Gregory D. Gillispie

Intramolecular hydrogen bonding. IX. Theoretical geometries of substituted anthraquinones relevant to proton transfer studies

Abstract Equilibrium geometries have been determined at the STO-3G RHF level for the ground electronic states of 1,4-, 1,5-, and 1,8-dihydroxyanthraquinone (DHAQ) and 1-aminoanthraquinone. Calculated bond lengths agree well with the limited available experimental data. The structure around the intramolecular hydrogen bonds is very similar in all three dihydroxyanthraquinones, including 1,8-DHAQ where both hydrogen bonds are to the same oxygen atom. The form arising from double intramolecular proton transfer in 1,4-DHAQ (quinizarin) is 10.4 kcal/mol higher in energy. Even though the hydrogen bond in 1-aminoanthraquinone is weak by the usual spectroscopic criteria, it appears to play a major role in organizing the molecules structure. The oxygen and nitrogen atoms of the NH…OC hydrogen bond are moved in opposite directions out of the mean molecular plane. This, along with rotation about the CN bond, brings the proton of the hydrogen bond close to the plane in which the carbonyl oxygen lone pair electrons are expected to lie.

Intramolecular hydrogen bonding. X. Comparison of free jet and Shpol'skii matrix electronic spectra of 1,8-dihydroxyanthraquinone

Abstract Site-selected fluorescence and excitation spectra in n -octane Shpolskii matrix, the free jet excitation spectrum, and the gas phase emission spectrum from the vibrationless level in S 1 are reported for 1,8-dihydroxyanthraquinone. The qualitative agreement of the gas phase and matrix spectra rules out any major influence of environment on the excited state proton transfer. However, the higher resolution of the jet excitation spectrum reveals important new and unique information on the excited state vibrational manifold. At about 600 cm −1 above the S 1 origin the excitation spectrum becomes very congested and the free jet rotational band contours also change. Combination bands and overtones of lower frequency fundamentals expected in this region are not found. Similar behavior occurs at 1100 cm −1 excess S 1 energy in 1,5-dihydroxyanthraquinone. We critically assess how reliably the excited state proton transfer potential function can be extracted from the spectra. One quantum of the OH stretching vibration is identified in the jet emission spectrum of 1,8-DHAQ.

In-situ tunable laser fluorescence analysis of hydrocarbons

Laser-induced fluorescence with fiber optic light delivery and collection has been applied for remote analysis of aromatic hydrocarbons. We report laboratory and field data acquired with a transportable and fully wavelength tunable laser system.

Real-time monitoring of chlorinated aliphatic compounds in air using ion mobility spectrometry with photoemissive electron sources

Ion mobility spectrometry with a photoemissive electron source is a promising approach for monitoring vapors of highly electronegative species such as chlorinated solvents and explosives. Electrons are generated over well-defined intervals by ultraviolet irradiation of a metal plate or metal-coated window by either a flashlamp or a pulsed laser beam, so no gating of the drift process is required. The negative ion mobility spectrum of air exhibits features predominately due to clusters of oxygen anion with water molecules. These species readily transfer electrons to chlorinated aliphatic compounds that undergo dissociative electron attachment to generate chloride ions. The ion mobility spectra change in a predictable fashion, permitting real-time detection of chlorinated species at low ppmV concentration. In this presentation we shall describe our methodology, display the response characteristics of our instrument, and summarize our investigations of the relevant ion-molecule reactions.

Intramolecular hydrogen bonding. VI. The phosphorescence of 9‐hydroxyphenalenone in an N‐hexane Shpol’skii matrix

Luminescence spectra have been measured for 9‐hydroxyphenalenone and its hydroxy‐deuterated form in an n‐hexane Shpol’skii matrix. Fluorescence spectra from the zero‐point level in S1 agree well with those reported for neon matrix by Bondybey et al. [J. Chem. Phys. 80, 5432 (1984)]. However, there is no measurable unrelaxed emission when higher S1 vibrational levels are excited; vibrational relaxation is much faster in n‐hexane than in solid neon. Vibrationally resolved phosphorescence spectra are the main new results reported here. The phosphorescence quantum yield for 9‐DPO is an order of magnitude stronger than for 9‐HPO and the origin band is red shifted by the deuterium substitution. It is proposed that the lowest triplet state is nonplanar.

