June McCombie

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.

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.

Photochemistry in jet-cooled aniline derivatives

Abstract High-resolution laser-induced fluorescence excitation spectra of four para-substituted pyrrolidino- and dimethylamino-benzoic acid nitriles and esters have been compared under jet-cooled conditions and the fluorescence decay profiles of jet-cooled dimethylaminobenzoic acid nitrile (DMABN) and methyl ester (DMABME) have been determined using synchrotron radiation. The resonant two-photon ionization mass spectra (REMPI-TOF-MS) of DMABME revealed the presence of monomeric and dimeric DMABME and of van der Waals complexes with water. The loss of a well-resolved B-type rotational structure in the LIF excitation spectra for DMABME as compared to DMABN is indicative of state mixing between the initially populated 1 L b and the energetically close-lying 1 L a excited state and is paralleled by the onset of continua in the excitation spectra and associated ‘anomalous’ red-shifted emission and non-exponentiality in the fluorescence decays. However, the REMPI-TOF-MS spectra of jet-cooled DMABME show that complexes with water make a major contribution to the underlying continuum and the associated red-shifted fluorescence and serve to emphasise the essential role of the ‘solvent’. A further contribution comes from DMABME dimers. The results regarding the enhancement of red fluorescence of DMABME with respect to DMABN are discussed in terms of an adiabatic photochemical reaction, i.e. formation of a rotamer on the excited state hypersurface and nearly full electron transfer from the donor to the acceptor moeity, the so-called twisted intramolecular charge transfer (TICT) state. Microsolvation and state mixing in DMABME facilitate this process.

Complex‐forming reactions in neutral noble gas clusters

The complex‐forming reaction between CH3F and HCl imbedded in medium–large argon clusters has been monitored using molecular beam infrared spectroscopy. A ‘‘pickup’’ technique has been used which consists of exposing the (CH3F)mArn clusters, formed by expansion of a dilute mixture of CH3F in Ar from a supersonic nozzle, to a cross flux of HCl molecules. The clusters are characterized, with and without the reactant flow, by bolometric photoevaporation IR spectroscopy using line tunable CO2 lasers. Our data show that the products can be clearly distinguished from the reactant species. A plot of reactivity vs nozzle pressure, which is directly related to cluster size, indicates that, within our range of measurements, the HCl diffusion into the clusters is completed in less than 100 μs.

VLT/UVES and WHT/UES absorption spectroscopy of the circumstellar envelope of IRC +10° 216 using background stars: First results and a search for DIBs

A unique and novel set of observations has been undertaken to probe the circumstellar envelope (CSE) of the nearby (130 pc) carbon star IRC +10 216 using optical absorption spectroscopy towards background stars lying beyond the envelope. The primary aim of the observations is to search for diuse band (DIB) carriers in the CSE, for which the mass-losing envelopes of carbon stars are a likely place of origin. Our principal target is a V = 16 G-type star located 37 00 from IRC +10 216 and was observed with VLT/UVES. A detailed model atmosphere and abundance analysis shows that it is somewhat metal-poor and has conrmed that it lies far beyond IRC +10 216. The circumstellar H+2H2 column density expected along the line of sight towards this target is relatively high, 2 10 21 cm 2 , and is large compared to that derived from the small interstellar extinction estimated in the zone of IRC +10 216 at b =+ 43, EB V < 0:03 mag. The CSE is certainly detected in the K i resonance lines, which are centred at the heliocentric velocity of IRC +10 216 and have FWHM 30 km s 1 , consistent with twice the terminal expansion velocity of the circumstellar gas. The data show also that circumstellar Na i is very probably detected, as seen towards two background stars. The strongest DIB (6284 A) present in the UVES wavelength coverage is detected but very probably arises in the foreground ISM. No DIB is detected at 6614 A, or elsewhere. Overall, the data suggest that the DIB carriers, if present in the CSE, have a low abundance relative to H in the C-rich envelope of IRC +10 216, in comparison with this ratio in the ISM.

Spatial distribution and interpretation of the 3.3 μm PAH emission band of the Red Rectangle

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Infrared spectroscopy and dimer formation at the surface of medium-large argon clusters

M.R. Hockridge, S.M. Knight, E.G. Robertson, J.P. Simons, J. McCombie and M. Walker Phys. Chem. Chem. Phys. 1, 407-413 1999.

HIGH RESOLUTION ELECTRONIC SPECTROSCOPY OF MOLECULAR CONFORMERS. METHYL- AND ETHYL-3-AMINOBENZOIC ACID ESTERS

The spatial distribution of 3.3m polycyclic aromatic hydrocarbon (PAH) and associated emission in the inner region of the Red Rectangle nebula has been determined using the UIST imager spectrometer at the United Kingdom Infrared Telescope (UKIRT). Interpretation of the 3.3m feature as comprising two spectroscopic components centred at 3.30 and 3.28m, as put forward by Song et al., is supported by these data which reveal that they have different spatial distributions. It is deduced that there are two classes of 3.3m band carrier with a peak wavelength separation of ∼0.02m. From comparison of the 3.3m observations with laboratory and theoretical spectra for a range of PAH molecules, it is proposed that the 3.28 and 3.30m components arise from ‘bay’ and ‘non-bay’ hydrogen sites, respectively, on the periphery of small neutral PAHs. Observational data are also obtained for L-band continuum emission and for the Pfund ɛ hydrogen recombination line.