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

The band shape of the v(1) hydrogen fluoride stretch in H2O center dot center dot center dot HF and H2O center dot center dot center dot 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 v(1)( HF) absorption band were obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectra. The asymmetric v(1) band of H2O center dot center dot center dot HF has a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. The v(1) band shape was reconstructed nonempirically as a superposition of rovibrational bands of the v(1)(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 v(6)(B-2) = 1 state. This leads to a value of 3633.8 cm(-1) for the v(1)( HF) stretch frequency of H2O center dot center dot center dot HF, which is higher than the commonly adopted value of 3608 cm(-1). Similar calculations of H2O center dot center dot center dot DF predict a value of 2689 cm(-1) for the v(1)( 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 center dot center dot center dot HCl predicted a value of 2726.5 cm(-1) for the v(1) fundamental, a lower frequency for the hot transition from the v(6)( B-2) = 1 excited state, and a v(1)(HCl) band shape in agreement with the results of recent low-temperature experiments.