Ab Initio Electron Propagator Research Articles

Electron Propagator Calculations Show that Alkyl Substituents Alter Porphyrin Ionization Energies

The effect of alkyl substituents on the four lowest vertical ionization energies of porphyrins is determined with ab initio electron propagator calculations on porphine and octamethylporphyrin. With the use of the partial third-order approximation, predicted ionization energies are in close agreement with recent photoelectron spectra. These data and the associated Dyson orbitals, which describe changes in electronic structure that accompany photoionization, enable assignment of photoelectron spectra and determination of alkyl-induced shifts. Hyperconjugation is most evident in the Dyson orbitals associated with the third and fourth ionization energies of octamethylporphyrin and is least prominent in the Dyson orbital of the second ionization energy. There is a positive correlation between the shift in an ionization energy produced by alkyl substitution and the degree of hyperconjugation in the associated Dyson orbital. Alkyl substitutions, therefore, may be employed to adjust the ionization energies of porphyrins and, consequently, their reactivity patterns that depend on charge-transfer capabilities and disposition to electrophilic attack.

Silicon−Nitrogen Bonding in Silatranes: Assignment of Photoelectron Spectra

Silicon-nitrogen bonding and the photoelectron spectra of hydro-silatrane and methyl-silatrane, XSi[OCH2CH2]3N (X = H and Me), were studied with ab initio electron propagator theory, many-body methods, and density functional models. A linear vibronic coupling (LVC) model was employed to estimate vibrational widths of the ionization bands and to study the dependence of the ionization energies on the molecular geometry. Particular attention was given to coordinates that change the Si-N distance and the strength of the donor-acceptor interaction between these two atoms. The ionization energy of the highest occupied molecular orbital has a very strong geometrical dependence which leads to an unusually large vibrational width in the corresponding photoelectron band. The assignment of this band in methyl-silatrane, which was controversial for a long time, is resolved by the present study. The calculated photoelectron spectra allow for clear assignment of at least three more bands in the observed spectra. The present results demonstrate the important role of electrostatic interactions in Si <-- N bonding and in the outer-valence ionization energies of the silatranes.

Conformational Effects on Glycine Ionization Energies and Dyson Orbitals

Five conformational isomers corresponding to minima in potential energy surfaces calculated with the B3LYP/6-311G** and MP2/6-311G** models lie within 6 kcal/mol of each other. The vertical ionization energies of the most stable isomer calculated with ab initio electron propagator theory in the P3/6-311G** approximation are in excellent agreement with experimental data from photoelectron spectroscopy. The chief components of the first four Dyson orbitals are a N lone pair, an O lone pair, πCO, and C−H lobes, respectively. Conformationally induced shifts of the vertical ionization energies are explained in terms of electrostatic and phase relationships in the Dyson orbitals.

Products of the addition of water molecules to Al3O3- clusters: structure, bonding, and electron binding energies in Al3O4H2-, Al3O5H4-, Al3O4H2, and Al3O5H4.

Two stable products of reactions of water molecules with the Al3O3- cluster, Al3O4H2- and Al3O5H4-, are studied with electronic structure calculations. There are several minima with similar energies for both anions and the corresponding molecules. Dissociative absorption of a water molecule to produce an anionic cluster with hydroxide ions is thermodynamically favored over the formation of Al3O3-(H2O)n complexes. Vertical electron detachment energies of Al3O4H2- and Al3O5H4- calculated with ab initio electron propagator methods provide a quantitative interpretation of recent anion photoelectron spectra. Contrasts and similarities in these spectra may be explained in terms of the Dyson orbitals associated with each transition energy.

Ground and excited states of NH4: Electron propagator and quantum defect analysis.

Vertical excitation energies of the Rydberg radical NH4 are inferred from ab initio electron propagator calculations on the electron affinities of NH4+. The adiabatic ionization energy of NH4 is evaluated with coupled-cluster calculations. These predictions provide optimal parameters for the molecular-adapted quantum defect orbital method, which is used to determine Einstein emission coefficients and radiative lifetimes. Comparisons with spectroscopic data and previous calculations are discussed.

Oxidative Damage to Cytosine: Implication for the Study of Radiation-Induced Damage to DNA

A radical observed in irradiated single crystals of cytidine, 3‘-CMP, and 5‘-dCMP is characterized by three anisotropic proton hyperfine couplings and is called the 3αH radical. The 3αH radical shares properties with the allyl-like radical observed in thymine derivatives. The goal of the present work is to show that the previously observed 3αH radicals are cytosine base radicals formed on 5-methylcytosine impurities in these crystals. Ab initio electron propagator calculations in the partial third-order (P3) approximation with the 6-311G(d,p) basis set have been used to show that these 5-methylcytosines are good hole traps, having an ionization potential comparable to that of guanine. The importance of these cytosine oxidation sites to the radiation chemistry of DNA is discussed.

Ionization Energies and Dyson Orbitals of 1,2-Dithiin

Photoelectron spectra of 1,2-dithiin are assigned with ab initio electron propagator calculations of vertical ionization energies and corresponding Dyson orbitals. Close agreement with experimental peaks obtains for the first nine final states. Dyson orbitals for the first three vertical ionization energies are composed chiefly of sulfur 3p and butadiene π fragment orbitals. Constructive interference between the second highest occupied π orbital of butadiene and sulfur 3p orbitals is seen in the Dyson orbitals pertaining to the fourth and seventh ionization energies. Weak σ bonding between sulfur atoms is found in the Dyson orbital for the fifth final state.

