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A combined study of experimental measurement and theoretical calculation of NMR property

Xiongjian Wu, University of Nebraska - Lincoln

Abstract

Solid state NMR has been proven to be an efficient probe for identification, structure and dynamics of molecules because the properties measured by NMR, such as chemical shifts and quadrupolar interactions, are remarkably sensitive to provide extensive information. In parallel to experimental studies, progress has also been made to theoretical calculation of NMR properties. This combined theoretical and experimental approach is not only to check the validity of computational methodology but also provide a useful tool for interpreting experimental NMR data. ^ Tetramethylsilane (TMS) is widely used as a reference for measuring chemical shifts in 1H, 13C, and 29Si NMR spectroscopy. However, in variable temperature NMR measurements, the magnetic shielding of reference substance itself changes with the temperature. This shift is generally neglected and could lead to errors in measurements. The temperature dependence of chemical shieldings of nuclei in TMS were calculated through a combination of ab initio calculations, density functional theory (DFT) and statistical mechanics. In addition, the temperature dependent coefficients of the TMS magnetic shielding in the gas were also measured. ^ The temperature dependence of NMR resonance of N-acetylglycine (NAG) was investigated through a combination of ab initio calculations and statistical mechanics. We have calculated vibrationally averaged NMR property as a function of temperature based on calculation results of standard quantum chemical software such as Gaussian and GAMESS. ^ By using 2H solid state NMR techniques, quadrupole coupling constants (QCC) and asymmetry parameters have been measured for the deuterium nuclei in uracil. To compare with the experimental results, a computational study was also performed to obtain electric field gradients (EFG). By using an optimized truncated seven-molecule model, we were able to reproduce the experimental QCC to a reasonably high degree of accuracy. Our results demonstrate that it is very important to have an accurate model in order to do EFG calculations and this truncated model provides a compromise that limits computation time while retaining accuracy. ^ Computing accurate electric field gradients (EFGs) is essential to the quantitative analysis of quadrupole couplings. We have recently extended our studies to the second row diatomics HCl, LiCl, NaCl and AlCl. As observed in the first row before, the ordinarily accurate and convergent aug-cc-pV nZ series introduces large systematic errors in EFGs. The same series modified to permit core-valence correlation (aug-cc-pCVnZ) performs far better giving accuracies of the order of 1%, and also converges more rapidly. We also compare various electron correlation methods (MP2, CCSD and DFT) to evaluate how well empirical and perturbation methods approach the “gold-standard” of CCSD for EFG calculations. ^

Subject Area

Chemistry, Analytical|Chemistry, Physical

Recommended Citation

Wu, Xiongjian, "A combined study of experimental measurement and theoretical calculation of NMR property" (2011). ETD collection for University of Nebraska - Lincoln. AAI3487115.
http://digitalcommons.unl.edu/dissertations/AAI3487115

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