Infrared Multiphoton Dissociation Spectroscopy of Protonated 1,2-Diaminoethane-water Clusters: Vibrational Assignment via the MP2 Method

摘要

Infrared multiphoton dissociation (IRMPD) spectra of various protonated 1,2-diaminoethane-water clusters DAE-H~+-(H2O)_n (n = 1-6) were measured in the wavelength range of 3 000-3 800 cm~(-1). The IRMPD spectra of the well separated ionic clusters were simulated by the MP2 method employing various basis sets. Comparison of the IRMPD spectra with the theory indicates that each cluster may exist as several low-lying conformers, and the sum spectra of the various conformers reveal almost one to one correspondence between theory and experiment. Free N-H and O-H stretches are observed in the ranges of 3400-3500 and 3600-3800 cm~(-1), respectively. The O-H···N and N-H···O stretches are, however, observed in the broad region of 3000-3600 cm~(-1). The theoretical calculations on DAE-H~+-(H2O)_n (n = 1-4) show gradual decrease of the average binding energy between DAE-H~+ and H2O as the cluster size increases, attaining the lowest value of 55 kJ/mol when n = 4. We found a low energy barrier of 21 kJ/mol to the isomerization converting the lowest energy cluster of DAE-H~+-H2O to the second lowest one.