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A study of Chemical Shielding Constants of Biomolecules by Theoretical And Experimental NMR

Ebrahimi, Hossein Pasha | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41028 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Tafazzoli, Mohsen; Parsafar, Gholam Abbas
  7. Abstract:
  8. Prediction and calculation of shielding tensors of H, C, F and P nuclei of different molecules has been the subject of much research, because these nuclei have the greatest importance in NMR experiments. The optimum wave functions and calculation method were obtained using empirical models and factorial design. Based on preliminary experiences, the following four factors at two levels were selected: electron correlation, triple-ξ valence shell, diffuse function and polarization function. The wave functions for calculating gas phase 1H and 13C chemical shifts of various types of alcohols, amines and simple hydrocarbons were optimized using empirical model. The B3LYP/6-311+G wave function is proposed as the best level of theory for calculating 1H chemical shifts. For the calculation of the 13C chemical shifts, B3LYP/6-311G is recommended. This model has also been used for the 13C chemical shifts of gaseous hydrocarbons in relation to two different references. An additional series of molecules were used as test sets and their results confirmed the validity of approaches. An extensive GIAO calculation has been performed within HF, MP2, B3LYP and PBEPBE of 31P chemical shielding constants for different phosphines and compared with each other. For the test set of molecules that were studied, triple-zeta is the most effective factor which used in PBEPBE calculations. For molecules containing only phosphorous and carbon atoms with sp3 hybridization, the PBEPBE/6-311G(d,p) level of theory has been shown to provide reliable 31P chemical shifts. A systematic computational study was carried out to find the optimum levels and basis sets for small fluorine molecules in different groups. The wave functions for calculation of gas phase 19F chemical shifts were optimally selected using the factorial design and linear correlation. A detailed comparison between GIAO and CSGT methods has been reported. The effects of electron correlation, triple-ξ valance shell, diffuse function, and polarization function on calculated 19F chemical shifts were discussed. As expected, this report verify that GIAO provides results that are often more accurate than those calculated with CSGT approach, at the same basis set size. Of the four factors, electron correlation and polarization function were the greatest effect on the results and B3LYP/6-31+G(d,p) wave function was proposed as the best and the most efficient level of theory for calculation of the 19F magnetic properties. An additional series of fluorine compounds were used as the test sets the results of which confirmed the validity of the approaches
  9. Keywords:
  10. Gas Phases ; Chemical Shift ; Chemical Shielding ; Nuclear Magnetic Resonance

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