Temperature Dependence of Non-Condon Effects in Two- Dimensional Vibrational Spectroscopy of Water

Non-Condon effects in vibrational spectroscopy refers to the dependence of a molecule's vibrational transition dipole and polarizability on the coordinates of the surrounding environment. Earlier studies have shown that such effects can be pronounced for hydrogen-bonded systems like liquid water. Here, we present a theoretical study of two-dimensional vibrational spectroscopy under the non-Condon and Condon approximations at varying temperatures. We have performed calculations of both two-dimensional infrared and two-dimensional vibrational Raman spectra to gain insights into the temperature dependence of non -Condon effects in nonlinear vibrational spectroscopy. The two-dimensional spectra are calculated for the OH vibration of interest in the isotopic dilution limit where the coupling between the oscillators is ignored. Generally, both the infrared and Raman line shapes undergo red shifts with decrease in temperature due to strengthening of hydrogen bonds and decrease in the fraction of OH modes with weaker or no hydrogen bonds. The infrared line shape is further red-shifted under the non-Condon effects at a given temperature, while the Raman line shape does not show any such red shift due to non-Condon effects. The spectral dynamics becomes slower on decrease of temperature due to slower hydrogen bond relaxation and, for a given temperature, the spectral diffusion occurs at a faster rate upon inclusion of non-Condon effects. The time scales of spectral diffusion extracted from different metrics agree well with each other and also with experiments. The changes in the spectrum due to non-Condon effects are found to be more significant at lower temperatures.

Automated summary

Non-Condon effects in vibrational spectroscopy refer to the dependence of a molecule's vibrational transition dipole and polarizability on the coordinates of the surrounding environment. In this study, we examined the temperature dependence of these effects using two-dimensional vibrational spectroscopy. Our calculations of two-dimensional infrared and Raman spectra showed that the line shapes of both spectra undergo red shifts with decreasing temperature. Additionally, we found that the non-Condon effects further red shift the infrared line shape, while the Raman line shape remains unaffected. The spectral dynamics also become slower at lower temperatures, and the inclusion of non-Condon effects leads to faster spectral diffusion.