No 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.

The equilibrium geometry of the NH3···trans-HONO complex and the harmonic vibrational frequencies and intensities are calculated in the MP2/aug-cc-pVTZ approximation with the basis set superposition error taken into account. Effects of anharmonic interactions on spectroscopic parameters are studied by solving vibrational Schrödinger equations in 1-4 dimensions using the variational method. Anharmonic vibrational equations are formulated in the space of normal coordinates of the complex. Detailed analysis is performed for the H-bond stretching vibration and internal vibrations of the trans-HONO isomer in the complex. The intermode anharmonicity and anharmonic coupling between two, three, and four vibrational modes are studied on the basis of correct ab initio potential energy surfaces calculated in the above approximation. The combinations of normal modes of the complex most strongly coupled to one another are examined. The calculated frequencies and intensities of vibrational bands are compared with the experimental data on the NH3···trans-HONO complex in an argon matrix and results of earlier calculations of monomeric HONO. In this calculation the strong resonance between the first excited state of the OH stretching vibration and the doubly excited state of the NOH bending vibration of trans-HONO isomer in the complex is thoroughly studied by solving vibrational equations in two and four dimensions.