Valence shell electronic spectroscopy of isoprene studied by theoretical calculations and by electron scattering, photoelectron, and absolute photoabsorption measurements.

The first ab initio calculations (vertical energies and oscillator strengths) are reported for the neutral electronic transitions of isoprene (2-methyl-1,3-butadiene), CH(2)CHC(CH(3))CH(2). The VUV photoabsorption spectroscopy of the molecule is presented in the energy range 4.6 to 10.8 eV (270-125 nm) with the highest resolution yet reported above 6.05 eV, revealing new spectral features. Valence and Rydberg transitions have been assigned in accordance with the theoretical results and the associated vibronic series have been analysed. The absolute photoabsorption cross sections at energies below 6.89 eV have been used to calculate the photolysis lifetime of isoprene in the upper stratosphere (20-50 km). Electron energy loss spectroscopy (EELS) measurements have enabled further photoabsorption cross sections to be derived in the range 9-28 eV. The first ab initio calculations have been carried out to determine excitation energies to the lowest energy ionic states of isoprene. The calculations are compared with the He(i) photoelectron spectrum (8 to 17 eV) and new vibrational structure is observed in the first photoelectron band.

Electronic basis of the comparable hydrogen bond properties of small H2CO/(H2O)n and H2NO/(H2O)n systems (n = 1, 2).

The electronic and structural properties of dihydronitroxide/water clusters are investigated and compared to the properties of formaldehyde/water clusters. Exploring the stationary points of their potential energy surfaces (structurally, vibrationally, and energetically) and characterizing their hydrogen bonds (by both atoms in molecules and natural bond orbitals methods) clearly reveal the strong similarity between these two kind of molecular systems. The main difference involves the nature of the hydrogen bond taking place between the X-H bond and the oxygen atom of a water molecule. All the properties of the hydrogen bonds occurring in both kind of clusters can be easily interpreted in terms of competition between intermolecular and intramolecular hyperconjugative interactions.