Electronic spectra of linear isoelectronic species HC6H+, C6H, HC5N+

Multireference configuration interaction calculations have been used to determine term energies of low-lying doublet electronic states of isoelectronic carbon chains HC6H+, C6H and HC5N+. Calculations on relevant excited states in the energy range up to 6 eV show that (2)Pi and (2)Phi excited states due to the pi-pi excitation are comparable in energy pattern among such species, but that there are significant differences in the properties of (2)Sigma and (2)Delta excited state series of these isoelectronic species. Similarities and discrepancies are discussed based on molecular orbital energy patterns and electron correlation. The strongest transitions for HC6H+, C6H and HC5N+ are predicted to correspond to X(2)Pi -->2(2)Pi transitions at 2.22, 2.56 and 2.17 eV, respectively. For HC6H+, no (2)Sigma or (2)Delta states arising from sigma-pi electron promotion are found in the energy range up to 6 eV. However, in C6H the first excited state 1(2)Sigma (+) due to the 13 sigma -->3 pi excitation is found to be only slightly higher (0.22 eV) than the ground state X(2)Pi. Calculated transition energies in the present study show good agreement with available experimental results

New Strategies in Modeling Electronic Structures and Properties with Applications to Actinides

This chapter discusses contemporary quantum chemical methods and provides general insights into modern electronic structure theory with a focus on heavy-element-containing compounds. We first give a short overview of relativistic Hamiltonians that are frequently applied to account for relativistic effects. Then, we scrutinize various quantum chemistry methods that approximate the $N$-electron wave function. In this respect, we will review the most popular single- and multi-reference approaches that have been developed to model the multi-reference nature of heavy element compounds and their ground- and excited-state electronic structures. Specifically, we introduce various flavors of post-Hartree--Fock methods and optimization schemes like the complete active space self-consistent field method, the configuration interaction approach, the Fock-space coupled cluster model, the pair-coupled cluster doubles ansatz, also known as the antisymmetric product of 1 reference orbital geminal, and the density matrix renormalization group algorithm. Furthermore, we will illustrate how concepts of quantum information theory provide us with a qualitative understanding of complex electronic structures using the picture of interacting orbitals. While modern quantum chemistry facilitates a quantitative description of atoms and molecules as well as their properties, concepts of quantum information theory offer new strategies for a qualitative interpretation that can shed new light onto the chemistry of complex molecular compounds.Comment: 43 pages, 3 figures, Version of Recor

MRD-CI characterization of electronic spectra of isoelectronic species C-6(-), NC4N+, and CNC3N+

The structure and stabilities of linear and cyclic isomers of Ce-6(-) and N2C4+ were investigated by DFT, MP2, CISD, and CCSD methods. The linear isomers of C-6(-) and NC4N+ are predicted to be the most stable forms. Multireference configuration interaction methodology was used for the calculation of the doublet and quartet excited states. Assignments to observed transitions in matrix spectroscopy of these species are made. The first X (2)Pi --> (2)Pi transitions for C-6(-), NC4N+, and CNC3N+ occur at 1.98, 2.14, and 2.65 eV, respectively, showing similar features with large oscillator strengths. On the other hand, a significant difference exists in the X (2)Pi (u) --> 1 (2)Sigma (+)(g) band system between the C-6(-) and NC4N+. Correlation between the relative molecular orbital energy and spectroscopic properties is discussed. The predicted electronic spectra agree well with available experimental data

Study of possible photodissociation channels in linear carbon clusters C-n (n=4-6)

Ab initio calculations are used to determine molecular properties of linear carbon clusters C-n (n=3-6) in ground and electronically excited states relative to photodissociation processes. MRD-CI calculations predict that in C-3 and C-5 the singlet-triplet splitting between the (1)Pi(u) and (3)Pi(u) arising from the same configuration is about 1 eV similar as in C-2. The energy differences between (1)Delta(g) and (1)Sigma(g)(+) corresponding to the same pi(g)(2) or pi(u)(2) configuration in C-4 and C-6 are less than 0.2 eV. Calculations support experimental finding that the energetically most favorable fragmentation channel for linear carbon clusters C-n (n=4-6) corresponds to the loss of C-3 to give its partner fragment Cn-3. Further fragmentation channels are discussed, (C) 2002 Published by Elsevier Science B.V

Ab initio study of the electronic spectrum of 7-hydroxyquinoline

The electronic spectrum of 7-hydroxyquinoline has been studied by using ab initio Multi-State Multi-Reference M??ller-Plesset Second-Order Perturbation Theory (MSMRMP2). The energy range up to 6.5 eV is taken into consideration. The lowest ??-??* transition is calculated to have an energy of 3.65 eV in good agreement with experimental data. Several other ??-??* transitions are predicted to have excitation energies of 4.93, 5.20, 5.47, and 6.08 eV. Excitations out of the highest occupied ?? orbital into the lowest unoccupied ??* orbitals are computed to occur at 5.28 and 6.44 eV. The ??-??* transition is predicted at 5.87 eV. The ??* orbital involved is identified as the Rydberg 3s orbital having antibonding character along the OH bond.close1

Ab initio multireference configuration interaction study of the electronic spectra of carbon chain anions C-2n+1(-) (n=2-5)

Multireference configuration interaction calculations on the vertical transition energies of the low-lying excited states in carbon chain anions C-2n+1(-) (n=2-5) are carried out. Calculated vertical term energies confirm the previously suggested assignments to the first and second (2)Pi <--X (2)Pi band systems in matrix isolation spectroscopy. The lowest 1 (2)Pi <--X (2)Pi electronic transitions of C-5(-), C-7(-), C-9(-), and C-11(-), are calculated at 2.66, 2.27, 1.90, and 1.54 eV, respectively, with large oscillator strengths. Relevant theoretical evidences for a more detailed assignment of the observed transitions are presented. Generally, corresponding excitation energies in the C-2n+1(-) carbon chain anions are found descending with an increase of the chain. The equilibrium geometries and harmonic vibrational frequencies of these linear carbon chains determined by the density functional theory approach also show reasonable agreement with available experimental data. (C) 2001 American Institute of Physics