Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the 13C Chemical Shift of the Carboxyl Carbon (δ					CO) of Benzoic Acid

Susanta K. Sen Gupta; Rajendra Prasad

doi:10.1071/CH09210

RESEARCH ARTICLE

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Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the ¹³C Chemical Shift of the Carboxyl Carbon (δ _CO) of Benzoic Acid

Susanta K. Sen Gupta ^A ^B and Rajendra Prasad ^A

+ Author Affiliations

- Author Affiliations

^A Department of Chemistry, Banaras Hindu University, Varanasi-221005, India.

^B Corresponding author. Email: sksg@bhu.ac.in

Australian Journal of Chemistry 63(2) 321-328 https://doi.org/10.1071/CH09210
Submitted: 10 April 2009 Accepted: 26 September 2009 Published: 26 February 2010

Abstract

Studies of substituent effects on NMR chemical shifts are of great benefit in determining fine details of electron distribution in molecules. Interestingly, NMR substituent effects are often different and even opposite to those associated with chemical reactivity. Among molecules exhibiting anomalous (reverse) substituent effects is benzoic acid, the standard model for studying substituent effects. The substituent effect on the ¹³C chemical shift of its carboxyl carbon (δ _CO) is just the opposite of that on its acid strength or reactivity. To develop insights into the origin of the anomalous effect of a substituent on δ _CO, occupancies of natural atomic orbitals at the carboxyl and ring carbons of a set of 10 meta-substituted benzoic acids have been calculated at the density functional theory level using the B3LYP function with split valance 6–311G++** basis set. Statistical correlations obtained for the ¹³C chemical shifts, δ _CO and δ _C-ring of these benzoic acids with the natural atomic orbital occupancies calculated for respective carbon atoms on one hand and with Taft’s inductive and resonance parameters (σ _I and σ _R ^BA ) of the substituents on the other hand have been critically analyzed. The findings have established firmly that a meta-substituent’s anomalous effect on δ _CO is caused by the substituent-induced changes in the total occupancy of only the p _z natural atomic orbitals at the carboxyl carbon. The study has demonstrated further that the transmission of the anomalous effect can be successfully interpreted by a 5.5:–2.5:1 combination of the localized, extended, and resonance-induced π-polarization effects.

Acknowledgements

One of the authors (R.P.) acknowledges financial support in the form of scientist’s pool scheme from CSIR, New Delhi. Thanks to Mr. R. C. P. Bipin for his meticulous assistance in recording ¹³C NMR spectra.

References

Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the 13C Chemical Shift of the Carboxyl Carbon (δ CO) of Benzoic Acid

Abstract

Acknowledgements

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Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the ¹³C Chemical Shift of the Carboxyl Carbon (δ _CO) of Benzoic Acid