The valence shell spectroscopic and thermodynamic properties of thiophene have been investigated through photoabsorption and ion fragmentation studies. The absolute photoabsorption cross-section has been measured between the ionisation threshold and 35 eV using a double ion chamber and synchrotron radiation. New Rydberg series converging onto either the B∼2A1 or the G∼2A1 limits have been observed. The photoabsorption spectrum is dominated by broad features due to intravalence transitions, and tentative assignments have been proposed based upon a term value analysis. Time-of-flight mass spectra have been recorded in the photon energy range 12–28 eV and these have allowed the appearance energies for 18 fragment ions and the doubly charged parent ion to be determined. Molecular orbital calculations have been performed to compute the energies of several neutral or ionic species relevant to the lowest energy fragmentation channel. These energies, when compared with the experimental data, help to define the ion formation mechanisms.

The unimolecular decomposition of internal-energy-selected furan molecular ions has been studied by means of threshold-photoelectron–photoion coincidence spectroscopy. Monochromatic synchrotron radiation was used as the ionisation source, and the molecular ion internal energy was established through the detection of a threshold electron. A pulsed electric field was applied to extract the ions from the interaction region and direct them towards a time-of-flight mass spectrometer. Breakdown curves were measured for photon energies up to 30 eV, and these have allowed appearance energies for a wide range of fragment ions to be determined. In the threshold region the breakdown curves have been measured for various ion residence times by introducing electronic delays between the detection of the threshold electron and the application of the ion extraction field. The breakdown curves have been modelled using the RRKM (Rice, Ramsperger, Kassel and Marcus)/QET (quasi-equilibrium theory) approach, and this has allowed activation energies and transition state geometries to be deduced. The threshold photoelectron spectra of furan-h4 and furan-d4 have been measured from the ionisation threshold to 28 eV, and vibrational structure has been observed and assigned in the bands due to the X2A2, the A2B1 and the G2A1 states. Vibrational progressions discernible between 16.2 and 17.3 eV have been attributed to autoionisation from a p-type Rydberg series converging onto the G2A1 state ionisation threshold.

The design, construction and performance of a threshold-photoelectron–photoion coincidence (TPEPICO) spectrometer for the study of unimolecular decomposition in polyatomic ions is described. The spectrometer incorporates a hemispherical electrostatic energy analyser and a time-of-flight (TOF) mass spectrometer. The entrance lens to the hemispherical analyser has been designed to have a high collection efficiency for low energy electrons but to discriminate strongly against energetic electrons. This arrangement has resulted in a resolution of about 3.5 meV being achieved for the threshold electron peak recorded at the krypton 2P3/2 ionisation limit. A pulsed electric field is used to extract the ions from the interaction region and propel them towards the TOF analyser. Computer modelling has been used to trace the electron and ion trajectories through their respective analysers. These simulations have enabled the effective interaction volume to be defined, and this has allowed the transmission efficiency of energetic fragment ions, formed through a process which also yielded a threshold electron, to be quantified. The ion TOF peak shape has been examined as a function of initial kinetic energy and as a function of ion residence time. The contribution of energetic fragments, having specific initial spatial and directional properties, to the TOF peak shape has been determined by tracing the paths of individual ions. The actual performance of the spectrometer is illustrated by a TPEPICO spectrum of the krypton isotopes. Experimental breakdown curves for furan are presented as an example of the use of the apparatus to study unimolecular decomposition in polyatomic ions. By introducing a delay between the detection of the threshold electron and the application of the ion extraction field, breakdown curves can be recorded as a function of ion residence time in the source region. The procedure for analysing the data is described, and the experimental factors that need to be taken into account to obtain a meaningful comparison with theoretical predictions are discussed.

The absolute photoabsorption cross-section of pyrrole has been measured between the ionisation threshold and 35 eV using a double ion chamber and monochromated synchrotron radiation. An interpretation has been proposed for some of the observed structure in terms of Rydberg series converging onto the (A) over tilde (2)B(1) state threshold. In addition, the possibility that several broad features may be attributed to pi --> pi* transitions is discussed. Time-of-flight mass spectra have been recorded for excitation energies between 11.8 and 77.5 eV, and appearance energies have been determined for seventeen fragment ions and the doubly charged parent ion. These have enabled, previously unknown, heats of formation to be estimated for five ions, including the doubly charged parent ion. A high resolution mass spectrometer, equipped with an electron impact ionisation source, has been used in B/E and B(2)/E linked scanning modes to identify reactant/product pairings. Fragmentation processes leading to the production of several high intensity fragment ions have been modelled using ab initio and semi-empirical methods. (C) 1999 Elsevier Science B.V. All rights reserved.

