Theoretical study of the electronic spectrum of p-benzoquinone

The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet–singlet and singlet–triplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the ππ∗ 1 1Ag→1 1B1u and ππ∗ 1 1Ag→3 1B1u transitions, computed to be at 5.15 and 7.08 eV, respectively. Assignments of the observed low-energy Rydberg bands have been proposed: An n→3p transition for the sharp absorption located at ca. 7.4 eV and two n→3d and π→3s transitions for the broad band observed at ca. 7.8 eV. The lowest triplet state is computed to be an nπ∗ 3B1g state, in agreement with the experimental [email protected] ; [email protected] ; [email protected]

A combined theoretical and experimental determination of the electronic spectrum of acetone

A combined ab initio and experimental investigation has been performed of the main features of the electronic spectrum of acetone. Vertical transition energies have been calculated from the ground to the ny→π∗, π→π∗, σ→π∗, and the n=3 Rydberg states. In addition, the 1A1 energy surfaces have been studied as functions of the CO bond length. The 1A1 3p and 3d states were found to be heavily perturbed by the π→π∗ state. Resonant multiphoton ionization and polarization‐selected photoacoustic spectra of acetone have been measured and observed transitions were assigned on internal criteria. The calculated vertical transition energies to the ny→π∗ and all Rydberg states were found to be in agreement with experiment. This includes the 3s‐, all three 3p‐, and the A1, B1, and B2 3d‐Rydberg states. By contrast, there is little agreement between the calculated and experimental relative intensities of the A1 and B2 3d‐Rydberg transitions. In addition, anomalously intense high vibrational overtone bands of one of the 3p‐Rydberg transitions have been observed. These results confirm the strong perturbation of the 3p‐ and 3d‐Rydberg states by the π→π∗ state found in the theoretical calculation and support the calculated position of this unobserved [email protected]

Theoretical study of the electronic spectrum of magnesium-porphyrin

Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepancy is the approximate treatment of dynamic correlation (second order perturbation theory), limitations in the basis set and the fact that all measurements have been made on substituted magnesium [email protected] ; [email protected]

A theoretical study of the electronic spectrum of bithiophene

The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is possible there is agreement between theory and [email protected] ; [email protected] ; [email protected]

Towards an accurate molecular orbital theory for excited states : Ethene, butadiene, and hexatriene

A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 1Bu state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 [email protected] ; [email protected] ; [email protected]

The chemical bonds in CuH, Cu2, NiH, and Ni2 studied with multiconfigurational second order perturbation theory

The performance of multiconfigurational second order perturbation theory has been analyzed for the description of the bonding in CuH, Cu2, NiH, and Ni2. Large basis sets based on atomic natural orbitals (ANOS) were employed. The effects of enlarging the active space and including the core‐valence correlation contributions have also been analyzed. Spectroscopic constants have been computed for the corresponding ground state. The Ni2 molecule has been found to have a 0+g ground state with a computed dissociation energy of 2.10 eV, exp. 2.09 eV, and a bond distance of 2.23 Å. The dipole moments of NiH and CuH are computed to be 2.34 (exp. 2.4±0.1) and 2.66 D, [email protected] ; [email protected] ; [email protected]

Selected dissociation‐ and correlation‐consistent configuration interaction by a perturbative criterion

We propose a perturbative criterion to select the most important dissociation‐ or correlation‐consistent type of contributions to perform generalized valence bond‐configuration interaction (GVB‐CI) calculations, dissociation‐consistent configuration interaction (DCCI) or correlation‐consistent configuration interaction (CCCI) approach, respectively. The procedure presented is computationally less demanding than the CCCI proposed by Goddard and co‐workers. To ensure the distance consistency of the MOs used, the nonvalence virtual orbitals are obtained by a projection technique. The results obtained for a few test calculations show the ability of the suggested approach to get close results to full CI, DCCI, and CCCI values using a small CI expansion. It seems to be a promising way to treat correlation changes in large molecular systems which would be inaccessible by other [email protected] ; [email protected]