Electron Propagator Theory of the Photoelectron Spectrum of Methanesulfenic Acid

Assignment of the photoelectron spectrum of methanesulfenic acid, H3CSOH, is facilitated by ab initio electron propagator calculations. Close agreement exists between experimental peak positions an...

Ionization energies of benzo[a]pyrene and benzo[e]pyrene

Photoelectron spectra of benzo[a]pyrene and benzo[e]pyrene have been assigned on the basis of ab initio electron propagator theory. Electron correlation effects are included in the partial third order approximation and agreement with experimental peaks is very close. To each ionization energy, there corresponds a Feynman–Dyson amplitude that exhibits the change in electronic structure associated with the removal of an electron. Correlation corrections to the Koopmans description of the cationic states are large for many states, but the qualitative validity of this model remains valid. The lowest final states with σ holes occur around 11.0 eV.

Ionization Energies of Acridine, Phenazine, and Diazaphenanthrenes

Ab initio electron propagator calculations on the lowest vertical ionization energies of acridine, phenazine, and five diazaphenanthrene isomers have been performed with the partial third order and outer valence Green's function approximations of electron propagator theory. Agreement with photoelectron spectra is very close, enabling the clarification of previous assignments. Numerous misorderings of final states from Koopmans's theorem are revealed. With the present approximations, Feynman−Dyson amplitudes are equivalent to canonical molecular orbitals. Plots of these one-electron functions aid in the interpretation of the spectra by revealing patterns of localization. Many final-state holes exhibit considerable mixing between nitrogen, lone-pair hybrids, and σ-bonding lobes.

Ionization energies of anthracene, phenanthrene, and naphthacene

Ab initio electron propagator calculations on the lowest vertical ionization energies of anthracene, phenanthrene, and naphthacene have been performed with the partial third order and outer valence Green’s function approximations. Agreement with photoelectron spectra is very close, enabling the clarification of previous assignments, some of which were contradictory. With the present approximations, Feynman–Dyson amplitudes are equivalent to canonical molecular orbitals. Plots of these one-electron functions aid in the interpretation of the spectra by revealing patterns of delocalization and locations of nodes.

Electron binding energies of TCNQ and TCNE

Ab initio electron propagator calculations on tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) produce accurate predictions of vertical ionization energies and electron affinities. Plots of Feynman–Dyson amplitudes associated with each ionization process represent how the electron distribution changes from initial to final states. Calculated electron detachment energies of the TCNQ dianion imply that two states of the TCNQ anion are bound with respect to the neutral molecule. Configuration interaction calculations on the TCNQ anion confirm this result.

Conformationally induced localization in the electronic structure of polysilanes

Ab initio electron propagator calculations with a double {lambda} effective core potential basis on the vertical electron binding energies of oligomeric models of polysilane chains reveal the effects of gauche conformational defects on electronic structure. When a gauche defect lies between two all-trans segments of unequal lengths, most of the probability density for the Feynman-Dyson amplitudes corresponding to the lowest ionization energy or the largest electron affinity resides on the longer segment. The energy gap between cationic and anionic states increases when a gauche defect is introduced, but the effect is larger when the defect is in the middle of the chain. These trends are explained in terms of simple bond function models suggested by examination of the Feynman-Dyson amplitudes.

Electron propagator calculations on the electron affinity of C 5

The vertical electron detachment energy of C − 5 and the adiabatic electron affinity of C 5 are calculated with ab initio electron propagator methods. The former energy difference is 2.86 eV and the latter is 2.82 eV. These values are in close agreement with recent experimental determinations. Structural changes associated with the addition of an electron to C 5 are explained in terms of electron repulsions. New techniques for the calculation and interpretation of vertical and adiabatic electron binding energies are employed.

Electron propagator theory of bonding in saturated silicon chains

Ab initio electron propagator calculations produce accurate electron binding energies and Feynman-Dyson amplitudes for oligosilanes as a function of bond angle and dihedral angle distortions. Ground state energies are also calculated for equilibrium and distorted structures. While the ground state energies are fairly insensitive to the distortions, changes in electron binding energies can be much larger. Examination of the Feynman-Dyson amplitudes leads to an electronic structure model that is based on phase relationships between bond orbital and antibond orbitals that are localized in SiSi bond regions. This model is successful in explaining the calculated shifts in electron binding energies and changes in polymer absorption spectra.

Electron binding energies of anionic alkali metal atoms from partial fourth order electron propagator theory calculations

Ionization energies of Li−, Na−, K−, Rb−, and Cs− are calculated with ab initio electron propagator theory. Effective core potentials are employed for K, Rb, and Cs. Third order diagonal self-energy corrections to Koopmans’s theorem are augmented with fourth order terms that are easily determined from by-products of a third order calculation. This partial fourth order procedure is derived using superoperator theory. The superiority of this method to an outer valence diagonal approximation is demonstrated for these systems. Covergence of results with respect to the order of electron interaction in the propagator self-energy and with respect to basis set improvement is studied. Agreement to within 0.1 eV of experiment is achieved. Remaining sources of error are analyzed.

Electron binding energies of anionic alkali metal triatomics from partial fourth order electron propagator theory calculations

Vertical ionization energies of Li−3, Na−3, LiNa−2, and Li2Na− are calculated with ab initio electron propagator theory. D∞h and C∞v isomers for the heteronuclear triatomics are considered. Two doublet final states with Σ symmetry are considered for each case. Koopmans’s theorem, second order, third order, and partial fourth order results form a steadily converging series. Outer valence approximation results are not similar and are probably inferior to the partial fourth order results. Convergence of results with respect to the order of electron interaction in the propagator self-energy and with respect to basis set saturation is achieved to within 0.1 eV.