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We show that first-principle calculations with an economic basis set can predict reliable geometric structures and accurate vibrational frequencies for small proton-bound carboxylic acid clusters. this leads to the potential of using limited computational resources to study large molecular systems. further applications using density functional (dft) theory on a series of proton-bound formic and acetic acid clusters have found reasonable evaporation energies in comparison with experiments. our dft calculations have exactly reproduced the changeover from monomer loss to dimer loss at n = 6 found in an experiment for proton-bound formic acid clusters. (c) 2000 elsevier science b.v. all rights reserved.

The photodissociation of o-bromotoluene at 266 nm has been investigated using the universal crossed laser-molecular beam technique. The angle-resolved time-of-flight (TOF) spectra corresponding to Br photofragment are measured at different lab angles. The observed translational energy distribution and anisotropy parameters of the Br photofragment indicate that o-bromotoluene dissociates via two channels. In the first channel, the anisotropy parameter beta is determined to be 0.5 +/- 0.2 and the average translational energy is only 9% of the available energy. In the other photofragmentation channel, beta is determined to be - 0.4 +/- 0.2 and 44% of the available energy is assigned to translational energy. Possible mechanisms are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

The photodissociation o -dichlorobenzene at 266 nm has been investigated using the universal crossed molecular beam technique. Photofragment translational energy distribution P ( E t ) and angular distribution of photofragments have been obtained and it is estimated that 23% of the available energy is assigned to translational energy. The anisotropy parameter is determined to be 0.4. From P ( E t ) and β we deduce that o -C 6 H 4 Cl 2 photodissociation is a slow process. Ab initio calculation has been performed and it shows that the parent molecule has a larger geometry deformation in its excited states comparing with that of the ground state. The possible dissociation mechanism has also been proposed.

Ab initio SCF Hartree-Fock theory and Density Functional Theory calculations were carried out to investigate the structure and vibrational spectra of phenyl radicals (neutral, anionic, cationic). All the calculations predicted a planar structure with C-2v symmetry. We identified the fundamental frequencies for phenyl radical by comparison with the frequencies and intensities between different theoretical methods and the experimental results. The structure and vibrational spectra for anionic and cationic radicals are also predicted. This study showed that the density functional theory was a powerful tool for understanding the vibrational spectra of phenyl radical. (C) 1999 Elsevier Science B.V. All rights reserved.

The photofragmentation of C6H5] at 266 nm is investigated on the nuiversal crossed molecular beam ma-chine, and the translational spectroscopy as well as the angular distribution of I atom is measured. The results reveal that under the laser intensity of 108 W/cm2 the single-photon dissociation competes with multi-photon processes. In singlephoton dissociation the anisotropy parameter β is 0.4 and the average translational energy is only 1.04 kcal/mol, which indicates that this process is a slow predissociation. In two-photon photofragmentation the average translational energy is 51.64 kcal/mol, which accounts for about 35% of the available energy. Another photofragmentation channel is even more faster, whose peak in time-of-flight spectra corresponds to four or five photon absorptions. The branching ratio of these three channels is determined to be about 3:3:4.

Photodissociation of p -bromotoluene at 266 nm has been investigated on the universal crossed molecular beam machine, and translational energy distribution P ( E t ) as well as the anisotropy parameter β have been obtained. Photofragment translational energy distribution P ( E t ) reveals that ∼38.5% of the available energy is partitioned into translational energy. The anisotropy parameter β is determined to be −0.4±0.2. From P ( E t ) and β , we deduce that p -bromotoluene photodissociation is a fast process and the perpendicular transition plays a central role at this wavelength. The possible mechanism has been discussed and comparison of p -bromotoluene with bromobenzene, o -bromotoluene has also been made.

The photodissociation of CH2BrCH2Cl at 266 nm has been investigated on the universal crossed molecular beam machine. The primary dissociation step leads exclusively to the formation of CH2CH2Cl radicals and Br atoms in the electronic ground state as well as in the spin-orbit excited state, with a branching ratio 2 +/- 1:8 +/- 1. Photofragment total c.m. translational energy distribution P(E-t) has been obtained and about 64% of the available energy is partitioned into translational energy for Br channel and about 28.5% of the available energy is partitioned into translational energy for Br* channel. The anisotropy parameters are determined to be beta(Br*) = 0.8 +/- 0.2 and beta(Br) = -0.6 +/- 0.2, respectively. Some CH2CH2Cl radicals with large internal excitation (corresponding to formation of ground state Br channel) may undergo secondary dissociation to form CH2CH2 +/- Cl. The experimental results are discussed in terms of a model that involves the initial excitation of two repulsive electronic states: one from an parallel transition to the (3)Q(0) state, and the other from a perpendicular transition to the (3)Q(1), (1)Q states. (C) 1999 Elsevier Science B.V. All rights reserved.

The X1Σ ground state and the low-lying 1,3Σ, 1,3Π excited states of the Ca–Ar and Sr–Ar systems have been studied using pseudopotentials at MCSCF(CASSCF)-SDCI level. The results indicate that the 1,3Σ excited states are essentially repulsive, but the X1Σ ground state and the 1,3Π excited states have shallow potential minima at large equilibrium distance (Re) values. Spectroscopic constants of the X1Σ ground state and the 1,3Π excited states have been calculated and compared to those obtained by experiment.

Calculations at both the SCF and MP2 levels including electron correlation effects on both the geometries and energies of different stationary points encountered along the decomposition pathways for thioformic acids are reported. Four conformers of monothioformic acid and two conformers of dithioformic acids have been investigated using ab initio electronic structure calculations with a series of basis sets. Results show that the keto tautomer of monothioformic acid, HC(:O)SH is the more stable form with respect to the thioketo tautomer, HC(:S)OH, Furthermore, all of the trans isomers of monothioformic and dithioformic acids are slightly more stable in energy,oy than the corresponding cis isomers, which are in agreement with experimental results, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the unimolecular rate constants for pyrolysis of these acids are carried out.