 1 ring mode for the axial (804 cm −1) and the equatorial (811 cm −1) conformers have been measured as a function of concentration. The ratio of integral Raman intensities and IR absorptions of the axial and the equatorial conformers, the band widths and their concentration dependence for the CBr stretching mode differ considerably from those obtained for the ring mode ν 1. To make this discrepancy clearer we have studied solvent effect on vibrational dynamics of bromocyclohexane by using various models of vibrational dephasing.

A Raman study of solvent effects on vibrational dynamics of axial and equatorial conformers in chlorocyclohexane

Raman spectra of chlorocyclohexane dissolved in carbon tetrachloride, methylcyclohexane, methanol have been measured. The isotropic line widths of the CCl stretching modes of the axial (684 cm −1) and equatorial (730 cm −1) conformers in these solvents have been analyzed by using various models of vibrational dephasing. For comparison, we have analyzed the v 1 modes of the axial (807 cm −1) and equatorial (817 cm −1) conformers describing the symmetric vibrations of the ring. The conformational equilibrium of chlorocyclohexane has been determined as a function of solvent.

EPR monitoring of sol-gel processes

Electron Paramagnetic Resonance (EPR) spectra of paramagnetic probes (free radical, 4-oxo-Tempo for non-doped glasses and vanadyl sulfate for doped silica glass) provide a non-intrusive means of monitoring of the dynamics of the sol-gel process. The free radical spectra parameters (rotation correlation time, isotropic hyperfine interaction parameter and line width) reflect changes in viscosity and polarity in the vicinity of the spin label. The EPR spectra of vanadyl can be used for monitoring the sol-gel process in that different types of mixed vanadyl complexes are formed during glass molding.

Rotational relaxation times from Raman band profiles for benzonitrile in different solvents

The Raman band profiles of two vibrational bands of benzonitrile belonging to the a1 species (2243 and 1000 cm-1) were analysed by separating the isotropic and anisotropic components. Spectra of benzonitrile were recorded, using argon laser excitation, in solutions of CC14, CS2, cyclohexane, cyclohexanol and tertiary butyl alcohol-all having different viscosities. The rotational correlation times were evaluated from the band profiles. The results indicate that (a) the isotropic (and not the anisotropic) linewidth increases with increasing viscosity, which is attributed to the greater effect of intermolecular interactions on vibrational relaxation; and (b) the two modes of the same species give different relaxation times.

High pressure Raman study of the CO stretching mode in liquid N,N-dimethylacetamide

The Raman ν4 symmetric C=O stretching mode of N,N-dimethylacetamide (DMA) is reported as a function of pressure from 1 to 4000 bar within the temperature range 40 to 100 °C. As in acetone, the isotropic line width ω1/2 (ISO) decreases with increasing density, in contrast to the opposite trend usually found in other molecular liquids. The noncoincidence effect, δω, which is the frequency difference between the peak maxima of the isotropic and the anisotropic Raman bands, increases with increasing density. These trends indicate the presence of strong intermolecular dipolar interactions between DMA molecules. Two theories of the noncoincidence effect due to McHale and Logan are used to interpret the density dependence of δω for DMA. In addition, we also used these two theoretical models to calculate δω for acetone using the results obtained earlier in our laboratory. McHale’s theory can predict the correct density dependence for the noncoincidence effect when the dielectric screening factor is ignored. Logan’s theory can account for the density behavior of the noncoincidence effect in DMA and in acetone.

Solvent dependence of vibrational frequencies and linewidths of the υ1 mode in group iva methylmetal trichlorides

Abstract The polarized Raman spectra of the υ 1 (CH 3 str.) mode in 1,1,1-trichloroethylsilane (TMSi), 1,1,1-trichloromethylgermane (TMGe) and 1,1,1-trichloromethylstannane (TMSn) were recorded in a number of solvents. The vibrational peak frequency displacements in TMSi were approximately linearly dependent on solvent polarizability. However, the correlation was substantially lower for TMGe and TMSn, indicating that forces other than dispersion interactions contribute significantly to the net frequency shifts in the heavier members of the series. Isotropic linewidths in TMSi and TMGe were observed to remain approximately constant in non-polar solvents, but increased markedly in polar media, In contrast to the results of an earlier investigation of 1,1,1-trichloroethane in solution, the isolated Binary Collision model of homogeneous dephasing does not correctly predict the experimental variations in linewidth. Therefore, it is concluded that vibrational linebroadening in these compounds is probably caused by inhomogeneous dipolar interactions.

