The VCD spectrum of 2-methylthietane: measured (800–1500 cm−1) and computed with semi-empirical models

Abstract

The vibrational circular dichroism (VCD) spectrum (800 to 1500 cm−1) is reported for 2-methylthietane (2MT). Fixed partial charge (FPC), atomic polar tensor (APT), and charge flow (CF) methods are used to compute the VCD spectra for both stable conformers (CH3 in the axial or equatorial orientation) for the complete range of fundamental vibrations, based on 3-21G and 6-31G* harmonic force fields. The experimental and computed absorption and VCD spectra of R-(+)-2MT are compared to the previously reported spectra of R-(−)-2-methyloxetane. The success of the APT and CF models for reproducing the dominant ring stretching IR and VCD features of that compound is rationalized by a comparison of APT and FPC intensity expressions. Further comparison of the APT and FPC calculated electric dipole moment derivatives are interpreted to suggest values for the diagonal CS and CH stretching charge flow terms for 2MT. These investigations then provide a rationale for previously reported successes of the FPC model, and permit identification of specific vibrational modes of 2MT that are amenable to FPC-VCD modeling. The previously established conformational mixture of ~30% axial and 70% equatorial is shown to be in accord with FPC-VCD predictions for the most appropriate (β-CH2 wagging) mode. Key words: vibrational circular dichroism, 2-methylthietane, fixed partial charge, atomic polar tensor, charge flow