Vibrational dynamics of medium strength hydrogen bonds: Fourier transform infrared spectra and band contour analysis of the DF stretching region of (CH2)2S-DF.

Fourier transform infrared spectra of the nu(s) band of the (CH2)(2)S-DF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell and in a supersonic jet expansion seeded with argon. A sufficient density of (CH(2))(2)S-DF heterodimers is produced by a double injection nozzle device, which limits the possibility of reaction between thiirane and DF before the expansion. The observation of partially resolved PQR branch structures at cell temperatures as high as 252 K indicates relatively small effective line widths, which allow a detailed analysis of the underlying vibrational couplings and of the structural properties of the complex. The analysis of cell and free jet spectra in the temperature range 50-250 K is performed with a software package for the simulation and fitting of multiple hot band progressions in asymmetric rotors. The analysis reveals that the three low frequency hydrogen-bond modes are strongly coupled to the DF stretch with anharmonic coupling constants, which indicates a strengthening of the hydrogen bond upon vibrational excitation of DF. Rovibrational parameters and a reliable upper bound for the homogeneous line width have been extracted.