On the statistical behaviour of molecular vibronic energy levels

Electronic spectra of polyatomic molecules often exhibit a high density of complicated energy levels, making a detailed analysis of the individual levels unfavourable. In these cases, statistical tests provide an appropriate means for analysing the spectra. Fluctuation measures are presented and evaluated for calculated and experimental molecular spectra as examples. The results are compared with the predictions of random matrix theory.     

The electronic structure of molecules by a many-body approach III. Ionization potentials and one-electron properties of furan and thiophene

The valence ionization potentials (IP's) of furan and thiophene are studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the Hartree—Fock approximation. For both molecules it is found that the ordering of the IP's as obtained in the Hartree—Fock approximation is correct. The assignment made for furan agrees with the ab initio calculation of Siegbahn, but it does not agree with the ordering proposed by Derrick et al. from their experimental investigations. For thiophene both the ordering of Derrick et al. and the one of Gelius et al. is shown to be incorrect concerning the position of the 1b₁(π) IP. For both molecules the first two IP's are due to the 1a₂(π) and the 2b₁(π) molecular orbitals. For furan four orbitals of σ-type symmetry are placed between the 2b₁ and the 1b₁ π-orbitals, for thiophene there is only one. Several one-electron properties are calculated in the one-particle approximation and compared with experimental and other theoretical data. The localized molecular orbitals are also discussed.     

PtF6(2-) dianion and its detachment spectrum: a fully relativistic study

In this work we calculate the photoelectron spectrum of the PtF(6)2- dianion by application of the third-order Dirac-Hartree-Fock one-particle propagator technique. Relativistic effects and electron correlation are hereby treated on a consistent theoretical basis which is mandatory for systems containing heavy elements. A PtF6(2-) gas phase photoelectron spectrum is not yet available and our calculations therefore have predictive character. As it is characteristic for dianionic systems a strong dependence on basis set size and molecular geometry is observed. In contrast to the already calculated PtCl(6)2- photoelectron spectrum no valence orbital inversion due to strong interplay of spin-orbit coupling and electron correlation is observed. Furthermore an unusually strong spin-orbit splitting was found for the sigma-type subvalence 1t1u molecular spinor despite its very small platinum p population. The double ionization threshold is strongly lowered by relativistic effects now enabling an interatomic Coulombic decay process after ionization from the sigma-bonding orbitals. The results stress the importance of spin-orbit coupling for the understanding of the spectral structure which cannot be reproduced by a scalar-relativistic treatment only.     

A one-step four-bond-breaking reaction catalyzed by an electron

Molecular demolition: Long-lived compound states involving a low-energy electron and a molecule generally involve the breaking of only one σ?bond in the first elementary reaction step. Using high-level ab?initio quantum-chemical methods, four-bond breaking in an elementary reaction step has been identified. Upon electron impact, a bicyclic molecule is shown to break four σ?bonds in a concerted and essentially barrierless reaction step, producing three closed-shell neutral molecules.     

Impact of sulfur vs oxygen on the low-lying excited states of trans-p-coumaric acid and trans-p-coumaric thio acid

The low-lying excited singlet states of trans-p-coumaric acid (CA) and trans-p-coumaric thio acid (CTA) are investigated in view of characterizing the chromophore of the photoactive yellow protein (PYP), with particular regard to the impact of sulfur on the chromophore's electronic structure. The comparative ab initio study, performed with the highly accurate EOM-CCSD method, shows that the electronic state ordering upon vertical excitation and following in-plane geometry relaxation indeed depends in a very sensitive fashion on the presence of either sulfur or oxygen. The study identifies three relevant excited singlet states, two of which are of pi-pi type while the third state is of n-pi character. The study highlights the role of the latter n-pi state which is shown to be the lowest-lying excited state of CTA at all in-plane geometries under consideration, whereas this is not the case for CA.     

Peierls instabilities in one-dimensional systems with Jahn-Teller-active monomers

A one-dimensional system is investigated which consists of monomers with Jahn-Teller-active vibrational coordinates. Peierls distortions in the ground states of the half- and quarter-filled systems are studied as a function of the electron-lattice and Jahn-Teller coupling constants. For both degrees of filling and for all the values of the coupling constants the band structure of the ground state is doubly degenerate. The interrelation is discussed between the present system and systems where the active internal degrees of freedom are not of the Jahn-Teller type.