THE CONFORMATION AND RING-PUCKERING VIBRATION OF 2-METHYLOXETANE FROMMICROWAVE SPECTROSCOPY AND ABINITIO COMPUTATIONS

The conformation and ring-puckering vibration of 2-methyloxetane have been studied using microwave spectroscopy and ab initio computations. The microwave spectra of the ground and first four excited states of t he ring-puckering vibration have been observed in the frequency range 8-40 GHz and the rotational and quartic centrifugal distortion constan ts have been determined. Vibrational energy separations have been obta ined from relative intensity measurements. The electric dipole moment (in D) of the ground vibrational state has been determined from Stark effect measurements as mu(a) = 0.0187 (5), mu(b) = 1.852(2), mu(c) = 0 .08(3), and mu(t) = 1.854(3). The vibrational energy separations and t he vibrational dependence of the rotational constants suggest that the ring-puckering vibration has an asymmetric single minimum potential f unction and also gave a revised assignment of the far infrared spectru m (J. Mol. Struct. 56 (1979) 157). A combined fit to the rotational co nstants, vibrational energy separations and far infrared vibrational f requencies has been used to determine a reduced potential for the ring puckering vibration. The partial derivatives of the rotational consta nts with respect to the reduced ring-puckering coordinate show that th e equilibrium conformation has the methyl group in the equatorial posi tion. Ab initio computations of the ring-puckering potential function have been made using 6-31G orbitals and full geometry optimization. T hese computations also show the molecule to have an asymmetric single minimum potential function with an equatorial equilibrium conformation .