Understanding the Infrared Multiple Photon Dissociation Spectra of Hydrogen-Tagged Protonated Betaine: Vibrational Confinement Counteracts the Hydrogen Bonding Induced OH Stretching Frequency Downshift.

Finite-temperature vibrational spectra of protonated betaine and its noncovalently bonded clusters with molecular hydrogen are modeled using Lagrangian dynamics with the atom-centered density matrix propagation (ADMP) scheme. The focus is put on the OH stretching mode, which serves as a primary indicator of the type and strength of the noncovalent intermolecular interactions. The computed anharmonic OH stretching vibrational frequency shifts in the case of protonated betaine upon tagging with H at the OH group site at 40 K are in quantitative agreement with the experimental infrared multiple photon dissociation data. The shifts computed from simulations at 4 K contain only the harmonic contributions. It is found that this is a consequence of vibrational confinement of the OH oscillator caused by the H tagger, which remains close to the vibrating atoms throughout the simulation and counteracts the frequency redshift induced by the weak hydrogen bonding interaction. Changes in the OH stretching potential, along with a small but observable confinement relaxation at 40 K leads to OH stretching frequency downshift as compared to 4 K. Application of the two-trace 2D correlation analysis of the computed vibrational density of states spectra enables a clear distinction between bands of different origin to be made.