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

The triphenylmethane and all-meta tert-butyl triphenylmethane dimers, (TPM) and (T  BuPM) , 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  BuPM) 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  BuPM) 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) , it displays an approximately S -symmetric tail-to-tail arrangement.