Microwave measurements and ab initio dynamics of the large amplitude ring puckering motion in 2-sulpholene

Abstract

Microwave measurements were made on the rotational spectrum of 2-sulpholene using a modified Flygare–Balle pulsed beam Fourier transform spectrometer. Analysis and calculations provided information on the large amplitude ring puckering vibration of this system. Twelve and six rotational transitions were measured for the v=0 and v=1 states of the ring puckering vibration, respectively. The transitions for each vibrational state were fitted to a Watson’s A reduced Hamiltonian including terms for quartic distortion yielding for v=0 the values B=2125.96(6), C=1983.28(8), ΔJK=0.664(4), ΔK=−0.34(4) MHz, and for v=1 the values A=3995(26), B=2128.3(1), C=1984.6(1), ΔJK=−0.8(1), ΔK=−32(6) MHz. Subsequently, ab initio calculations were performed at the self-consistent-field (SCF)/3-21G*, MP2/6-31G*, and MP4/6-31G* levels of theory to determine the barrier to inversion. The MP4/6-31G* barrier was ΔE=116 cm−1, and can be considered to be the most accurate barrier value calculated in this study. An ab initio potential energy curve was calculated at the SCF/3-21G* level in terms of a single parameter (ω) describing the large amplitude motion of the ring puckering. Vibration-coordinate dependence of the effective reduced mass associated with this large amplitude motion and the resultant kinetic energy expression was determined. The solutions of a one-dimensional Schrödinger equation solved within this double well potential yield a separation between the v=0 and v=1 large amplitude motion vibrational states of 8 cm−1 when the effective reduced mass was assumed constant, and a separation of 9 cm−1 when the effective reduced mass was expressed as a function of the ω coordinate. The v=0 and v=1 eigenfunctions for the SCF ring puckering potential were found to give vibrationally averaged rotational constants in good agreement with those obtained from the microwave spectrum.