Rotational Spectroscopy Probes Lone Pair???-Hole Interactions in Hexafluorobenzene-Tertiary Alkylamines Complexes

We employed microwave spectroscopy to investigate the1:1 complexesof hexafluorobenzene with trimethylamine and quinuclidine, respectively.These complexes exhibit a C (3v ) symmetry and are stabilized by nitrogen lone pair center dot center dot center dot pi-holeinteractions along the C (3) axes. The N center dot center dot center dot pi-centerdistances were determined to be 3.110(1) and 3.040(2) angstrom, respectively,which are shorter than that of hexafluorobenzene-ammonia at 3.2685(3)angstrom. Additionally, the strength of the intermolecular interactionincreases with cluster size. While it was initially expected thatthe electron-donating effect of alkyl groups was responsible for changingthe N center dot center dot center dot pi interaction, the symmetry-adapted perturbationtheory analysis revealed that, from hexafluorobenzene-ammonia to bothhexafluorobenzene-alkylamines, electrostatic interaction actuallydecreases while dispersion interaction increases and becomes dominant.Interestingly, dispersion interaction decreases while electrostaticinteraction increases from C6F6-N(CH3)(3) to C6F6-NC7H13. The splitting pattern of the spectra indicateshexafluorobenzene rotates freely relative to its partners along theaxis of the N center dot center dot center dot pi-hole interactions.