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

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=983.28(8), Δ_JK=0.664(4), Δ_K=-0.34(4) MHz, and for v=1 the values A=3995(26), B=2128.3(1), C=1984.6(1), Δ_JK=-0.8(1), Δ_K=- 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.