Single Excitation Configuration Interaction Research Articles

Ab initio study of molecular structures and excited states in anthocyanidins

The structural and electronic characters of four types of hydroxyl group-substituted anthocyanidins (pelargonidin, cyanidin, delphinidin, and aurantinidin) were examined using quantum chemical calculations. For these cationic molecules, both the planar and non-planar structures in the electronic ground state were determined at the B3LYP/D95 level of theory. We revealed that the planar structure is slightly more stable than the non-planar structure for each molecule. For the optimized planar structures, single excitation–configuration interaction (SE–CI) based on the restricted Hartree–Fock (RHF) wave function was evaluated and the electronic character in the low-excited states was discussed in terms of the MO theory. Symmetry adapted cluster (SAC)/SAC-CI calculations were also carried out to estimate the excitation energies precisely. The results showed that hydroxylation of the phenyl group causes a change in the excitation energies without taking the solvent effects into account. The results are in agreement with spectral experiments and previous MO calculations.

Potential energy surface, van der Waals motions, and vibronic transitions in phenol-argon complex

The structure and intermolecular vibrational energy levels of the phenol-Ar complex are calculated from its potential energy surface. This surface is constructed from a large set of the interaction energy values computed using second-order Moller-Plesset perturbation theory with the augmented correlation consistent polarized valence double-zeta basis set. The global minimum in the potential energy surface corresponds to a cluster structure with Ar located over the geometric center of the phenol ring at a distance of 3.510 A and shifted by 0.1355 A towards oxygen. The calculated dissociation energy of 371 cm(-1) is in accordance with the experiment. Additional local minima higher in energy are with Ar placed in the phenol plane. However, they are too shallow to form the bound states corresponding to planar isomers. The deformation of the potential energy surface shape, created by the interaction of Ar with the phenolic oxygen, is responsible for a pronounced intermode mixing. As a result, a set of hybrid stretching-bending states appears which cannot be described in terms of the standard models. The intermode coupling is reflected in the vibronic structure of the S1-S0 electronic transition. The intensities of the vibronic bands are calculated from the electronic transition dipole moment surfaces determined using the ab initio single-excitation configuration interaction method. They allow us to correct and complete the assignment of the spectra observed in phenol-Ar, as well as in the analogous complexes of phenol with Kr and Xe.

Experimental and Theoretical Studies on (p-methoxyphenyl)thiosemicarbazide

The title compound, (p-methoxyphenyl)thiosemicarbazide, has been characterized by elemental analysis, IR, electronic spectroscopy, and single-crystal X-ray diffraction. Ab initio calculations of the structure, atomic charges, natural bond orbital, topological analysis, and thermodynamic functions of the title compound were performed at HF/6-311G** and B3LYP/6-311G** levels of theory. The calculated results show that the sulfur atom and nitrogen atoms have bigger negative charges, which result in that they are the potential sites to react with the metallic ions. Electronic absorption spectra were calculated by the time-dependent density functional theory (TD-DFT) and configuration interaction single-excitation (CIS) methods and they are corresponding to the experimental values. The calculation of the second-order optical nonlinearity was carried out, and the molecular hyperpolarizability was 2.592×10−30 esu, indicating it is a potential candidate as second-order nonlinear optical material.

Theoretical investigations on the structure and vibrational spectra of N-(2-hydroxy-1-naphthylidene)threonine

Hartree–Fock (HF) and hybrid density functional theory (DFT) have been used to investigate the electronic structure and energetics of enol-imine and keto-amine tautomers of N-(2-hydroxy-1-naphthylidene)threonine. Both these tautomers engender six-membered ring due to intramolecular hydrogen bonded interactions. The enol-imine tautomer turns out to be ∼9 kJ mol −1 lower in energy in the HF framework owing to strong O–H⋯N (1.832 Å) interactions compared to N–H⋯O interactions (H⋯O=1.906 Å) in keto-amine tautomer. Consequently the downshift of the C N stretching vibration to 1560 cm −1 in the enol-imine tautomer has been predicted. The intense C O stretching, insensitive to the tautomeric transformation, turns out to be at 1715 cm −1. The effect of solvents on the tautomeric stability has been investigated. Self-consistent reaction field (SCRF) calculations have shown that unlike in the gas phase structure protic polar solvents facilitate the keto-amine form to be of lower energy. Configuration interaction single excitation (CIS) calculations reveal relatively strong O–H⋯N (1.768 Å) interactions in the excited state of enol-imine tautomer.

