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Prediction of Gas Phase NMR Chemical Shifts Using Gas Phase NMR and Quantum Calculations in Optimally Selected Level of Theory by Factorial Design

Shaghaghi, Hoora | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39763 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Tafazzoli, Mohsen; Jalali Heravi, Mehdi
  7. Abstract:
  8. The optimum wave functions and calculation method were obtained using a 24 factorial design. Based on preliminary experiences, the following four factors at two level was selected: electron correlation, triple-ξ valence shell, diffuse function and polarization function.
    The wave functions for calculating gas phase 1H chemical shifts of primary and secondary alcohols were optimized using factorial design as multivariate technique. Gas-phase experimental 1H chemical shifts of 18 alcohols were used to establish the best levels of theory for obtaining 1H chemical shift, among them the new experimental values of 1H chemical shifts of 10 alcohols were obtained in our laboratory. HF/6-31G(d,p) wave function is proposed as the best level of theory for calculating 1H chemical shifts of primary alcohols. For secondary alcohols, ONIOM(B3LYP/6-31G(d,p):HF/6-31G(d,p)) are recommended. An additional series of primary and secondary alcohols were used as test sets and their results confirmed the validity of approaches. The wave functions for calculating gas phase 13C nuclear magnetic shielding constants of 22 molecules were optimized using 24 factorial design. GIAO and CSGT methods were used for computation of shielding constants. Different optimized wave functions for different types of carbons were recommended. A wave function as the best level of the theory is proposed for almost similar carbons. ONIOM approach for molecules with different types of carbons is applied. The results of GIAO method using optimized wave function are in very good agreement with the experimental values. An additional series (21 carbons) were used as test sets and their results confirmed the validity of the approaches. The wave functions for calculating gas phase 19F chemical shifts were optimally selected using the factorial design. The effects of electron correlation, triple-ξ valance shell, diffuse function, and polarization function on calculated 19F chemical shifts were discussed. Of the four factors, electron correlation and polarization function were the greatest effect on the results and B3LYP/6-31+G(df,p) wave function was proposed as the best and the most efficient level of theory for calculating 19F chemical shifts. An additional series of fluoro compounds were used as the test sets the results of which confirmed the validity of the approaches.

  9. Keywords:
  10. Chemical Shift ; Gauge-Indepent Atomic Orbital (GIAO) ; Continuouse Set of Gauge Transformation ; Our Own N‐layer Integrated Molecular Orbital Molecular Mechanics (ONIOM)Method ; Factorial Design ; Nuclear Magnetic Resonance

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