Rotational Spectrum, Tunneling Motions, and IntramolecularPotential Barriers in Benzyl Mercaptan

The rotational spectrum of benzyl mercaptan (parent and four isotopologues) has been assigned in a supersonic jet expansion using chirped-pulsed Fourier transform microwave spectroscopy. The spectrum is characterized by torsional tunneling doublings, strongly perturbed by Coriolis interactions. The experimental rotational constants reveal that the sulfur atom is located above the ring plane in a gauche conformation. The torsion dihedral θ0 = φ(SCα-C1C2) is approximately 74°, according to a flexible molecular model calculation reproducing the energy separation (ΔE01~2180.4 MHz) between the first two torsional sub-states. The global minimum configuration is four-fold degenerate, corresponding to potential minima with θ0~ ±74° and ± (180-74)°. The four equivalent minima are separated by potential barriers at θ = ± 90°, 0° or 180°. The tunneling splittings are caused by the potential barrier at θ = ± 90°, while the barriers at torsions of 0° or 180° are too large to generate detectable splittings. The tunnell...