A new mechanism for the production of highly vibrationally excited OH in the mesosphere: An ab initio study of the reactions of O2(AΣu+3andA′Δu3)+H

In an attempt to explain the observed nightglow emission from OH(v=10) in the mesosphere that has the energy greater than the exothermicity of the H+O(3) reaction, potential energy surfaces were calculated for reactions of high lying electronic states of O(2)(A (3)Sigma(u) (+) and A' (3)Delta(u)) with atomic hydrogen H((2)S) to produce the ground state products OH((2)Pi)+O((3)P). From collinear two-dimensional scans, several adiabatic and nonadiabatic pathways have been identified. Multiconfigurational single and double excitation configuration interaction calculations show that the adiabatic pathways on a (4)Delta potential surface from O(2)(A' (3)Delta)+H and a (4)Sigma(+) potential surface from O(2)(A (3)Sigma(u) (+))+H are the most favorable, with the zero-point corrected barrier heights of as low as 0.191 and 0.182 eV, respectively, and the reactions are fast. The transition states for these pathways are collinear and early, and the reaction coordinate suggests that the potential energy release of ca. 3.8 eV (larger than the energy required to excite OH to v=10) is likely to favor high vibrational excitation.

Determination of theg-Tensors and Their Orientations forcis,trans-(L-N2S2)MoVOX (X = Cl, SCH2Ph) by Single-Crystal EPR Spectroscopy and Molecular Orbital Calculations

A single-crystal study of cis,trans-(L-N2S2)MoVOCl (1) doped into cis,trans-(N2S2)MoVIO2 (3) has enabled the g-tensor of 1 and its orientation with respect to the molecular structure to be determined. The EPR parameters (g1, 2.004; g2, 1.960; g3, 1.946; A1, 71.7 x 10(-4) cm(-1); A2, 11.7 x 10(-4) cm(-1); A3, 32.0 x 10(-4) cm(-1)) of cis,trans-(L-N2S2)MoVOCl [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine] mimic those of the low-pH form of sulfite oxidase and the "very rapid" species of xanthine oxidase. The principal axis that corresponds to g1 is rotated approximately 10 degrees from the Mo[triple bond]O vector, while the principal axis that corresponds to g3 is located in the equatorial plane and approximately 38 degrees from the Mo-Cl vector. Independent theoretical calculations of the g-tensor of 1 were performed using two types of techniques: (1) the spectroscopically parametrized intermediate neglect of differential overlap technique (INDO/S) combined with single-excitation configuration interaction (CIS); (2) a scalar relativistic DFT (BP86 and B3LYP functionals) treatment using the zeroth order regular approximation to relativistic effects (ZORA) in combination with recently developed accurate multicenter mean field spin-orbit operators (RI-SOMF) and the estimation of solvent effects using dielectric continuum theory at the conductor-like screening model (COSMO) level. The excellent agreement between experiment and theory, as well as the high consistency between the INDO/S and BP86/ZORA results, provides a sound basis for analysis of the calculated orientation of the g-tensor for cis,trans-(L-N2S2)MoVO(SCH2Ph) (2), for which single-crystal EPR data are not available but which contains three equatorial sulfur donor atoms, as occurs in sulfite oxidase and xanthine oxidase. The implications of these results for the EPR spectra of the Mo(V) centers of enzymes are discussed.

A theoretical study on the inner-valence photoelectron spectra lying between 21 and 26 eV of the O2 molecule

Ab initio calculations were carried out to study the structures of the photoelectron spectra of the O(2) molecule using complete active space self-consistent field and multireference single and double excitation configuration interaction methods. We focused our attention primarily on the inner-valence ionized states lying below 26 eV. Vibrational levels were calculated using adiabatic potential curves and vibrational wave functions were used to obtain Franck-Condon factors. We show that not only discrete vibrational levels of (3) (2)Pi(u), but also the continuum of nuclear motion in (1) (2)Sigma(u) (-) contribute to the bands of vibrational progression and the broad peak of the continuum between 22.5 and 26 eV. Other states with weak intensity lying between 19 and 26 eV are also discussed.

Spectral properties of minimal‐basis‐set orbitals: Implications for molecular electronic continuum states

AbstractEarly studies of the electronically excited states of molecules by John A. Pople and coworkers employing ab initio single‐excitation configuration interaction (SECI) calculations helped to simulate related applications of these methods to the partial‐channel photoionization cross sections of polyatomic molecules. The Gaussian representations of molecular orbitals adopted by Pople and coworkers can describe SECI continuum states when sufficiently large basis sets are employed. Minimal‐basis virtual Fock orbitals stabilized in the continuous portions of such SECI spectra are generally associated with strong photoionization resonances. The spectral attributes of these resonance orbitals are illustrated here by revisiting previously reported experimental and theoretical studies of molecular formaldehyde (H2CO) in combination with recently calculated continuum orbital amplitudes. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

High-level ab initio studies of unimolecular dissociation of the ground-state N3 radical

A comprehensive study of the unimolecular dissociation of the N(3) radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the N(3) radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear N(3). Three equivalent C(2v) minima of cyclic N(3) are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic N(3) is metastable with respect to ground state products, N((4)S)+N(2), and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N(3) minima. This strongly suggests that cyclic N(3) possesses a long collision-free lifetime even if formed with substantial internal excitation.

