MICROWAVE AND AB INITIO INVESTIGATION OF $(CH_{3})_{3}N \cdots HF \cdots HF$

Author Institution: Department of Chemistry, University of Minnesota; Lehrgebiet Physikalische Chemie A, Institut f\""{u}r Physikalische Chemie und Elektrochemie, Universt\""{a}t HannoverRotational 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 $(CH_{3})_{3}N \cdots HF$. The rotational spectra of $(CH_{3})_{3}N \cdots HF \cdots HF$ and five of its isotopically substituted derivatives have been observed by Fourier transform microwave spectroscopy. This follows a previous study by our group on $H_{3}N \cdots HF \cdots HF$, in which the addition of the second HF decreased the $N \cdots H$ hydrogen bond distance in $H_{3}N \cdots HF$ by $0.21(6) {\AA}$. The present study investigates the effect of the increased basicity of the amine on the $N \cdots H$ hydrogen bond. We observe a simple asymmetric rotor spectrum with strong a- and b- type transitions, consistent with a ring structure for the $N \cdots HF \cdots HF$ frame. No evidence of internal rotation is observed. Structural analysis is underway and will be discussed, but preliminary analysis indicates an $N \cdots H$ hydrogen bond distance of about $1.4 {\AA}$, approximately halfway between the hydrogen bond distance in $H_{3}N \cdots HF$ of $1.7 {\AA}$ and the N-H covalent length of $1.1 {\AA}$. Ab initio calculations concur with experiment, indicating that, as with $H_{3}N \cdots HF \cdots HF$, the trimethylamine complex forms a ring in which both the $N \cdots H$ 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 $(CH_{3})_{3}N \cdots HF$ and is useful in understanding the role of local environment in promoting proton transfer