Calculation of vibrational spectroscopic and geometrical characteristics of the [F(HF)2]- and [F(DF)2]- complexes using the second-order vibrational perturbation theory and a 6D variational method.

Vibrational spectroscopic and average geometrical parameters of the strong H-bonded complexes [F(HF)2]- and [F(DF)2]- are determined for the first time from nine-dimensional (9D) perturbative and 6D variational calculations. The frequencies and intensities for all fundamental and some combination and overtone transitions obtained by the method of second-order vibrational perturbation theory (VPT2) are reported. A two-fold decrease in the H-F (D-F) stretching band frequency and a more than ten-fold increase in the intensity of this band upon complexation are predicted. The theoretical frequencies for both isolated isotopologues are in satisfactory agreement (to better than 70 cm-1) with the scarce experimental data obtained in condensed phases. The main purpose of variational calculations is to analyze the intermode anharmonic coupling and the changes in the geometrical parameters upon vibrational excitation and H/D isotopic substitution. The equilibrium nuclear configuration and the 2D potential energy surface (PES) of [F(HF)2]- for H-F stretches are calculated in the MP2/6-311++G(3df,3pd), CCSD(T)/6-311++G(3df,3pd), CCSD(T)/aug-cc-pVTZ, and CCSD(T)/d-aug-cc-pVTZ approximations with the basis set superposition error taken into account. Anharmonic vibrational problems are solved by the variational method for 2D, 4D, and 6D systems of H-bond and H-F (D-F) stretches and in-plane bends. The VPT2 calculations and calculations of the PESs for 4D and 6D systems are performed in the MP2/6-311++G(3df,3pd) approximation. Comparison of variational anharmonic solutions for different vibrational subsystems demonstrates the influence of intermode anharmonic coupling on the mixing of wave functions and spectroscopic and geometrical characteristics. The inverse Ubbelohde effect is predicted and substantiated.

Quantum-Mechanical Prediction of the Averaged Structure, Anharmonic Vibrational Frequencies, and Intensities of the H2O···trans-HONO Complex and Comparison with the Experiment.

The geometrical parameters, the frequencies, and absolute intensities of vibrational transitions of H2O···trans-HONO hydrogen-bonded complex are calculated using the approach earlier tested in calculations of isolated molecules of nitrous acid and complexes of this acid with ammonia. The equilibrium nuclear configuration and potential energy and dipole moment surfaces are calculated by the MP2/aug-cc-pVTZ method with the basis set superposition error taken into account. The fundamental transition frequencies and intensities of the complex are first obtained in the harmonic approximation, and then the energy values, vibrational wave functions, and transition frequencies and intensities are determined from variational solutions of one- to four-dimensional anharmonic equations. The results obtained are compared with the data calculated in the same approximation for an isolated trans-HONO molecule and the NH3···trans-HONO complex. The average discrepancy between the anharmonic frequency values and five available experimental data is 15 cm-1. Three absorption bands of trans-HONO with the highest intensity are recommended for detecting the presence of H2O···trans-HONO.

Anharmonic Calculation of the Structure, Vibrational Frequencies, and Intensities of the NH3···cis-HONO and NH3···cis-DONO Complexes.

The geometrical parameters, the frequencies, and absolute intensities for transitions between vibrational states of NH3···cis-HONO and NH3···cis-DONO hydrogen-bonded complexes are calculated using the approach earlier tested in calculations of isolated molecules of nitrous acid and the NH3···trans-HONO and NH3··trans-DONO complexes. Vibrational wave functions and energy values of the complexes are derived from variational solutions of anharmonic equations in one to four dimensions. The equilibrium nuclear configuration and potential energy surfaces are calculated by the MP2/aug-cc-pVTZ method with the basis set superposition error taken into account. Comparison of the obtained results with the analogous data calculated in the same approximation for isolated cis- and trans-HONO (DONO) molecules and the NH3···trans-HONO (DONO) complexes provides information about the changes in the spectroscopic and geometrical parameters of nitrous acid upon cis-trans transition, H/D substitution, and H-bond formation.

Anharmonic Calculation of Structural and Spectroscopic Parameters of the cis-DONO Molecule.

