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 upsilon1 hydrogen fluoride stretch in H20...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 upsilon1(HF) absorption band were obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectra. The asymmetric upsilon1 band of H2O...HF has a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. The upsilon1 band shape was reconstructed nonempirically as a superposition of rovibrational bands of the upsilon1 (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 upsilon6 (B2) = 1 state. This leads to a value of 3633.8 cm(-1) for the upsilon1 (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 upsilon1(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 upsilon1 fundamental, a lower frequency for the hot transition from the upsilon6 (B2) = 1 excited state, and a upsilon1(HCl) band shape in agreement with the results of recent low-temperature experiments.