Conformation of biphenyl in a cyclohexane host at 10 K

Abstract The site-selected fluorescence, phosphorescence, and luminescence excitation spectra of biphenyl in a cyclohexane Shpolskii matrix have been measured. There are three principal sites. The relative population of the sites depends on the freezing protocol. The S 1 ↔ S 0 0&.sbnd;0 band, a forbidden gg transition in D 2h planar biphenyl, is weakly observed for one of the sites. For this site the zero-phonon lines in fluorescence and phosphorescence are accompanied by prominent sidebands but such sidebands are nearly completely absent in the excitation spectrum. The other two sites have negligibly weak 00 bands in fluorescence and minimal sidebands. We attribute the sidebands to translational and/or librational motion of biphenyl in the cyclohexane cage.

Compound identification in multicomponent mixtures via REMPI at ambient conditions

Our efforts to chemically analyze aromatic hydrocarbons by resonance enhanced multiphoton ionization (REMPI) spectroscopy has been extended to mixtures. Indene has been detected in the headspace over coal tar and creosote via a characteristic band at 287.9 nm. Both sample and data collection occur for ambient pressure and temperature conditions. The indene spectra have also been examined for the effects of pressure broadening, which are discernible but small. High resolution absorbance spectra establish the feasibility for a similar REMPI strategy to detect benzene and toluene via an on-off resonance strategy, similar to that practiced in differential absorbance LIDAR (DIAL) and differential absorbance optical spectroscopy. Angular/wavelength acceptance criteria for the frequency doubling process, by which the tunable ultraviolet light is generated, are considered. Options for ultraviolet tunable solid state lasers, which are the key to real-time monitors based on REMPI technology, are assessed.

Chemical vapor detection in ambient air via REMPI

The 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectra of gas phase indene near the origin of the first electronic transition at 287.9 nm are reported. This work relates to chemically-specific measurements of trace organic constituents in ambiant air. The spectra were acquired to help assess options for on- and off-resonance REMPI, similar in concept to differential absorbance LIDAR (DIAL) and differential optical absorbance spectroscopy (DOAS). The 1+1 REMPI spectrum for indene at 50 ppbv concentration in air compares well with published high resolution absorbance spectra obtained for much longer pathlength and higher sample concentration. Extensive sequence band structure is observed near the origin. The rotational contours of the origin and sequence bands consist of narrow, nearly symmetric features, accompanied by a lower intensity broad sideband shading to the red. The narrow feature is not as narrow as in the highest resolution absorbance spectrum and cannot be explained in terms of the laser linewidth or intensity (saturation) broadening at high laser power; pressure broadening may be the source. We also report styrene 1+1 REMPI spectrum to document instrument improvements made since our previous study.

Real-time monitoring of BTEX in air via ambient-pressure MPI

We have developed and begun to field test a very sensitive method for real-time measurements of single-ring aromatic hydrocarbons in ambient air. In this study, we focus on the efficient 1 + 1 resonance enhanced multiphoton ionization (REMPI) of the BTEX species in the narrow region between 266 and 267 nm. We particularly emphasize 266.7 nm, a wavelength at which both benzene and toluene exhibit a sharp absorbance feature and benzene and its alkylated derivatives all absorb. An optical parametric oscillator system generating 266.7 nm, a REMPI cell, and digital oscilloscope detector are mounted on a breadboard attached to a small cart. In the first field test, the cart was wheeled through the various rooms of a chemistry research complex. Leakage of fuel through the gas caps of cars and light trucks in a parking lot was the subject of the second field test. The same apparatus was also used for a study in which the performance of the REMPI detector and a conventional photoionization detector were compared as a BTEX mixture was eluted by gas chromatography. Among the potential applications of the methodology are on-site analysis of combustion and manufacturing processes, soil gas and water headspace monitoring, space cabin and building air quality, and fuel leak detection.

REMPI detection of volatile aromatic hydrocarbons in ambient air

Volatile aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes (BTEX), and styrene, aniline, and phenol can be directly and sensitively detected in ambient air by resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The REMPI spectra closely resemble conventional absorbance spectra, but the REMPI technique is far more sensitive. Detection limits for directly focused tunable laser light into the ionization cell are less than 10 ppbv for BTEX and less than 1 ppbv for styrene, aniline, and phenol. Benzene in aqueous solution was remotely detected down to concentrations at the (mu) g/L level by a headspace analysis in which light was delivered to the ionization cell over a 20-meter long optical fiber.