Ab Initio Potential Energy Research Articles

Reionization mechanism of neutralized He in low energy ion scattering spectroscopy

The reionization mechanism of neutralized He in low energy ion scattering spectroscopy is clarified on the basis of an ab initio CI potential energy calculation for some diatomic systems. It is newly reported that the antibonding interaction between the orbital and the core orbitals of the target atoms is important in the reionization mechanism.

Ab-initio SCF potential energy surfaces for the nucleophilic attack of hydride on coordinated carbon monoxide

Ab-initio SCF calculations are reported for the nucleophilic addition of the hydride ion H− on the iron pentacarbonyl Fe(CO)5 complex. The stereochemistry of the attack has been established from the determination of two potential energy surfaces. The reaction is shown to be highly exothermic and with no activation barrier. This activation of CO towards the nucleophilic addition is rationalized in terms of molecular orbital interactions.

Isomerism of the activated complex in water dimer interconversion

Abstract The structural and vibrational characteristics of three transition states with point group symmetry C s , S 2 and C 2v were found on the basis of recent ab initio SCF CI potential energy hypersurfaces of (H 2 0) 2 . These structures are used as activated complexes in the conversion between superimposable minimum energy structures (trans near-linear dimers). The activation characteristics, rate constants and tunnel-effect corrections corresponding to the partial rate processes going through the individual activated complexes were calculated from the activated complex theory. A weighting treatment was then applied, leading to summary characteristics connected with the overall rate process (H 2 0) 2 → (H 2 0) 2 . The role of the individual structures of the activated complex in this weighting treatment is discussed for a broad temperature range.

Isomerism, energetics, and thermodynamics of (H2O)2 and its role in real gas phase of water

Recent ab initio SCF CI potential energy hypersurfaces of (H2O)2 have been systematically investigated with the use of an automatic optimization procedure working with analytically constructed energy gradient with the aim to localize all their stationary points lying in the relevant region. For these stationary points the vibrational analysis has been carried out based on analytically constructed second derivatives of energy with correct respecting of the eight redundancy conditions introduced into the problem by the original choice of the coordinates. Out of the six localized stationary points only one has been found to be a minimum, three of them represent transition states, and the remaining two are higher types of stationary points. Relation of the analytically and the numerically constructed force constant matrices and possibility of the occurrence of extraneous stationary points have been studied. The calculated harmonic vibrational frequencies corresponding to the energy minimum show reasonable agreement with available experimental data. On the basis of the generated molecular parameters of the minimum the thermodynamics of formation of the water dimer has been described within a broad temperature interval, and a good agreement with experimental findings has been obtained. Role of the water dimer in real gas phase of water has been analyzed, and contribution of (H2O)2 to value of the second virial coefficient of water vapour has been evaluated. Within the framework of the given quantum-chemical methodological level, this study has led to negation of possibility of isomerism of (H2O)2 with respect to equilibrium problems, it has, however, shown the isomerism in the case of kinetics of interconversion of (H2O)2.

Theoretical study of the photodissociation of HOCl

A theoretical study of the uv photodissociation of HOCl is presented based on ab initio SCF-CI calculations. Results are reported for the low-lying singlet and triplet states of the HOCl molecule using both double-zeta and double-zeta-plus-polarization basis sets of contracted Cartesian Gaussian functions. The calculated equilibrium geometry for the ground electronic state [(1)1A′] is in excellent agreement with experiment. The vertical excitation energies to the two lowest singlet states are determined to be: (1)1A′→ (1)1A″ =4.50 eV and (1)1A′→ (2)1A′=5.66 eV. Electronic transition dipole matrix elements have been calculated for these two excitations. The (1)1A′→ (2)1A′ transition moment is more than a factor of 10 larger than that for the (1)1A′→ (1)1A″ transition. The photoabsorption spectrum for HOCl (vapor) between 200 and 400 nm has been computed based on the ab initio potential energies and transition moments. This spectrum exhibits a single peak, at 220 nm, with an extinction coefficient of 123 l mole−1 cm−1. The only previous experimental measurement is that of Fergusson, Slotin, and Style, who reported two peaks at 220 and 320 nm. Possible explanations for this discrepancy are discussed in light of recent unpublished experimental data. By examination of the topographies of the (1)1A″ and (2)1A′ potential energy surfaces, the photodissociation products are predicted to be Cl+OH. Since HOCl does not have significant photoabsorption cross section in the wavelength range 300–500 nm we conclude that it would be stable to photodissociation if formed in the stratosphere. The implications of HOCl as a reservoir for stratospheric chlorine are discussed.

Theoretical studies of the chemistry of singlet and triplet species. II. Cycloaddition reaction

Ab initio SCF potential energy surfaces for the cycloaddition reactions C2H4(1Alg) + O(1D,3P) are reported. The lowest energy singlet path corresponds to the symmetric, concerted addition reaction yielding ethylene oxide. The lowest energy triplet path corresponds to an asymmetric approach of the reactants to form an open ring adduct [Formula: see text] The spatial distribution of the spin density in this adduct conforms to that expected for a diradical. The barrier to internal rotation of the terminal methylene is calculated to be ∼5 kcal mol−1. The lowest intersystem crossing from the triplet to the singlet surface, as required for ring closure to yield ground state product, is found to occur ∼36 kcal mol−1 above the energy of the triplet reactants.The differences in singlet and triplet charge distributions and energies for similar approach geometries may be rationalized in terms of the additional restraints imposed on the course of the triplet reaction by the Pauli principle, restraints which are made evident by the spatial distribution of the spin density. Two principal mechanisms of spin polarization, spin unpairing and spin decoupling, are found to provide a rationale for the observed differences of singlet and triplet reactants in abstraction, insertion and addition reactions. The favoured course of a triplet reaction is found to be governed by the axiom that localized distributions of spin density are to be maximally separated in space.