Nonadiabatic coupling and diabatic electronic population dynamics on 11A2 and 11B1 states of ozone molecule

In this work, we examine nonadiabatic population dynamics for 1¹B₁ and 1¹A₂ states of ozone molecule (O₃). In O₃, two lowest singlet excited states, ¹A₂ and ¹B₁, can be coupled. Thus, population transfer between them occurs through the seam involving these two states. At any point of the seam (conical intersection), the Born-Oppenheimer approximation breaks down, and it is necessary to investigate nonadiabatic dynamics. We consider a linear vibronic coupling Hamiltonian model and evaluate vibronic coupling constant, diabatic frequencies for three modes of O₃, bilinear and quadratic coupling constants for diabatic potentials, displacements, and Huang-Rhys coupling constants using ab initio calculations. The electronic structure calculations have been performed at the multireference configuration interaction and complete active space with second-order perturbation theory with a full-valence complete active space self-consistent field methods and augmented Dunning's standard correlation-consistent-polarized quadruple zeta basis set to determine ab initio potential energy surfaces for the ground state and first two excited states of O₃, respectively. We have chosen active space comprising 18 electrons distributed over 12 active orbitals. Our calculations predict the linear vibronic coupling constant 0.123 eV. We have obtained the population on the 1¹B₁ and 1¹A₂ excited electronic states for the first 500 fs after photoexcitation.     

Possible cyclic triatomic molecules including N2O and O3

Proposed metastable cyclic conformations of N₂O⁺ and O₃, have been examined by INDO and ab initio calculations. INDO is found to exaggerate the stability of possible cyclic species. In ab initio calculations a multi-dimensional energy surface must be explored. With a [5, 3] basis SCF calculations yield a trivariate local minimum for cyclic O₃. However, for N₂O, N₂O⁺ and O²⁺₃, starting from cyclic “bonded” structures, paths involving asymmetric deformations run downhill in energy to a diatomic molecule and a separated atom. A paradox concerning the removal of an electron from an antibonding orbital in a cyclic molecule is resolved.     

Molekulares PO2Br Matrix-IR-Untersuchung und ab initio SCF-Rechnungen

Molecular PO₂Br. Matrix IR Investigations and ab initio SCF Calculations Monomeric PO₂Br produced by a photochemical reaction between the high-temperature molecule OPBr and O₃ in solid Ar has been studied by IR spectroscopy. IR spectra including ¹⁶O/¹⁵O-shifts of the planar molecule with C_2v symmetry show that the OPO angle is about 135° and that the PBr bond is unexpectedly strong (f(PBr) = 2.8 mdyn/Å). These results are confirmed by ab initio SCF calculations.     

Pair Potentials for Alumina from AB Initio Results on the Al2O3 Molecule

Abstract

We use results from an ab initio investigation by Chang et al., on energetically low-lying stationary points of the Al₂O₃ molecule to determine interionic potentials for the Al—O, O—O and Al—Al pairs. Our results are discussed in the perspective of previous studies of the condensed phases of alumina, with special regard to the structure of its molten state.     

Ab initio investigations of structure and stability of the LiBeF3 molecule

Ab initio calculations of the potential energy surface of LiBeF₃ have been performed using the basis set of Roos and Siegbahn. The extremum and saddle points were made more precise with Huzinaga-Dunning basis sets in double-and triple-zeta contractions The “bidentate” structure (symmetry group C_2v) is found to have the lowest energy and is much more advantageous than the others, and the LiBeF₃ molecule turns out to be rigid with respect to migration of the cation around the anion. The calculated internuclear distances and the energy of complex formation are in agreement with experimental values within 0.03 Å and 2 kcal/mole. The results are compared with similar ab initio data for LiBeH₃ and LiNO₃.     

Density functional and ab initio study of the free radical MgNC

A new “non-terrestrial” molecule present in the envelope of the carbon star IRC + 10216 was described for the first time in 1986. Recently, this molecule was identified as the free radical MgNC, the first Mg-containing molecule in space. We present here the first density functional study performed on this radical, as well as on its isomer MgCN and the transition state connecting these species. It is shown that the optimum geometry obtained at the Becke3LYP/6-311+G(3df) level leads to the most exact rotational constants B_e and B_o calculated up to now. It is also shown that the energy differences between the three species are completely in agreement with the best ab initio calculations available. Furthermore, it is shown that the popular MP2 method fails for this system in the same way that has been demonstrated for other radicals.     

ChemInform Abstract: High Pressure Superconducting Phase of BI3: An ab initio Study.

The high pressure crystal structure of BI₃ with space group P2₁/c consisting of B₂I₆ dimers is determined using an ab initio evolutionary algorithm for crystal structure prediction in conjunction with density functional theory.     

