Vibrational dynamics of the hydrogen bonded complexes (CH2)2O–HF and –DF investigated by combined jet- and cell-Fourier transform infrared spectroscopy

Abstract

Fourier transform infrared spectra of the ν_s stretching bands of HF and DF bonded to (CH₂)₂O have been recorded at 0.5 cm⁻¹ resolution in a cooled cell and in a supersonic expansion seeded with argon. The analysis of the congested spectra of this type of medium strength hydrogen bonded complexes exploits a combination of controlled temperature effects in the ranges 25–80 K and 200–300 K and a band contour simulation program accounting for homogeneous and inhomogeneous contributions. Significant anharmonic couplings between the donor stretch mode and three of the low frequency intermolecular modes are found to be responsible for the characteristic hot band patterns in the ν_s fundamental region of cell spectra. A global analysis of sum and difference combination bands involving ν_s provides reliable values of intermolecular frequencies, anharmonic coupling constants and a good estimate of the dissociation energy of the complex which compares favorably with ab initio results. The effective linewidth provides a lower bound for the predissociation lifetime of 1.5 ps for HF and 7 ps for DF containing complexes, respectively. The correlation between effective linewidths and vibrational densities of states for (CH₂)₂O–HF and –DF underlines the important role of intramolecular vibrational redistribution in the vibrational dynamics of these complexes while the lifetime decrease for HF (or DF) bonded to oxygenated cyclic ethers with respect to sulfured homologues might be explained by the change in the arrangement of the acid relative to the plane of the acceptor subunit.