Configuration interaction calculations of the valence ionization energies of the water dimer

The eight vertical valence ionization energies of the water dimer are calculated by the ΔCI method. Excellent agreement with measurements of the first and second ionization energies is found. Calculations of the remaining six ionization energies is found. Calculations of the remaining six ionization energies are sufficiently accurate to be of value in the identification and assignment of the dimer photoelectron spectrum.     

On the structure and valence ionization energies of o-benzyne

The valence ionization energies of o-benzyne, computed using a Green function method and by CI calculations at a molecular geometry optimized with the 6-31G^* basis using a two-term GVB wavefunction, suggest an assignment of the photoelectron spectrum of this molecule which differs from that given by a previous MNDO calculation. The first three ionization energies are found to be nearly degenerate.     

键能的分子轨道理论研究2: 键能与Mulliken布居对键强度的 判断的比较

用键能E~A~B和Mulliken布居对化学键强度的判别进行了分析比较。结果表明,键能判据比Mulliken布居判据所得结论更符合实际情况。作为衡量原子间化学键强度的尺度,不仅应考虑原子轨道间的布居因素,还应考虑分子轨道(或原子轨道)的能量因素。     

The valence orbital ionization potential of the first transition-metal atoms and ions

The 3d, 4s and 4p valence orbital ionization potentials (VOIP) are determined for the atoms and the ions with the electron configuration 3d 4s p , using the Anno-Teruya values of the average energies of the configurations and the experimental ionization potentials. Not all the configurations of this type are treated for lack of the data. As far as the data are available, a quadratic equation in terms of the atomic number Z is fitted to the VOIP's of an isoelectronic series: VOIP=A ₀+A ₁ Z+A ₂ Z ². The coefficients A ₀'s, A ₁'s and A ₂'s thus obtained are analysed in the light of Slater's simple expression for the total energy of an atom with the idea of screening effect due to inner electrons.

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Zusammenfassung Mit den Anno-Teruya-Werten für die durchschnittliche Energie der Konfigurationen und den experimentellen Ionisationspotentialen werden für Atome und Ionen der Konfigurationen 3d 4s p die 3d-, 4s- und 4p-VOIPs bestimmt. Mangels verfügbarer Daten werden nicht alle Konfigurationen dieses Typs behandelt. Sofern Daten verfügbar sind, wird den VOIPs einer isoelektronischen Reihe eine in der Kernladungszahl Z quadratische Gleichung angepa\t: VOIP=A ₀+A ₁ Z+A ₂ Z ². Dies aus dieser Gleichung gewonnenen RegelmÄ\igkeiten in den Koeffizienten A ₀, A ₁ und A ₂ werden mit der einfachen Slaterformel für die Gesamtenergie eines Atoms als Abschirmeffekte der inneren Elektronen erklÄrt.

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Résumé Les potentiels d'ionisation des orbitales de valence (VOIP) 3d, 4s et 4p sont déterminés pour les atomes et les ions de configuration électronique 3d 4s p en utilisant les valeurs de Anno-Teruya des énergies moyennes des configurations et les potentiels d'ionisation expérimentaux. Par suite d'absence de données toutes les configurations de ce type ne sont pas étudiées. Dans la mesure des données existantes, une relation quadratique en fonction du numéro atomique Z est ajustée pour les VOIP d'une série isoélectronique: VOIP=A ₀+A ₁ Z+A ₂ Z ². Les coefficients A ₀, A ₁ et A ₂ ainsi obtenus sont analysés à la lumière des expressions simples de Slater pour l'énergie totale d'un atome avec l'idée d'un effet d'écran dû aux électrons internes.

     

Relativistic effective valence shell Hamiltonian method: excitation and ionization energies of heavy metal atoms

The relativistic effective valence shell Hamiltonian H(v) method (through second order) is applied to the computation of the low lying excited and ion states of closed shell heavy metal atoms/ions. The resulting excitation and ionization energies are in favorable agreement with experimental data and with other theoretical calculations. The nuclear magnetic hyperfine constants A and lifetimes tau of excited states are evaluated and they are also in accord with experiment. Some of the calculated quantities have not previously been computed.     

