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The thermal decomposition kinetics of 2-chloroethylsilane and derivatives in the gas phase has been studied computationally using density functional theory, along with various exchange-correlation functionals (UM06-2x and ωB97XD) and the aug-cc-pVTZ basis set. The calculated energy profile has been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the UM06-2x/aug-cc-pVTZ approach are in good agreement with the experimental data. The decomposition of 2-chloroethyltriethylsilane species into the related products [C2H4 + Et3SiCl] is characterized by 6 successive structural stability domains associated to the sequence of catastrophes C8H19SiCl: 6-C†FCC†[FF]-0: C6H15SiCl + C2H4. Breaking of Si–C bonds and formation of Si–Cl bonds occur in the vicinity of the transition state. All calculations presented in this work have been performed at the Flemish Supercomputer Center (Vlaams Supercomputer Centrum). This cluster has been financed by budgets obtained from the Katholieke Universiteit Leuven, as well as from individual contributions by users, and funding obtained from the Hercules foundation and the Flemish government.

The kinetics and mechanisms of pyrolysis of 1,3-dihydroisothianaphthene-2,2-dioxide toward benzocyclobutene have been theoretically studied using canonical transition state theory (CTST), statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and bonding evolution theory (BET) in conjugation with M06-2X/aug-cc-pVTZ calculations. The CTST slightly breaks down to estimate the reaction rate of the cheletropic extrusion. RRKM results indicated that the cheletropic extrusion and electrocyclic reaction require energy barriers of 171.3 and 122.2 kJ/mol to be overcome; and can be characterized respectively by 7 and 3 phases associated to the sequence of catastrophes C 8 H 8 SO 2 ( 1 ): 7-[FF]C † C † FFF-0: C 8 H 8 + SO 2 and C 8 H 8 ( 2 ): 3-[F † F † ]C-0: C 8 H 8 ( 3 ). For the cheletropic extrusion, breaking of the C 7 –S and C 8 –S bonds begins respectively at Rx = −2.7434 amu 1/2 Bohr and Rx = −1.7458 amu 1/2 Bohr, and formation of the sulfur dioxide is completed at Rx = −0.2494 amu 1/2 Bohr. For the electrocyclic reaction, formation of new C 7 –C 8 bond occurs at Rx = 1.6214 amu 1/2 Bohr from C- to C- coupling between the generated pseudoradical centers at Rx = 0.1474 amu 1/2 Bohr on the terminal carbon atoms.

A comprehensive study of the He (I) ultra-violet photoelectron spectra of fluorene, carbazole and dibenzofuran is presented with the aid of one-particle Green’s Function calculations employing the outer-valence Green’s Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with Dunning’s correlation consistent basis sets of double and triple zeta quality (cc-pVDZ, cc-pVTZ). Extrapolations of the ADC(3) results for the outermost one-electron π-ionization energies to the cc-pVTZ basis set enable theoretical insights into He (I) measurements within ∼0.15 eV accuracy, up to the σ-ionization onset. The lower ionization energy of carbazole is the combined result of mesomeric and electronic relaxation effects. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital picture of ionization at the ADC(3) level. Comparison is made with calculations of the lowest doublet–doublet excitation energies of the radical cation of fluorene, by means of time-dependent density functional theory (TDDFT).

The vertical and adiabatic singlet–triplet energy gaps, electron affinities and ionization energies of azulene, phenanthrene, pyrene, chrysene, and perylene are computed by applying the principles of a focal point analysis onto a series of single-point calculations at the level of Hartree–Fock theory, second-, third-, and fourth-order Moller–Plesset perturbation theory, as well as coupled cluster theory including single, double and perturbative triple excitations, in conjunction with correlation consistent basis sets of improving quality. Results are supplemented with an extrapolation to the limit of an asymptotically complete basis set. According to our best estimates, azulene, phenanthrene, pyrene, chrysene, and perylene exhibit adiabatic singlet–triplet energy gaps of 1.79, 2.92, 2.22, 2.79 and 1.71 eV, respectively. In the same order, the corresponding adiabatic electron affinities (EAs) amount to 0.71, −0.08, −0.40, 0.24, and 0.87 eV, whereas benchmark values equal to 7.43, 8.01, 7.48, 7.66 and 7.15 eV, are found for the adiabatic ionization energies.

