Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF: Experiment and theory

The rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate $\text{(0.1\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}})$ and high resolution $\text{(0.0024\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}}$ full bandwidth, half-maximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at $\text{200\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}}$ to the near infrared $\text{(12\hspace{0.05em}000\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}})$ covering all the fundamentals and CD stretching overtones up to polyad $\text{N=5.}$ The spectra are completely analyzed in terms of their vibrational assignments to fundamentals, combinations and overtones. At high excitation the analysis reveals the dominant anharmonic coupling between four high frequency vibrational modes; the CD stretching ${\mathrm{(\nu }}_1\mathrm{),}$ two CD bending ${\mathrm{(\nu }}_2{\mathrm{,\nu }}_3\mathrm{),}$ and the CF stretching mode ${\mathrm{(\nu }}_4\mathrm{).}$ The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom. We also present vibrational variational calculations on a grid in a four-dimensional normal coordinate subspace. The potential energy and the dipole moment function are calculated ab initio on this grid using self-consistent field second order Møller–Plesset perturbation theory (MP2). Experimental and theoretical results for band positions and integrated intensities as well as effective spectroscopic parameters are found to be in good agreement. The important anharmonic coupling between the CD chromophore and the CF stretching vibration can be described by an effective cubic Fermi resonance coupling constant $k_{\mathrm{sff}}^{\prime }\text{≈(50±10)\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ which leads to intramolecular vibrational redistribution between the CD and CF chromophores on the femtosecond time scale. Time dependent intramolecular vibrational redistribution processes in CDBrClF are derived in various representations, including time dependent probability densities (“wave packets”) in coordinate space and finally time dependent entropy.