Biomedical subjects

Wolfgang Eisfeld

Publications and source records attributed to Wolfgang Eisfeld.

8 recordsLinked to original sources

PubMed2006

Experimental and theoretical study of the electronic spectrum of the methylene amidogen radical (H2CN): verification of the 2A1 <-- 2B2 assignment.

A collaborative experimental and theoretical study of the electronic spectrum and excited-state photochemistry of H(2)CN has been carried out. The absorption spectrum, in the range of 287-278 nm, was measured through cavity ring-down spectroscopy. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features were observed in the 34800-35800 cm(-1) spectral range, along with the A-X (1,0) band of the OH cofragment. The broad features were assigned through high-level ab initio calculations as vibronic transitions to the ground and 2b(1) (umbrella mode) levels of the second excited B (2)A(1) state from the ground X (2)B(2) state of H(2)CN. Rotational constants for the lower and upper levels of these transitions were computed from the expectation values of the moments of inertia tensor, using the appropriate vibrational wave functions. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric dipole-allowed (2)A(1) <-- (2)B(2) transition appropriate to this transition. The former assignment to the dipole-forbidden (2)B(1) <-- (2)B(2) transition can be ruled out by these results. A theoretical investigation of the dissociation pathways for electronically excited H(2)CN is also presented. The upper states of the observed bands cannot dissociate directly but rather decay through internal conversion and subsequent dissociation to H + HCN fragments; higher b(1) levels are above the excited-state dissociation limit.

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PubMed2006

Photoionization-induced dynamics of ammonia: ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation.

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs.

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PubMed2006

Resonance Raman spectrum of the solvated electron in methanol: simulation within a cluster model.

The microsolvation of the CH(3)OH(2) hypervalent radical in methanol clusters has been investigated by density functional theory. It is shown that the CH(3)OH(2) radical spontaneously decomposes within methanol clusters into protonated methanol and a localized solvated electron cloud. The geometric and electronic structures of these clusters as well as their vibrational frequencies have been characterized. Resonance Raman intensities, associated with the s --> p transition of the unpaired electron, have been estimated for CH(3)OH(2)M(n) (M = CH(3)OH, n = 1-3) clusters. It is shown that with increasing cluster size the simulated spectra converge toward the resonance Raman spectrum of the solvated electron in methanol measured recently by Tauber and Mathies (J. Am. Chem. Soc. 2004, 126, 3414). The results suggest that CH(3)OH(2)M(n) clusters are useful finite-size model systems for the computational investigation of the spectroscopic properties of the solvated electron in liquid methanol.

Computer Simulation↗

PubMed2006

Calculation of the vibrationally resolved electronic absorption spectrum of the propargyl radical (H2CCCH).

The absorption spectrum of the propargyl radical in the region from 180 to 400 nm is investigated in detail by means of theory. Vertical excitation energies and potential energy surfaces are determined by highly accurate multireference configuration interaction (MRCI) calculations. The vibrational dynamics of several electronic states is studied, and line intensities and positions are calculated with respect to the electronic and vibrational ground state. Four electronic states absorb in the region of interest: 1 2B2, 2 2B1, 2 2B2, and 3 2B1. However, electronic excitations into the 2B2 states are dipole-forbidden from the X 2B1 ground state and corresponding vibronically allowed transitions are shown to be weak. The spectrum is dominated by the strong 2 2B1 <-- 1 2B1 band which is computed in overall good agreement with available experiments. A strong absorption at 242 nm, which has been assigned to propargyl, is not confirmed by the calculations, and only very weak absorptions are found at wavelengths shorter than 280 nm. The present results strongly suggest that the 242 nm feature must be due to a different species.

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PubMed2005

Higher order (A+E) multiply sign in circle e pseudo-Jahn-Teller coupling.

