Microwave Investigation and the Ring Puckering Potential Function of 3-Methylthietan.

Abstract The microwave spectrum of 3-methylthietan was investigated in the frequency range 26.5–40 GHz. The rotational spectra were measured for the ground state, five puckering states (including the lowest state of the second conformer), and two methyl torsion excited states. The conformer with the methyl group in the equatorial orientation (the ground state, v p = 0) is more stable than the axial counterpart ( v p = 1) by 100(17) cm −1 (1.2(2) kJ/mol). The electric dipole moment was determined for both conformers by measuring the Stark effect on several transitions. A V 3 barrier of 1470(30) cm −1 (17.6(4) kJ/mol) was obtained for the internal rotation of the axial methyl group from the A - E doubling of several rotational transitions in the first torsional excited state. Earlier far-infrared assignments were revised on the basis of microwave intensity measurements. The flexible model approach was used to reproduce the spacings between puckering levels simultaneously with the variation of the rotational constants with puckering excitation, allowing some structural relaxation accompanying the ring deformation. Comparisons are made with the results obtained when an asymmetric one-dimensional puckering Hamiltonian is used.