John P. Simons

Intramolecular charge transfer in jet-cooled aromatic amines

Abstract LIF excitation spectra have been recorded for a family of molecules whose dual fluorescence in solution has been ascribed to twisted intramolecular charge transfer (TICT) state formation, promoted through torsional motion in the initially photoexcited state. To eliminate the influence of solvation, the LIF spectra have been recorded under jet-cooled isolated-molecule conditions: the molecules include para-substituted nitrile or ester derivatives of dialkylamino benzene compounds. Evidence both for torsional motion and TICT state formation has been gained for the ester derivatives where access into the TICT state correlates with the onset of a quasi-continuous component in the LIF system and the appearance of a red-shifted fluorescence emission.

Electronic spectral shifts of large van der Waals molecules

Abstract Experimental values for the red spectral shifts in the fluorescence excitation spectra of large van der Waals complexes of aromatic molecules and rare gas atoms have been used, in conjunction with calculated equilibrium distances between the aromatic and rare gas atom, to estimate differences between the dispersion forces in the ground and excited states. Theoretical red-shifts calculated by a formalism given by Longuet-Higgins and Pople are an order of magnitude too large and this discrepancy is traced to incorrect assumptions concerning values chosen for average excitation energies. Modifications of the theory for a van der Waals complex are explored. A simpler and more easily used formula to correlate the magnitudes of the spectral shifts is derived and tested.

Laser-induced fluorescence spectroscopy and structure of microsolvated molecular clusters. Part 2.—Laser-induced fluorescence spectroscopy of jet-cooled ethyl 4-aminobenzoate, methyl 4-aminobenzoate, 4-aminobenzonitrile and their dimethylamino and pyrrolidino derivatives

An iterative band contour simulation strategy has been applied to the analysis of partially resolved rovibronic bands in the laser-induced fluorescence (LIF) spectra of a range of jet-cooled, substituted aromatic amines (4-dimethylaminobenzonitrile, ethyl 4-dimethylaminobenzoate, methyl 4-dimethylaminobenzoate, 4-aminobenzonitrile, ethyl 4-aminobenzoate, methyl 4-aminobenzoate, 4-pyrrolidinobenzonitrile, ethyl 4-pyrrolidinobenzoate and methyl 4-pyrrolidinobenzoate). A internally consistent set of structural parameters has been obtained which, by appeal to the predictions of molecular orbital and/or molecular mechanics calculations, provides an experimentally based set of molecular structures as well as the resolution and identification of alternative molecular conformers.

Reactions of Xe(3 P 2) and Xe(3 P 1) with HCl, HBr and HI; energy utilization, energy disposal, product rotational polarization and reaction dynamics

We report a systematic study of the reactions of Xe(3 P 1, 3 P 2) with HCl, HBr and HI using both spectroscopic and molecular beam techniques. No reaction is observed at room temperatures for Xe(3 P 2) with HCl; analysis of the Xe(3 P 1)/HCl data yields an estimate of the endoergicity of the 3 P 2 reaction which agrees within experimental error with the threshold obtained from deconvolution of the excitation function measured in the molecular beam experiments, thus indicating little or no barrier in the entrance channel. Surprisal analysis of the vibrational population distributions obtained by inversion of the rare gas halide spectra yields estimates for the fraction of the available energy disposal into vibration . The results are in broad agreement with earlier estimates and are roughly independent of the spin orbit component for a given reaction. Deviations from the purely kinematic limit in the product rotational alignment ( ) increase across the series HCl, HBr, HI while ...

Solvent shifts in van der waals complexes of perylene

Abstract The solvation and structure of vdW complexes formed in supersonic free-jet expansions of perylene with rare gas atoms and organic “solvent” molecules, using helium as carrier gas, have been investigated. The complexes were probed and analysed by combining laser-induced fluorescence spectroscopy with approximate summed pair-potential calculations. Theoretical examination of the bathochromic spectral shifts for the isolated 1:1 vdW complexes is consistent between solvent-shift theory and equilibrium configuration calculations. Analysis of the spectral shifts assumes they arise solely through the change in dispersion energies on electronic excitation and that this difference can be estimated using average excitation energies.

Laser-induced fluorescence spectroscopy and structure of microsolvated molecular clusters. Part 3.—Methyl and ethyl 4-amino- and 4-pyrrolidino-benzoates, and 4-amino-and 4-pyrrolidino-benzonitrile with Ar, Kr, CH4, H2O or CH3OH

An iterative band contour simulation strategy has been extended to the analysis of partially resolved rovibronic bands in the laser-induced fluorescence (LIF) spectra of complexes of jet-cooled, substituted aromatic amines (4-aminobenzonitrile, ethyl 4-aminobenzoate, methyl 4-aminobenzoate, 4-pyrrolidinobenzonitrile, ethyl 4-pyrrolidinobenzoate and methyl 4-pyrrolidinobenzoate) with both polar and non-polar solvents. The pattern which emerges on complexation with non-polar solvents is of a high degree of site selectivity. With polar solvents several distinct conformers are observed, with different solvent shifts, for the same complex stoichiometry.

Laser-induced fluorescence spectroscopy and structure of microsolvated molecular clusters. Part 1.—Structure Determined via rotational band contour analysis and semi-empirical calculation

An iterative rotational band contour simulation strategy has been applied successfully to the analysis of partially resolved rovibronic bands in the laser-induced fluorescence (LIF) spectra of 4-pyrrolidinobenzonitrile (PYRBN) and its clusters with argon, and of the ethyl 4-pyrrolidinobenzoate (PYRBEE) 1 ∶ 1 complex with water. An internally consistent set of structural parameters has been obtained which, in conjunction with the predictions of molecular mechanics (MM2) and MNDO calculations, provides experimentally based structures for the host molecules as well as their individually resolved van der Waals complexes. A broad range of simulations associated with alternative ‘trial’ structures is used to quantify the precision of the structural parameters and to indicate the sensitivity of the strategy.

Van der Waals complexes of perylene with rare-gas atoms

Abstract Fluorescence excitation spectra of weakly bound complexes between perylene (P) and rare-gas atoms (R) have been observed. Model calculations of potential surfaces of vdW complexes consisting of perylene and rare-gas atoms show that geometrical re-organisation is important and that thermal population of intermolecular P-R vibrational states may account in part for unresolved broadening at moderate stagnation pressures.

The dynamics of electronically excited states in the rare-gas–halogen systems

Detailed studies of the reactions of electronically excited IBr[E(0+)] with Xe, translationally excited Xe(3P2) with I2, Br2 and ICl, and Kr(3P2) with Br2 are reported.Vibrationally excited levels in the E(0+) ion-pair state of IBr were pumped using tunable synchrotron radiation and the ‘vibrational’ excitation function for XeBr(B) formation determined for energies ⩽66 kJ mol–1 above threshold. Branching ratios for XeBr(B)vs. XeI(B) formation, together with data for the alternative excitation-transfer channel have also been determined over the same energy range.Fully dispersed chemiluminescence spectra and their polarizations have been measured under superthermal atomic beam [Xe(3P2), Kr(3P2)]–Maxwellian gas (I2, Br2, ICl) conditions at average collision energies Ēcm < 120 kJ mol–1. These have revealed distinctive patterns of behaviour for the collisional energy dependence of branching into alternative atom- and excitation-transfer channels and of vibrational energy and rotational angular momentum disposals.A simple global model is presented to reconcile the varying patterns of behaviour found in the rare-gas–halogen systems under both ‘collisional’ and ‘complexed’ conditions, and in particular the dependence on the initial reagent state preparation.