Rotational Spectroscopy Probes Lone Pair···π-Hole Interactions in Hexafluorobenzene-Tertiary Alkylamines Complexes.

We employed microwave spectroscopy to investigate the 1:1 complexes of hexafluorobenzene with trimethylamine and quinuclidine, respectively. These complexes exhibit a C 3 v symmetry and are stabilized by nitrogen lone pair···π-hole interactions along the C 3 axes. The N···π-center distances were determined to be 3.110(1) and 3.040(2) Å, respectively, which are shorter than that of hexafluorobenzene-ammonia at 3.2685(3) Å. Additionally, the strength of the intermolecular interaction increases with cluster size. While it was initially expected that the electron-donating effect of alkyl groups was responsible for changing the N···π interaction, the symmetry-adapted perturbation theory analysis revealed that, from hexafluorobenzene-ammonia to both hexafluorobenzene-alkylamines, electrostatic interaction actually decreases while dispersion interaction increases and becomes dominant. Interestingly, dispersion interaction decreases while electrostatic interaction increases from C 6 F 6 -N(CH 3 ) 3 to C 6 F 6 -NC 7 H 13 . The splitting pattern of the spectra indicates hexafluorobenzene rotates freely relative to its partners along the axis of the N···π-hole interactions.