Barrierless reactions between two closed-shell molecules. I. Dynamics of F(2)+CH(3)SCH(3) reaction

A detailed experimental and theoretical investigation of the first-reported barrierless reaction between two closed-shell molecules [J. Chem. Phys. 127, 101101 (2007)] is presented. The translational energy and angular distributions of two product channels, HF+CH(2)SFCH(3) and F+CH(3)SFCH(3), determined at several collision energies, have been analyzed to reveal the dynamics of the studied reaction. Detailed analysis of the experimental and computational results supports the proposed reaction mechanism involving a short-lived F-F-S(CH(3))(2) intermediate, which can be formed without any activation energy. Other possible reaction mechanisms have been discriminated. The decay of the intermediate and competition between the two product channels have been discussed.     

Double-exchange contributions to the first-order interaction energy between closed-shell molecules

Explicit formulas are presented for the double-exchange first-order interaction energy of two closed-shell molecules. Calculations on He₂ show that the double-exchange terms contribute significantly for R < 3.5 au.     

On the role of higher-order correlation effects on the induction interactions between closed-shell molecules

High-order correlation contributions to the second-order induction energy were studied for various representative van der Waals complexes. It was found that the induction energy obtained by the truncation of the relaxed M?ller-Plesset expansion in the second or third order is in most cases quite close to the induction energy computed with the coupled-cluster method (restricted to single and double excitations). Also, the effect of triples excitations on this perturbation term is usually small. However, given an oscillatory behaviour of the M?ller-Plesset induction corrections, the coupled-cluster method seems to be better suited to a reliable calculation of the induction energy. The sources of the remaining differences between the interaction energies computed by symmetry-adapted perturbation theory and those computed by the supermolecule coupled-cluster method (restricted to single, double, and noniterative triple excitations) are examined. It has been found that they can be attributed to the higher-order correlation terms in the second-order dispersion and exchange-induction corrections.     

Relativistic Dirac–Fock calculations for closed-shell molecules

A new code has been written to perform relativistic Dirac–Fock self-consistent field (SCF) calculations on closed-shell molecules of any symmetry. The choice of the basis set allows us to work at different levels of approximation depending on the precision required. Calculations on the H₂Po molecule show that accurate results on specific problems like geometry optimization can be obtained by evaluating the two-electron integrals on half the basis spinors. © 1994 by John Wiley & Sons, Inc.     

Variational-Perturbation theory in the LCAO-MO method for closed-shell molecules

Using the variational principle, equations have been obtained for density matrix of molecules with the external perturbation of the common type present. The perturbed wave function was taken as a superposition of the ground and single-excited configurations made of the Hartree-Fock molecular spin orbitals. On the basis of these equations a series of variants of static perturbation theory have been worked out for the ground and excited states of closed-shell molecules.     

Direct calculation of ionization potentials of closed-shell atoms and molecules

Vertical ionization potentials, electron affinities and information about quasi-particles can be obtained by using the technique of the single-particle propagator. The expansion of the self-energy part up to third order perturbation theory can be evaluated numerically, but does not lead, in most cases, to satisfying results. A theoretical and numerical analysis of the diagrammatic expansion of the self-energy part requires the introduction of a renormalized interaction and renormalized hole and particle lines.     

Molecular design of ambipolar redox-active molecules II: closed-shell systems

This mini-review highlights key structural features that should be taken into account when creating ambipolar redox-active closed-shell metal-free molecules. This type of compound is strongly required for the fabrication of all-organic ‘poleless’ batteries and semiconductors. The suggested strategies aimed at stabilization of both oxidized (cationic) and reduced (anionic) redox-states are based on the comprehensive analysis of the most successful structures taken from the recent publications.     

Single-determinantal open- and closed-shell FSGO calculations for some diatomic molecules

Open-shell single-determinantal calculations are reported here for the molecular species H₂, Li₂⁺, N₂, O₂ (triplet), O₂^2?;, O₂^?, O₂²⁺, O₂⁺, and F₂; corresponding closed-shell calculations are reported for the species H₂, N₂, O₂ (singlet), O₂^2?, O₂²⁺, and F₂. The floating spherical Gaussian orbital (FSGO ) method was employed. The calculated trend in bond lengths of isonucleic diatomic molecules is in agreement with experiment. For heteronucleic diatomic molecules, however, the experimental trend in bond lengths is not obtained; in this connection, the effect of lone pairs on bond length is discussed. The dissociation energies of H₂ and Li₂⁺ are evaluated. The energy gap between the triplet and singlet states of the oxygen molecule is calculated to be 8.96 eV compared to the experimental value of 4.54 eV.     

Dynamics of bimolecular reactions of vibrationally highly excited molecules: quasiclassical trajectory studies

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.     

