Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis

Mobli, Mehdi and Abraham, Raymond J. (2005) Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis. Journal of Computational Chemistry, 26 4: 389-398. doi:10.1002/jcc.20177


Author Mobli, Mehdi
Abraham, Raymond J.
Title Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis
Journal name Journal of Computational Chemistry   Check publisher's open access policy
ISSN 1096-987X
0192-8651
Publication date 2005-03-01
Sub-type Article (original research)
DOI 10.1002/jcc.20177
Open Access Status Not Open Access
Volume 26
Issue 4
Start page 389
End page 398
Total pages 10
Place of publication New York
Publisher Wiley
Language eng
Subject 030606 Structural Chemistry and Spectroscopy
Abstract A model based on classical concepts is derived to describe the effect of the nitro group on proton chemical shifts. The calculated chemical shifts are then compared to ab initio (GIAO) calculated chemical shifts. The accuracy of the two models is assessed using proton chemical shifts of a set of rigid organic nitro compounds that are fully assigned in CDCl3 at 700 MHz. The two methods are then used to evaluate the accuracy of different popular post-SCF methods (B3LYP and MP2) and molecular mechanics methods (MMX and MMFF94) in calculating the molecular structure of a set of sterically crowded nitro aromatic compounds. Both models perform well on the rigid molecules used as a test set, although when using the GIAO method a general overestimation of the deshielding of protons near the nitro group is observed. The analysis of the sterically crowded molecules shows that the very popular B3LYP/6-31G(d,p) method produces very poor twist angles for these, and that using a larger basis set [6-311++G(2d,p)] gives much more reasonable results. The MP2 calculations, on the other hand, overestimate the twist angles, which for these compounds compensates for the deshielding effect generally observed for protons near electronegative atoms when using the GIAO method at the B3LYP/6-311++G(2d,p) level. The most accurate results are found when the structures are calculated using B3LYP/6-311++G(2d,p) level of theory, and the chemical shifts are calculated using the CHARGE program based on classical models. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 389-398, 2005
Formatted abstract
A model based on classical concepts is derived to describe the effect of the nitro group on proton chemical shifts. The calculated chemical shifts are then compared to ab initio (GIAO) calculated chemical shifts. The accuracy of the two models is assessed using proton chemical shifts of a set of rigid organic nitro compounds that are fully assigned in CDCl3 at 700 MHz. The two methods are then used to evaluate the accuracy of different popular post-SCF methods (B3LYP and MP2) and molecular mechanics methods (MMX and MMFF94) in calculating the molecular structure of a set of sterically crowded nitro aromatic compounds. Both models perform well on the rigid molecules used as a test set, although when using the GIAO method a general overestimation of the deshielding of protons near the nitro group is observed. The analysis of the sterically crowded molecules shows that the very popular B3LYP/6-31G(d,p) method produces very poor twist angles for these, and that using a larger basis set [6-311++G(2d,p)] gives much more reasonable results. The MP2 calculations, on the other hand, overestimate the twist angles, which for these compounds compensates for the deshielding effect generally observed for protons near electronegative atoms when using the GIAO method at the B3LYP/6-311++G(2d,p) level. The most accurate results are found when the structures are calculated using B3LYP/6-311++G(2d,p) level of theory, and the chemical shifts are calculated using the CHARGE program based on classical models. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 389-398, 2005
Keyword Proton chemical shifts
Nitro compounds
Charge
GIAO
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collections: Excellence in Research Australia (ERA) - Collection
Institute for Molecular Bioscience - Publications
 
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Created: Wed, 11 Feb 2009, 01:33:33 EST by Joanne Mellor on behalf of Institute for Molecular Bioscience