The development of a third-generation electron momentum spectrometer with significantly improved energy and momentum resolutions at Tsinghua University (ΔE = 0.45–0.68 eV, Δθ = ±0.53° and Δϕ = ±0.84°) has enabled a reinvestigation of the valence orbital electron momentum distributions of H2O with improved statistical accuracy. The measurements have been conducted at impact energies of 1200 eV and 2400 eV in order to check the validity of the plane wave impulse approximation. The obtained ionization spectra and electron momentum distributions have been compared with the results of computations carried out with Hartree Fock [HF] theory, density functional theory in conjunction with the standard B3LYP functional, one-particle Green’s function [1p-GF] theory along with the third-order algebraic diagrammatic construction scheme [ADC(3)], symmetry adapted cluster configuration interaction [SAC-CI] theory, and a variety of multi-reference [MR-SDCI, MR-RSPT2, MR-RSPT3] theories. The influence of the basis set on the computed momentum distributions has been investigated further, using a variety of basis sets ranging from 6-31G to the almost complete d-aug-cc-pV6Z basis set. A main issue in the present work pertains to a shake-up band of very weak intensity at 27.1 eV, of which the related momentum distribution was analyzed for the first time. The experimental evidences and the most thorough theoretical calculations demonstrate that this band borrows its ionization intensity from the 2a1 orbital.

A comprehensive study of the He (I) ultra-violet photoelectron spectra of the 1.2,3.4; 1.2,5.6 and 1.2,7.8 isomers of dibenzanthracene up to the double ionisation threshold at ∼18 eV is presented with the aid of one-particle Green’s Function calculations performed using the outer-valence Green’s Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with basis sets of improving quality. Suited extrapolations of the ADC(3) results for the one-electron energies characterising the π-band system ( e b

Calculations using ab initio Hartree–Fock and Density Functional theories, the latter employing the B3LYP functional, in combination with a number of large standard basis sets ranging from 6-31G** to cc-pVDZ, have been performed on a series of ten and twelve vertex closo-carborane isomer species. Results obtained for optimized structural parameters and molecular properties are presented for 1,2-, 1,6- and 1,10-C2B8H10 and 1,2-, 1,7- and 1,12-C2B10H12 and compared, where possible, with both earlier theoretical data and experiment. Irrespective of the model chemistry chosen, the para-isomer in each class of carborane cluster is found to be the most stable species, corresponding to a structure in which the cage carbon atoms are positioned at diametrically opposed ends of the respective polyhedron. Boron–hydrogen and carbon–hydrogen bond lengths are found to change little on going from isomers of one particular cage size to another, supporting analogous conclusions previously established for small closo-carborane cages possessing five, six and seven vertices. The calculated vibrational spectra of the isomers of both decacarborane and dodecacarborane are seen to be similar to each other and reflect a high degree of rigidity within each cluster. Key polyhedral skeletal breathing modes along with characteristic boron–hydrogen and carbon–hydrogen stretching frequencies are identified in the spectra and compared with experiment. Thermochemical data relating to each species are also analyzed. 2002 Elsevier Science B.V. All rights reserved.

An extensive high-level computational study of various possible isomers of five, six and seven vertex closo -carboranes has been carried out using ab initio self-consistent field and density functional (DFT) theories. In total nine different cage structures have been investigated at the Hartree–Fock and B3LYP DFT levels in conjunction with the 6-31G** and 6-31++G** basis sets. Energies, optimized geometries, Mulliken charges, harmonic frequencies and electric dipole and quadrupole moments have been computed and compared with previous calculations and experimental data where applicable. The inclusion of diffuse functions in the basis set is shown not to significantly affect the results obtained for structural parameters and molecular properties. For a fixed cluster size, and for whichever model chemistry is chosen, the energetically most stable isomer is computed to be 1,5-C 2 B 3 H 5 , 1,6-C 2 B 4 H 6 and 2,4-C 2 B 5 H 7 . It is also found that irrespective of cluster vertex number, boron–hydrogen and carbon–hydrogen bond lengths remain relatively constant. The simulated vibrational spectra are analysed in detail and characteristic boron–hydrogen and carbon–hydrogen stretching frequencies and skeletal breathing modes are identified.

With the aim of improving and validating theoretical quantum mechanical tools for tracking conformational signatures in the XPS spectra of n -alkane compounds, the XPS spectra of model structures are simulated by means of direct Green's function schemes and compared to available experimental records obtained in the gas and solid phases. Calculations are performed using the minimal STO-3G and 3-21G basis sets, and self-energy expansions derived at several levels of decoupling approximation: second-order, shifted Born collision (SBC) and diagonal two-particle—hole Tamm—Dankoff approximation (2ph-TDA). The latter scheme is shown to be accurate enough to generate reliable relative line positions and convoluted photoionization intensities in the inner valence region, which is of fundamental importance for the identification of secondary molecular structures from XPS spectra. Preventing overcounting of self-energy diagrams containing a diagonal ladder part is crucial in that respect. As a byproduct, this method of simulation is applied to some particular conformations of C 9 H 20 modeling the topmost layer of polyethylene in crystalline form, enabling the identification of fold structures at the extreme surface of lamellar crystals of polyethylene.