Dispersion‐Bound Isolated Dimers in the Gas Phase: Observation of the Shortest Intermolecular CH⋅⋅⋅H−C Distance via Stimulated Raman Spectroscopy

Abstract

The triphenylmethane and all-meta tert-butyl triphenylmethane dimers, (TPM)₂ and (T ^t BuPM)₂ , respectively, were studied with ionization loss stimulated Raman spectroscopy in molecular beam experiments to resolve structure sensitive vibrations. This answers the question whether the recently reported linear head-to-head arrangement in (T ^t BuPM)₂ results from crystal packing or prevails also in the gas phase, and therefore must result from extraordinarily strong London dispersion (LD) interactions. Our study clearly demonstrates that the head-to-head arrangement is maintained even under isolated molecular beam conditions in the absence of crystal packing effects. The central Raman-active aliphatic C-D vibration of appropriately deuterated (T ^t BuPM)₂ associated with an unusually short C-D⋅⋅⋅D-C distance exhibits a strong blue-shift compared to the undisturbed case. As the LD stabilizing tert-butyl groups are absent in (TPM)₂ , it displays an approximately S₆ -symmetric tail-to-tail arrangement.