C. S. BRAUER, M. B. CRADDOCK, S. W. HUNT AND K. R. LEOPOLD, Department of Chemistry, University of Minnesota, Minneapolis, MN 55455; JENS UWE-GRABOW, Lehrgebiet Physikalische Chemie A, Institut für Physikalische Chemie und Elektrochemie, Universtät Hannover, Callinstr. 3-3a, D-30167 Hannover.
Rotational spectroscopy and ab initio calculations have been used to examine the effect of a single HF solvent molecule on the gas phase proton transfer in the hydrogen bonded complex (CH3)3N HF. The rotational spectra of (CH3)3N HF HF and five of its isotopically substituted derivatives have been observed by Fourier transform microwave spectroscopy. This follows a previous study by our group on H3N HF HF, in which the addition of the second HF decreased the N H hydrogen bond distance in H3N HF by 0.21(6) Å. The present study investigates the effect of the increased basicity of the amine on the N H hydrogen bond. We observe a simple asymmetric rotor spectrum with strong a- and b- type transitions, consistent with a ring structure for the N HF HF frame. No evidence of internal rotation is observed. Structural analysis is underway and will be discussed, but preliminary analysis indicates an N H hydrogen bond distance of about 1.4 Å, approximately halfway between the hydrogen bond distance in H3N HF of 1.7 Å and the N-H covalent length of 1.1 Å. Ab initio calculations concur with experiment, indicating that, as with H3N HF HF, the trimethylamine complex forms a ring in which both the N H hydrogen bond and the HFH angle are significantly perturbed. In addition, two of the methyl groups of the trimethylamine assume an eclipsed conformation, apparently participating in a bifurcated interaction with the fluorine atom of the second HF. This complex provides the first step in microsolvation of (CH3)3N HF and is useful in understanding the role of local environment in promoting proton transfer.