Outer Valence Green Research Articles

Coexistence of 1,3-butadiene conformers in ionization energies and Dyson orbitals

The minimum-energy structures on the torsional potential-energy surface of 1,3-butadiene have been studied quantum mechanically using a range of models including ab initio Hartree-Fock and second-order Møller-Plesset theories, outer valence Green's function, and density-functional theory with a hybrid functional and statistical average orbital potential model in order to understand the binding-energy (ionization energy) spectra and orbital cross sections observed by experiments. The unique full geometry optimization process locates the s-trans-1,3-butadiene as the global minimum structure and the s-gauche-1,3-butadiene as the local minimum structure. The latter possesses the dihedral angle of the central carbon bond of 32.81 degrees in agreement with the range of 30 degrees-41 degrees obtained by other theoretical models. Ionization energies in the outer valence space of the conformer pair have been obtained using Hartree-Fock, outer valence Green's function, and density-functional (statistical average orbital potentials) models, respectively. The Hartree-Fock results indicate that electron correlation (and orbital relaxation) effects become more significant towards the inner shell. The spectroscopic pole strengths calculated in the Green's function model are in the range of 0.85-0.91, suggesting that the independent particle picture is a good approximation in the present study. The binding energies from the density-functional (statisticaly averaged orbital potential) model are in good agreement with photoelectron spectroscopy, and the simulated Dyson orbitals in momentum space approximated by the density-functional orbitals using plane-wave impulse approximation agree well with those from experimental electron momentum spectroscopy. The coexistence of the conformer pair under the experimental conditions is supported by the approximated experimental binding-energy spectra due to the split conformer orbital energies, as well as the orbital momentum distributions of the mixed conformer pair observed in the orbital cross sections of electron momentum spectroscopy.

A study of the valence shell photoelectron and photoabsorption spectra of CF3SF5

The outer valence shell photoelectron spectrum of CF3SF5 has been studied experimentally and theoretically. Synchrotron radiation has been used to record angle-resolved outer valence shell photoelectron spectra of CF3SF5 in the photon energy range 18–60 eV. These spectra have allowed photoelectron asymmetry parameters and branching ratios to be derived. The Outer Valence Green's Function approach has been employed to calculate the molecular orbital configuration and associated binding energies. A charge distribution analysis has also been obtained. Assignments have been proposed for the peaks observed in the photoelectron spectrum. The absolute photoabsorption cross section of CF3SF5 has been measured from threshold to 40 eV, and strongly resembles that of SF6. Assignments, involving intravalence transitions, have been proposed for some of the principal features appearing in the photoabsorption spectrum of CF3SF5.

Allene and pentatetraene cations as models for intramolecular charge transfer: Vibronic coupling Hamiltonian and conical intersections

We consider the vibronic coupling effects involving cationic states with degenerate components that can be represented as charge localized at either end of the short cumulene molecules allene and pentatetraene. Our aim is to simulate dynamically the charge transfer process when one component is artificially depopulated. We model the Jahn-Teller vibronic interaction within these states as well as their pseudo-Jahn-Teller coupling with some neighboring states. For the manifold of these states, we have calculated cross sections of the ab initio adiabatic potential energy surfaces along all nuclear degrees of freedom, including points at large distances from the equilibrium to increase the physical significance of our model. Ab initio calculations for the cationic states of allene and pentatetraene were based on the fourth-order Møller-Plesset method and the outer valence Green's function method. In some cases we had to go beyond this method and use the more involved third-order algebraic diagrammatic construction method to include intersections with satellite states. The parameters for a five-state, all-mode diabatic vibronic coupling model Hamiltonian were least-square fitted to these potentials. The coupling parameters for the diabatic model Hamiltonian are such that, in comparison to allene, an enhanced preference for indirect charge transfer is predicted for pentatetraene.

Theoretical investigation of the absorption and ionization spectrum of the super greenhouse gas SF5CF3

SF5CF3, recently found in the Earth's atmosphere, is an extremely potent greenhouse gas. Its behaviour under irradiation, as in the upper atmosphere, is of great importance for the possible impact on the global climate. The vertical absorption spectrum of the title compound is studied by various single- and multi-reference ab initio methods and the results are compared to experiments. The best results for valence states are obtained by the multi-state complete active space second order perturbation theory (MS-CASPT2) approach. In contrast, the popular time-dependent density functional theory (TDDFT) gives very poor results. Multi-reference configuration interaction (MRCI) does not yield very accurate energies because only a limited number of valence electrons can be included in the correlation treatment. The Rydberg states are calculated very accurately and efficiently by a frozen-core multi-configuration self-consistent field (FC-MCSCF) method. The accuracy is assessed by quantum defect theory and the experimental and ab initio calculated first ionization potential. The photoelectron spectrum is investigated by the outer-valence Green's functions (OVGF) method. The simulated spectrum is in excellent agreement with a recent experiment.

