Abstract
Significant changes in 13C chemical shifts of benzoic acid (BA) were observed for the carboxyl and quarternary carbon atoms on going from non-polar benzene to solvents with different hydrogen bond donor or acceptor abilities. Shielding effects of 6.145 and 5.413 ppm at the carboxyl carbon in dimethyl sulfoxide (DMSO) and acetone, respectively, are due to the redistribution of electron density upon the formation of new intermolecular hydrogen bonds with solvent molecules. 1H NMR upfield shifts of the carboxyl proton in DMSO and chloroform solutions in comparison with benzene solution, and shifts in the CO stretching frequency, ν CO, observed in infrared spectra of BA complexes are consistent with these results. Structural parameters and interaction energies of the hydrogen-bonded complexes determined by quantum-chemical calculations substantiate the experimental observations. Using the semiempirical PM3 hamiltonian, hydrogen bonding was treated as the donor-acceptor mechanism within the natural bond orbital (NBO) formalism. Two correlations, which reflect hydrogen bonding in different solvents, are proposed: one between the 13C chemical shifts and the change in p-character of the carboxyl carbon atom and the other between the carboxyl CO stretching frequency and the calculated interaction energy of the hydrogen-bonded complexes.
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