The equilibrium nuclear configuration of the cis-DONO molecule is calculated in the MP2/aug-cc-pVTZ approximation. The main objective of this work is to calculate the frequencies and absolute intensities for fundamental transitions of cis-DONO, to examine the influence of H/D substitution on the form of vibrational modes and structural and spectral parameters, and to formulate a feasible and reliable approach for calculating such parameters of H-bonded complexes formed by cis-DONO. Vibrational wave functions and energy values of cis-DONO are derived from variational solutions of anharmonic equations in one to four dimensions with the potential energy surfaces calculated by the MP2/aug-cc-pVTZ method. The equilibrium geometry and one-dimensional anharmonic frequencies and intensities of cis-DONO are also computed using the ccsd(t)/aug-cc-pVQZ approximation. All the calculated results are compared with the experimental data, the most accurate results of other authors, and the values obtained earlier for trans-DONO and cis- and trans-HONO using the MP2/aug-cc-pVTZ approximation.

IR spectroscopic study of the HCl···O3 molecular complex in liquid argon.

Infrared spectra are reported of ozone and HCl dissolved in liquid argon (86-134 K) at concentrations varying from 5×10(-5) to 9×10(-6) M for HCl, and from 1×10(-3) to 5×10(-5) M for ozone. At low concentrations of O3 and of HCl, no spectral features due to O3-HCl complex were found. At higher concentrations (1×10(-3) of ozone vs 5×10(-5)-9×10(-6) of HCl), a new band near 2840 cm(-1) due to the HCl···O3 complex was observed. FWHM of νHCl of the complex is 8 cm(-1). From the temperature dependence of the absorption band intensity, the enthalpy of the complex formation was estimated, ΔH°=4.7±0.4 kJ mole(-1). The optimized geometry of the cis-HCl···O3 complex and a value of 6.3 kJ mole(-1) for its binding energy were determined by ab initio calculations.

Experimental and theoretical study of absorption spectrum of the (CH3)2CO···HF complex. Influence of anharmonic interactions on the frequency and intensity of the C=O and H-F stretching bands.

IR absorption spectra of mixtures (CH3)2CO/HF and free (CH3)2CO molecules are recorded in the region of 4000-900 cm(-1) with a Bruker IFS-125 HR vacuum Fourier spectrometer at room temperature with a resolution up to 0.02 cm(-1). Spectral characteristics of the 2ν(C=O) overtone band of free acetone are reliably measured. The ν1(HF) and ν(C=O) absorption bands of the (CH3)2CO···HF complex are obtained by subtracting the absorption bands of free HF and acetone and absorption lines of atmospheric water from the experimental spectrum of mixtures. The experimental data are compared with theoretical results obtained from variational solutions of 1D-4D vibrational Schrödinger equations. The anharmonic potential energy and dipole moment surfaces used in the calculations were computed in the MP2/6-311++G(2d,2p) approximation with corrections for the basis set superposition error. Comparison of the data derived from solutions for different combinations of vibrational degrees of freedom shows that taking the inter-mode anharmonic interactions into account has different effects on the transition frequencies and intensities. Particular attention has been given to elucidation of the influence of anharmonic coupling of the H-F and C=O stretches with the low-frequency intermolecular modes on their frequencies and intensities and the strength of resonance between the fundamental H-F and the first overtone C=O transitions.

Isotope effects in hydrogen-bonded complexes. Calculation of geometrical and vibrational characteristics of asymmetric isotopologues of [F(HF)2]-.

The geometrical and vibrational characteristics of isolated H-bonded anionic complexes [FHFDF](-), [FHFTF](-), and [FDFTF](-) are calculated quantum-mechanically. The four-dimensional anharmonic vibrational problems are solved by the variational method using the potential energy and dipole moment surfaces calculated in the MP2/6-311++G(3df,3pd) approximation with the basis set superposition error taken into account. Changes in the bond lengths of molecular fragments LF (L = H, D, T) and in the distances between the F(-) anion and the centers of mass of LF are used as the vibrational coordinates. For each isotopologue, the vibrational energy levels, the transition frequencies and absolute intensities for the H-bond and L-F stretching vibrations are determined. To study the isotope effects on the geometrical parameters, the values of internuclear separations and the asymmetry parameter of the F(-)···L-F bridges, averaged over the ground state and several excited vibrational states, are calculated, as well as their standard deviations. The calculations revealed an extremely strong influence of anharmonic coupling between different vibrations on the absorption intensities and a significant mass-dependence of spectroscopic and structural parameters. The geometry and harmonic frequencies of KH(2)F(3), KD(2)F(3), and KHDF(3) are also calculated at a lower ab initio level. The results obtained for [FHFDF](-), [FHFTF](-), and [FDFTF](-) are compared with the available experimental data and the results of earlier calculations of the symmetric complexes [F(HF)(2)](-), [F(DF)(2)](-), and [F(TF)(2)](-) and complexes containing a positive K-meson.