Raman study of peak frequencies and linewidths of the v i mode of trichloroethane in solution

The vibrational displacements and isotropic Raman linewidths of the v 1 (CH 3 symmetric stretching) mode of 1,1,1-trichloroethane (TCE) have been measured in a number of solvents of diverse molecular properties. It was determined that the gas-solution frequency shifts are proportional to the solvent's polarizability, and thus could be interpreted on the basis of solution variations in the London dispersion forces. The isotropic linewidths, on the other hand, were found to be uncorrelated to solvent polarizability, indicating that dispersion interactions are not an important relaxation mechanism. On the other hand, a qualitative correlation was observed between bandwidth and the dipole moment of the solvent, suggesting that dipolar interactions may play a significant role in the line broadening of methyl group vibrations. An alternative explanation of the observed behavior was furnished by the Isolated Binary Collision model, which assumes that vibrational relaxation arises from collisionally modulated repulsive interactions in the liquid. A good correlation was observed between experimental linewidths and widths predicted by the IBC model.

High pressure isotropic bandwidths and frequency shifts of the C–H and C–O modes of liquid methanol

The Raman bands of the C–H ν3 and C–O ν8 stretching modes of liquid methanol have been measured at temperatures ranging from 273 to 363 K and pressures from 10 bar to 4 kbar. The effects of density and temperature on the isotropic linewidth, peak frequency of the isotropic band, ν0 (ISO), and the difference δν between anisotropic ν0(VH) and ν0(ISO) band frequencies are reported and discussed qualitatively in terms of available theoretical models. It appears that repulsive interactions are responsible for the observed C–H line changes, and the Schweizer–Chandler (SC) model is used to evaluate frequency shift and bandwidth of the ν3 C–H mode after the effect of Fermi resonance coupling to the 2ν4 overtone was removed. In particular, the anomalous temperature and density dependence of the Raman band of the C–O stretching mode is noted and compared to the relatively typical behavior observed for the Raman line shape of the C–H mode.

Raman linewidth study of hydration structure of p-dioxane in aqueous solution

Abstract The isotropic linewidth of the CO stretching Raman line of p-dioxane at 835 cm −1 has been measured at a variety of concentration in aqueous solution and in nonpolar tetrachloroethylene solution. The hydration structure of p-dioxane in aqueous solution has been investigated by a comparison of the experimental results obtained here with those for t-butyl alcohol in the preceding study. As a result, the present data prefer to the hydrogen-bond model rather than the clathrate model, proposed for the p-dioxane-water system.

ChemInform Abstract: RAMAN STUDY OF PRESSURE EFFECTS ON FREQUENCIES AND ISOTROPIC LINE SHAPES IN LIQUID ACETONE

The Raman line shape of the symmetric C = O stretching band at 1710 cm−1 has been measured in liquid acetone as a function of pressure from 1 bar to 4 kbar over the temperature range from −25 to 50 °C. The experimental data obtained show several unusual features. First, there is a frequency difference 〈δν〉 of about 7 cm−1 between the polarized and depolarized components. Sceond, the isotropic linewidth Γiso decreases with increasing density, in contrast to the opposite trend usually found in other liquids. Third, the second moment M2(V) of the isotropic band appears to decrease with increasing density. The consideration of the experimental linewidth and frequency data leads to a conclusion that intermolecular dipole–dipole coupling between polar acetone molecules are responsible for the observed unusual behavior of 〈δν〉, Γiso, and M2(V).

Raman study of pressure effects on frequencies and isotropic line shapes in liquid acetone

The Raman line shape of the symmetric C = O stretching band at 1710 cm−1 has been measured in liquid acetone as a function of pressure from 1 bar to 4 kbar over the temperature range from −25 to 50 °C. The experimental data obtained show several unusual features. First, there is a frequency difference 〈δν〉 of about 7 cm−1 between the polarized and depolarized components. Sceond, the isotropic linewidth Γiso decreases with increasing density, in contrast to the opposite trend usually found in other liquids. Third, the second moment M2(V) of the isotropic band appears to decrease with increasing density. The consideration of the experimental linewidth and frequency data leads to a conclusion that intermolecular dipole–dipole coupling between polar acetone molecules are responsible for the observed unusual behavior of 〈δν〉, Γiso, and M2(V).