Electrical anharmonicity and dampings contributions to Cl-[formula omitted] stretching band in gaseous (CH3)2O…HCl complex: Quantum dynamic study and prediction of the temperature effects

Abstract

In a previous work (Rekik et al., 2017), we demonstrated the ability of a simple anharmonic model of the dipole moment function of the X-H stretching band to explain a set of spectroscopic features of hydrogen bonding formation. Within the context of this model, we have shown that the origins of the broadening of the X-H→ stretching band is attributed to large terms in the expansion of the autocorrelation functions due to the electrical anharmonicity. However, the question remained as to the ability of this model to treat the more complex situation in which we take into account the relaxation mechanisms that look at the effect of the surroundings and thereby gives rise to signatures of the medium to the X-H→ stretching band lineshapes. Thus, in the present study, we investigated this situation by envisaging that the direct relaxation mechanism is due to the coupling between the fluctuating local electric field and the dipole moment of the complex as rationalized by Rosh and Ratner and the indirect damping resulting from the interaction of the X-H→ stretch with its environment via the H-bond bridge mode. Theoretical experiments show that mixing of all these effects results in a speard and complicated structure. Using an ensemble of physically sound parameters as input into this approach, we have captured the main features in the experimental Cl-H→ band in gaseous (CH3)2O…HCl complex and shown that the direct relaxation entrains a broadening of the spectra and is capable of qualitatively capturing the main features in the experimental spectra and quantitatively capturing the characteristic time scale of the vibrational dynamics of the Cl-H→ stretching band. Furthermore, due to the decent agreement obtained between the theoretical and experimental line shapes at 226K, the evolution of the IR spectra with the varaiation of temperature is proposed. The findings gained herein underscore the utility of combining simultaneously the effects of the electrical anharmonicity and the damping mechanisms for efficiently modeling the IR spectra of H-bonded systems and predicting its evolution with the temperature.