Rotational Spectrum, Tunneling Motions, and Intramolecular Potential Barriers in Benzyl Mercaptan

The rotational spectrum of benzyl mercaptan (parent and four isotopologues) has been assigned in a supersonic jet expansion using chirped-pulse 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 substates. The global minimum configuration is 4-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 tunnelling barrier has been determined as 248 cm^-1, similar to the value obtained with high-level MP2 ab initio calculations (259 cm^-1), but smaller than in benzyl alcohol (280 cm^-1).