Equilibrium structure of gas phase o-benzyne

An equilibrium structure has been derived for o-benzyne from experimental rotational constants of seven isotopomers and vibration–rotation constants calculated from MP2 (full)/6-31G(d) quadratic and cubic force fields. In the case of benzene, this method yields results that are in excellent agreement with those obtained from high quality ab initio force fields. The ab initio-calculated vibrational averaging corrections were applied to the measured A₀, B₀ and C₀ rotational constants and the resulting experimental, near-equilibrium, rotational constants were used in a least squares fit to determine the approximate equilibrium structural parameters. The C–C bond lengths for this equilibrium structure of o-benzyne are, beginning with the formal triple bond (C₁–C₂): 1.255, 1.383, 1.403 and 1.405 Å. The bond angles obtained are in good agreement with most of the recent ab initio predictions.     

Nuclear quantum and H/D isotope effects on three-centered bonding diborane: Path integral molecular dynamics simulations

Nuclear quantum and H/D isotope effects of bridging and terminal hydrogen atoms of diborane (B₂H₆) molecules were systematically studied by classical ab initio molecular dynamics (CLMD) and ab initio path integral molecular dynamics (PIMD) simulations with BHandHLYP/6-31++G** level of theory at room temperature (298.15 K). Calculated results clearly show that H/D isotope effect appears in the distribution of hydrogen (deuterium) of B₂H₆ (B₂D₆). Geometry of B₂H₆ also plays a significant role in the nuclear quantum effect proved by PIMD simulations, but slightly deviated from its equilibrium structure when simulated via CLMD simulation. The bond lengths between boron atoms R (B1 … B2) and the bridging hydrogen atoms R_HH (H_B1 … H_B2) of the B₂H₆ molecule obtained from PIMD simulations are slightly longer than those of the deuterated form of the diborane (B₂D₆) molecule. The principal component analysis (PCA) was also employed to distinguish the important modes of bridging hydrogen as related to the nuclear quantum and H/D isotope effects. The highest level of contribution obtained from PCA of PIMD simulations is bending, while various mixed vibrations with less contribution were also found. Therefore, the nuclear quantum and H/D isotope effects need to be taken into account for a better understanding of diborane geometry.     

A diatomics-in-molecules model for singly-ionized argon clusters

A DIM model using ab initio input for the diatomic interactions predicts a collinear bound Ar₃ ⁺ molecule (in agreement with ab initio calculations) and stable clusters Ar_n ⁺ consisting of an Ar₃ ⁺ ion embedded in (n?3) neutral atoms. These results support existing theories that dynamical size selection may be more relevant in interpreting experimental results than the relative stabilities of clusters in their minimum energy configurations.     

A theoretical study of the stability and the molecular electronic structure of methylene peroxide (CH2O2)

In relation to several postulates of the ozonolysis mechanism, the stability and conformation of the hypothetical methylene peroxide molecule (CH₂O₂) is investigated by an ab initio SCF MO calculation using a gaussian lobe function basis set of double zeta accuracy. The structure which has a three membered ring is found to be the most stable one amongst the conformations considered. The molecular electronic structure is discussed with the help of the electronic charge distribution in this species.     

The properties and possible transformation path for C12B24N24

The structure and frequencies of C₁₂B₂₄N₂₄ have been calculated by means of an ab initio method. By comparing the average bond energies with C₆₀, the calculated results predict that the cage C₁₂B₂₄N₂₄ is a stable molecule. The calculated results indicate that the cage molecule C₁₂B₂₄N₂₄ has a relative large HOMO–LUMO energy gap and a low rigidity. The structures and stability of six possible isomers of C₂B₄N₄ are used to suggest a possible transformation path from the pentagon CB₂N₂ to the C₁₂B₂₄N₂₄ materials. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 363–368, 2001     

The structures of the [M−H] anions of the CH3NO2 isomers nitromethane and methyl nitrite

The 60-u anion observed in the ion/molecule chemistry of methyl nitrite is shown to have the structure O=NCH₂O^?, and not that of the expected deprotonation product ^?CH₂ONO, by the use of mass-analyzed ion kinetic energy spectrometry, ion/molecule reactivity, and ab initio calculations.

     

Bismuth–Boron Multiple Bonding in BiB2O− and Bi2B−

Despite its electron deficiency, boron is versatile in forming multiple bonds. Transition‐metal–boron double bonding is known, but boron–metal triple bonds have been elusive. Two bismuth boron cluster anions, BiB₂O⁻ and Bi₂B⁻, containing triple and double B−Bi bonds are presented. The BiB₂O⁻ and Bi₂B⁻ clusters are produced by laser vaporization of a mixed B/Bi target and characterized by photoelectron spectroscopy and ab initio calculations. Well‐resolved photoelectron spectra are obtained and interpreted with the help of ab initio calculations, which show that both species are linear. Chemical bonding analyses reveal that Bi forms triple and double bonds with boron in BiB₂O⁻ ([Bi≡B−B≡O]⁻) and Bi₂B⁻ ([Bi=B=Bi]⁻), respectively. The Bi−B double and triple bond strengths are calculated to be 3.21 and 4.70 eV, respectively. This is the first experimental observation of Bi−B double and triple bonds, opening the door to design main‐group metal–boron complexes with multiple bonding.     

