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Theoretical absorption spectra of HCl trihydrate, HCl hexahydrate, and HNO 3 trihydrate in the mid-IR have been calculated using the S iesta suite of programs, a DFT method especially designed for periodic systems of large size. Infrared intensities are obtained from the macroscopic polarization changes per normal mode. The theoretical results can account globally for the observed experimental features. In all spectra, the largest absorption intensities are due to stretching vibrations of hydrated oxonium ions. Oxonium stretching absorptions appear as broad bands in the experimental spectra, with band widths growing with increasing number of water molecules coordinated to the H 3 O + ion.

A combined theoretical and experimental study of crystalline hydrogen chloride hexahydrate HCl AE 6(H2O) has been performed. This species is stable in the range of atmospheric pressure and temperature, and some apparent discrepancies existed between previous crystallographic and vibrational studies. This work presents theoretical calculations based on the SIESTA method that allow a refinement of the geometrical structure and a prediction of the vibrational modes of the crystal. The experimental part consists in new reflection–absorption infrared spectra, that are analysed and assigned with the help of the theoretical results, and compared to previous assignments of infrared transmission spectra. This research supports the equivalence of the samples prepared by different methods in previous experimental investigations. � 2004 Elsevier B.V. All rights reserved.

Ab initio calculations and reflection–absorption infrared (RAIR) spectroscopy have been used for the study of the structure and vibrational frequencies of crystalline nitric acid. The crystallographic data on the unit cell structure have been refined and the vibrational frequencies of the crystal have been determined in the calculation. A good general coincidence is found between the location of the calculated vibrational frequencies and the absorption bands in the IR spectra. Some discrepancies with previous empirical assignments are discussed. The results provide a useful base to support studies of more complex hydrous phases of nitric acid of great relevance for atmospheric chemistry. 2003 Elsevier B.V. All rights reserved.

19 pags. 3 figs., The rotational relaxation of N2 in collisions with He has been investigated in free jets with a combination of resonance-enhanced-multiphoton ionization spectroscopy and time-of-flight techniques. From the measured data, a global cross-section for rotational relaxation has been derived. The cross section grows from a value of ≈8 Å2 at 100 K to ≈18 Å2 at 5-15 K. For the weakest supersonic expansions investigated, a breakdown of the translational equilibrium between He and N2 has been observed, that could be well accounted for by an isentropic jet model and classical collision cross-sections. © 2002 Elsevier Science B.V. All rights reserved., Financed by the Ministry of Science and Technology of Spain under Grants PB98-0762-C03 and REN 2000-1557 CLI and by the European Union through the RTN Reaction Dynamics (HPRN-CT-1999-0007). Support from the Plan Andaluz de Investigación (group FQM-205).

The dependence of the cross sections for the F+H 2 and F+D 2 reactions on the collision energy has been determined by means of quasiclassical trajectory calculations (QCT) on the Stark and Werner (SW) ab initio potential energy surface (PES). From these data rate constants as a function of temperature have been evaluated. For both reactions, fairly good agreement between the calculated and experimental rate constants is obtained. Our results are compared with previous QCT and quantum mechanical (QM) calculations on other PES, as well as with recent approximate QM results on the SW-PES. The rate constants are in close agreement for the F+D 2 reaction, while for F+H 2 our values are larger than the ones obtained on the 6SEC-PES. The more orienting character of the SW-PES can explain these differences, when the nuclear spin statistics of H 2 and D 2 are taken into account.

Integral and differential reaction cross sections resolved in the final vibrational states of the products have been calculated for the F+H2 (D2)→HF (DF)+H (D) reactions by the method of quasi-classical trajectories (QCT) on the DMBE-5SEC potential energy surface. The results are compared to experimental measurements and to recent quantum-mechanical calculations on a related potential surface. Forward peaks appearing selectively in the HF (ν′ = 3) and DF (ν′ = 4) center of mass angular distributions and attributed to a quantum-mechanical resonance are shown to be also present in the classical calculation. However, the QCT branching ratio to the different final vibrational states is found to be in contradiction with the experimental results. © 1993., Financed by DGICYT of Spain under Grants PB90-0622-CO3 and PB-9 1-O 128.

Integral and differential state-resolved cross sections have been calculated by the method of quasiclassical trajectories for the H+D2→HD+D reaction in the range of initial conditions of interest to recent experiments. When possible, the results are compared to exact quantum-mechanical calculations. The differential cross sections reported here should be useful for the evaluation of current molecular beam experiments on this reaction. © 1992 Elsevier Science Publishers B.V., Partially financed by the CICYT of Spain under Grant PB890041.

Relative values of the total reaction cross section σ R for the crossed molecular beam reaction K + C 2 H 5 I → KI + C 2 H 5 have been measured over the translational energy ( E T ) range 0 17–0.55 eV. It is found that σ R decreases monotonically with E T over this range, any maximum in σ R ( E T ) is presumed to lie below 0.17 eV.