MICROWAVE SPECTRA OF $HCN-SO_{3}$ AND $CH_{3}CN-SO_{3}$

Abstract

The complexes of HCN and $CH_{3}CN$ with the strong Lewis acid $SO_{3}$ have been studied by pulsed-nozzle Fourier transform microwave spectroscopy. Both have the expected symmetric top structure with the nitrogen lone pair directed toward the sulfur of the $SO_{3}$. In $HCN-SO_{3}$, the N-S bond distance is $2.58 {\AA}$, and the NSO angle is $92^{\circ}$. In the $CH_{3}CN-SO_{3}$, the N-S distance is $2.4 {\AA}$ and the NSO angle is $95^{\circ}$. The shorter bond length and wider NSO angle in the $CH_{3}CN$ adduct indicate that the dative bond is significantly more developed than that in the HCN complex, in good accord with chemical intuition. Nonetheless, it is remarkable that the N-S distances differ by only $0.2 {\AA}$, since B-N bond distances in $HCN-BF_{3}$ and $CH_{3}CN-BF_{3}$ differ by $0.46 {\AA}$. The nitrogen quadrupole coupling constant in $HCN-SO_{3}$ is similar to that in most nitrogen-bound van der Waals complexes of HCN, giving an apparent average excursion angle of $18.8^{\circ}$ away from the intermolecular axis. However, the deuterium quadrupole coupling constant in $DCN-SO_{3}$ is reduced by only a few kHz from that of free DCN, indicating an excursion angle of only about $6^{\circ}-9^{\circ}$ and suggesting a much more rigid structure. Thus, it appears that the similarity of the nitrogen quadrupole coupling constant to that of a weakly bound system results from a fortuitous cancellation of electronic and projective contributions. Further work is in progress on the $CH_{3}CN$ complex, and a comparison of the bonding in the two systems will be presented.