Valence One-Electron and Shake-Up Ionization Bands of Polycyclic Aromatic Hydrocarbons. III. Coronene, 1.2,6.7-Dibenzopyrene, 1.12-Benzoperylene, Anthanthrene

A comprehensive theoretical study of the He(I) UV photoionization spectra of coronene, 1.2,6.7-dibenzopyrene, 1.12-benzoperylene, and anthanthrene up to electron binding energies of ∼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, using Dunning's correlation-consistent polarized valence basis set of double-ζ quality and the 6-31G basis set, respectively. The deviations from the one-electron OVGF/cc-pVDZ binding energies and experimental results most generally do not exceed 0.3 eV. OVGF/cc-pVDZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital (or one-electron) picture of ionization at the ADC(3)/6-31G level. A comparison has been made with calculations of the lowest doublet−doublet excitation energies of the related radical cations, by means of time-dependent density functional theory (TDDFT) and the Becke−Lee−Yang−Parr (BLYP) functional: Because of systematic and significant underestimations of the lowest [π0* ← σ] transition energies in the cations, this approach has led to erroneous identifications of the σ-ionization onset of the neutral molecules.

Reaction of AgN3 with SOCl2: Evidence for the Formation of Thionyl Azide, SO(N3)2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reaction of AgN3 with SOCl2: evidence for the formation of thionyl azide, SO(N3)2.

Pure thionyl azide SO(N(3))(2), which is the only gaseous reaction product, has been generated in a vacuum by the heterogeneous reaction of SOCl(2) vapor with AgN(3) at room temperature at a SOCl(2) vapor pressure of 1 x 10(-3) Torr. Evidence for the formation of SO(N(3))(2) is given by on line photoelectron spectroscopy (PES) combined with outer valence Green's function (OVGF) calculations with the 6-311++G(2df) basis set. The good agreement between the PES experiment and the OVGF calculation shows that SO(N(3))(2) has C(1) symmetry. The first ionization energy of SO(N(3))(2) is 10.18 eV.

Jahn–Teller effect in van der Waals complexes; Ar–C6H6+ and Ar–C6D6+

The two asymptotically degenerate potential energy surfaces of argon interacting with the X (2)E(1g) ground state benzene(+) cation were calculated ab initio from the interaction energy of the neutral Ar-benzene complex given by Koch et al. [J. Chem. Phys. 111, 198 (1999)] and the difference of the geometry-dependent ionization energies of the complex and the benzene monomer computed by the outer valence Green's function method. Coinciding minima in the two potential surfaces of the ionic complex occur for Ar on the C(6v) symmetry axis of benzene(+) (the z axis) at z(e)=3.506 A. The binding energy D(e) of 520 cm(-1) is only 34% larger than the value for the neutral Ar-benzene complex. The higher one of the two surfaces is similar in shape to the neutral Ar-benzene potential, the lower potential is much flatter in the (x,y) bend direction. Nonadiabatic (Jahn-Teller) coupling was taken into account by transformation of the two adiabatic potentials to a two-by-two matrix of diabatic potentials. This transformation is based on the assumption that the adiabatic states of the Ar-benzene(+) complex geometrically follow the Ar atom. Ab initio calculations of the nonadiabatic coupling matrix element between the adiabatic states with the two-state-averaged CAS-SCF(5,6) method confirmed the validity of this assumption. The bound vibronic states of both Ar-C(6)H(6) (+) and Ar-C(6)D(6) (+) were computed with this two-state diabatic model in a basis of three-dimensional harmonic oscillator functions for the van der Waals modes. The binding energy D(0)=480 cm(-1) of the perdeuterated complex agrees well with the experimental upper bound of 485 cm(-1). The ground and excited vibronic levels and wave functions were used, with a simple model dipole function, to generate a theoretical far-infrared spectrum. Strong absorption lines were found at 10.1 cm(-1) (bend) and 47.9 cm(-1) (stretch) that agree well with measurements. The unusually low bend frequency is related to the flatness of the lower adiabatic potential in the (x,y) direction. The van der Waals bend mode of e(1) symmetry is quadratically Jahn-Teller active and shows a large splitting, with vibronic levels of A(1), E(2), and A(2) symmetry at 1.3, 10.1, and 50.2 cm(-1). The level at 1.3 cm(-1) leads to a strong absorption line as well, which could not be measured because it is too close to the monomer line. The level at 50.2 cm(-1) gives rise to weaker absorption. Several other weak lines in the frequency range of 10 to 60 cm(-1) were found.

