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

Abstract

We employed microwave spectroscopy to investigate the 1:1 complexes of hexafluorobenzene with trimethylamine and quinuclidine, respectively. These complexes exhibit a C 3v 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 C6F6–N(CH3)3 to C6F6–NC7H13. The splitting pattern of the spectra indicates hexafluorobenzene rotates freely relative to its partners along the axis of the N···π-hole interactions.

Structure and dynamics of 3′-aminoacetophenone and 4′-aminoacetophenone from rotational spectroscopy

“…In addition to intramolecular effects, also intermolecular interactions can prevent the amine group inversion: when ammonia-like molecules form a non-covalent bond through their nitrogen lone pair the motion is blocked. This has been clearly observed in several molecular complexes, for instance, the hexafluorobenzene complexes with ammonia 7 and trimethylamine, 8 where the nitrogen lone pair points toward the σ -hole at the centre of the ring. Interestingly, in these species, another large amplitude motion is generated and observed: the free rotation between the monomers, which results in an overall symmetric-top geometry of the complex.…”

A rotational spectroscopy study of microsolvation effects on intramolecular proton transfer in trifluoroacetylacetone–(H2O)1–3

“…Rotational spectroscopy has inherently high resolution and is exceptionally sensitive to molecular mass distribution and geometry, making it capable of providing precise experimental evidence for tautomerism equilibrium. 19–21 This technique has been widely used to characterize the structural and/or dynamic properties of microsolvated clusters. 15,22–24 In this work, the structures and dynamic properties of PT reactions in the TFAA–(H 2 O) 1–3 clusters were investigated using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy 24–27 complemented with theoretical calculations.…”