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.     

Comparison of ribonucleic acid homopolymer ionization energies and charge injection barriers

Thin films of guanosine and uridine ribonucleic acid (RNA) homopolymers (poly rG, poly rU) were grown in high vacuum in several steps on highly oriented pyrolytic graphite (HOPG) using electrospray deposition. Between deposition steps, the sample surface was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). The resulting spectra series allowed the determination of the orbital alignment at the HOPG interface, as well as the ionization energies of the homopolymer thin films. Comparison with earlier results on cytidine and adenosine RNA homopolymers (poly rC, poly rA) indicates significant ionization energy and charge injection barrier differences between purines and pyrimidines.     

Structure determination of protein-ligand complexes by transferred paramagnetic shifts

Rational drug design depends on the knowledge of the three-dimensional (3D) structure of complexes between proteins and lead compounds of low molecular weight. A novel nuclear magnetic resonance (NMR) spectroscopy strategy based on the paramagnetic effects from lanthanide ions allows the rapid determination of the 3D structure of a small ligand molecule bound to its protein target in solution and, simultaneously, its location and orientation with respect to the protein. The method relies on the presence of a lanthanide ion in the protein target and on fast exchange between bound and free ligand. The binding affinity of the ligand and the paramagnetic effects experienced in the bound state are derived from concentration-dependent (1)H and (13)C spectra of the ligand at natural isotopic abundance. Combined with prior knowledge of the crystal or solution structure of the protein and of the magnetic susceptibility tensor of the lanthanide ion, the paramagnetic data define the location and orientation of the bound ligand molecule with respect to the protein from simple 1D NMR spectra. The method was verified with the ternary 30 kDa complex between the lanthanide-labeled N-terminal domain of the epsilon exonuclease subunit from the Escherichia coli DNA polymerase III, the subunit theta, and thymidine. The binding mode of thymidine was found to be very similar to that of thymidine monophosphate present in the crystal structure.     

Cations of halogenated methanes: adiabatic ionization energies, potential energy surfaces, and ion fragment appearance energies

The DFT-B3LYP and G3X model chemistry were used to predict the cation structures and energetics of fluorinated, chlorinated, and brominated methanes. Ion–complex structures between methylene cations and HX (X = F, Cl, Br) were found for all H-containing cations, and [CHF–FH]⁺, [CF₂–FH]⁺, [CCl₂–ClH]⁺, and [CCl₂–FH]⁺ structures are more stable than their normal tetravalent structures. Several cations should also be better described as ion–complex structures between methyl cations and halogen atoms, e.g., [CF₃–Br]⁺. Transition states connecting normal and ion–complex structures were also located, and potential energy diagrams were constructed for decomposition of methane cations and to predict the fragmentation pathways. The G3X energies were used to predict the adiabatic ionization energies (IE_as) and ion fragment appearance energies (AEs) from methanes. Many of the experimental AEs correspond to the energies of transition states instead of the thermodynamic dissociation limits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hydrogen-bonding-induced shifts of the excitation energies in nucleic acid bases: an interplay between electrostatic and electron density overlap effects

