Ab initio study of the methylsulfonate and phenylsulfonate anions

Abstract

Owing to the importance of the sulfonate group in organic and pharmaceutical chemistry a theoretical study at an ab initio level has been carried out for this ionic group. The geometric, electronic and energy properties of the methyl- and phenyl-sulfonate anions have been calculated by optimizing with the Hartree–Fock (HF) 6-31G* and 6-31 + G* basis sets and including correlation effects at the second-order Moller–Plesset (MP2) level. In addition, the harmonic vibrational frequencies and the zero-point vibrational energy of the different conformers of these compounds have been evaluated. The results show that the inclusion of diffuse functions does not affect the geometrical description of the sulfonate group. However, both optimizations provide a flatter pyramidal disposition for this hypervalent sulfur atom when compared with the experimental values. It has been found that the inclusion of correlation effects seems to be necessary for a good electronic description of these anions. In the case of methylsulfonate a minimum and a transition structure were located and the rotational barrier evaluated. Phenylsulfonate has a flatter rotational profile and only two structures were fully optimized and characterized. Using the assumption that there is a relation between the population of conformers in the crystalline phase and that in the gas phase, a rotational barrier for the phenylsulfonate anion is suggested.