Metastable-atom-stimulated desorption from dodecanethiolate self-assembled monolayers

The potential curves for the dissociation of dodecane cation are calculated in order to discuss the indirect mechanism of metastable-atom-stimulated desorption of CH x + from dodecanethiolate self-assembled monolayers. A single excitation configuration interaction method is used for drawing up the potential diagrams in two different reaction coordinates: C H and C C. The potential barrier height obtained for the ejection of CH 3 + was smaller than that of H +, suggesting that CH x + desorption can be triggered by the excitation of molecular orbitals even if a molecule is only locally stimulated.

Theoretical study of lower electronic excitation spectra of [(Re 6S 8)Cl 6] 3−

We report the electronic structure of [(Re 6S 8)Cl 6] 3− using theoretical calculations. Since the lowest 2A u and 2A g states were energetically very close to each other, we employed single and double excitation configuration interaction method and coupled pair approximation to specify the ground state. It was settled to the 2A u state. Wavefunctions of many lower excited states were obtained by the Tamm–Dankoff type CI. Calculated oscillator strength distribution was in good agreement with observed absorption intensity curve. Those excited states lower than 2.3 eV were described mainly as intra ligands transition and those higher than 2.3 eV were charge transfer transition from ligands to metals.

Ab initio potential energy surfaces of the ion-molecule reaction: C2H2 + O+.

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double zeta) and cc-pVTZ (triple zeta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Sigmag+) + O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+ (2Piu) + O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+ (2Piu) + O(3P) (CT1) and C2H2+ (4A2) + O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.

The effect of spin-orbit coupling on fast neutral chemical reaction O(3P)+CH3-->CH3O.

The effect of nonadiabatic transitions through the spin-orbit couplings has been investigated on the fast neutral reaction, O((3)P)+CH(3)-->CH(3)O. Adiabatic potential energies and the spin-orbit coupling terms have been evaluated for the four electronic states of CH(3)O ((2)E, (2)A(2), (4)E, and (4)A(2)) that correlate with the O((3)P)+CH(3) asymptote, as a function of CO distance and OCH angle under the C(3v) symmetry, by ab initio electronic structure calculations using multireference internally contracted single and double excitation configuration interaction method with the 6-311G(2df,2pd) basis sets. Multistate quantum reactive scattering calculations have been carried out with the use of thus obtained potential energies and spin-orbit coupling matrices, based on the generalized R-matrix propagation method. The calculated thermal rate constants show a slight positive dependence on temperature in a range between 50 and 2000 K, supporting the previous experimental results. It is shown that the spin-orbit coupled excited states give rise to reflections over the centrifugal barrier due to the quantum interference. Classical capture calculations yield larger rate constants due to the neglect of quantum reflections. It is concluded that the effect of nonadiabatic transitions is of minor importance on the overall reactivity in this reaction.

Calculation of the electronic spectra of molecules in solution and on surfaces

An approach is presented to calculate the electronic spectra of molecules in solution or adsorbed on surfaces. Through limiting electronic excitations to include only those between orbitals localized on the solute or adsorbant, large computational savings are achieved with minimal additional error. This approach has been implemented within time-dependent density functional theory and single excitation configuration interaction. Calculations of the electronic spectrum of formamide in water and carbon monoxide on the Ni(1 1 1) surface illustrate the method.

Electronically excited states of sodium–water clusters

The lowest electronically excited state of small Na(H2O)n clusters has been investigated experimentally and theoretically. The excitation energy as determined by the depletion spectroscopy method drops from 16 950 cm−1 for the sodium atom down to 9670 cm−1 when only three water molecules are attached to the Na atom. For larger clusters the absorption band shifts back towards higher energies and reaches 10 880 cm−1 for n=12. The experimental data are compared to quantum-chemical calculations at the Møeller–Plesset second-order perturbation and multireference single and double excitation configuration interaction levels. We found that the observed size dependence of the transition energy is well reproduced by the interior structure where the sodium atom is surrounded by water molecules. The analysis of the radial charge distribution of the unpaired electron in these interior structures gives a new insight into the formation of the “solvated” electron.