Theoretical Study of the Structure and Bonding in Phosphatrane Molecules

The molecular structures of a representative selection of phosphatranes were studied by means of the ab initio MP2 method. The calculated results reasonably matched the available X-ray data. The special properties of the intrabridgehead interaction were described in terms of hybridization, bond order index, force constant, and vibrational frequency. The atom-in-molecules analysis of the electronic charge density showed the presence of a CP(3,-1) betwen the bridgehead centers in the cationic species but not in all of the neutral phosphatranes. The proton affinities of bases with no apical substituent on the phosphorus were accurately calculated for both the gas phase and solution (DMSO). The unique electronic structure of phosphatranes was theoretically investigated by means of their spectroscopic properties. The NMR chemical shifts and indirect nuclear spin-spin coupling constants, computed by DFT-based methods, were fairly consistent with experimental evidence, in particular for the observables involving the central phosphorus. The photoelectron spectrum of azaphosphatrane, having NMe equatorial groups, was interpreted by means of ab initio outer valence Green's function calculations, which gave a consistent reproduction of the energies and splittings of the uppermost bands, associated with the lone pairs of the bridgehead and equatorial heteroatoms. Thus, computations provided reliable predictions of the variations of the NMR parameters and ionization energies with change of the equatorial centers and apical substitution at phosphorus.

Structural and spectroscopic characterization of oxo-sparteines

Abstract The equilibrium structures of the 11 possible oxo-derivatives of sparteine were investigated with the density functional theory, using the B3LYP functional. The conformational preferences of the seven amino-ketonic sparteines are consonant with those of the parent free base, whose lowest energy conformer consists of a chair–chair quinolizidine A/B-trans system and a boat–chair quinolizidine C/D-trans system. A similar situation also occurs for lactams 2-oxo- (lupanine), 15-oxo-, and 17-oxo-sparteine, whose ketonized ring has a half-chair/sofa, half-chair, and sofa shape, respectively. Unlike the other isomers, the most stable conformer of lactam aphylline (10-oxo-sparteine) adopts the chair–sofa A/B-transoid, chair–chair C/D-cisoid disposition of the two quinolizidine moieties. These theoretical predictions are consistent with the available X-ray experimental results. The electronic structure of the oxo-sparteines was examined by measuring and calculating relevant parameters of their NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, calculated by means of DFT formalisms, correlate well with observation. Notably, the effective manifestation of stereoelectronic hyperconjugative effects on Δδ(Heq/Hax) and Δ1J(CHeq/CHax) of the lactams is correctly accounted for by the DFT results. The ab initio outer valence Green's function calculations yielded a consistent description of the main features in the photoelectron spectra, i.e., location, splitting, and sequence of the n(N) and n(O) ionization energies, which also reflect the competing through-bond and through-space interactions within the sparteine framework.

THEORETICAL STUDY OF ADIABATIC AND VERTICAL ELECTRON AFFINITY OF RADIOSENSITIZERS IN SOLUTION PART 2: ANALOGUES OF TIRAPAZAMINE

Tirapazamine is a radiosensitizer, whose biological activity is associated to its electron affinity (EA). The electron affinity can be divided in two main processes: Vertical and Adiabatic. In this work, we calculated the EAs of nitroimidazoles (Fig. 2) using HF and DFT methods and evaluated solvent effects (water and carbon tetrachloride) on EAs. For water, we combined the Polarized Continuum Model (PCM) and free energy perturbation (Finite Difference Thermodynamic Integration, FDTI) methods. For carbon tetrachloride, we used the FDTI method. The values of adiabatic EA obtained are in agreement with experimental data (deviations of 13.25 meV). The vertical EA were calculated according to Cederbaum's Outer Valence Green Function (OVGF) method. This study, which relays on theoretical aspects of free energy calculations on charged molecules in solution, could be helpful in the rational design of new and more selective bioreductive anticancer drugs.

