UNDERSTANDING THE IRMPD SPECTRA OF HYDROGEN-TAGGED PROTONATED BETAINE: VIBRATIONAL CONFINEMENT COUNTERACTS THE HYDROGEN BONDING INDUCED O-H STRETCHING FREQUENCY DOWNSHIFT.

Abstract

Finite - temperature vibrational spectra of protonated betaine and its noncovalently bonded clusters with molecular hydrogen were modeled using Lagrangian dynamics with the atom-centered density matrix propagation scheme (ADMP). The focus was 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 H2 at the OH group site at 40 K is in quantitative agreement with the experimental IRMPD data. The shifts computed from simulations at 4 K contain only the harmonic contributions. We have found out that this is a consequence of vibrational confinement of the O-H oscillator caused by the H2 tagger, which remains close to the vibrating atoms throughout the simulation and counteracts the frequency red shift induced by the weak hydrogen bonding interaction. Confinement relaxation at 40 K enables the OH oscillator to span a wider range of the vibrational potential curve, sampling significantly the more anharmonic parts thereof. Application of the two-trace two-dimensional correlation analysis of the computed vibrational density of states spectra enables a clear distinction between bands of different origin to be made.