Origin of threefold methyl torsional potential in methylindoles

In this article, we present a systematic study on mono-methylindoles to investigate the electronic origin of the threefold symmetric component (V ₃) of the methyl torsional potential barrier in the ground electronic state (S₀). The structures and the torsional potential parameters of these molecules were evaluated from ab initio calculation using Hartree-Fock (HF), second order Mollar Plesset perturbation (MP2) and B3LYP density functional level of theories and Gaussian type basis set 6-31G(d, p). Natural bond orbital (NBO) analysis of these molecules were carried out using B3LYP/6-31G(d, p) level of calculation to understand the formation of the threefold V₃ term arising from the changes of various non-covalent interactions during methyl rotation. Our analysis reveals that the contributions from π orbitals play a dominant role in the barrier height determination in this class of molecules. The threefold term in the barrier arises purely from the interactions non-local to the methyl group in case when the methyl group has two single bonds vicinal to it. On the other hand, it is the local interaction that determines the potential energy barrier when the methyl group has one single bond and one double bond vicinal to it. However, in all these cases, the magnitude of the energy barrier depends on the resonance structure formation in the benzene ring frame upon rotation of the methyl group and, therefore, the energetics of the barrier cannot be understood without considering the molecular flexing during methyl rotation.