Theoretical study of the He-HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces.

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He-HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X2Pi ground state of the HF+monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF+ geometry. The well depth and equilibrium separation are 2.240 A and 1631.3 cm(-1), respectively, at an HF+ bond length r=1.0012 A, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 A.

Near-threshold photoionization spectroscopy of the mono-terpenes limonene and carvone

The photoionization of two representative mono-terpenes (R-limonene and R-carvone) has been investigated using low energy synchrotron radiation in the region from the threshold for ionization of these molecules up to 11 eV. Data from threshold photoelectron spectra are interpreted with the aid of Outer Valence Green’s Function (OVGF) calculations to identify the character of the outermost orbitals and improved ionization potentials are deduced. Fragmentation of the parent ions is studied using threshold photoelectron-photoion coincidence (TPEPICO) time-of-flight (TOF) mass spectrometry and breakdown diagrams identifying the major products in this near-threshold region are presented.

Valence and C 1s Core-Level Photoelectron Spectra of Butan-2-ol

Photoelectron spectra of the chiral molecule butan-2-ol spanning the full inner and outer valence regions and the C 1s core region are presented. The valence spectra are interpreted with the aid of outer valence Green's function calculations of the ionization energies. These were performed for each of nine possible conformational structures, each structure having been optimized by density functional calculations. The preferred gas-phase conformations can hence be identified. An assignment of the C 1s core spectrum is also achieved, and the vibrational structure observed on one band is fitted with two progressions. These are identified as C−H stretching and C−OH bending motions localized on the chiral −CHOH− carbanol carbon atom.

Two-Dimensional Penning Ionization Electron Spectroscopy of Adamantanes and Cyclohexanes: Electronic Structure of Adamantane,1-Chloroadamantane, Cyclohexane, and Chlorocyclohexane and Interaction Potential with He*(23S)

The electronic structures of adamantane, 1-chloroadamantane, cyclohexane, and chlorocyclohexane and their interaction potentials with He*(23S) metastable atoms are studied by two-dimensional (collision- and electron-energy-resolved) Penning ionization electron spectroscopy and model potential calculations. The model potential calculations are performed for the target-Li system based on a well-known resemblance between a metastable excited He*(23S) atom and a ground-state Li(22S) atom. Assignments of the electron spectra are made on the basis of ab initio and outer-valence Green's function calculations. Strong repulsive interactions between the saturated hydrocarbons and the metastable atom and strength sequence σCC > σCH observed experimentally are consistent with the anisotropic potential calculations. It is noted that attractive interaction in the region where lone-pair nCl electrons are distributed for 1-chloroadamantane is weaker than that for chlorocyclohexane, because of the stronger steric shielding effect by the larger adamantane skeleton. Through comparison with the previous results on the saturated chloro-hydrocarbons, we find that the attractive interaction around the nCl electron distribution does not become stronger as the energy level of the highest occupied molecular orbital becomes higher, which is the result of competition between effects of low exterior electron distributions and high energy levels.

HeI Photoelectron Spectroscopy of Trialkylaluminum and Dialkylaluminum Hydride Compounds and Their Oligomers

The HeI photoelectron spectra of trimethylaluminum, triethylaluminum, dimethylaluminum, and diethylaluminum hydrides were recorded as a function of temperature. From the spectra observed at different temperatures the spectra of the pure monomers and the trimethylaluminum, dimethylaluminum, and diethylaluminum hydride dimers and some of the bands related to the dimethylaluminum hydride trimer have been obtained. The spectra were interpreted with the aid of ab initio quantum chemical calculations, including Hartree−Fock/Koopmans, outer valence Green's function, and equation of motion coupled-cluster ionization energy calculations. The vertical ionization energies predicted by the latter two methods are in excellent agreement with the experimental values. Furthermore, the association capability of these compounds, the effects of the substituents, and the degree of association on the electronic structure are also discussed.

