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

The rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate (0.1 cm(-1)) and high resolution (0.0024 cm(-1) f ull bandwidth, half-maximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at 200 cm(-1) to the near i nfrared (12 000 cm(-1)) covering all the fundamentals and CD stretching ove rtones up to polyad 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 be tween four high frequency vibrational modes; the CD stretching (nu(1)), two CD bending (nu(2),nu(3)), and the CF stretching mode (nu(4)). The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom. We also present vibrational variational ca lculations 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 Moller-Plesset perturba tion theory (MP2). Experimental and theoretical results for band positions and integrated intensities as well as effective spectroscopic parameters ar e found to be in good agreement. The important anharmonic coupling between the CD chromophore and the CF stretching vibration can be described by an e ffective cubic Fermi resonance coupling constant k(sff)' approximate to (50 +/- 10) cm(-1), which leads to intramolecular vibrational redistribution b etween the CD and CF chromophores on the femtosecond time scale. Time depen dent intramolecular vibrational redistribution processes in CDBrClF are der ived in various representations, including time dependent probability densi ties ("wave packets") in coordinate space and finally time dependent entrop y. (C) 2000 American Institute of Physics. [S0021-9606(00)02008-0].