THE METHANOL CENTER-DOT TRIMETHYLAMINE COMPLEX - STRUCTURE AND LARGE-AMPLITUDE MOTIONS

Rotational transitions were measured for the methanol . trimethylamine (methanol . TMA) complex and four of its isotopomers, using Fourier t ransform microwave spectroscopy. A partial analysis of the spectrum re vealed that both the TMA subunit and the methyl group in methanol unde rgo hindered internal rotations. The rotational transitions associated with the ground torsional state of TMA and the two lowest A and E sta tes of methanol were assigned and fit. A simplified two-top torsion-ro tation Hamiltonian was used in the analysis. From the fitted parameter s, a nearly linear hydrogen-bonded structure was obtained for the comp lex. The N ... H distance was determined as 1.92 Angstrom. The TMA sym metry axis was found to be tilted 5.5 degrees from the N ... H hydroge n bond. The apparent methyl group torsional barrier height (in methano l) was determined to be 174 +/- 10 cm(-1), from which the OH bending p otential barrier height was estimated to be 1178 +/- 200 cm(-1). The p otential barrier that hinders the TMA internal rotation was estimated as 32 +/- 5 cm(-1). The dipole moment of the complex was mu(T) = 2.87 +/- 0.15 D. Comparison of the hydrogen bond strength of this complex w ith some related methanol complexes was made using hydrogen bond lengt hs and the apparent methyl torsional barrier heights. (C) 1996 Academi c Press, Inc.