Microwave spectroscopic and ab initio studies of the hydrogen-bonded trimethylamine–hydrogen sulfide complex

Rotational spectra have been recorded for six isotopomers of the trimethylamine–hydrogen sulfide complex using a Fourier-transform microwave spectrometer. The spectra are characteristic of a symmetric top (B+C)/2=1395.463 (1) MHz, and are indicative of free internal rotation of H2S about the trimethylamine symmetry axis. A structure with a single, linear hydrogen bond, with an S–N distance of 3.36 (5) Å, best reproduces the moments of inertia of six isotopomers, including three distinct deuterated complexes. The experimental structure is compared to an ab initio structure optimized at the MP2/6-31+G(d,p) level which predicts an S–N distance of 3.328 Å. MP2/aug′-cc-pVTZ calculations were used to determine the binding energy of the complex (ΔEe=−5.8 kcal/mol) and the barrier to an internal tunneling motion which exchanges the two H2S hydrogens (3.0 kcal/mol).