Isotopic substitution of a hydrogen bond: A near infrared study of intramolecular vibrations in (DF)(2)

High resolution near infrared spectra of the two high frequency intramolecular modes in (DF)(2) have been characterized using a slit-jet infrared spectrometer. In total, four pairs of vibration-rotation-tunneling (VRT) bands are observed, corresponding to K=0 and K=1 excitation of both the nu(2) (''bound'') and nu(1) (''free'') intramolecular DF stretching modes. Analysis of the rotationally resolved spectra provides vibrational origins, rotational constants, tunneling splittings and upper state predissociation lifetimes for all four states. The rotational constants indicate that the deuterated hydrogen bond contracts and bends upon intramolecular excitation, analogous to what has been observed for (HF)(2). The isotope and K dependence of tunneling splittings for (HF)(2) and (DF)(2) in both intramolecular modes is interpreted in terms of a semiclassical 1-D tunneling model. High resolution line shape measurements reveal vibrational predissociation broadening in (DF)(2): 56(2) and 3(2) MHz for the nu(2) (bound) and nu(1) (free) intramolecular stretching modes, respectively. This 20-fold mode specific enhancement parallels the greater than or equal to 30-fold enhancement observed between analogous intramolecular modes of (HF)(2), further elucidating the role of nonstatistical predissociation dynamics in such hydrogen bonded clusters. (C) 1996 American Institute of Physics.