Theoretical studies of thermal decomposition of anhydrous cadmium and silver oxalates

The results of first principles calculations of band structure, density of states and electron density topology of CdC₂O₄ and Ag₂C₂O₄ crystals are presented. The calculations have been performed with WIEN2k ab initio program, using highly precise full potential linearized augmented plane wave (FP LAPW) method within Density Functional Theory formalism. The obtained SCF electron density has been used in calculations of Bader’s AIM (atoms in molecules) topological properties of the electron density in crystal. The obtained results show important similarities in electronic structure and electron density topology of both compounds and allow supposing, that during the thermal decomposition process these compounds should behave similarly, which is in agreement with the experiment.     

Variational transition‐state theory study of the atmospheric reaction OH + O3 → HO2 + O2

We report variational transition‐state theory calculations for the OH + O₃→ HO₂ + O₂ reaction based on the recently reported double many‐body expansion potential energy surface for ground‐state HO₄ [Chem Phys Lett 2000, 331, 474]. The barrier height of 1.884 kcal mol^?1 is comparable to the value of 1.77–2.0 kcal mol^?1 suggested by experimental measurements, both much smaller than the value of 2.16–5.11 kcal mol^?1 predicted by previous ab initio calculations. The calculated rate constant shows good agreement with available experimental results and a previous theoretical dynamics prediction, thus implying that the previous ab initio calculations will significantly underestimate the rate constant. Variational and tunneling effects are found to be negligible over the temperature range 100–2000 K. The O₁? O₂ bond is shown to be spectator like during the reactive process, which confirms a previous theoretical dynamics prediction. © 2007 Wiley Periodicals, Inc. 39: 148–153, 2007     

Reactions between a superoxide anion and alkyl bromides in dimethyl sulfoxide

The activation parameters of the reactions between a superoxide anion (O₂^·−) and alkyl bromides are measured. An ab initio study of the transition states for various mechanisms of this reaction is performed. The mechanism of radical separation in a polar solvent becomes competitive upon an increase in the number of alkyl groups in an alkyl bromide molecule and depends on their arrangement relative to a reaction center.     

Character of the electronic ground state and of charge-transfer excited states in ionic solids: An ab initio cluster model approach

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A1₂O₃, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF₃. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A1₂O₃, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF₃. © 1994 John Wiley & Sons, Inc.     

Molecular calculations with the MODPOT,VRDDO, and MODPOT/VRDDO procedures. IV. Boron hydrides and carboranes

Reference completely ab initio 6–3G and nonempirical 3G/MODPOT (ab initio effective core model potential) LCAO -MO -SCF calculations (using the same valence atomic orbital basis) were performed for a series of boron hydrides (B₄H₁₀, B₅H₉, B₅H₁₁, and B₆H₁₀) and a test 3G/MODPOT + VRDDO (variable retention of diatomic differential overlap for charge conserving integral prescreening) calculation were also performed for B₅H₉, B₆H₁₀, and B₁₀H₁₄. The agreement between the ab initio 6–3G and the corresponding 3G/MODPOT calculations was excellent for valence orbital energies, gross atomic populations, and dipole moments. The results also compared favorably to previous ab initio minimum STO basis results of Lipscomb and coworkers. The 3G/MODPOT + VRDDO calculations verified that for such spatially compact molecules (such as boron hydrides, which are fragments of polyhedra), the VRDDO procedure does not result in a noticeable savings in computer time for molecules of the size and shape of B₅H₉ and B₆H₁₀, in contrast to the savings previously realized for organic molecules of comparable atomic size. However, the agreement in calculational results between the 3G/MODPOT and the 3G/MODPOT +VRDDO results was still as extremely close as it had been for the organic molecules. 3G/MODPOT calculations were also carried out for B₈H₁₂, B₉H₁₅, B₁₀H₁₄, B₁₀H₁₄^?2, 1,2-C₂B₄H₆, and 1,6-C₂B₄H₆ and the results compared to the previous minimum STO basis results. For B₁₀H₁₄, the 3G/MODPOT +VRDDO method led to savings in computer time of 28% over the 3G/MODPOT method itself. The agreement of the 3G/MODPOT results with available experimental photoelectron spectral data for B₅H₉ and 1,6-C₂B₄H₆ was as good as that of the previous ab initio minimum STO basis calculations.     

Ab initio Study of the Structure of the Molecular Forms of C3H4O2

The geometrical parameters, force fields, and vibration frequencies were calculated by the ab initio SCF MO LCAO technique using extensive basis sets of Cartesian Gaussian functions for a number of structural isomers of the C₃H₄O₂ molecule. The relative energies of all the isomers were refined in terms of second-order Möller–Plesset (MP2) perturbation theory including electron correlation. For the most energetically favorable forms of the C₃H₄O₂ molecule, geometry optimization was fulfilled in the MP2 approximation. For the main conformer, -hydroxypropenal, possessing a chelate OCCCO fragment, the data are compared with the experimental and theoretical data available in the literature. The MP2 calculated internuclear distances and bond angles are in good agreement with the experimental values. For each form of the C₃H₄O₂ molecule, the geometrical and electronic structure is analyzed. It is shown that the presence of an intramolecular hydrogen bond is the characteristic feature of the structure of the isomers and an additional stabilizing factor.