Ab initio SCF computation of force constants for CO2

Force constants for CO₂ have been evaluated using SCF wave functions. The effect ofd basis functions and geometry are investigated. Comparison with experimental values shows that a large error, due to neglect of electron correlation, occurs for theK ₁₂ interaction stretch force constant.     

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 calculation of the vibrational and electronic spectra of trans- and cis-azobenzene

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order M?ller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.     

Ab initio calculations of the electronic structure of helical polymers

An ab initio self-consistent-field (SCF ) algorithm taking into account all the features of the one-dimensional translational periodicity and the helical symmetry is presented. This algorithm includes the long-range correction to the Coulomb potential and is designed to calculate the band structure of periodic one-dimensional polymers (planar or helical). Its efficiency in terms of computing time and numerical accuracy is tested via applications on a (LiH)_n chain, polyethylene, and four conformers of polypropylene.     

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.     

宝塔烷电子结构与张力的ab initio研究

利用MNDO程序优化出宝塔烷、宝塔烷双正离子及宝塔二烯的构型参数。从头算的电荷分布及分子轨道均表明: 宝塔烷与其价键异构体宝塔二烯性质相近。双正离子的中间四个碳原子则形成了一种与环丁烷双正离子类似的四中心双电子大π键。张力研究表明, 宝塔烷的键张力要比不能稳定存在的小螺桨烷小得多, 说明了宝塔烷骨架的刚性及稳定性。     

Ab initio Calculations for the electronic structure of carbazole and trinitrofluorenone

The results of the SCF-LCAO-MO calculations for the electronic structure of carbazole and trinitrofluorenone are reported. Comparison is made with previous computations and with experimental data. A qualitative calculation tends to explain satisfactorily the first electro-absorption peak in amorphous films of polyvinylcarbazole and trinitrofluorenone on the basis of a complete charge transfer model. The singlet-triplet splitting predicted for carbazole also compares favorably with the experimental results.     

Ab initio Calculation of the Electronic Structure and Equilibrium Geometry of Cortisol and Its Derivatives

The molecular geometries of a series of steroid hormones including cortisol, 9-fluorocortisol, 6-fluorocortisol, and 9-chlorocortisol were optimized by 3-21G and 6-31G* ab initio calculations. The results of calculations on both levels are well consistent with each other and with the experiment. The conformational changes and electron density redistribution occurring in going from cortisol to its 6- and 9-halo derivatives were discussed. A conclusion was made that the O and F atoms can participate in hydrogen bonding with the corresponding structural groups of glucocorticoid receptors.     

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 investigation of the electronic structure of malonic dialdehyde derivatives

An ab initio quantum chemical investigation is performed for a series of ligands of Β-diketonate metal complexes: neutral and anionic forms of malonic dialdehyde and its nitrous analogs. The nature and sequence of molecular orbitals are established, and the influence of the basis set used on the results of electronic structure calculations of the compounds are analyzed. The electronic effects of oxygen substitution by the NH group are analyzed. The results are compared with the photoelectron spectroscopy data of the ligands. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 6, pp. 1061–1066, November–December, 1997.     

Ab initio study of the structure, force fields, and vibrational spectra of alkali metal tellurites

The equilibrium geometrical parameters, force fields, vibration frequencies, and intensities in the IR molecular spectra of M₂TeO₃ (M-Li, Na, K) are found by the Hartree — Fock ab initio method in the bases complemented with polarization and diffusion functions using relativistic effective core potentials. The relative energies of alternative molecular configurations and the energies of the dissociation M₂TeO₃→ M2O + TeO2 are refined using Möller — Plessett second-order perturbation theory. It is found that the chemical bond in these molecules can be approximated by the scheme (M⁺)₂[TeO₃]^2- The equilibrium nuclear configuration has C_s symmetry and corresponds to the bis-bidentate coordination of M₊ cations by the pyramidal anion [TeO₃]^2-. The mono-bidentate structures of C_s symmetry correspond to the saddle points on the potential energy surface of the molecules. The calculated IR molecular spectrum of K₂TeO₃ agrees with the experimental spectrum obtained by the matrix isolation method by J. Ogden et al. [J. Chem. Soc. Dalton Trans., 1957 (1997)]. Based on the results of the previous ab initio studies of alkali metal sulfites and selenites, the barrier of the intramolecular rearrangement (bb)→(mb)→(bb)’ was found to lower in the series Li₂XO₃→Na₂XO₂→K₂XO₃ (X = S, Se, Te) and M₂TeO₃→M₂SeO₃→M₂SO₃. The degree of deformation of the XO fragment in the M₂XO₃ molecules decreases when the M atom is replaced by its heavier analog (Li→Na→K).     

Ab initio calculation of the electronic structure of TCNQ and its ions

Ab initio SCF-LCAO-MO calculations have been performed for TCNQ and its positive and negative ions in various electronic states. A basis set consisting of 412 primitive Gaussian type orbitals contracted to 180 was used in the investigation. The electron density distribution in TCNQ and the negative ions, and the redistribution upon ionization has been illustrated by plotting difference density contour maps. The quinone structure of the neutral TCNQ system undergoes a transformation to a benzenoid structure when electrons are added. Electronic transitions, ionization potentials and electron affinities have also been calculated.     

