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  Home > JCE Print > Journal of Chemical Education > Issues > 2002  > May  >
In the Classroom
Fun with Computational Chemistry: Solving Spectral Problems Using Computed 13C NMR Chemical Shifts. A Comparison of Empirical and Quantum Mechanical Methods

Leon J. Tilley and Shawn J. Prevoir
Department of Chemistry, Stonehill College, Easton, MA 02357

David A. Forsyth
Department of Chemistry, Northeastern University, Boston, MA 02115
Cover
May 2002
Vol. 79 No. 5
p. 593
Abstract
The viability of using 13C NMR shift prediction as an aid to solving undergraduate spectral problems was tested. The chemical shifts of structures corresponding to four typical problems were calculated on a PC using two approaches: (i) a quantum mechanical method for calculating carbon shieldings and (ii) empirically based shift predictions. The quantum mechanical approach used GIAO B3LYP/3-21G (X, 6-31+G*) calculations of isotropic shieldings based on molecular mechanics geometries (MM3). Four commercial programs were used for the empirical predictions. The average shift differences between the predicted and known values were compiled and used to determine the correct structure. Results indicated that the quantum mechanical method could be used to identify the correct structure in all cases. The empirically based shift prediction software was much less effective. In particular, the important issue of stereochemistry is ignored in most of the empirically based predictions.

The process of computing chemical shifts is fun and relatively easy to follow. While primarily serving as an aid in solving problems, it can also reinforce visualization and molecular modeling skills and pique student interest in computational and other branches of chemistry.

Supplement
The complete data for problems B-D (Tables 2-4) are available.
*  Contents Folder (JCE2002p593W) containing three (3) Microsoft Excel documents (JCE2002p593W_B.xls, JCE2002p593W_C.xls, JCE2002p593W_D.xls).
*  Download
JCE2002p0593W.zip

JCE2002p0593W.sit
More Information
*  Citation
 Tilley, Leon J.; Prevoir, Shawn J.; Forsyth, David A. J. Chem. Educ. 2002 79 593.
*  Keywords
 CER Problem Solving; Computational Chemistry; Molecular Modeling / Dynamics; NMR Spectrometry; Organic Chemistry; Theoretical Chemistry
*  History
 Created:
Last Updated:		 April 1, 2002
March 16, 2005
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