Excited doublet and quartet states of SiP: a high level theoretical investigation

Abstract Doublet and quartet states of the SiP molecule dissociating into the four lowest dissociation channels are characterized theoretically at a high-level of correlation treatment (multireference single and double excitation configuration interaction). Potential energy curves give a global view of the manifold of possible electronic states. For selected states, dipole and transition moment functions, and transition probabilities and radiative lifetimes are also reported as well as an extensive set of spectroscopic constants. A new 2 Π state offers another likely route for exploring transitions to excited vibrational states of both X 2 Π and A 2 Σ + states. A detailed set of data for the quartet states is expected to provide valuable information for the experimental identification of these states.

Correlated ab initio calculation of electronic g-tensors using a sum over states formulation

An implementation of a sum-over-states (SOS) formulation for the electronic g-tensor is reported and evaluated through test calculations. The calculations include contributions from the relativistic mass correction, the gauge correction and the second-order orbital Zeeman/spin–orbit coupling part. Five different configuration interaction (CI) based methods ranging from single excitation CI to accurate multireference methods were evaluated. The results were compared to experimental data and other high-level calculations.

Ab initio theoretical studies of potential energy surfaces in the photodissociation of the vinyl radical. I. Ã state dissociation

The mechanism of photodissociation of the vinyl radical, C2H3, starting from the first doublet excited (D1,Ã) state was studied with high-level ab initio methods as well as with ab initio direct dynamics. Geometry optimizations of stationary points and surface crossing seams were performed with complete active space self-consistent field (CASSCF) method, and the energies were re-evaluated with single-point multireference single and double excitation configuration interaction (MRCISD) calculations. Both internal conversion and intersystem crossing channels, which could bring the excited vinyl radical down to the ground state potential energy surface leading to dissociation on the ground state, have been identified within planar Cs, twisted Cs and C2v symmetry. Direct dynamics calculation indicates that the most feasible reaction channel is the direct internal conversion from D1 to the ground state (D0) within planar Cs symmetry, through a minimum of seam of crossing (conical intersection) at an energy of about 80 kcal/mol (with respect to the ground-state equilibrium geometry). The other internal conversions from D1 to D0 through conical intersections within twisted Cs symmetry require energies of about 80 and 75 kcal/mol at the two minima of seam of crossing, respectively, and they are not favored dynamically without initial out-of-plane vibrational excitation. The intersystem crossing channels between D1 and the lowest quartet state (Q1) and D0 and Q1 within twisted Cs and C2v symmetry are not efficient due to the high energy of the minima of seam of crossing as well as the small spin–orbit coupling.

1,2-, 1,3-dioxetanes, 그리고 1,3-cyclodisiloxane의 분자구조, 에너지와 진동주파수에 대한 순 이론 양자 역학적 연구

1,2-와 1,3-dioxetane<TEX>$(C_2O_2H_4)$</TEX>, 그리고 1,3-cyclodisiloxane<TEX>$(Si_2O_2H_4)$</TEX>에 대하여 높은 이론 수준에서 분자구조, 진동주파수, 그리고 에너지 등을 계산하였다. 위의 모든 분자들에 대하여 TZ2P CCSD(T)의 이론 수준까지 분자구조를 최적화 하였으며. 진동주파수는 여러 basis set에서 SCF 방법으로 계산하였다. 본 연구에서 최적화된 분자구조들에 대한 진동주파수가 모두 실수(real number)로 예측됨으로서, 제안된 모든 분자구조가 local minimum 구조임을 확인하였다. 1,2- 및 1,3-dioxetane들과 cyclodisiloxane이 두 분자의 aldehyde와 silanone으로 해리 될 때의 중합에너지를 zero-point vibrational energy(ZPVE)를 고려하여 계산하고, 안정성을 비교하였다. The geometrical parameters, vibrational frequencies, and relative energies for 1,2-, 1,3-dioxetanes, and 1,3-cyclodisiloxane have been investigated using high level ab initio quantum mechanical techniques with large basis sets. The geometries have been optimized at the self-consistent field(SCF), the single and double excitation configuration interaction(CISD), the coupled cluster with single and double excitation(CCSD), and the CCSD with connected triple excitations[CCSD(T)] levels of theory. The highest level of theory employed in this study is TZ2P CCSD(T). Harmonic vibrational frequencies and IR intensities are also determined at the SCF level of theory with various basis sets and confirm that all the optimized geometries are true minima. Also zero-point vibrational energies have been considered to predict the dimerization energies for 1,2- and 1,3-isomers.

A theoretical study on lower electronic states of CoN

In order to identify the ground state of CoN, we carried out calculations of multi-reference single and double excitation configuration interaction with Davidson-type correction and multi-reference coupled pair approximation (MRCPA). MRCPA predicts that 1Σ + is the ground state of CoN. The 5 Δ state lies only 0.223 eV above 1Σ +. Internuclear equilibrium distance, harmonic frequency, and rotational constants of 1Σ + agree well with experimental results.