The theoretically calculated dimerization-induced shifts of the lowest excitation energies in two model systems, adenine-thymine and guanine-cytosine base pairs, are analyzed. The applied formalism is based on first principles and allows one to study the influence of the microscopic environment of a given molecule on its ground- [Wesolowski, T. A.; Warshel, A. J. Phys. Chem. 1993, 97, 8050] and excited-state [Casida, M. E.; Wesolowski, T. A. Int. J. Quantum Chem. 2004, 96, 577] properties. The assessment of the relative importance of such effects as (a) Coulomb interactions, (b) orbital interactions, (c) electronic polarization of the environment, and (d) electron density overlap effects is straightforward in this formalism. In the applied formalism, electron density overlap effects can be further decomposed into the exchange-correlation component which provides a small attractive contribution and the repulsive kinetic energy-dependent component. It is shown that the shifts can be attributed to the electrostatic interactions and the repulsive overlap-dependent term in the embedding potential. The electronic polarization of the environment plays a significant role (up to 30% of the total shift) only in transitions involving the orbitals localized on hydrogen bond donor groups. For all analyzed shifts, the contribution of the intermolecular orbital interactions is negligible. The analysis of this work provides strong evidence supporting the use of the widely applied embedding-molecule strategy in computational studies of chromophores in a condensed phase even in such cases where only one end of the hydrogen bond is included in the quantum mechanical part.     

An efficient algorithm for energy gradients and orbital optimization in valence bond theory

An efficient algorithm for energy gradients in valence bond theory with nonorthogonal orbitals is presented. A general Hartree-Fock-like expression for the Hamiltonian matrix element between valence bond (VB) determinants is derived by introducing a transition density matrix. Analytical expressions for the energy gradients with respect to the orbital coefficients are obtained explicitly, whose scaling for computational cost is m(4), where m is the number of basis functions, and is thus approximately the same as in HF method. Compared with other existing approaches, the present algorithm has lower scaling, and thus is much more efficient. Furthermore, the expression for the energy gradient with respect to the nuclear coordinates is also presented, and it provides an effective algorithm for the geometry optimization and the evaluation of various molecular properties in VB theory. Test applications show that our new algorithm runs faster than other methods.     

Electronic structure and ionization energies of palladium and platinum N-heterocyclic carbene complexes

Density functional methods have been used to calculate the geometries, electronic structure and ionization energies (IE) of N-heterocyclic carbene complexes of palladium and platinum, [M(CN2R2C2H2)2](M = Pd, Pt; R = H, Me, Bu t). Agreement with X-ray structures (R = Bu t) was good. Calculated IE agreed well with the photoelectron (PE) spectra (R = Bu t); metal bands were calculated to be within 0.25 eV of the experimental values, whereas the higher lying ligand bands deviated by up to 0.9 eV. Spin-orbit methods were needed to achieve this level of agreement for the Pt complex, but the calculations were found to underestimate the spin-orbit splitting somewhat. The principal metal-ligand bonding is between the carbene lone pair HOMO and a (d(z2)+ s) hybrid on the metal. The metal p(z) orbital contributes very little to the bonding. The metal d(xz,yz) orbitals mix primarily with the filled pi3 orbitals on the ligands and secondarily with the empty pi5 orbitals. Consequently they are little stabilized in comparison to the metal d(xy,x2- y2) orbitals, which are non-bonding in the complex. The first PE band for both the Pd and Pt complexes is from ionization of a (s - d(z2)) hybrid orbital. The IE is greater for Pt than for Pd on account of the post-lanthanide relativistic stabilization of the Pt 6s orbital.     

Accurate ab initio potential energy curve of F2. I. Nonrelativistic full valence configuration interaction energies using the correlation energy extrapolation by intrinsic scaling method

The recently introduced method of correlation energy extrapolation by intrinsic scaling (CEEIS) is used to calculate the nonrelativistic electron correlations in the valence shell of the F(2) molecule at 13 internuclear distances along the ground state potential energy curve from 1.14 A to 8 A, the equilibrium distance being 1.412 A. Using Dunning's correlation-consistent double-, triple-, and quadruple-zeta basis sets, the full configuration interaction energies are determined, with an accuracy of about 0.3 mhartree, by successively generating up to octuple excitations with respect to multiconfigurational reference functions that strongly change along the reaction path. The energies of the reference functions and those of the correlation energies with respect to these reference functions are then extrapolated to their complete basis set limits. The applicability of the CEEIS method to strongly multiconfigurational reference functions is documented in detail.     

