Experimental and theoretical study of the ν(HF) absorption band structure in the H2O…HF complex

Abstract

The νHF absorption band shape of the H2O…HF complex is studied in the gas phase at a temperature of 293 K. The spectra of H2O/HF gaseous mixtures in the range 4000–3400 cm−1 are recorded at a resolution of 0.2–0.02 cm−1 with Bruker IFS-113v and Bruker IFS-120 HR vacuum Fourier spectrometers in a 20-cm cell. The spectra of the H2O…HF complex in the region of the ν1(HF) absorption band are obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectrum. The ν1 band of the H2O…HF complex has an asymmetric shape with a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. Two approaches are used to calculate the ν1 band shape as a superposition of rovibrational bands of the fundamental and hot transitions involving the low-frequency modes of the complex. The first approach is based on a simplified semiempirical procedure. The second approach relies on a nonempirical anharmonic calculation of the vibrational energy levels, the frequencies and intensities of the corresponding transitions, and the rotational constants. These parameters are obtained by calculating ab initio the potential energy and dipole moment surfaces in the second-order Moller-Plesset approximation and using the variational method to solve one-, two-, and three-dimensional anharmonic vibrational problems. The absorption spectrum of the complex in the range 3600–3720 cm−1, reconstructed using the nonempirical electro-optical parameters, reproduces rather well the main features of the experimental spectrum, including the relative intensities of peaks of the vibrational structure. However, the interpretation of most of the structural features of the spectrum differs from that adopted in the semiempirical scheme. First of all, it follows from the results of nonempirical calculation that the central, most intense, maximum of the experimental spectrum should correspond to the v 1=1←0 transition from the ground vibrational state. This fact gives rise to a new value of the vibrational transition frequency ν 1 0 in the H2O…HF complex equal to 3635 cm−1, which is higher than the commonly accepted value of 3608 cm−1.