The Methanol·Trimethylamine Complex: Structure and Large Amplitude Motions

Abstract

Rotational transitions were measured for the methanol·trimethylamine (methanol·TMA) complex and four of its isotopomers, using Fourier transform microwave spectroscopy. A partial analysis of the spectrum revealed that both the TMA subunit and the methyl group in methanol undergo hindered internal rotations. The rotational transitions associated with the ground torsional state of TMA and the two lowestAandEstates of methanol were assigned and fit. A simplified two-top torsion–rotation Hamiltonian was used in the analysis. From the fitted parameters, a nearly linear hydrogen-bonded structure was obtained for the complex. The N···H distance was determined as 1.92 Å. The TMA symmetry axis was found to be tilted 5.5° from the N···H hydrogen bond. The apparent methyl group torsional barrier height (in methanol) was determined to be 174 ± 10 cm−1, from which the OH bending potential barrier height was estimated to be 1178 ± 200 cm−1. The potential barrier that hinders the TMA internal rotation was estimated as 32 ± 5 cm−1. The dipole moment of the complex was μT= 2.87 ± 0.15D. Comparison of the hydrogen bond strength of this complex with some related methanol complexes was made using hydrogen bond lengths and the apparent methyl torsional barrier heights.