Cryogenic ion vibrational predissociation (CIVP) spectroscopy of a gas-phase molecular torsion balance to probe London dispersion forces in large molecules.

We report a gas-phase molecular torsion balance that uses a conformational equilibrium to "weigh" London dispersion against a competing cation-π interaction, for which the readout is the shift in an N-H stretching frequency measured by cryogenic ion vibrational predissociation (CIVP) spectroscopy of electrosprayed pyridinium cations in a Fourier-transform ion cyclotron resonance trap. While frequency calculations with DFT, within the harmonic approximation, assist in the interpretation of the spectra, the observed complex spectrum most likely comes from a Fermi resonance of the N-H stretch with otherwise "dark" overtones of in-plane C-H wagging modes, as argued on the basis of comparison of the spectrum to those for a range of related cations with systematically varied substitution. An equilibrium in favor of the asymmetric conformer would suggest that the dispersion-corrected DFT calculations tested in this work appear to overestimate significantly the stability of the compact conformations favored by London dispersion in the gas phase, which would then pertain to the use of dispersion energy donors in the design of stereoselective reactions.