Biomedical subjects

Heinz-Peter Liebermann

Publications and source records attributed to Heinz-Peter Liebermann.

7 recordsLinked to original sources

PubMed2006

Complex self-consistent field and multireference single- and double-excitation configuration interaction calculations for the 2Pi(g) resonance state of N2-.

Self-consistent field and multireference single- and double-excitation configuration interaction calculations employing the complex basis function technique are carried out for the (2)Pi(g) resonance state of the N(2) (-) molecule as well as several other anionic resonance states in the neighboring energy region. The results of calculations employing the same method for the (1)S (2s(2)) state of the He atom and the (1)Sigma(g) (+) (sigma(u) (2)) state of the H(2) molecule are found to be in good agreement with those of earlier work. The present theoretical treatment has succeeded for the first time in satisfying the rigorous criterion of the complex variational principle in computing the N(2) (-) resonance states, namely, a cusp in the plots of real versus imaginary components of the corresponding complex energies has been located at each internuclear distance. On this basis, it is found that the open-shell orbital in the lowest-energy adiabatic N(2) (-) resonance state of (2)Pi(g) symmetry changes its character from quite compact at large internuclear distance to relatively diffuse for r<2.3a(0). This is in contrast to all previous theoretical treatments of this system that have not rigorously satisfied the complex variational principle in their determination of this wave function.

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PubMed2006

Ab initio MRD-CI study of the spectrum of the TeO molecule employing relativistic effective core potentials.

Potential energy curves and properties of the low-lying electronic states of tellurium oxide have been computed using a configuration interaction treatment that includes the spin-orbit coupling interaction. Relativistic effective core potentials (RECPs) are used to describe the inner shells of both the Te and O atoms. Good agreement is obtained for the spectroscopic constants of the X1-X2(3)sigma-, a1delta, and b1sigma+ states for which experimental data are available. The ratio of the parallel and perpendicular b-X transition moments, as well as the radiative lifetime of the b state, was computed, and both results were also found to be in good agreement with measurement. The energetic order of the electronic states in TeO appears to be very similar to that observed for the isovalent O2 molecule, but the Rydberg valence-mixing effects that are so prominent in the latter's spectrum (e.g., for the Schumann-Runge bands) are totally absent in TeO.

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PubMed2005

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface.

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.

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PubMed2005

Positron binding energies for alkali hydrides.

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) calculations are carried out to study the interactions of positrons with the members of the alkali hydride class of molecules. A new computer program has been constructed for this purpose that makes use of the Table-Direct-CI method for construction of the required Hamiltonian matrixes and electronic/positronic wave functions. The calculations indicate that the binding energy (positron affinity PA) of a single positron to these systems increases by an increment of 0.2-0.3 eV as the atomic number of the alkali atom is increased. It is found that the positron prefers a location in the more electronegative regions of such molecules, similarly as has been found in earlier calculations for the urea and acetone molecules. The positron orbital itself possesses a diffuse charge distribution with relatively small expectation values of the kinetic energy in all four systems considered. Each of the four positronic molecules is stable with respect to formation of either positronium (Ps) or HPs according to the present calculations. Relatively large changes in the equilibrium bond distance of the hydrides occur as a result of the positron interaction. The importance of bond dipole moments in producing the binding of positrons to molecules is discussed, as well as the role that the electronegativity of the constituent atoms plays in determining the magnitude of the PA for a given system.

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PubMed2005

Analysis of the effect of spin-orbit coupling on the electronic structure and excitation spectrum of the Bi2(2-) anion in (K-crypt)2Bi2 on the basis of relativistic electronic structure calculations.

The Bi2(2-) anions that have been characterized in (K-crypt)2Bi2 are isoelectronic with O2 but are diamagnetic and EPR-silent, unlike O2. The UV-vis spectrum measured for (K-crypt)2Bi2 shows two broad absorption peaks located at 2.05 and 2.85 eV, but no absorption at lower energies down to 0.62 eV. To account for these observations, the electronic structures of the isoelectronic diatomic dianions Q2(2-) (Q = N, P, As, Sb, Bi) were compared on the basis of relativistic density functional theory calculations, and the electronic excitations of Bi2(2-) were analyzed on the basis of relativistic configuration interaction calculations. The extent of spin-orbit coupling, brought about by the relativistic effect, increases steadily in the order N < P < As < Sb < Bi such that the "closed-shell" state is more stable than the "open-shell" state for Bi2(2-), while the opposite is the case for N2(2-), P2(2-), As2(2-), and Sb2(2-). The nature of the electronic excitations of Bi2(2-) was assigned and discussed from the viewpoint of molecular orbitals in the absence of spin-orbit coupling.

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PubMed2004

On the ultraviolet photodissociation of H(2)Te.

The photodissociation of H(2)Te through excitation in the first absorption band is investigated by means of multireference spin-orbit configuration interaction (CI) calculations. Bending potentials for low-lying electronic states of H(2)Te are obtained in C(2v) symmetry for Te-H distances fixed at the ground state equilibrium value of 3.14a(0), as well as for the minimum energy path constrained to R(1)=R(2). Asymmetric cuts of potential energy surfaces for excited states (at R(1)=3.14a(0) and theta;=90.3 degrees ) are obtained for the first time. It is shown that vibrational structure in the 380-400 nm region of the long wavelength absorption tail is due to transitions to 3A('), which has a shallow minimum at large HTe-H separations. Transitions to this state are polarized in the molecular plane, and this state converges to the excited TeH((2)Pi(1/2))+H((2)S) limit. These theoretical data are in accord with the selectivity toward TeH((2)Pi(1/2)) relative to TeH((2)Pi(3/2)) that has been found experimentally for 355 nm H(2)Te photodissociation. The calculated 3A(')<--XA(') transition dipole moment increases rapidly with HTe-H distance; this explains the observation of 3A(') vibrational structure for low vibrational levels, despite unfavorable Franck-Condon factors. According to the calculated vertical energies and transition moment data, the maximum in the first absorption band at approximately 245 nm is caused by excitation to 4A("), which has predominantly 2(1)A(") ((1)B(1) in C(2v) symmetry) character.

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PubMed2004

Ab initio study of the BiSe and BiTe electronic spectra: what happens with X2-X1 emission in the heavier Bi chalcogenides?

A series of spin-orbit configuration interaction calculations has been carried out for the BiSe and BiTe molecules and analyzed in comparison with data obtained earlier for the isovalent BiO and BiS systems. An avoided crossing caused by the spin-orbit interaction between the X2Pi and A4Pi electronic states is shown to have a decisive effect on the lower-energy spectrum in each case. Irregularities in the X2 3/2 state vibrational manifold occur as a consequence of this nonadiabatic interaction, and the v vibrational number for the onset of these perturbations is found to gradually decrease in going from BiO to BiSe, in agreement with experiment. In BiTe the shape of the X2 potential curve is so altered by the avoided crossing that its minimum becomes shifted to a significantly larger distance than for the X1 state, unlike the case for BiSe or the lighter Bi chalcogenides. This characteristic appears to be the root cause for the fact that the X2 state has not yet been found experimentally in the BiTe spectrum, despite careful searches in the expected energy range. Radiative lifetimes have also been calculated for the low-lying states of both the BiSe and BiTe molecules, and these results are found to be consistent with experimental observations.

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