Gas phase infrared spectra of small polyatomic ions as a benchmark for theory

Infrared multiple photon dissociation (IRMPD) provides a method for examining vibrational spectroscopy of ions in the gas phase. Comparison of such experiments with the IR spectra of neutral molecules shows that the strength of a bond between a trigonal center and an attached methyl group tends to diminish after fluoride attacks to make the central atom tetrahedral. For selected main group species, the change becomes more pronounced as the central atom becomes more electropositive. Trimethylboron displays a more dramatic alteration than do silicon analogs, which indicates an effect beyond that of electronegativity by itself. In comparing gaseous trimethylboron with the negative ions derived from it, CH hyperconjugation ought to operate to a much greater extent in the neutral than in the anions. Anharmonic vibrational calculations using Density Functional Theory (DFT) match the experimental vibrational spectra and confirm that interpretation, providing evidence in favor of substantial hyperconjugative stabilization. However, further simplification of vibrational spectroscopy in terms of “atomic force constants” does not show much promise. Atoms in molecules (AIM) comparison of C–C bond orders in cations with B–C bond orders in neutral boranes is consistent with hyperconjugation.