Natural bond orbital (NBO) and topological electron density analyses have been used to investigate the electronic structure of phosphazenes [N3P3R6] (R = H, F, Cl, Br, CH3, CF3, N(C2H4); 2R = O2C6H4), [N4P4Cl8], and H[NPCl2]4H. Using the former, the two most likely phosphazene bonding alternatives, negative hyperconjugation and ionic bonding have been critically evaluated. Ionic bonding, as suggested by topological analysis, was found to be the dominant bonding feature, although contributions from negative hyperconjugation are necessary for a more complete bonding description. Substituent effects on the P-N bond have been assessed and cases of bond length alternation have been rationalized using this combined bonding model, which supersedes previous models involving d-orbital participation, leading to an explanation for the observed bond length alternation found in some linear polyphosphazenes. In addition, common aromaticity indicators, nucleus independent chemical shifts (NICS) and para-delocalization indices (PDI), have been determined for the cyclophosphazenes.

Single-photon infrared emission spectroscopy (SPIRES) has been used to measure emission spectra from polycyclic aromatic hydrocarbons (PAHs). A supersonic free-jet expansion has been used to provide emission spectra of rotationally cold and vibrationally excited naphthalene and benzene. Under these conditions, the observed width of the 3.3-micrometers (C-H stretch) band resembles the bandwidths observed in experiments in which emission is observed from naphthalene with higher rotational energy. To obtain complete coverage of IR wavelengths relevant to the unidentified infrared bands (UIRs), UV laser-induced desorption was used to generate gas-phase highly excited PAHs. Lorentzian band shapes were convoluted with the monochromator-slit function in order to determine the widths of PAH emission bands under astrophysically relevant conditions. Bandwidths were also extracted from bands consisting of multiple normal modes blended together. These parameters are grouped according to the functional groups mostly involved in the vibration, and mean bandwidths are obtained. These bandwidths are larger than the widths of the corresponding UIR bands. However, when the comparison is limited to the largest PAHs studied, the bandwidths are slightly smaller than the corresponding UIR bands. These parameters can be used to model emission spectra from PAH cations and cations of larger PAHs, which are better candidate carriers of the UIRs.

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The photodissociation of O3 in the Hartley band has been investigated by high-resolution photofragment translational spectroscopy (PTS). At λdiss=248 nm we determined the quantum yield of the dominant decay channel leading to O2(1Δg)+O(1D) and the fragment vibrational state distribution. The fragment recoil anisotropy (β=1.25±0.15) was found to be independent of the fragment vibrational states. Between λdiss=275 and 295 nm β assumes a value of 1.6±0.2, which exceeds the value expected for a simple impulsive process. Photofragment yield measurements carried out by PTS between 272 and 286 nm revealed a strong fluctuation of the vibrational state distribution with λdiss. Based on the small but distinct structure superimposed on the broad continuum of the Hartley band and the findings of recent 3D wave packet calculations, we propose this fluctuation, a manifestation of wavelength-dependent partial cross sections, to arise predominantly from an interference effect. The latter occurs between the part of the initially prepared wavepacket which propagates directly into the exit channel and a (minor) part which is temporarily trapped by the motion of the bending and symmetric stretching modes in the excited O3 molecule.