Understanding the Infrared Multiple Photon Dissociation Spectra of Hydrogen‐Tagged Protonated Betaine: Vibrational Confinement Counteracts the Hydrogen Bonding Induced OH 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 2 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 2 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.