Knowledge Bank

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 $(CH3)3N\cdots HF$. The rotational spectra of $(CH3)3N\cdots HF\cdots HF$ and five of its isotopically substituted derivatives have been observed by Fourier transform microwave spectroscopy. This follows a previous study by our group on $H3N\cdots HF\cdots HF$, in which the addition of the second HF decreased the $N\cdots H$ hydrogen bond distance in $H3N\cdots HF$ by $0.21(6)\backslash AA$. 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 $1.4\backslash AA$, approximately halfway between the hydrogen bond distance in $H3N\cdots HF$ of $1.7\backslash AA$ and the N-H covalent length of $1.1\backslash AA$. Ab initio calculations concur with experiment, indicating that, as with $H3N\cdots HF\cdots HF$, 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 $(CH3)3N\cdots HF$ and is useful in understanding the role of local environment in promoting proton transfer.