Three experimental techniques (photoabsorption, photoelectron, and photoion spectroscopy) have been used to study the spectroscopic and thermodynamic properties of furan. The absolute photoabsorption cross-sections of furan-h4 and furan-d4 have been measured using a double ion chamber and a new Rydberg series converging onto the G2A1 ionisation threshold has been observed. HeI-excited photoelectron spectra of the X2A2, the A2B1 and the G2A1 states of furan-d4 have been recorded. Vibrational structure has been observed in all three bands and has allowed the energies of the ν3, ν4, ν6 and ν8 vibrational modes to be determined. Time-of-flight mass spectra have been recorded using monochromatic synchrotron radiation, and appearance energies have been measured for 19 fragment ions and the doubly charged parent ion. The fragmentation processes leading to the production of several high-intensity fragment ions have been modelled using ab initio and semi-empirical methods.

The absolute photoabsorption, photoionisation and photodissociation cross-sections and the photoionisation quantum efficiency of benzene and hexadeuterobenzene have been measured from the ionisation threshold to 350 Angstrom using a double ion chamber and monochromated synchrotron radiation. The HeI excited photoelectron spectrum of C6D6 has been recorded and the vibrational structure exhibited in the (XE1g)-E-2, A(2)E(2g), (EB1u)-B-2 and F(2)A(1g) bands has been analysed with the aid of theoretical predictions and by analogy with the recently reported high-resolution photoelectron spectrum of C6H6. The photoabsorption spectrum displays extensive vibrational structure extending from the ionisation threshold to similar to 730 Angstrom, and many of these absorption features have been arranged into Rydberg series. A detailed assignment of the vibrational progressions associated with some Rydberg states has been accomplished by making use of the corresponding photoelectron spectrum. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining wavelength regions. (C) 1998 Elsevier Science B.V.

Three experimental techniques – photoabsorption, photoelectron and photoion spectroscopy – have been combined with many-body Green's function calculations to investigate the spectroscopic and thermodynamic properties of toluene. The absolute photoabsorption cross section has been measured from the ionisation threshold to 350 Å using a double ion chamber and monochromated synchrotron radiation. Some of the structure has been arranged into Rydberg series. He I excited photoelectron spectra of toluene-h8 and toluene-d8 have been recorded and vibrational progressions have been observed in three bands. Fragmentation processes have been studied by measuring time-of-flight spectra and appearance energies have been determined for many small fragments, and the doubly-charged parent ion. The many-body Green's function approach, specially adapted for the outer valence region, has been used to calculate ionisation energies and pole strengths.

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Molecular mass distributions of coal-tar pitch fractions separated by planar chromatography (P-C) have been compared by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, carried out in the absence of added matrix. Solvent pairs used in the P-C separation were pyridine-acetonitrile, pyridine-N,N-dimethylformamide and tetrahydrofuran-toluene. Molecular complexity and average molecular masses were found to increase with decreasing mobilities of the fractions. UV-fluorescence spectroscopy showed shifts of peak intensities to longer wavelengths and decreasing quantum yields, suggesting the presence of progressively greater concentrations of large polynuclear aromatic systems with decreasing mobility of samples in P-C. Fractions immobile in pyridine gave MALDI mass spectra of low intensity, indicating that larger molecules are less easily desorbed or ionized, The observed similarity of upper-mass limits of planar-chromatographic fractions of different mobility is thought to suggest the presence of relatively large-molecular mass (MM) materials of variable polarity in coal-tar pitch. The results were consistent with earlier investigations showing the presence of high molecular mass materials in coal-derived liquids, MM-distributions determined by size exclusion chromatography (SEC) were also observed to increase with decreasing mobility of the fractions on P-C plates. This qualitative agreement with results from MALDI-MS indicates that reported shifts of polar molecules to shorter elution times in SEC (i.e. to apparently larger-MMs) were not of a sufficient magnitude to distort the relative ordering of MM-distributions of PC-fractions observed by SEC. A number of problems relating to the refinement of MALDI-MS determinations on complex mixtures are discussed.

