The subtle trans-gauche equilibrium in the ethanol molecule is affected by hydrogen bonding. The resulting conformational complexity in ethanol dimer manifests itself in three hydrogen-bonded OH stretching bands of comparable infrared intensity in supersonic helium expansions. Admixture of argon or nitrogen promotes collisional relaxation and is shown to enhance the lowest frequency transition. Global and local harmonic frequency shift calculations at MP2 level indicate that this transition is due to a gauche-gauche dimer, but the predictions are sensitive to basis set and correlation level. Energetically, the homochiral gauche-gauche dimer is predicted to be the most stable ethanol dimer conformation. The harmonic MP2 predictions are corroborated by perturbative anharmonicity contributions and CCSD(T) energies. Thus, a consistent picture of the subtle hydrogen bond energetics and vibrational dynamics of the ethanol dimer is starting to emerge for the first time.

H+3 potential energies and multipole moments are calculated from a full CI with a 10s, 4p, 2d GTO hydrogen basis. 69 calculated energy points with energies of up to 25 000 cm−1 above the minimum are fitted by a power series expansion in terms of a Morse-type coordinate with a mean square error of less than 1 cm−1. Rotationless vibrational states with energies of up to 12 000 cm−1 above equilibrium are calculated variationally for ten isotopomers. The resulting band origins for the seven analyzed fundamental transitions show a mean deviation of less than 2 cm−1. For the other predicted frequencies, the errors are expected to be below 0.1% also. The equilibrium bond length of H+3 is predicted to be 0.8732(2) Å.

Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X̃ and à states of NH3 yielding NH2 (X̃ 2B1,Ã 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The à state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 Å (in the NH2 fragment), and RNH =1.323 Å (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out-of-plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X̃–à conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X̃ and à states in planar structures depends strongly on αHNH and varies from about 1.68 Å (60°) to infinity (180°). The photodissociation process NH3(Ã) →NH2(X̃ 2B1) +H(2S) is discussed on the basis of the calculated potential energy surfaces.

The conical intersection regions on the potential energy functions of the valence excited 1,3Σu−, 1,3Δu, 3Σu+, and 1,3Πg states of CO2 have been investigated by ab initio calculations. Using large scale multireference configuration interaction the ordering of the lowest valence excited states of CO2 has been calculated to be 3B2, 3A2, 1A2 followed by 1B2. All these states have bent equilibrium structures and lie energetically below their dissociation asymptotes. The near equilibrium parts of the potential energy functions have been mapped in three dimensions by multiconfiguration self-consistent field calculations. The 1,3B2 and 1,3A2 states differ in their equilibrium angles (118° and 127°, respectively), and have much longer equilibrium distances (around 1.26 Å) than the electronic ground state. Anomalously low values of ca. 800 cm−1 have been calculated for the wave number of the antisymmetric stretching vibrations of the 1A2, 3B2, and 1B2 states. The crossings between the bent valence excited states in the geometry region 90°≤αOCO≤150° and 1.1 Å≤RCO≤1.4 Å have been located.

The light-induced (266 nm) ultrafast decarboxylation of two peroxides R 1 -C(O)O-OR 2 , with R 1 ) phenyl and R 2 ) benzoyl or tert-butyl, in solution has been studied on the picosecond time scale by absorption spectroscopy with a time resolution typically of 100 to 200 fs. The reaction was investigated in various solvents of different polarity and viscosity to elucidate the influence of the solvent environment on the decarboxylation rate. Transient intermediate benzoyloxy radicals, R 1 -CO 2 , were monitored at wavelengths between 300 and 1000 nm. While the primary dissociation of the peroxide is too fast to be resolved, the dissociation of intermediate benzoyloxy radicals is clearly detected on the picosecond time scale. The mechanism of light-induced two-step dissociation is discussed, as is the dependence of reaction dynamics on the type of substituent R 2 as well as the branching ratio between prompt and delayed CO 2 formation. A model for the decarboxylation process is presented that is based on molecular structure parameters and energies. The latter quantities, which are obtained from density functional theory calculations, serve as input data for calculations of the specific decomposition rate coefficients of benzoyloxy intermediates via statistical unimolecular rate theory. The predicted benzoyloxy radical decay data are compared with corresponding experimental concentration versus time traces.