Study of the ν1 band shape of the H2O⋯HF, H2O⋯DF, and H2O⋯HCl complexes in the gas phase

Abstract

The band shape of the ν₁ hydrogen fluoride stretch in H₂O⋯HF and H₂O⋯DF complexes was studied in the gas phase. The spectra of H₂O/HF mixtures at 293 K in cells 20 and 1200 cm long were recorded in the range 4200–3000 cm⁻¹ at a resolution of 0.2−0.02 cm⁻¹. The spectra of the 1 : 1 complex in the region of the ν₁(HF) absorption band were obtained by subtracting the calculated spectra of free H₂O and HF molecules from the experimental spectra. The asymmetric ν₁ band of H₂O⋯HF has a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. The ν₁ band shape was reconstructed nonempirically as a superposition of rovibrational bands of the ν₁(HF) fundamental transition and hot transitions from excited states of low-frequency modes. The reconstruction was based on an ab initio calculation of the potential energy and dipole moment surfaces and subsequent variational multidimensional anharmonic calculations of the vibrational energy levels, the frequencies and intensities of the transitions considered, and the rotational constants. The calculated spectrum reproduces the structure of the experimental spectrum, in particular, the relative intensities of the peaks. However, the assignment of spectral features differs from that generally accepted. The central, most intense, peak is associated with the transition from the ground state, while the lowest-frequency peak with the P branch head of transition from the v₆(B₂) = 1 state. This leads to a value of 3633.8 cm⁻¹ for the ν₁(HF) stretch frequency of H₂O⋯HF, which is higher than the commonly adopted value of 3608 cm⁻¹. Similar calculations of H₂O⋯DF predict a value of 2689 cm⁻¹ for the ν₁(DF) stretch and a less structured band shape. On formation of a 1 : 1 complex with water the frequency is shifted by −331.8 cm⁻¹ and −229.4 cm⁻¹ and the intensity is increased by a factor of 3.87 and 3.51 for HF and DF, respectively. Similar calculations of H₂O⋯HCl predicted a value of 2726.5 cm⁻¹ for the ν₁ fundamental, a lower frequency for the hot transition from the v₆(B₂) = 1 excited state, and a ν₁(HCl) band shape in agreement with the results of recent low-temperature experiments.