László Udvardi

Electronic excitations in fullerenes: Jahn-Teller distorted structures of C60

Abstract The ground state wavefunction of the neutral icosahedral C 60 molecule belongs to the total symmetric one-dimensional A g representation. However, the degeneracy of the HOMO is five-fold while the LUMO is triply degenerate. This means that the lowest excited and ionized many-electron states will also be degenerate, and thus they are subjected to Jahn-Teller distortions. In this work we used a simple model Hamiltonian to study the extent and energy of the distortions arising due to excitations. For the π-electrons we used a Pariser-Parr-Pople type wavefunction augmented by an empirical potential to describe the σ cores. The π-electron part of the Hamiltonian depends on the bond lengths which can be optimized by means of simple gradient techniques. The excited states are described by the Tamm-Dancoff approximation (all single CI). In order to locate the distorted states on the energy hypersurface, the degenerate excited states were reduced according to those subgroups of the I h , group that contain one-dimensional (that is, Jahn-Teller inactive) irreducible representations in an excitation subspace. Distorted structures of D 2h , D 3d , D 5d and T h symmetries were determined. The extent of the distortions is small, the largest change in bond lengths being 0.02 A. The Jahn-Teller distortion energies were found to be typically 1-3 kcal mol −1 . Singlet and triplet excited state spectra were computed using the CNDO/S-CI method. Comparison of the calculated spectra with experimental results shows evidence of Jahn-Teller distortions.

Excited states of the C60 dimer

Abstract Recent investigations are reported on two different sorts of fullerene dimer. The first type is a simple cluster of two C 60 molecules, i.e., a van der Waals complex. The second one is a covalently bound system. The nature of the electronic excitations has been studied for these model systems. It is found that for the van der Waals complex the low-lying excitations remain localized on one monomer and, thus, they are precursors of the Frenkel-type excitons in the crystal. At significantly higher energies, one obtains so-called charge transfer type excitations, which are precursors of the Wannier excitons. The picture of different excitations resembles that of a very weakly bound system, e.g., two benzene molecules at 5 A from each other. For the covalent dimer, we considered the binding situation where there are two single bonds between the monomers. The extent of the conjugation between the two C 60 molecules is monitored by the change in the excitation energies. The lowest-lying transitions were observed at higher energies than those in the monomer. This feature is quite unusual for a system exhibiting at least partial conjugation, and it is in agreement with recent band structure calculations and hybridization studies in the fullerene polymer.

Optical spectra of diels-alder adducts of C60

Abstract The UV-Vis spectra of Diels-Alder adducts of C 60 show characteristic differences with respect to the spectra of isolated C 60 . New bands appear between 400 and 500 nm, as well as at around 700 nm. In order to explain the origin of these bands we performed quantum chemical calculations for model molecules: C 60 H 2n ( n =1,2,3). The upper bands correspond to singlet-singlet transitions while the lowest energy band can originate only from a triplet level and spin dependent interactions should be responsible for this excitation. Symmetry arguments indicate that such transitions remain forbidden for pure C 60 .

Jahn-Teller Distorted Excited States of C60

The geometrical and electronic structure of various excited states of the C60 molecule is determined by a semi-empirical extended Hubbard model. The Jahn-Teller distortions are found to be small in terms of bond length changes (max. 0.02 A). Triplet excited states of symmetries D 5d and D 2h have been found to be 0.089 eV and 0.040 eV below the icosahedral state. Other symmetries as D 3d can also be located but they are of higher energies. The triplet ground state in solution is predicted to have D 5d symmetry, while the D 2h state is likely the lowest triplet in the solid phase.

A decomposition of the total energy at the HF-SCF level and at several levels of correlation: IV. A study of the interaction in H2O-AlH2OH + NH3 at the correlated level

Abstract The method of separated molecular orbitals (SMOs) combined with the localized many-body perturbation theory (LMBPT) has already proved useful for partitioning the energy at several levels of correlation in various weakly interacting systems. The energy decomposition into different contributions according to the interacting units is discussed for a zeolite model system in the present paper. The effect of basis set superposition is also considered, using the SMO–LMBPT scheme.

Jahn-Teller Distorted Excited States of the C60 Cluster

The geometry and the electronic structure of the ground state of the isolated cluster have been the subject of several theoretical investigations1–4. Similarly, the ionized structures C 60 - ,C 60 + have recently been studied in detail5–8. While the ground state of this cluster is known to have an icosahedral nuclear configuration, the geometry will be distorted in the degenerate ionized and excited states, according to the Jahn-Teller theorem. The present paper aims to discuss the energy and geometry of excited states C60 . The low-lying triplet and singlet states have been evaluated in some distorted states.