Intermolecular vibrations of (CH2)2O–HF and –DF hydrogen bonded complexes investigated by Fourier transform infrared spectroscopy and ab initio calculations

Abstract

A series of Fourier transform infrared spectra (FTIR) of the hydrogen bonded complexes (CH₂)₂O–HF and –DF have been recorded in the 50–750 cm⁻¹ range up to 0.1 cm⁻¹ resolution in a static cell maintained at near room temperature. The direct observation of three intermolecular transitions enabled us to perform band contour analysis of congested cell spectra and to determine reliable rovibrational parameters such as intermolecular frequencies, rovibrational and anharmonic coupling constants involving two l₁ and l₂ librations and one σ stretching intermolecular motion. Inter-inter anharmonic couplings could be identified between ν_l1, ν_l2, ν_σ and the two lowest frequency bending modes. The positive sign of coupling constants (opposite with respect to acid stretching intra-inter ones) reveals a weakening of the hydrogen bond upon intermolecular excitation. The four rovibrational parameters ν_σ and x_σj (j = σ, δ₁, δ₂) derived in the present far-infrared study and also in a previous mid-infrared one [Phys. Chem. Chem. Phys. 2005, 1, 592] make deviations appear smaller than 1% for frequencies and 12% for coupling constants which gives confidence to the reliability of the data obtained. Anharmonic frequencies obtained at the MP2 level with Aug-cc-pvTZ basis set agree well with experimental values over a large set of frequencies and coupling constants. An estimated anharmonic corrected value of the dissociation energy D^CP₀ for both oxirane–HF (2424 cm⁻¹) and –DF (2566 cm⁻¹) has been derived using a level of theory as high as CCSD(T)/Aug-cc-pvQZ, refining the harmonic value previously calculated for oxirane–HF with the MP2 method and a smaller basis set. Finally, contrary to short predissociation lifetimes evidenced for acid stretching excited states, any homogeneous broadening related to vibrational dynamics of (CH₂)₂O–HF and –DF has been observed within the three highest frequency intermolecular states, as expected with low excitation energies largely below the dissociation limit as well as a negligible IVR contribution.