Georg Hohlneicher

Facts and artifacts about aromatic stability estimation

Abstract The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗.

Importance of initial and final states as intermediate states in two-photon spectroscopy of polar molecules

Two‐photon transitions may proceed via a mechanism not involving virtually excited intermediate states. For some examples, it is shown numerically that this mechanism may account for the largest contribution to the two‐photon cross section in polar molecules, if the dipole moment strongly alters with excitation. Possible consequences for the two‐photon detection of ’’forbidden’’ states in polar polyene systems are discussed.

Two‐photon spectroscopy of dipole‐forbidden transitions. II. Calculation of two‐photon cross sections by the CNDO–CI method

In the first paper of this series we investigated the applicability of a CNDO/S scheme including double excited configurations for the calculation of excitation energies of larger unsaturated molecules. In this paper we show that the same scheme is very useful for the prediction of two‐photon transition probabilities. If the proper expansion is used, the results converge quite well with increasing number of intermediate states. We also show that the inclusion of double excited configurations is not only necessary to obtain better energies for dipole‐forbidden transitions to ’’covalent’’ excited states but also to obtain the correct order of magnitude for two‐photon cross sections.

TWO-PHOTON SPECTROSCOPY OF THE LOW-LYING SINGLET STATES OF NAPHTHALENE AND ACENAPHTHENE

Abstract Two-photon excitation spectra of naphthalene and acenaphthene have been measured up to 50000 cm −1 . In naphthalene. three two-photon allowed states are observed for which the symmetry assignment is confirmed by polarization. The corre- sponding transitions are also seen in acenaphthene. The experimental data are in excellent agreement with theoretical predictions.

EXPERIMENTAL DETERMINATION OF THE LOW-LYING EXCITED A STATES OF trans-STILBENE

Abstract The low-lying A states of trans -stilbene are investigated by means of two-photon excitation spectroscopy. The experimental findings are compared with theoretical results. From the combined information of one- and two-photon spectroscopy we can assign at least seven excited singlet states in the energy range below 50 000 cm −1 . The newly obtained information is discussed with regard to the mechanism proposed for the photoisomerization of stilbene.

11Bu–21Ag conical intersection in trans-butadiene: ultrafast dynamics and optical spectra

Abstract The potential-energy functions of the 1 1 B u and 2 1 A g excited valence states of trans-butadiene have been characterised by the CASPT2 method. Based on these ab initio data, a vibronic-coupling model describing the conical intersection of the 1 1 B u and 2 1 A g states has been constructed. UV resonance-Raman and absorption spectra have been calculated, employing the time-dependent approach. The time-dependent wave-packet calculations reproduce the expected ultrafast (≈30 fs) radiationless decay of the optically bright 1 1 B u state into the dark 2 1 A g state.

Two-photon spectroscopy of dipole-forbidden transitions: The low-lying singlet states of anthracene

Abstract The two-photon excitation spectrum of anthracene in solution is reported from 29000 to 47000 cm −1 . Two-photon al- lowed transitions to 1B 3g (35800 cm −1 ), 2A g (38000 cm −1 ) and 3A g (43000 cm −1 ) are assigned by polarization measure- ments. These results, with those from one-photon spectroscopy, agree with calculations. Theoretical data suggest assignment of a B 3g state to a shoulder at 39000 cm −1 in the two-photon spectrum.

Two-photon spectroscopy of dipole-forbidden transitions

We have investigated the applicability of CNDO/S-type methods for the calculation of optical spectra of molecules with the special implication that the calculations should not only describe the intense, dipole-allowed transitions which dominate the one-photon absorption spectrum but also those transitions which are one-photon forbidden in first-order approximation. We show that such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if double excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still two types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approximation by which we describe the R-dependence of the Coulomb repulsion gains much more importance than in cases where mainly dipole allowed transitions are of interest. These findings confirm results obtained earlier from theories for which the pairing theorem is valid. The calculated data show an excellent stability with respect to further increase of the number of configurations if at least about 200 energy selected configurations are taken into account.

Theoretical prediction of the vibrational spectrum of naphthalene in the first excited singlet state

Complete harmonic force fields have been calculated for the ground state (S0) and the first excited singlet state (S1) of naphthalene using the multiconfiguration self‐consistent field (MCSCF) approach. Identical calculations were performed for benzene to test the methodology with already available theoretical and empirical force fields. Two different basis sets were applied (STO‐3G and near double‐zeta) and all π‐orbitals included in the active space. The geometries of ground and excited states were separately optimized. Following the ideas of Pulay, the force constants were scaled before calculating frequencies and normal modes. For the ground states the influence of correlation is discussed by comparison with Pulay’s results. Except for special vibrations where correlation effects turn out to be important, the use of Pulay’s scaling factors leads to a satisfactory description of the in‐plane‐vibrations. In the case of benzene the calculated frequency shifts between S0 and S1 are in complete qualitative...

High-resolution one- and two-photon spectra of matrix-isolated anthracene

Abstract Single-site fluorescence and one- and two-photon fluorescence excitation spectra of anthracene in an argon matrix at about 12 K are presented. The spectra exhibit a resolution of less than 4 cm −1 and show almost no site contamination. The excitation spectra are discussed with respect to the transitions to the first and second excited singlet state ( 1 L a and 1 L b ). The spectra confirm the results of earlier investigations that the onset of the hidden 1 L b transition to the second excited singlet state lies at about 28000 cm −1 in nonpolar solvents. By choosing different sites for the excitation spectra the solvent shifts for the two transitions were measured. Extrapolation of these shifts leads to an estimate of the separation between the origins of the two transitions in the free molecule of less than 500 cm −1 . For one-photon absorption a two-dimensional fluorescence/fluorescence excitation spectrum is presented. This spectrum shows that in the one-photon case none of the bands observed up to an energy of about 1600 cm − above the origine of the 1 L a band can be assigned to the 1 L b transition with any confidence. All observed long axis polarized vibrational bands are false origins of the 1 L a transition.