Biomedical subjects

James E Boggs

Publications and source records attributed to James E Boggs.

7 recordsLinked to original sources

PubMed2006

Orbital disproportionation and spin crossover as a pseudo Jahn-Teller effect.

It is shown that in systems with electronic half-closed-shell configurations of degenerate orbitals, e(2) and t(3) (which have totally symmetric charge distribution), ground state distortions from high-symmetry geometries may occur due to a strong pseudo Jahn-Teller effect (PJTE) in the excited states, resulting also in a novel phenomenon of PJT-induced spin crossover. There is no JTE neither in the ground state term nor in the excited terms (including degenerate terms) of these configurations but a strong PJT mixing between two excited states [((1)E+(1)A) [cross-filled circle] e and ((2)T(1)+(2)T(2)) [cross-filled circle] e in the e(2) and t(3) cases, respectively] pushes down the lower term to cross the ground state of the undistorted system and to form the global minimum with a distorted geometry. The analysis of the electronic structure of this distorted configuration shows that it is accompanied by orbital disproportionation: instead of proportional population of all degenerate orbitals by one electron each (as in the ground state of the undistorted system that follows Hund's rule), two electrons with opposite spins occupy one orbital, resulting in transformations of the type (e(theta);e(epsilon))-->(e(theta)e(theta)) for e(2) and (t(x);t(y);t(z))-->(t(x);t(x);t(z)) for t(3) systems. Since the two geometry configurations, undistorted and distorted, appertain to different electronic terms that have different spin states, the formation of the global minimum with the distorted configuration is accompanied by a spin crossover. Distinguished from the known spin-crossover phenomenon in some transition metal compounds, the two states with different spin in the PJT-induced spin crossover have also different nuclear configurations, undistorted and distorted, that coexist with a relatively small energy difference. The change of configuration reduces significantly the rate of relaxation between the two states; the relaxation is further reduced by the lower spin-orbital coupling in the light-atom systems as compared with transition metal compounds. This means that there may be systems for which the switch between the two states (in both directions) under perturbations may be observed as a single-molecule phenomenon. Systems with half-closed-shell electronic configurations e(2) and t(3) are available in a variety of molecules from different classes, organic and inorganic; the theory is illustrated here by ab initio calculations for a series of molecular systems, including Si(3), Si(3)C, CuF(3), Na(3), Si(4), Na(4), Na(4) (-), and C(60) (3-), which are in agreement with the experimental data available.

Journal Article↗

PubMed2006

Extensive ab initio study of the valence and low-lying Rydberg states of BBr including spin-orbit coupling.

The electronic states of the BBr molecule, including 12 valence states and 12 low-lying Rydberg states, have been studied at the theoretical level of MR-CISD+Q with all-electron aug-cc-pVQZ basis sets and Douglas-Kroll [Ann. Phys. (N.Y.) 82, 89 (1974)] scalar relativistic correction. The spin-orbit coupling effect in the valence states was calculated by the state interaction approach with the full Breit-Pauli Hamiltonian. This is the first multireference ab initio study of the excited electronic states of BBr. Potential energy curves of all states were plotted with the help of the avoided crossing rule between electronic states of the same symmetry. The structural properties of these states were analyzed. Computational results reproduced most experimental data well. The transition properties of the a (3)Pi(0(+) ), a (3)Pi(1), and A (1)Pi(1) states to the ground state X (1)Sigma(0(+) ) (+) transitions were obtained, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes. The evaluated radiative lifetime of the a (3)Pi(0(+) ), and a (3)Pi(1) states are near 1 ms, much longer than that of the A (1)Pi(1) state.

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PubMed2006

Lost topological (Berry) phase factor in electronic structure calculations. Example: the ozone molecule.

It is shown that standard computations of electronic structures of polyatomic systems that yield the global minimum configuration and vibrational frequencies may be faulty if the symmetry of this configuration is lower than the highest possible one and the origin of this distortion, which is always due to the Jahn-Teller effect, is neglected; this may lead, in particular, to the loss of the Berry phase factor that changes the vibronic energy level spectrum and which we show to be present even when there are no apparent conical intersections. The general case and the ozone molecule are analyzed.