Quantum chemistry calculations and experimental investigations of the unimolecular dissociation step of the molecular radical cation of tetramethylsilane losing a methyl radical to give the m/z 73 trimethyl silyl cation have been carried out. There is an apparent lack of an ion/neutral complex in the minimum energy reaction pathway, where in quaternary carbon compounds there is evidence of such complexes. It is suggested that at interfragment separation distances at which an ion/neutral complex would be expected the electrostatic interactions are “swamped” by the strength of the conventional covalent bonding interactions which are an order of magnitude greater than in the quaternary carbon systems at similar separations.

This chapter discusses the molecular mass distributions of coal products relevant to pyrolysis, liquefaction, and combustion. The two techniques commonly employed to determine the molecular mass distributions (MMDs) of products derived from coal are mass spectroscopy (MS) and size exclusion chromatography (SEC). The recent advent of laser MS techniques suggested that the MMD of coal derivatives extends beyond 10,000 daltons. The chapter describes chromatograms and MMDs from SEC, using the traditionally employed tetrahydrofuran (THF) solvent, which are compared to those, obtained using a new mobile phase N-methyl-2- pyrrolidinone (NMP) and to mass spectra from laser induced mass analysis (LIMA). From the results reported, it is clear that NMP surpasses THF as a mobile phase for coal-derived materials. It offers the opportunity to analyze more of the sample and show no adsorption effects. Polystyrene calibration for these materials remains a source of error, particularly for higher mass materials. The combination of NMP–SEC and mass analysis by LIMA and other new MS techniques provides a possible route to the manufacture of relevant coal calibrants and more accurate MMDs.

Coal pyrolysis tars and liquefaction extracts prepared from the set of eight Argonne coal samples have been characterized by MALDI-MS. A Kratos Kompact MALDI III linear time-of-flight mass spectrometer was used, with a nitrogen laser operating at 337 nm and sinapinic acid as matrix. Spectra were collected by summing 50 laser pulses at low laser fluence to avoid fragmentation of desorbed ions. At low mass, 200-500 u, spectra from the pyrolysis tars and liquefaction extracts showed common features: intense peaks originating from the sinapinic acid matrix, m/z 205, 224 and 246, and, a range of peaks in the mass range from about 250 u to 400 u, which probably corresponds to overlapping homologous series of apparently polar material. A peak of ion intensity between 1000 and 5000 u was systematically observed, which was sample dependent and not always similar for the tar and extract prepared from the same coal. At high masses, separate trends were observed for coal pyrolysis tars and liquefaction extracts: molar mass ranges of pyrolsis tars were smaller, and showed no particular trend with carbon content of the original coal; these findings are consistent with size-exclusion chromatography derived findings. Spectra of liquefaction extracts extended over wider ranges of molar masses and increased with increasing coal rank; the highest masses extended to 50 000 u. Overall, pyrolysis tars appeared structurally different from liquefaction extracts and more difficult to desorb. The effect of changes in laser fluence and ion extraction voltage on mass spectra have been investigated: high-mass regions of the spectra were found to be very sensitive to the magnitude of the ion extraction voltage. No carbon clusters or fullerene structures were detected.

The Argonne set of coals cover the rank range from lignite to semi-anthracite; these samples have been studied by matrix-assisted laser desorption mass spectrometry (MALDI-MS) in a time-of-flight mass spectrometer equipped with a nitrogen laser at 337 nm, using sinapinic acid as matrix. The coal particle size was less than 5 microns. The characteristics of the MALDI-MS spectra of the set of coals were found to be rank-related; desorption from high-rank coals was found to take place with greater relative ease than from low-rank coals. Two major features were found in all spectra: a homologous series of peaks in the 200-500 u mass range and an intense peak between 1000 and 5000 u, the particular shape of the peak depending on coal rank. A continuum of lower intensity peaks extending to very large molecular masses was found in all spectra, the upper limit of molecular masses increasing with coal rank at the same laser fluence. The effect of changes in laser power on spectra was investigated: upper mass limits were found to increase with power up to the detection limit of the instrument but low-mass parts of spectra were found to distort, possibly due to detector overloading. A maximum laser fluence value acceptable over the coal-rank range represented by these samples could therefore not be easily defined. None of the mass spectra showed evidence of the presence of either carbon clusters or fullerene formation, indicating that laser fluences did not reach intensities high enough to induce substantial secondary reactions. Comparing molecular mass distributions detected by MALDI of coal pyrolysis tars and directly from coals suggests the MALDI and pyrolytic mechanisms of volatile release to be structurally different; in particular, the preferential evaporation of lighter species which occurs during pyrolytic tar evolution (and during field-ionization mass spectroscopy) appears to evolve material with a more restricted range of molecular masses compared to laser desorption mechanisms.

