Inhomogeneity of the amorphous solid water dangling bonds.

Amorphous solid water (ASW) is one of the most widely studied molecular systems because of its importance in the physics and chemistry of the interstellar medium and the upper layers of the Earth's atmosphere. Although the global structure of this material, i.e. the bulk and the surface, is well characterised, we are far from having an overall understanding of the changes induced upon chemical or physical perturbation. More specifically, the behaviour of the surface and the immediate sublayers upon mid-infrared irradiation must be understood due to its direct effect on the adsorption capacities of the ASW surface. Small molecules can accrete or form at the surface, adsorbed on the dangling OH groups of surface water molecules. This behaviour allows further reactivity which, in turn, could lead to more complex molecular systems. We have already demonstrated that selective IR irradiations of surface water molecules induce a modification of the surface and the production of a new monomer species which bonds to the surface via its two electronic doublets. However, we did not probe the structure of the dangling bands, namely their homogeneity or inhomogeneity. The structure and orientation of these surface molecules are closely linked to the way the surface can relax its vibrational energy. In this work, we have focussed our attention on the two dH dangling bonds, carrying out a series of selective irradiations which reveal the inhomogeneity of these surface modes. We have also studied the effects of irradiation duration on the surface reorientation, determining that the maximum photoinduced isomerisation yield is ∼15%.

Photo-induced hydrogen exchange reaction between methanol and glyoxal: formation of hydroxyketene.

We study the structure and photochemistry of the glyoxal-methanol system (G-MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non-hydrogen-bonded complex, G-MeOH-1, is present in an inert environment of solid argon. MP2/aug-cc-pVDZ calculations indicate that G-MeOH-1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry-adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non-hydrogen-bonded G-MeOH-1 complex than compared to the hydrogen-bonded ones. The irradiation of G-MeOH-1 with the filtered output of a mercury lamp (lambda>370 nm) leads to its photo-conversion into the hydroxyketene-methanol complex HK-MeOH-1. The identity of HK-MeOH-1 is confirmed by both FTIR spectroscopy and MP2/aug-cc-pVDZ calculations. An experiment with deuterated methanol (CH(3)OD) evidences that hydroxyketene is formed in a photo-induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo-conversion of G-MeOH-1 to HK-MeOH-1 is studied by a coupled-cluster method [CR-CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.

Structure, spectra and stability of a tetrafluoromethane-water complex.

The complex formed between water and tetrafluoromethane has been studied by infrared matrix isolation spectroscopy and ab initio calculations. The geometries of the CF4-H2O complexes were optimized in two steps at the MP2/aug-cc-pVTZ level of theory. The structure found at this level was reoptimized on the CP-corrected potential energy surface. The interaction energy was partitioned according to the SAPT scheme and the topological analysis of the electron density was performed. The optimized structure corresponds to the nonhydrogen bonded complex with an oxygen atom of water oriented toward the carbon atom of CF4. The infrared spectra of CF4-H2O /Ne(Ar) matrices demonstrate the presence of a well defined CF4-H2O structure in accord with theoretical prediction. Two complex vibrations were identified in the spectra of neon matrices and four vibrations were observed in the spectra of argon matrices. The available experimental data are in accord with the CP-corrected calculated data.

Ultrafast dynamics of acetylacetone (2,4-pentanedione) in the S2 state.

The dynamics of the enolic form of acetylacetone (E-AcAc) was investigated using a femtosecond pump-probe experiment. The pump at 266 nm excited E-AcAc in the first bright state, S2(pi pi*). The resulting dynamics was probed by multiphoton ionization at 800 nm. It was investigated for 80 ps on the S2(pi pi*) and S1(n pi*) potential energy surfaces. An important step is the transfer from S2 to S1 that occurs with a time constant of 1.4 +/- 0.2 ps. Before, the system had left the excitation region in 70 +/- 10 fs. An intermediate step was identified when E-AcAc traveled on the S2 surface. Likely, it corresponds to an accidental resonance in the detection scheme that is met along this path. More importantly, some clues are given that an intramolecular vibrational energy relaxation is observed, which transfers excess vibrational energy from the enolic group O-H to the other modes of the molecule. The present multistep evolution of excited E-AcAc probably also describes, at least qualitatively, the dynamics of other electronically excited beta-diketones.