Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Ab initio study of the electronic spectrum of dichlorocarbene CCl2

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for seven low-lying electronic states X ¹A₁, ¹B₁, ³B₁, ¹A₂, ³A₂, ¹B₂ and ³B₂ of dichlorocarbene CCl₂ have been calculated at the MRSDCI level with a double-zeta plus polarization basis set. Our calculated equilibrium geometry for the X ¹A₁ state, excitation energy for X ¹A₁ → ¹B₁ and vibrational frequencies for the X ¹A₁ and ¹B₁ states are in good agreement with experimental data. The electronic transition dipole moments, oscillator strengths for the ¹B₁ → X ¹A₁ and ¹B₂ → X ¹A₁ transitions, radiative lifetimes for the ¹B₁ and ¹B₁ states are calculated using MRSDCI wavefunctions, predicting results in reasonable agreement with experiment.     

硝酸乙酯分子间相互作用的ab initio研究

在abinitio-HF/6-31G水平上求得硝酸乙酯二聚体势能面上的四种优化构型和电子结构。经MP2电子相关校正和基组叠加误差(BSSE)以及零点能(ZPE)校正，求得二聚体的最大结合能为11.46kJ.mol^-^1，还进行HF/6-311G和HF/6-311++G水平的总能量比较计算，发现6-31G基组对计算结合能比较适合，二子体系间的电荷转移很少，对优化构型进行振动分析，并基于统计热力学求得从单体形成二聚体的热力学性质变化。     

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4sigma- symmetry featuring three bonds, with a recommended dissociation energy of D0 = 70.0 kcal/mol and r(e) = 1.640 angstroms. The first and second excited states, which also correlate to ground state atoms, are of 6sigma- and 8sigma- symmetry, respectively, and lie 17.7 and 28.2 kcal/mol above the X state at the MRCI level of theory.     

Ab initio molecular orbital caclulations,the electronic structure and electron spectrum of norbornadiene

Two LCGO minimal basis (7/3/3) calculations are reported using best atom and scaled gaussian functions. The electron spectrum (ESCA) of the core and valency shell orbitals is obtained, for the title compound norbornadiene.     

Ab initio study of low-lying electronic states of the FNO2 molecule

Ab initio electronic structure calculations are reported for low-lying electronic states, ¹A₁, ¹A₂, ³A₂, ¹B₁, ³B₁, ¹B₂, and ³B₂ of the FNO₂ molecule. Geometric parameters for the ground state ¹A₁ are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.     

Ab initio study of the electronic structure and bonding of aluminum nitride

For the diatomic aluminum nitride (AlN), we have constructed potential energy curves for 45 states employing multi-reference variational methods and quantitative basis sets. Thirty-six states are relatively strongly bound, five present local minima, and four are of repulsive nature. Almost all states are of intense multi-reference character rendering their calculation and interpretation quite problematic. Our tentative assignment of the ground state is 3Pi, while a 3Sigma- state is above by less than 1 kcal/mol. Our best estimate for the binding energy of the X3Pi state is D0 = 56.0 +/- 0.5 kcal/mol at re = 1.783 A, in good agreement with the experimental values of D = 66 +/- 9 kcal/mol and re = 1.7864 A. The binding energy of the A3Sigma- state is very similar to the X state because they both correlate to the ground-state atoms, but the bond distance of the former is 0.13 A longer. The first seven states can be tagged as follows: X3Pi, A3Sigma-, a1Sigma+, b1Pi, c1Delta, B3Sigma+, and d1Sigma+, a rather definitive order with the exception of X and A states.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

Ab initio spectrograms of the molecular vibrational spectrum

Theoretical spectrograms of the vibrational spectrum of 3,3-dimethylcyclopropene were constructed and juxtaposed with the experimental Raman and IR spectrograms. The theoretical spectrograms are represented as sets of vertical lines starting from the points corresponding to the values of the vibrational frequencies calculated from the scaled quantum-mechanical (QM) force field obtained at the HF/6-31G*//HF/6-31G* level. Two theoretical Raman spectrograms were constructed. In the first case, the heights of the vertical lines correspond to the QM values of the Raman scattering activities. In the second case they represent the relative differential Raman cross-sections calculated using the QM values of Raman scattering activities. The initial vibrational mode matrix remains virtually unchanged upon scaling of the QM force constant matrix because the dispersion of the scale factor values is low. Therefore, the heights of the theoretical lines for the IR spectrogram represent the QM intensities directly. The theoretical spectrogram based on the relative differential Raman cross-sections was shown to depict the experimental Raman spectrum more adequately. This makes it possible to use the results of the corresponding QM calculations more completely and obtain well-substantiated assignments of the vibrational frequencies.     