Journal Article↗

PubMed2006

Pseudo-Jahn-Teller origin of geometry and pseudorotations in second row tetra-atomic clusters X4 (X=Na, Mg, Al, Si, P, S).

Experimentally determined or ab initio calculated molecular geometries carry no information about their origin. Employing the Jahn-Teller (JT) vibronic coupling effects as the only source of instability and consequent distortions of high-symmetry molecular configurations, we have worked out a procedure that allows us to trace the origin of particular geometries and determine the detailed electronic mechanism of their formation. This procedure is illustrated by considering a series of X(4) clusters with X=Na, Mg, Al, Si, P, and S. It shows explicitly why Na(4), Si(4), and Al(4) have a rhombic geometry in the ground state, while Mg(4) and P(4) are tetrahedral, whereas S(4) is a trapezium. Even when the minimum-energy geometries are the same (as in the case of rhombic Na(4), Si(4), and Al(4)), the electronic mechanism of their formation is quite different. In particular, in Na(4) and Si(4) the rhombic minima are produced by a strong pseudo JT coupling between two excited states in the square-planar configuration (different in the two cases) that stabilizes one of them and makes it the ground state by rhombic distortions. The rhombic configuration of Al(4) is due to the pseudo JT effect in its ground-state square-planar configuration, and the trapezium in S(4) is formed by two pseudo JT couplings essentially involving excited states. In several cases this analysis shows also the tunneling paths between equivalent configurations.

Journal Article↗

PubMed2005

Extensive ab initio study of the electronic states of SCl including spin-orbit coupling.

The effect of different basis sets for calculation of the spectroscopic constants of the ground state of sulfur monochloride (SCl) was analyzed using scalar relativistic multireference configuration interaction with single and double excitations plus Davidson correction. Then the generally contracted all-electronic correlation-consistent polarized valence quintuple zeta basis sets were selected to compute the electronic states of SCl including 12 valence and 9 Rydberg lambda-S states. The spin-orbit coupling effect was calculated via the state interaction approach with the full Breit-Pauli Hamiltonian. This effect splits these lambda-S states into 42 omega states. Potential-energy curves of all these states are plotted with the help of the avoided crossing rule between the electronic states of the same symmetry. The structural properties of these states are analyzed. Spectroscopic constants of bound excited states that have never been observed in experiment are obtained. The transition dipole moments and the Franck-Condon factors of several transitions from low-lying bound excited states to the ground state were also calculated.

Journal Article↗

PubMed2005

Ground and valence-excited states of SF: a multireference configuration interaction with single and double excitations+Q study.

Ab initio calculations on the ground and valence-excited states of the sulfur monofluoride radical have been performed using entirely uncontracted all-electron augmented correlation consistent polarized valence quintuple zeta basis sets and the internally contracted multireference configuration interaction with single and double excitations method and Davidson correction (+Q). Potential-energy curves of all valence electronic states and the spectroscopic constants of several bound states are fitted. It is the first time that the entire 27-omega states generated from the 12 valence lambda-S states which come from the S(3P(g)) and F(2P(u)) atomic states of SF radical have been studied theoretically. The effects of spin-orbit coupling and the avoided crossing rule between omega states of the same symmetry are analyzed. The calculated results reproduce well the available experimental values and predict the properties of several bound excited states that have never been observed in experiment. The transition properties of the dipole-allowed transitions from bound excited states to the ground state are predicted for the first time, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes.

Computer Simulation↗

PubMed2005

Approaching the basis set limit for transition metal compounds with highly polar bonds: a benchmark coupled-cluster study of the ScF3 and FeF3 molecular structures and spectra.

The molecular equilibrium geometries, quadratic and cubic force constants, vibrational frequencies, and infrared intensities of scandium and iron trifluorides are determined ab initio with a sequence of atomic natural orbital basis sets using the CCSD(T) treatment of electron correlation. The largest basis set of spdf ghi quality contains 462 contracted Gaussian functions. Relativistic corrections are applied to compute the equilibrium geometries and vibrational frequencies. The cubic force constants are used to estimate vibrational corrections to the effective r(g) internuclear distances determined in the gas electron diffraction experiments. The computed molecular properties are extrapolated to the complete basis-set limit. The predicted values are compared to the available experimental data; uncertainties and inconsistencies in these data are then discussed.

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