Sergey A. Varganov

The interaction of oxygen with small gold clusters

Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Aun and Aun− (n=2,3) clusters. Density functional theory (DFT), second order perturbation theory (MP2), and singles and doubles coupled cluster theory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to Aun. The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and...

A study of the reactions of molecular hydrogen with small gold clusters

This work presents a study of reactions between neutral and negatively charged Au(n) clusters (n=2,3) and molecular hydrogen. The binding energies of the first and second hydrogen molecule to the gold clusters were determined using density functional theory (DFT), second order perturbation theory (MP2) and coupled cluster (CCSD(T)) methods. It is found that molecular hydrogen easily binds to neutral Au(2) and Au(3) clusters with binding energies of 0.55 eV and 0.71 eV, respectively. The barriers to H(2) dissociation on these clusters with respect to Au(n)H(2) complexes are 1.10 eV and 0.59 eV for n=2 and 3. Although negatively charged Au(n) (-) clusters do not bind molecular hydrogen, H(2) dissociation can occur with energy barriers of 0.93 eV for Au(2) (-) and 1.39 eV for Au(3) (-). The energies of the Au(2)H(2) (-) and Au(3)H(2) (-) complexes with dissociated hydrogen molecules are lower than the energies of Au(2) (-)+H(2) and Au(3) (-)+H(2) by 0.49 eV and 0.96 eV, respectively. There is satisfactory agreement between the DFT and CCSD(T) results for binding energies, but the agreement is not as good for barrier heights.

Variational geminal-augmented multireference self-consistent field theory: Two-electron systems

We introduce a geminal-augmented multiconfigurational self-consistent field method for describing electron correlation effects. The approach is based on variational optimization of a MCSCF-type wave function augmented by a single geminal. This wave function is able to account for some dynamic correlation without explicit excitations to virtual molecular orbitals. Test calculations on two-electron systems demonstrate the ability of the proposed method to describe ionic and covalent electronic states in a balanced way, i.e., including the effects of both static and dynamic correlation simultaneously. Extension of the theory to larger systems will potentially provide an alternative to standard multireference methods.

Ab initio calculations of endo-and exohedral C60 fullerene complexes with Li+ ion and the endohedral C60 fullerene complex with Li2 dimer

The results of ab initio Hartree-Fock calculations of endo-and exohedral C60 fullerene complexes with the Li+ ion and Li2 dimer are presented. The coordination of the Li+ ion and the Li2 dimer in the endohedral complexes and the coordination of Li+ ion in the exohedral complex of C60 fullerene are determined by the geometry optimization using the 3–21G basis set. In the endohedral Li+C60 complex, the Li+ ion is displaced from the center of the C60 cage to the centers of carbon hexa-and pentagons by 0.12 nm. In the Li2 dimer encapsulated inside the C60 cage, the distance between the lithium atoms is 0.02 nm longer than that in the free molecule. The calculated total and partial one-electron densities of states of C60 fullerene are in good agreement with the experimental photoelectron and X-ray emission spectra. Analysis of one-electron density of states of the endohedral Li+@C60 complex indicates an ionic bonding between the Li atoms and the C60 fullerene. In the Li+C60 and Li+@C60 complexes, there is a strong electrostatic interaction between the Li+ ion and the fullerene.

Chiral Peropyrene: Synthesis, Structure, and Properties

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using p-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV-visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M-1 cm-1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations.

A study of the isomers of C36 fullerene using single and multireference MP2 perturbation theory

The role of electron correlation in the five lowest energy isomers of C36 fullerene is studied using the CASSCF method with single and multireference MP2 theories. It is shown that the distortion of the D6h isomer at HF levels of theory is an artificial effect caused by the oversimplified form of the wavefunction. The singlet state of the D6h isomer has diradical character, and it is the lowest energy state. The triplet states of the D6h and D2d isomers are 34 and 92 kJ/ mol and the singlet state of the D2d isomer is 148 kJ/mol higher in energy than the D6h singlet state. 2002 Elsevier Science B.V. All rights reserved.

