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Double-resonance IR/UV spectroscopy and quantum chemistry wee used to investigate the intrinsic character of the helicogenic properties on the Ac-Aib-Phe-Aib-N[H.sub.2] molecule under isolated conditions in the gas phase. The results obtained establish the formation of an incipient [3.sup.10] helix in the isolated conditions.

A measurement of the binding energy of indole-water and 1-methylindole-water gas-phase complexes using a photoionization-based approach is presented. The indole-water complex seems to be a conventional 'translinear' H-bonded species.

The photoionization behavior of the gas-phase hydrogen-bonded compounds of phenol with water, dimethyl ether and methanol was investigated using both experimental and molecular modeling techniques. Specifically, the onset of fragmentation after ionization was examined for phenol-S complexes (S = H2O, CH3OH and CH3OCH3) and the threshold for dissociative ionization was measured. Accurate binding energies for the neutral species were deduced for the first time. Results stressed the role of the dispersive interactions in the structure of the phenol-S complexes.

The first rotationally resolved observation of the infrared (IR) spectrum of the molecular complex C3H4–HCl in gas phase is reported. New IR spectra have been recorded at high resolution by means of a slit jet. Rotational constants and vibrational frequencies have been obtained from these spectra. These data are presented and compared with high level ab initio calculations (CCSD(T)/cc-pVTZ). The results obtained in this study are compared with those on acetylene and ethene-HCl complexes. We observe a broadening of the lines interpreted as a decrease of the lifetime of the excited vibrational state with the size of the partner of complexation and also a correlation between the vibrational shift of the H–Cl stretching mode and the proton affinity of the acceptor molecule. [ABSTRACT FROM AUTHOR]

Recent experiments have investigated the dissociative ionization of NaI and CsI, each aggregated with a molecule of acetonitrile CH[sub 3]CN, using two femtosecond laser pulses. The first, the pump pulse, excites the NaI or CsI diatom to a dissociative state. The second, the probe pulse, then ionizes the sodium or cesium atom after a variable delay time, and the resulting ion is detected. In the case of NaI, the ion signal is characterized by a single maximum. For CsI, however, an oscillatory signal is observed. By performing two-dimensional wave packet propagations, we are able to reproduce this behavior, which can be explained using simple physical arguments. [ABSTRACT FROM AUTHOR]

A simple yet accurate methodology to account for charge resonance in ionic clusters is proposed. The coupling elements between these structures are proportional to an overlap integral between relevant nonorthogonal monomer molecular orbitals.

A study was conducted to analyze the ionization dynamics of the C6H6-H2O and C6H6-D2O complexes near the ionization threshold. Experiment and molecular dynamics simulations were utilized to carry out the analysis. The neutral benzene-water complexes were prepared in a supersonic expansion of a gas mixture with the vapor pressure of benzene and water in helium. Experimental results identified that the conformations responsible for threshold photoionization correlated with one part of the set of conformations associated with ionization below the dissociation limit.