On the structure and spectroscopic properties of bispidine, N, N′-dimethylbispidine and a bis-bispidine macrocycle

The equilibrium conformations of bispidine and its N, N ′-dimethyl derivative were investigated with the ab initio MP2 formalism and the density functional theory (DFT) ansatz B3LYP. For bispidine, the chair–chair (cc) ( exo, endo) is the most stable conformer. On the contrary, for its N, N ′-dimethyl derivative, the energy difference between the cc and chair–boat (cb) conformers is small, allowing for conformational mobility of the bicyclo rings. For tetraaza-macrocycle with four nitrogen lone pair orbitals (LPOs) pointing into the molecular cavity, the C 2h structure with stepped ethylene bridges is preferred over the twisted structure of D 2 symmetry. Mono- and di-protonated forms of this base are characterized by intramolecular asymmetric hydrogen bonds. All the theoretical conformational predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations (CSGT) formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron (PE) spectra were measured and assigned by means of ab initio outer valence Green's function (OVGF) calculations. The predominance of the lowest energy conformation of bispidine and tetraaza-macrocycle is supported by the spectroscopic results. The conformational situation for N, N ′-dimethyl-bispidine is more complex. Whereas its NMR spectrum indicates a predominance of the cc conformer in THF-d 8 solution, the vibrational and PE spectra should be interpreted in terms of a mixture of the cc and cb conformers in the neat liquid and gas-phase.

An experimental and theoretical study of the valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene

The valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene have been studied, both experimentally and theoretically, in order to characterise the main bands due to single-hole states and to assess the importance of electron correlation in the formation of satellite states. Synchrotron radiation has been employed to measure photoelectron angular distributions and branching ratios in the photon energy range 13–115 eV, and the results indicate that the photoionisation dynamics of the (12 a ′) −1 B 2 A ′ , (2 a ″) −1 C 2 A ″ and ( 11 a ′) −1 D 2 A ′ states are affected by Cooper minima. The experimental data demonstrate that the 12a ′ and 2a ″ orbitals, corresponding to the Br 4p lone pairs, retain their atomic properties to a substantial degree, and that the same is true, although to a lesser extent, for the 11a ′ orbital which is related to the S 3p subshell. The results are compared with similar measurements on chlorothiophene and bromobenzene. The vertical ionisation energies and spectral intensities of the entire valence shell photoelectron spectrum have been computed using the third-order algebraic–diagrammatic construction approximation scheme for the one-particle Green's function. These theoretical predictions have allowed assignments to be proposed for all the prominent structure observed in the experimental spectra. The outer valence Green's function method has also been used and the calculated ionisation energies show good agreement with the experimental values. Mulliken atomic populations have been computed for some of the outer valence orbitals, and the calculated Br 4p and S 3p content possessed by the 12a ′, 2a ″ and 11a ′ molecular orbitals is in accordance with the strength of the Cooper minimum observed in the photoelectron asymmetry parameters associated with these orbitals.

Outer-Valence Green's Function Study of Cycloalkane and Cycloalkyl−Alkane Compounds

In this contribution, we report a quantitative analysis of gas-phase ultraviolet photoelectron spectra (HeI) of model compounds such as cyclopentane, cyclohexane, and cycloalkyl−1-alkane derivatives, by means of outer-valence Green's function (OVGF) calculations. These spectra and their OVGF theoretical simulations are also compared with UPS measurements on α-cycloalkyl−ω-alkanethiol monolayers adsorbed on gold. This theoretical study analyses the dependence of valence ionization spectra of saturated hydrocarbons on structural characteristics (configuration, conformation, and torsional flexibility).

Collision-Energy-Resolved Penning Ionization Electron Spectroscopy of Difluorobenzenes: Anisotropic Interaction of Difluorobenzenes with He*(23S) and Assignments of Ionic States

Penning ionization of difluorobenzenes upon collision with metastable He*(23S) atoms was studied by two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy. Collision energy dependence of the partial ionization cross sections (CEDPICS), which reflects interaction potential energy between the molecule and He*(23S), showed anisotropic interaction around the molecules. Assignments of the Penning ionization electron spectra and He I ultraviolet photoelectron spectra were discussed on the basis of different behavior of CEDPICS and calculated ionization potentials by the outer valence Green's function (OVGF) method. Furthermore, the ordering of the reactivity of the n∥ and σCF orbitals and the magnitude of attractive interaction around the F atoms with the metastable atom was found to be o- > m- ∼ p-C6H4F2. It indicates that the wider potential well brings the larger attractive effect for Penning ionization reaction.