Ab initio electronic structure study of one-electron reduction of polychlorinated ethylenes

Polychlorethylene radicals, anions, and radical anions are potential intermediates in the reduction of polychlorinated ethylenes (C(2)Cl(4), C(2)HCl(3), trans-C(2)H(2)Cl(2), cis-C(2)H(2)Cl(2), 1,1-C(2)H(2)Cl(2), C(2)H(3)Cl). Ab initio electronic structure methods were used to calculate the thermochemical properties, (298.15 K), S degrees (298.15 K,1 bar), and DeltaG(S)(298.15 K, 1 bar) of 37 different polychloroethylenyl radicals, anions, and radical anion complexes, C(2)H(y)Cl(3)(-)(y)(*), C(2)H(y)Cl(3)(-)(y)(-), and C(2)H(y))Cl(4)(-)(y)(*)(-) for y = 0-3, for the purpose of characterizing reduction mechanisms of polychlorinated ethylenes. In this study, 8 radicals, 7 anions, and 22 radical anions were found to have stable structures, i.e., minima on the potential energy surfaces. This multitude of isomers for C(2)H(y)Cl(4)(-)(y)(*)(-) radical anion complexes are pi*, sigma*, and -H...Cl(-) structures. Several stable pi* radical anionic structures were obtained for the first time through the use of restricted open-shell theories. On the basis of the calculated thermochemical estimates, the overall reaction energetics (in the gas phase and aqueous phase) for several mechanisms of the first electron reduction of the polychlorinated ethylenes were determined. In almost all of the gas-phase reactions, the thermodynamically most favorable pathways involve -H...Cl(-) complexes of the C(2)H(y)Cl(4)(-)(y)(*)(-) radical anion, in which a chloride ion is loosely bound to a hydrogen of a C(2)H(x)Cl(2)(-)(x))(*) radical. The exception is for C(2)Cl(4), in which the most favorable anionic structure is a loose sigma* radical anion complex, with a nearly iso-energetic pi* radical anion. Solvation significantly changes the product energetics with the thermodynamically most favorable pathway leading to C(2)H(y)Cl(3)(-)(y)(*) + Cl(-). The results suggest that a higher degree of chlorination favors reduction, and that reduction pathways involving the C(2)H(y)Cl(3)(-)(y)(-) anions are high energy pathways.     

Ab initio study on structural and electronic properties of BanOm clusters

Density-functional calculation within local density approximation, shows that the electronic property of a barium oxide cluster is strongly correlated with its equilibrium structure. The ground-state structures of BanOm (4 < or = n < or = 9,m < or = n) clusters can be classified into four categories: (a) compact, (b) dangling state, (c) F-center, and (d) stoichiometric. The compact cluster is metallic, almost no energy gap exists between the highest occupied and the lowest unoccupied molecular orbitals. The energy gap for the dangling state cluster is larger than that for the F-center cluster, while the stoichiometric cluster has the largest energy gap.     

Ab initio study of low-lying electronic states of SnCl2+

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), and restricted-spin coupled-cluster singles-doubles with perturbative triples [RCCSD(T)] calculations have been carried out on low-lying doublet and quartet states of SnCl2+, employing basis sets of up to aug-cc-pV5Z quality. Effects of core correlation and off-diagonal spin-orbit interaction on computed vertical ionization energies were investigated. The best theoretical estimate of the adiabatic ionization energy (including zero-point vibrational energy correction) to the X2A1 state of SnCl2+ is 10.093+/-0.010 eV. The first photoelectron band of SnCl2 has also been simulated by employing RCCSD(T)/aug-cc-pV5Z potential energy functions and including Duschinsky rotation and anharmonicity.     

Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck‐Condon region have been theoretically examined by means of ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations. The optimized structure of styrene in S₀ is planar but the torsional motion of the phenyl group is very floppy. The S₁ state is assigned to the local π–π* excitation within the benzene ring. On the other hand, S₂, above S₁ by 0.561 eV, is assigned to a state that resembles the so‐called V‐state of ethylene. The transition intensity of S₀–S₁ is weak, while that of S₀–S₂ is strong. This is in good agreement with the experimental absorption spectrum where the S₀–S₁ and S₀–S₂ transitions are in the energy range of 290–220 nm. The optimized geometry of S₁, characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S₁ state. © 2002 Wiley Periodicals, Inc. J Comput Chem 9: 928–937, 2002     

Ab initio study of the low lying electronic states of ZnF and ZnF-

Highly correlated ab initio calculations have been performed for an accurate determination of the electronic structure and of the spectroscopy of the low lying electronic states of the ZnF system. Using effective core pseudopotentials and aug-cc-pVQZ basis sets for both atoms, the potential curves, the dipole moment functions, and the transition dipole moments between relevant electronic states have been calculated at the multireference-configuration-interaction level. The spectroscopic constants calculated for the X(2)Sigma(+) ground state are in good agreement with the most recent theoretical and experimental values. It is shown that, besides the X(2)Sigma(+) ground state, the B(2)Sigma(+), the C(2)Pi, and the D(2)Sigma(+) states are bound. The A(2)Pi state, which has been mentioned in previous works, is not bound but its potential presents a shoulder in the Franck-Condon region of the X(2)Sigma(+) ground state. All of the low lying quartet states are found to be repulsive. The absorption transitions from the v=0 level of the X(2)Sigma(+) ground state toward the three bound states have been evaluated and the spectra are presented. The potential energy of the ZnF(-) molecular anion has been determined in the vicinity of its equilibrium geometry and the electronic affinity of ZnF (EA=1.843 eV with the zero energy point correction) has been calculated in agreement with the photoelectron spectroscopy experiments.     

Ab initio potential energy surfaces, bound states, and electronic spectrum of the Ar-SH complex

New ab initio potential energy surfaces for the (2)Pi ground electronic state of the Ar-SH complex are presented, calculated at the RCCSD(T)/aug-cc-pV5Z level. Weakly bound rotation-vibration levels are calculated using coupled-channel methods that properly account for the coupling between the two electronic states. The resulting wave functions are analyzed and a new adiabatic approximation including spin-orbit coupling is proposed. The ground-state wave functions are combined with those obtained for the excited (2)Sigma(+) state [D. M. Hirst, R. J. Doyle, and S. R. Mackenzie, Phys. Chem. Chem. Phys. 6, 5463 (2004)] to produce transition dipole moments. Modeling the transition intensities as a combination of these dipole moments and calculated lifetime values [A. B. McCoy, J. Chem. Phys. 109, 170 (1998)] leads to a good representation of the experimental fluorescence excitation spectrum [M.-C. Yang, A. P. Salzberg, B.-C. Chang, C. C. Carter, and T. A. Miller, J. Chem. Phys. 98, 4301 (1993)].     

Ab initio study on the electronic and mechanical properties of ReB and ReC

The structural, electronic, and mechanical properties of ReB and ReC have been studied by use of the density functional theory. For each compound, six structures are considered, i.e., hexagonal WC, NiAs, wurtzite, cubic NaCl, CsCl, and zinc-blende type structures. The results indicate that for ReB and ReC, WC type structure is energetically the most stable among the considered structures, followed by NiAs type structure. ReB-WC (i.e., ReB in WC type structure) and ReB-NiAs are both thermodynamically and mechanically stable. ReC-WC and ReC-NiAs are mechanically stable and becomes thermodynamically stable above 35 and 55 GPa, respectively. The estimated hardness from shear modulus is 34 GPa for ReB-WC, 28 GPa for ReB-NiAs, 35 GPa for ReC-WC and 37 GPa for ReC-NiAs, indicating that they are potential candidates to be ultra-incompressible and hard materials.     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.