Relativistic contributions to single and double core electron ionization energies of noble gases

We have performed relativistic calculations of single and double core 1s hole states of the noble gas atoms in order to explore the relativistic corrections and their additivity to the ionization potentials. Our study unravels the interplay of progression of relaxation, dominating in the single and double ionization potentials of the light elements, versus relativistic one-electron effects and quantum electrodynamic effects, which dominate toward the heavy end. The degree of direct relative additivity of the relativistic corrections for the single electron ionization potentials to the double electron ionization potentials is found to gradually improve toward the heavy elements. The Dirac-Coulomb Hamiltonian is found to predict a scaling ratio of ～4 for the relaxation induced relativistic energies between double and single ionization. Z-scaling of the computed quantities were obtained by fitting to power law. The effects of nuclear size and form were also investigated and found to be small. The results indicate that accurate predictions of double core hole ionization potentials can now be made for elements across the full periodic table.     

Esca study of some chromium complexes: ionization energies and multi-peak structure of the spectra

The chemical shifts of the core ionization energies of C₆H₆CrC₆H₆, C₆H₆Cr(CO)₃, COOCH₃C₆H₅Cr(CO)₃ and Cr(CO)₆ have been measured and information on the bonding of these complexes has been obtained. In the spectra of the complexes studied the main peaks have been frequently found to be accompanied by smaller ones, whose intensities and energies are structure dependent. In an attempt to assign these “extra” peaks, the energy separations from the main peak have been compared with the ultraviolet absorption energies of the neutral molecules.     

Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

As known, the electron density of covalently bound halogen atoms is anisotropically distributed, making them potentially able to establish many weak interactions, acting at the same time as halogen bond donors and hydrogen bond acceptors. Indeed, there are many examples in which the halogen and hydrogen bond coexist in the same structure and, if a correct bond analysis is required, their separation is mandatory. Here, the advantages and limitations of coupling the charge displacement analysis with natural orbital for chemical valence method (NOCV-CD) to separately analyze orthogonal weak interactions are shown, for both symmetric and asymmetric adducts. The methodology gives optimal results with intermolecular adducts but, in the presence of an organometallic complex, also intramolecular interactions can be correctly analyzed. Beyond the methodological aspects, it is shown that correctly separate and quantify the interactions can give interesting chemical insights about the systems.     

A theoretical investigation of electronic reorganizations accompanying core and valence ionization in simple hydrogen bonded systems

Non-empirical LCAO MO SCF calculations have been carried out on the ground state and core ionized states of some hydrogen bonded dimers, and in the particular case of H₂O the trimer has also been investigated. Comparison of absolute and relative binding energies and relaxation energies with respect to the corresponding monomers reveals that substantial changes occur in going to the associated species. The relaxation energies for a given core hole are shown to increase on going from monomer to dimer indicating that intermolecular contributions to relaxation energies are of the same sign irrespective of the sign for the shift in core binding energy. Creation of a core hole in the dimer species is shown to give rise to substantial changes in hydrogen bond energies compared with the neutral species. In the particular case of valence holes dominantly of 2s and 2p character it is shown that trends in shifts and relaxation energies parallel those for the core hole states.     

Generalized valence bond orbital interactions (GVB-OIs) and stabilization (resonance) energies: Part III--Nitrogenous heterocyclic compounds

In order to determine the stabilization (resonance) energies of nitrogenous heterocyclic compounds, the generalized valence bond orbital interactions (GVB-OIs) have been considered within the cyclic periphery of 2p_z-GVB orbitals. The overall process of GVB-OIs goes through a number of successive three-electron interactions (known as Pauli's orbital interactions (POIs)), each of which involves interaction between the two 2p_z-GVB bonding orbitals and the one 2p_z-GVB nonbonding orbital, and occurs following pauli's principle. After taking into account the total number of POIs involved and their associated minimization energies, the stabilization energies (SE)/resonance energies (RE) of mononitrogenous five- and six-membered heterocyclic compounds have been calculated by the formulae derived for them. The SE/RE values of polynitrogenous heterocyclic compounds have been calculated individually.