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Small carbon clusters (Cn, n = 2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry. The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C3. Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge-transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n = 4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n = 4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C10.

Small carbon clusters (Cn, n = 2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry. The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C3. Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge-transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n = 4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n = 4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C10.

This publication contains conference paper and poster abstracts on the following general topics: Techniques of molecular quantum mechanics; Analytic derivatives; Electron correlation; Structure and spectroscopy; Dynamics and reactions., This document contains the program and abstracts of the Conference on Molecular Quantum Mechanics: Analytic Gradients and Beyond held in Budapest, Hungary on 29 May-3 June 2007. Supported by ONR Proposal No. 034733-01. The conference proceedings were published in Molecular Physics, v105 n19-22 p2439-2926, 2007. ISBN 0-415-34264-3.

The isomers arachno-2-gallatetraborane(10) and arachno-1-gallatetraborane(10) have been studied by ab initio molecular quantum mechanical methods, following the synthesis of the 2-derivative by Downs and coworkers; substantial basis sets, e.g. Ga (14s 11p 6d/7s 5p 3d), have been used in conjunction with the self-consistent-field approximation and both structures are genuine minima, the terminal gallium structure being predicted to lie lower than its isomer by 24 kcal mol–1(1 cal = 4.184 J).

The dimers of the three chlorides of gallium(III) have been studied by using ab initio quantum-mechanical methods at high levels of theory for these relatively large molecules. The problem of accurately predicting the geometry of the bridge region is investigated. Large basis sets (including f functions) and electron correlation are required to obtain the Ga-Clb distance correctly (b denotes bridging). Theoretical bond angles converge much more quickly with respect to level of theory, leading us to suggest that the experimental Ga-Clb-Ga angle of 87.2° for Ga2Cl2H4 be revised slightly upward to 90°. The H-Ga-H angle is predicted to be 132°. The geometry and energy of all possible isomers of the monochloro- and dichlorogallane dimers are also reported, as well as those for Ga2Cl6. Agreement between theory and experiment for Ga2Cl6 is significantly better than for Ga2Cl2H4. For each Cl on a bridging site, the dimerization energy (relative to an analogous molecule with a H at that site) is increased by 6-8 kcal mol-1, and for each terminally bonded Cl the dimerization energy is decreased by 1-2 kcal mol-1 relative to the analogous molecule with a H bonded at that position. Harmonic vibrational frequencies for the monochlorogallane dimer are predicted and compare well with the experimental infrared and Raman fundamentals.

The isomers arachno-2-gallatetraborane(10) and arachno-1-gallatetraborane(10) have been studied by ab initio molecular quantum mechanical methods, following the synthesis of the 2-derivative by Downs and coworkers; substantial basis sets, e.g. Ga (14s 11p 6d/7s 5p 3d), have been used in conjunction with the self-consistent-field approximation and both structures are genuine minima, the terminal gallium structure being predicted to lie lower than its isomer by 24 kcal mol–1(1 cal = 4.184 J).

The dimers of the three chlorides of gallium(III) have been studied by using ab initio quantum-mechanical methods at high levels of theory for these relatively large molecules. The problem of accurately predicting the geometry of the bridge region is investigated. Large basis sets (including f functions) and electron correlation are required to obtain the Ga-Clb distance correctly (b denotes bridging). Theoretical bond angles converge much more quickly with respect to level of theory, leading us to suggest that the experimental Ga-Clb-Ga angle of 87.2° for Ga2Cl2H4 be revised slightly upward to 90°. The H-Ga-H angle is predicted to be 132°. The geometry and energy of all possible isomers of the monochloro- and dichlorogallane dimers are also reported, as well as those for Ga2Cl6. Agreement between theory and experiment for Ga2Cl6 is significantly better than for Ga2Cl2H4. For each Cl on a bridging site, the dimerization energy (relative to an analogous molecule with a H at that site) is increased by 6-8 kcal mol-1, and for each terminally bonded Cl the dimerization energy is decreased by 1-2 kcal mol-1 relative to the analogous molecule with a H bonded at that position. Harmonic vibrational frequencies for the monochlorogallane dimer are predicted and compare well with the experimental infrared and Raman fundamentals.

The development of efficient and safe conventional (i.e., nonnuclear) propellants and/or fuels is a goal of obvious technological significance. A desirable quality of such a propellant is clearly a high ratio of energy release to mass. The present hypothesis rests on a simple, but previously unrecognized, analogy between oxygen and sulfur. Preliminary studies showed that the oxygen ring systems are sufficiently promising to warrant the detailed, high-level theoretical research reported here. Keywords: Quantum chemistry; Propellants; Oxygen compounds; Cyclic compounds.

This research had two goals, the development of coupled cluster theory and the application of molecular electronic structure theory to the alkali dioxide molecules LiO2, NaO2, and KO2. The research on coupled cluster theory was successful. It was possible to take the single and double excitation coupled cluster method (CCSD) from an exotic, fringe technique to the very center of applied quantum chemistry. Critical to this remarkable transformation was the formulation of an analytic gradient method for the CCSD method. A sustained effort on the LiO2 and NaO2 molecules was begun, however, unusual difficulties (specifically symmetry-breaking) were discovered for LiO2. However, all these problems were eventually solved, resulting in a massive and truly definitive paper, to be submitted for publication very shortly. Work using the same methods is well underway for NaO2. Coupled Cluster method, Energetic materials, Alkali dioxide molecules, Analytic gradient method, Lithium dioxide, Sodium dioxide, Potassium dioxide.