Dispersion energy between closed-shell and half-closed-shell molecules.an ab inito calculation from a perturbation procedure on LiHe

A perturbation procedure previously used for closed-shell molecules is generalized to the case of the interaction between closed-shell and half-closed-shell molecules In particular, the use of different denominators is considered and special attention is paid to the effect of small basis sets for LiHe.     

Highly strained 2,3-bridged 2H-azirines at the borderline of closed-shell molecules

Substituted 1-azidocyclopentenes and 1-azidocyclohexenes were photolyzed to generate 2,3-bridged 2H-azirines. In the case of bridgehead azirines with a six-membered carbocycle, detection by NMR spectroscopic analysis was possible, whereas even kinetically stabilized bridgehead azirines with a five-membered ring could not be characterized by low-temperature NMR spectroscopic analysis. Thus, a recent report on the latter heterocycles was corrected. Depending on the substitution pattern, irradiation of 1-azidocyclopentenes either led to products that can be explained on the basis of short-lived 2,3-bridged 2H-azirines, or gave secondary products generated from triplet nitrenes. The diverse photoreactivity of 2,3-bridged 2H-azirines was also studied by quantum chemical methods (DFT, CCSD(T), CASSCF(6,6)) with respect to the singlet and triplet energy surfaces. The ring-opening processes leading to the corresponding vinyl nitrenes were identified as key steps for the observed reactivity.     

Calculation of vertical excitation energies of closed-shell molecules in the CNDO and INDO approximations

The Green's function method for the calculation of vertical excitation energies is adapted to the CNDO and INDO approximations by introducing an effective interaction into the irreducible vertex part. The computational scheme is explicitly developed for closed-shell molecules and applied to H₂O, H₂CO, HCOOH, HCONH₂.     

Energy transfer in collisions between two vibrating molecules

We study vibrational energy transfer in inelastic collinear collisions between two diatomic molecules. The system is represented by two linearly driven parametric oscillators with a bilinear, time-dependent residual coupling between them. We account for the time evolution of the linearly driven parametric oscillators with an operator algebra, and use perturbation theory and basis expansions to include the residual coupling. Results are presented for H₂FH and N₂CO. Direct two-quantum transitions are found to be important even for low relative collision energies.     

A Dirac–Fock self-consistent field method for closed-shell molecules including Breit interaction

We present a method for including the Breit interaction in relativistic self-consistent field calculations for closed-shell molecular systems using atomic basis spinors of kinetically balanced Gaussian-type functions. The method extends the formalism described in a previous paper [A. Mohanty and E. Clementi, Int. J. Quantum Chem. 39 , 487–517 (1991)] that dealt with the two-electron effect due to Coulomb interaction only. It is shown that both frequency-dependent and frequency-independent Breit interactions can be treated on equal footing, and the corresponding matrix elements are evaluated following the well-known Fourier transform technique applied to electron repulsion integral evaluation in nonrelativistic molecular calculations.     

Dynamics of chain molecules. Intramolecular diffusion controlled reactions for a pair of terminal reactive groups

The first order intramolecular rate constant for the reaction between the terminal groups of flexible macromolecules is calculated in the partial draining case, following the theory formulated by Wilemski and Fixman for diffusion controlled reactions. Substantial differences with respect to the nondraining and free-draining limits are evidenced, and it is shown that the rate constant in the partial-draining case has relevant contributions from all the modes of the bead-spring chain. The effects of chain flexibility and hydrodynamic interaction in macromolecules of increasing molecular weight are examined. The effective diffusion constant of the end groups increases with both the flexibility and the length of the chain. Numerical results for polystyrene (PS) and polydimethylsiloxane (PDMS) are presented and employed to calculate the relative quantum yield for fluorescence quenching. For highly flexible chains, like PDMS, quenching effects are expected in a range of molecular weight well above the limit of validity of the bead-spring model. On the contrary, for more rigid polymers, like PS, the quenching can be observed only at molecular weights lower than this limit. The calculated behavior is compared with some experimental results recently obtained by the authors.     

Tunneling in the proton transfer between two water molecules

SCF CNDO calculations were performed for the species H₅O⁺₂ at several positions of the intervening proton and at interoxygen distances of 2.65, 2.70 and 2.75 Å. The energy profile was fitted to a potential energy function containing a quadratic term plus a gaussian. The eigenvalues and eigenvectors were obtained by using the variational method with the eigenfunctions of the parabolic potential as basis set. The results indicate that at 2.65 Å the top of the barrier is below the first energy level and that at 2.75 Å the first two energy levels are below the top of the barrier with the splitting of the symmetric-antisymmetric pair of 0.00132 au indicating that tunneling occurs at a frequency of 10¹⁴ reciprocal seconds.