The sulfonation of pyrrole: Choice of relevant computational procedure for quantum chemical analysis of positional orientation

The influence of the solvent effect, zero-point energy correction, and electron correlation on conclusions about positional orientation of pyrrole sulfonation was studied by the B3LYP, HF, and MP2 methods with the 6-31G(d) and 6-31+G(d) basis sets. The most important role is played by the solvation effects. Basis set extension effects and the inclusion of zero-point energy correction contribute insignificantly. According to calculations using all the methods listed above, the formation of β-pyrrolesulfonic acid in polar solvent is thermodynamically more favorable and less kinetically hindered. B3LYP calculations seem to be more appropriate, because the method allows experimental data on positional orientation of the sulfonation of pyrroles to be interpreted in a simpler and more reliable fashion. Dedicated to Academician V. A. Tartakovsky on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1425–1430, August, 2007.