The pseudo-Jahn-Teller (PJT) coupling of a nondegenerate state A with a twofold degenerate state E by a degenerate vibrational mode e is studied for a general system with a C(3) main rotational axis. The PJT coupling terms up to sixth order are derived by symmetry considerations for this general (A+E) multiply sign in circle e case. The obtained expression for the 3 x 3 diabatic potential energy matrix is found to be closely related to the expression recently developed for the higher order Jahn-Teller case [A. Viel and W. Eisfeld, J. Chem. Phys. 120, 4603 (2004)]. The dynamical PJT coupling, which can arise for states of appropriate symmetry if one of the vibrational modes induces a change of the nuclear point group between D(3h), C(3v), C(3h), and C(3), is discussed. The effect of the higher order PJT coupling is tested by a two-dimensional model study based on the e bending mode of NH(3)(+). The models are analyzed by fitting the two-dimensional potential energy surfaces. The significance of the higher order terms on the nonadiabatic dynamics is demonstrated by quantum wave packet propagations.

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PubMed2004

Theoretical investigation of ground and excited states of the methylene amidogene radical (H(2)CN).

The excited states and the absorption spectrum of the methylene amidogene radical are studied by high-level ab initio calculations. The multireference configuration interaction method was used in combination with different basis sets and basis set extrapolation to compute equilibrium geometries, harmonic frequencies, and excitation energies of the four lowest doublet electronic states of the title species. Potential curves and transition dipole moment functions were determined along the normal mode coordinates of the electronic ground state. These functions were employed to determine vibronic absorption spectra. The intensities of dipole forbidden but vibronically allowed transitions were calculated by explicitly evaluating integrals over the vibrational wave functions and the transition dipole functions of the involved electronic states. By this method the oscillator strengths of the dipole allowed (2)A(1)<--(2)B(2) and the dipole forbidden (2)B(1)<--(2)B(2) bands were computed. It turns out that the dipole forbidden transition is two orders of magnitude weaker than the dipole allowed one. The 0-0 excitation energies are found to be 30 256 cm(-1) for the (2)B(1) state and 34,646 cm(-1) for the (2)A(1) state. From the combined results of the excitation energies and oscillator strengths it is concluded that the experimentally observed peaks must be due to the (2)A(1) state, in contradiction to earlier assignments.

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PubMed2004

Effects of higher order Jahn-Teller coupling on the nuclear dynamics.

In this paper effects of higher order Jahn-Teller coupling terms on the nonadiabatic dynamics are studied. Of particular interest is the case when the potential energy surfaces of the degenerate state show pronounced anharmonicity. In order to demonstrate the effects a two-dimensional E multiply sign in circle e Jahn-Teller model system is treated which is based on the e(') stretching vibration of the photoactive (2)E(') state of NO(3) as a realistic example. The sixth order E multiply sign in circle e Jahn-Teller Hamiltonian is derived in the diabatic representation which is valid for any system with a C(3) rotation axis. This diabatization scheme is compared to lower-order Jahn-Teller Hamiltonians and to symmetry adapted as well as ad hoc approximations. Lower-order potentials result in pronounced quantitative and qualitative differences in the dynamics, including differences in the evolution of mean values, the autocorrelation functions (and thus the corresponding spectra), and the electronic population evolution. In the particular example treated, the results of fourth and fifth order potentials are very similar to the sixth order reference system. In contrast, the approximate sixth order Hamiltonians, though the corresponding adiabatic surfaces seem to be nearly identical, results in pronounced differences. The possible consequences for the dynamics of realistic systems with higher dimensionality are briefly discussed.

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PubMed2002

Are helionitronium trications stable?

In a recent article [Olah, G. A., Prakash, G. K. S. & Rasul, G. (1999) Proc. Natl. Acad. Sci. USA 96, 3494-3495] the authors found that the helionitronium trication, HeNO(2)(3+), is an unusually stable helium-containing polyatomic ion. This result was based on second-order many-body perturbation (MP2) calculations that showed that strong binding should occur between the oxygen and helium atoms in the assumed singlet ground state. The dissociation energy with respect to NO(+) and HeO(2+) was predicted to be 7.95 eV. We show here by thorough multireference configuration interaction (MRCI) studies that the ground state for the helionitronium trication is a triplet (3)B(1) state with He binding to the N atom (C(2v)). The HeO bound structure of C(s) symmetry is not stable. Dissociation of the helionitronium trication occurs toward NO(2)(2+) and He(+), and the trication is bound by at most 0.25 eV. These results indicate that the helionitronium trication is unstable under ambient conditions. The discrepancies between our results and the previous study are explained by the strong multireference character of the wave function of the trication.

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