T. Frank Palmer

Chemiluminescence from reactions of bis(pentachlorophenyl)oxalate, hydrogen peroxide and fluorescent compounds. Kinetics and mechanism

The chemiluminescence arising from the reaction between bis(pentachlorophenyl)oxalate (PCPO) and hydrogen peroxide in the presence of the fluor 9,10-diphenylanthracene (DPA) has been examined in detail. The effects of varying reactant, base catalyst and fluor concentrations on chemiluminescence decay rates and quantum yields have been investigated and a mechanism which accounts for the kinetics of the reaction has been proposed. The rate-determining step is a reaction between PCPO and the hydroperoxide anion (OOH–) and does not involve the fluorescent activator. The lifetime of the reactive intermediate formed in the rate-determining step is short (5 × 10–7 s) and consistent with the proposed mechanism.

Vapour pressure measurements on some organic high explosives

Equilibrium vapour pressures of 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitro-m-xylene (TNX), 2,4,6-trinitromesitylene (TNM), 2,4,6-trinitroaniline (TNA), 2,4,6-trinitro-m-phenylene diamine (DATB), 2,4,6-N-tetranitro-N-methylaniline (Tetryl), 2,4,6-trinitrophenol (picric acid), 2,4,6-trinitroresorcinol (styphnic acid, TNR), 2,4,6-trinitro-m-cresol (TNC), pentaerythritoltetranitrate (PETN), cyclotrimethylenetrinitramine (RDX), sym-cyclotetramethylene-tetramitramine (HMX) and nitroguanidine have been measured over a range of temperature using the Knudsen cell technique. The data were found, within the limits of experimental error, to follow the Clausius–Clapeyron equation and values for the constants A and B in the integrated form of the equation log10p/N m– 2=A–B/T determined. The standard enthalpies, entropies and Gibbs energies for sublimation are quoted.

Atom—atom potential parameters for van der Waals complexes of aromatics and rare-gas atoms

Abstract The structure of the dispersion terms in the interaction for van der Waals complexes of aromatics and rare-gas atoms is examined and found to contain important three-body terms. Using calculations for benzene with a sequence of rare gases, the errors in approximating the three-body π-electron dispersion energy by atom—atom terms are found to be generally significant, but small for certain symmetrical positions (including the equilibrium position). For these positions, the total interaction coefficients are compared with empirically determined values using a Lennard-Jones model. Agreement is good after scaling to allow for higher-order terms.

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.

Chemiluminescence from reactions of bis(pentachloro-phenyl)oxalate, hydrogen peroxide and fluorescent compounds. Role of the fluor and nature of chemielectronic process(es)

The effect of fluorescence activators on the quantum yields and emission decay rates of the chemiluminescence arising from the reaction between bis(pentachlorophenyl)oxalate and hydrogen peroxide has been examined in detail. Chemiluminescence was detected for a range of fluors with excited-singlet-state energies <400 kJ mol–1. The chemiluminescence quantum yields were strongly dependent upon the nature and concentrations of the fluorescent activators, but the chemiluminescence decay rates were independent of the properties of the fluor. Delayed addition of the fluor had no effect upon both chemiluminescence quantum yields and decays.A quantitative relationship has been shown to exist between the normalised chemiluminescence quantum yields (ϕCL/ϕF) and the half-wave oxidation potential of the fluorescence activators. The results are discussed in terms of chemically initiated electron transfer between a reactive intermediate and the fluorescence activators. A mechanism involving 3-pentachlorophenoxy-3-hydroxy-1,2-dioxetanone as an intermediate is proposed.

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.