Evi Honegger

Intramolecular vibrations of small water clusters

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4‐31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for ...

Intermolecular bonding and vibrations of the carbazole⋅B complexes (B=H2O, D2O, NH3)

Carbazole⋅B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen‐bond vibrations and dissociation energies were studied by several laser‐spectroscopic techniques (fluorescence excitation and emission, resonance‐two‐photonionization with mass‐specific detection). The following results were obtained for both S0 and S1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen‐bond dissociation energies D0 for carbazole⋅H2O/D2O, and (5) electronic spectral shifts δν relative to the bare carbazole molecule. The latter are large (500–710 cm−1) and reflect an increase of the hydrogen‐bond energy by ≈40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b1 vibration of carbazole are observed and partially analyzed. To complem...

Solute-solvent interactions in microhydrate clusters: Carbazole - (H2O)n

Abstract We report the laser-induced fluorescence (LIF) and the resonant two-photon ionization spectra of the carbazole-(H2O)n (n = 1–3) hydrogen-bonded complexes. Use of carbazole as a symmetric “probe” molecule allows detailed insight into the symmetries, structures and intermolecular vibrations of these microhydrate complexes.

The analysis of conformational equilibria by variable temperature photoelectron spectroscopy: A comment

Abstract A numerical experiment which simulates the temperature dependence of the PE spectrum of thioanisole, i.e. a molecule with restricted internal rotation, shows that an analysis in terms of an assumed equilibrium between two “frozen” conformers can lead to substantial systematic errors in their energy difference.

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

Abstract The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C 3 H 6 O + isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.

Systematic regularities of molecular SCF energies

New formulae for the approximate computation of total molecular energies are developed based on ab initio calculations of n-alkanes. Their application to various kinds of molecules reveals that good expectation values for total molecular energies can be obtained by considering only the one-electron terms hi and the nuclear repulsion energy. It is further shown that very good agreement with SCF total energies is obtained by a relationship which connects the total energy with the sum of inner-shell (core) orbital energies. The results turn out to be better than those obtained using Ruedenbergs approximation, which takes both inner-shell and valence-shell orbital energies into account.

The ionisation energies of bent and twisted double bonds. Part I

Abstract The energies of the π orbital and the other molecular orbitals of ethylene have been calculated, using the ab initio STO-3G method, as a function of the three out-of-plane internal coordinates using a 33 complete factorial design. The analysis yields the significant terms needed for a complete description of the influence of the out-of-plane deformations of ethylene on its π−1 ionisation energies. In agreement with experimental evidence obtained from the photoelectron spectra of strained and twisted substituted ethylenes, it is found that the effect of nonplanar distortion upon the π-electron orbital energy is insignificant. This lack of sensitivity is due both to the small sizes of the individual contributions associated with each of the three internal coordinates and, in addition, to the compensation of such effects having opposite sign and/or the cross-terms between pairs of internal coordinates.

The Equivalent Bond Orbital Model and the Interpretation of PE Spectra

The low-energy part of the He(Iα) or He(IIα) PE spectrum of a hypothetical molecule M (shown in Fig. 1) consists of four bands, ① to ④, which — for simplicity — we assume not to overlap. The bands ① and ② exhibit resolved vibrational fine structure, whereas the fine structure of ③ and ④ cannot be resolved because of line broadening, of random noise and/or the resolution of the recording, the latter being typically of the order of 20 meV.