Laser-desorption mass spectrometry (LD-MS) has been used to investigate the mass spectra of a suite of standard polynuclear aromatic hydrocarbons and a sample of fullerenes (mixed C-60 and C-70), as a part of a study of high molecular mass ions in tars and liquid extracts derived from coal. Positive- and negative-ion LD-MS spectra of these standards were compared with electron impact spectra to elucidate the mechanism of laser desorption. The data indicate that ion-molecule reactions occur during the desorption step giving hydrogenated molecular ions via mechanisms analogous to self-chemical ionization; fullerenes gave no hydrogenated molecular ions. Carbon cluster ions (both positive and negative ions) are formed by the laser energy with fewer carbon atoms than the original molecule, suggesting their formation to result from the thermal destruction of parent molecules. However, none of the standard compounds gave cluster ions greater than the hydrogenated molecular ions. This finding provides confirmation that large molecular mass materials identified in coal-derived liquids originated from the sample itself and did not form from smaller molecular mass compounds under the power of the laser.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

This paper describes the application of matrix assisted laser desorption ionization (MALDI) to coals and coal-derived materials using sinapinic acid as the matrix. The mass range of molecules in coal and coal-derived materials has been extended by a factor 100 compared with prelaser desorption mass spectrometric measurements. A peak of intensity is observed for coals and coal-derived materials in the mass range 1000–5000 u which is sample dependent. The upper mass ranges of the spectra vary according to sample, ranging from over 260 000 u for Point of Ayr coal and 200 000 u for a coal tar pitch, to 20 000 u for a maceral concentrate liquefaction extract. These results confirm earlier results using laser desorption mass spectrometry and are in broad qualitative agreement with size exclusion chromatography results. Detailed quantitative agreement, however, requires further work. The implications of this work for the debate on coal structure and models of coal conversion are considerable.

Two kerogen samples from shales of different geological age, selected from each of Types I, II and III, have been characterized by matrix-assisted laser desorption ionization mass spectroscopy (MALDI-MS). A Kratos Kompact MALDI III time-of-flight mass spectrometer equipped with a nitrogen laser operated at 337 nm was used on samples mounted in a matrix of sinapinic acid. The spectrometer was used in linear mode with a mass range up to 270 000 u and an ion extraction voltage of 20 kV. Individual spectra from 50 laser shots at low laser fluence were summed for each spectrum. The present study is a first attempt at characterizing kerogens by MALDI-MS. At low mass (200-500 u) families of peaks consisting of overlapping homologous series of apparently polar compounds have been observed: these peaks appear to be distinct from those due to the matrix material. In higher mass regions of the spectra, continua of masses extending from 1000 u up to around 10 000 u were observed; ions at higher masses were separated to baseline up to masses of 50 000 u. Evidence was found for a relationship between the minimum laser power sufficient for activating the MALDI-ionization process and the geological age of the kerogens; the laser power decreased as the geological age increased. In the case of one kerogen, a high laser fluence was used to generate molecular ions up to the detection limit of the mass spectrometer, in excess of 260 000 u. This paper presents results from a preliminary study of kerogens which requires an in-depth examination of the desorption process in complex mixtures.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The production of carbon clusters (but not fullerenes) from coal-derived material by the use of laser desorption mass spectrometry is described. Benzo(e)pyrene, a standard polynuclear hydrocarbon found in coal tars, also gave carbon clusters despite using low laser power to avoid pyrolysis of target molecules. Positive- and negative-ion spectra of the standard included carbon clusters. These clusters are formed by the laser energy and can be confused with aromatic hydrocarbons of the same nominal masses. They have no relevance for studies of coal structure.