Ab initio structure and vibrational frequencies of lithium aromatic sulfonyl imide salts

The structure and vibrational frequencies of an aromatic lithium sulfonyl imide, i.e., lithium bis(4-nitrophenylsulfonyl)imide (LiNPSI) has been studied using self-consistent ab initio Hartree–Fock and hybrid density functional methods. These calculations engender two linkage isomers, which correspond to the local minima on the potential-energy surface. In the lowest-energy isomer, the ligand binds to the metal ion through two oxygens, one from each of the different SO₂ groups on the central nitrogen and forms a six-membered ring. Another LiNPSI isomer, wherein the anion coordinates through oxygen and nitrogen atoms and which is 55.9 kJmol⁻¹ higher in energy, has also been obtained. The S–N–S bond angle in the free anion as well as in the LiNPSI complex turns out to be nearly 121°. A comparison of the vibrational spectra of the free NPSI anion and that of the LiNPSI complex reveals that the SO₂ stretching vibrations at 1,239 and 1,205 cm⁻¹ can be used to differentiate between the two linkage isomers of the complex. The stronger complexation ability of the NPSI anion, compared to that for (CF₃SO₂)₂N⁻ has been explained in terms of the charge density within the molecular electrostatic potential isosurface encompassing both SO₂ groups of the anion. Received: 20 February 2002 / Accepted: 25 March 2002 / Published online: 3 June 2002     

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 on the stability,geometry and electronic structure of the cyclopropenyl system

Results of an LCAO-MO-SCF investigation into the stability and geometry of the cyclopropenyl cation (C₃H₃⁺), anion (C₃H₃^?) and radical (C₃H₃^o) are presented. By independently varying the two relevant bond angles, the shape of the potential energy curves in the corresponding two-parameter space for these three species has been obtained. It is found that the cation is most stable in the D_3h nuclear configuration, while the anion has minimum energy in the C_s symmetry. In the specific case of the C₃H₃ radical, which exhibits an orbital, as well as spin-degeneracy, in the D_3h configuration, a strong Jahn-Teller effect is observed, leading to estimates for the non-symmetrical equilibrium configuration at C_2v and for the distortion energy of 12 kcal/mole from the D_3h symmetry. In terms of the population analysis and the contour diagrams, the electronic charge distribution has also been studied for these species in their most stable configurations.     

Ab initio study of the structure,force field,and vibrational spectrum of the LiClO3 molecule

The geometrical structure and the vibrational spectrum of the LiClO₃ molecule are studied by the Hartree-Fock-Roothaan (HF) and configurational interaction (CI) methods taking into account single and double excitations and Davidson's correction for quadruple excitations. Double-zeta basis sets of Huzinaga-Dunning and McLean-Chandler complemented with polarization and diffuse functions are used. Potential surface sections corresponding to migration of the Li⁺ cation around the ClO ₃ ⁻ fragment are investigated. It was found that the LiClO₃ molecule has a single stable configuration of C_s symmetry with the bidentate coordination of the Li⁺ cation by the ClO ₃ ⁻ anion. The cyclic fragment is nonplanar (the dihedral angle θ(LiO₂Cl)=173°). The tridentate configuration of C_3v symmetry lies higher than the equilibrium configuration by 24.5 (HF) or 18.3 (CI) kJ/mole and is not an isomer. The ab initio force field of the molecule was refined by the scaling method. Some assignments of the IR bands of the matrix-isolated molecular forms existing in vapor over lithium chlorate are corrected. The vibration frequencies (cm⁻¹) and IR intensities (km/mole; in parentheses) are calculated with the refined force field: A′ type 1099(236), 856(81), 630(73), 557(119), 481(87), 156(66); A″ type 887(229), 459(35), 367(23). Ivanovo State Chemical Technological Academy. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 440–449, May–June, 1996.     

The structure of linear SiCC: An ab initio SCF CI study including vibrational effects

Large-scale SCF CI calculations have been performed for the ground ¹Σ⁺ state of linear SiCC. The calculation includes up to quadruple excitations in the valence space plus all single and double excitations from the valence localized orbitals of the single HF configuration. Vibrational wavefunctions have been derived from the CI potential surface. Vibrational frequencies including anharmonicity corrections are calculated together with the zero-point vibrational correction to the rotational constant. The large dipole moment, 4.62 D, should make this molecule suitable for radioastronomic searches.     

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 investigation of the electronic and geometric structure of magnesium diboride, MgB2

Employing multireference variational (MRCI) and coupled cluster (CC) methods combined with quadruple-zeta quality correlation-consistent basis set, we have studied 36 states of the magnesium diboride (MgB(2)) molecule as well as 17 states of the experimentally unknown diatomic MgB. For all states of MgB(2), we report geometries, atomization energies, and dipole moments, while for the first 5 states, potential energy profiles have been also constructed. The ground state is formally of (1)A(1) V-shaped symmetry with an atomization energy of 108.1(109) kcal/mol at the MRCI(MRCI + Davidson correction) level. The first excited state ((3)B(1)) is less than 1 kcal/mol above the X(1)A(1) state, with the next state of linear Mg-B-B geometry (b(3)Sigma(-)) located 10 kcal/mol higher. In all states, bent or linear, the bonding is complicated and unconventional because of the extraordinary bonding agility of the boron atom(s).