Ab Initio Multiple Spawning Method for Intersystem Crossing Dynamics: Spin-Forbidden Transitions between 3B1 and 1A1 States of GeH2

Dynamics at intersystem crossings are fundamental to many processes in chemistry, physics, and biology. The ab initio multiple spawning (AIMS) method was originally developed to describe internal conversion dynamics at conical intersections where derivative coupling is responsible for nonadiabatic transitions between electronic states with the same spin multiplicity. Here, the applicability of the AIMS method is extended to intersystem crossing dynamics in which transitions between electronic states with different spin multiplicities are mediated by relativistic spin-orbit coupling. In the direct AIMS dynamics, the nuclear wave function is expanded in the basis of frozen multidimensional Gaussians propagating on the coupled electronic potential energy surfaces calculated on the fly. The AIMS method for intersystem crossing is used to describe the nonadiabatic transitions between the (3)B1 and (1)A1 states of GeH2. The potential energies and gradients were obtained at the CASSCF(6,6)/6-31G(d) level of theory. The spin-orbit coupling matrix elements were calculated with the configuration interaction method using the two-electron Breit-Pauli Hamiltonian. The excited (3)B1 state lifetime and intersystem crossing rate constants were estimated by fitting the AIMS state population with the first-order kinetics equation for a reversible unimolecular reaction. The obtained rate constants are compared with the values predicted by the statistical nonadiabatic transition state theory with transition probabilities calculated using the Landau-Zener and weak coupling formulas.

Interfacing the Ab Initio Multiple Spawning Method with Electronic Structure Methods in GAMESS: Photodecay of trans-Azomethane

This work presents a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane, using the ab initio multiple spawning (AIMS) program that has been interfaced with the General Atomic and Molecular Electronic Structure System (GAMESS) quantum chemistry package for on-the-fly electronic structure evaluation. The interface strategy is discussed, and the capabilities of the combined programs are demonstrated with a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane. Energies, gradients, and nonadiabatic coupling matrix elements were obtained with the state-averaged complete active space self-consistent field method, as implemented in GAMESS. The influence of initial vibrational excitation on the outcome of the photoinduced isomerization is explored. Increased vibrational excitation in the CNNC torsional mode shortens the excited state lifetime. Depending on the degree of vibrational excitation, the excited state lifetime varies from ∼60-200 fs. These short lifetimes are in agreement with time-resolved photoionization mass spectroscopy experiments.

Accurate potential energy, dipole moment curves, and lifetimes of vibrational states of heteronuclear alkali dimers

We calculate the potential energy curves, the permanent dipole moment curves, and the lifetimes of the ground and excited vibrational states of the heteronuclear alkali dimers XY (X, Y = Li, Na, K, Rb, Cs) in the X(1)Σ(+) electronic state using the coupled cluster with singles doubles and triples method. All-electron quadruple-ζ basis sets with additional core functions are used for Li and Na, and small-core relativistic effective core potentials with quadruple-ζ quality basis sets are used for K, Rb, and Cs. The inclusion of the coupled cluster non-perturbative triple excitations is shown to be crucial for obtaining the accurate potential energy curves. A large one-electron basis set with additional core functions is needed for the accurate prediction of permanent dipole moments. The dissociation energies are overestimated by only 14 cm(-1) for LiNa and by no more than 114 cm(-1) for the other molecules. The discrepancies between the experimental and calculated harmonic vibrational frequencies are less than 1.7 cm(-1), and the discrepancies for the anharmonic correction are less than 0.1 cm(-1). We show that correlation between atomic electronegativity differences and permanent dipole moment of heteronuclear alkali dimers is not perfect. To obtain the vibrational energies and wave functions the vibrational Schrödinger equation is solved with the B-spline basis set method. The transition dipole moments between all vibrational states, the Einstein coefficients, and the lifetimes of the vibrational states are calculated. We analyze the decay rates of the vibrational states in terms of spontaneous emission, and stimulated emission and absorption induced by black body radiation. In all studied heteronuclear alkali dimers the ground vibrational states have much longer lifetimes than any excited states.

Effect of H2 Binding on the Nonadiabatic Transition Probability between Singlet and Triplet States of the [NiFe]-Hydrogenase Active Site

We investigate the effect of H2 binding on the spin-forbidden nonadiabatic transition probability between the lowest energy singlet and triplet electronic states of [NiFe]-hydrogenase active site model, using a velocity averaged Landau-Zener theory. Density functional and multireference perturbation theories were used to provide parameters for the Landau-Zener calculations. It was found that variation of the torsion angle between the terminal thiolate ligands around the Ni center induces an intersystem crossing between the lowest energy singlet and triplet electronic states in the bare active site and in the active site with bound H2. Potential energy curves between the singlet and triplet minima along the torsion angle and H2 binding energies to the two spin states were calculated. Upon H2 binding to the active site, there is a decrease in the torsion angle at the minimum energy crossing point between the singlet and triplet states. The probability of nonadiabatic transitions at temperatures between 270 and 370 K ranges from 35% to 32% for the active site with bound H2 and from 42% to 38% for the bare active site, thus indicating the importance of spin-forbidden nonadiabatic pathways for H2 binding on the [NiFe]-hydrogenase active site.