Calculation of the structure, potential energy surface, vibrational dynamics, and electric dipole properties for the Xe:HI van der Waals complex.

We report the structure and spectroscopic characteristics for the Xe:HI van der Waals binary isomers determined from variational solutions of two-dimensional and three-dimensional (3D) vibrational Schrödinger equations. The solutions are based on a potential energy surface computed at the coupled-cluster level of theory including single and double excitations and a non-iterative perturbation treatment of triple excitations [CCSD(T)]. The dipole moment surface was calculated using quadratic configuration interaction (QCISD). The global potential minimum is shown to be located at the anti-hydrogen-bonded Xe-IH isomer, 21 cm(-1) below the secondary local minimum associated with the hydrogen-bonded Xe-HI isomeric form. The dissociation energy from the global minimum is 245.9 cm(-1). 3D Schrödinger equations are solved for the rotational quantum numbers J = k = 0, 1, and 2, without invoking an adiabatic separation of high- and low-frequency degrees of freedom. The vibrational ground state resides in the Xe-HI potential well, while the first excited state, 8.59 cm(-1) above the ground, occupies the Xe-IH well. We find that intra-complex dynamics exhibits a sudden transformation upon increase of the r(HI) bond length, accompanied by abrupt changes in the geometric and dipole parameters. A similar chaotic behavior is predicted to occur for Xe:DI at a shorter r(DI) bond length, which implies stronger coupling between low- and high-frequency motions in the heavier complex. Our calculations confirm a strong enhancement for the r(HI) stretch fundamental and a significant weakening for the first overtone vibrational transitions in Xe:HI, as compared to those in the free HI molecule. A qualitative explanation of this, earlier experimentally detected effect is suggested.

Study of the H-F stretching band in the absorption spectrum of (CH3)2O...HF in the gas phase.

The absorption spectra of the (CH3)2O...HF complex in the range of 4200-2800 cm(-1) were recorded in the gas phase at a resolutions of 0.1 cm(-1) at T = 190-340 K. The spectra obtained were used to analyze their structure and to determine the temperature dependencies of the first and second spectral moments. The band shape of the (CH3)2O...HF complex in the region of the nu1(HF) stretching mode was reconstructed nonempirically. The nu1 and nu3 stretching vibrations and four bending vibrations responsible for the formation of the band shape were considered. The equilibrium geometry and the 1D-4D potential energy surfaces were calculated at the MP2 6-311++G(2d,2p) level with the basis set superposition error taken into account. On the basis of these surfaces, a number of one- and multidimensional anharmonic vibrational problems were solved by the variational method. Solutions of auxiliary 1D and 2D vibrational problems showed the strong coupling between the modes. The energy levels, transition frequencies and intensities, and the rotational constants for the combining vibrational states necessary to reconstruct the spectrum were obtained from solutions of the 4D problem (nu1, nu3, nu5(B2), nu6(B2)) and the 2D problem (nu5(B1), nu6(B1)). The theoretical spectra reconstructed for different temperatures as a superposition of rovibrational bands associated with the fundamental, hot, sum, and difference transitions reproduce the shape and separate spectral features of the experimental spectra. The calculated value of the nu1 frequency is 3424 cm(-1). Along with the frequencies and absolute intensities, the calculation yields the vibrationally averaged values of the separation between the centers of mass of the monomers Rc.-of-m., R(O...F), and r(HF) for different states. In particular, upon excitation of the nu1 mode, Rc.-of-m. becomes shorter by 0.0861 A, and r(HF) becomes longer by 0.0474 A.

Study of the upsilon1 band shape of the H2O...HF, H2O...DF, and H2O...HCl complexes in the gas phase.

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.