Stability and aromaticity of tautomers and kinetics of proton transfer in 6-methylpentafulvene and its exo-substituted derivatives: a computational study

Abstract

Thermodynamic and kinetic aspects of the tautomeric equilibrium of 6-methylpentafulvene and its exo-substituted derivatives were investigated at the B3LYP/6-311 ++G(d,p) level of theory for 17 substituents. It was found that in the case of CF3 and COCN derivatives, the most stable form is the tautomer in which the proton migrated from the exo-methyl group to a ring carbon atom. The activation energy for such proton transfer is in the range of 53–61 kcal/mol depending on the substituent, NH2 derivative being an exception with a very high value of 114 kcal/mol. Proton transfers between consecutive ring carbon atoms are associated with much lower activation energies, in general below 30 kcal/mol. The aromaticity of tautomers and transition states of substituted 6-methylpentafulvene was analyzed by employing geometric (HOMA) and electronic (pEDA) indices. It was found that the aromaticity of the nominal form of the molecule is the main driving force behind the stability order of the tautomers. In the case of most substituents, the aromatization of transition states is substantial comparing to the nominal form and is an important factor lowering the activation energy. Solvent effects were modeled by means of PCM and SMD methods.