AB-INITIO STUDY OF THE METHYLSULFONATE AND PHENYLSULFONATE ANIONS

Owing to the importance of the sulfonate group in organic and pharmace utical chemistry a theoretical study at an ab initio level has been ca rried out for this ionic group, The geometric, electronic and energy p roperties of the methyl- and phenyl-sulfonate anions have been calcula ted by optimizing with the Hartree-Fock (HF) 6-31G and 6-31+G* basis sets and including correlation effects at the second-order Moller-Ples set (MP2) level. In addition, the harmonic vibrational frequencies and the zero-point vibrational energy of the different conformers of thes e compounds have been evaluated, The results show that the inclusion o f diffuse functions does not affect the geometrical description of the sulfonate group. However, both optimizations provide a flatter pyrami dal disposition for this hypervalent sulfur atom when compared with th e experimental values. It has been found that the inclusion of correla tion effects seems to be necessary for a good electronic description o f these anions, In the case of methylsulfonate a minimum and a transit ion structure were located and the rotational barrier evaluated. Pheny lsulfonate has a flatter rotational profile and only two structures we re 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 phenylsu lfonate anion is suggested.