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Ultraviolet excitation of uranium atoms which promotes reaction with methane to form the methyl uranium hydride C[H.sub.3]-UH and its [alpha]-H transfer product uranium methylidene dihydride, C[H.sub.2]=U[H.sub.2] is described.

New basis sets of the atomic natural orbital (ANO) type are developed for the first, second, and third row transition metal atoms. The ANOs are obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field.

Author(s): Laura Gagliardi (corresponding author) [1, 2]; Björn O. Roos [1, 2] Covalent bonding is commonly described by Lewis's theory [1], with an electron pair shared between two atoms constituting [...]

The electronic spectrum of the UO2 molecule is determined using multiconfigurational wave functions together with the inclusion spin-orbit coupling. It is shown that the most intense transitions from the two lowest levels, 2u and 3u states, correspond to excitations from the U 7s to U 7p orbitals.

Research has been conducted on the neptunyl mono-, bi- and tricarbonato complexes. The study of the complexes' ground state in water solutions has been carried out and the structure of the systems and the water coordination have been examined and discussed.

Research was conducted to examine the electronically excited states of the two isomers 1,3 and 1,4-cycloohexadiene using multiconfigurational second-order perturbation theory with extended ANO-type basis sets. New optical and resonant-enhanced multiphoton ionization spectra of 1,3-CHD were calculated and analyzed. Computed and experimental vertical transition energy results were in excellent agreement.

Research was conducted to reexamine the products of the reaction of Sc and dioxygen. Additional information about the infrared spectra was achieved and new bands were observed by the addition of the electron-trapping molecule CCl4. Further ab initio calculations were also carried out on possible products of the Sc plus O2 reaction. The problems related to the computational study of ScO2 are presented.

The electronic spectra of the N(9)H and N(7)H tautomers of purine were investigated. The complete active space SCF method and the multiconfigurational second-order perturbation theory were used to calculate the vertical integration energies, oscillator strengths, dipole moments and transition moment directions in gas phase. The results showed that the electronic absorption spectra of purine can ben satisfactorily explained when the N(9)H tautomer is considered predominant in gas phase and nonpolar solvents while the N(7)H pure is assumed to predominate in polar solvents.

The electronic spectra of the nucleic acid base monomers guanine and two tautomers of adenine were examined using the complete active space SCF technique as well as the multiconfigurational second-order perturbation theory. The experiment with adenine was complicated by at least two tautomeric forms, which were hypothesized to influence the observed spectra in solution. It was found that the observed spectra may be the result of transitions belonging to the N(9)H-adenine tautomer. In comparison, the experiment with guanine was much simpler. The conformance with theory indicates that the crystal field effects may not be that relevant.

The electronic spectrum of the blue copper plastocyanin was investigated by ab initio multiconfigurational second-order perturbation theory. The six lowest electronic transitions were computed and assigned with an error of less than 2000/cm. Findings indicated that the bright blue color is due to an electron transfer to the singly occupied orbital in the ground state from the corresponding Cu 3d-S(sub Cys), 3p-pi bonding orbital.

A study has been conducted to examine the electronic spectrum of indole in the gas phase and in a solvent using the complete active space SCF technique and the multiconfigurational second-order perturbation theory. Calculations suggest that there are two low-lying and weaker singlet excited valence states in the gas phase, designated as 1Lb and 'La, with the 1La band origin above the 1Lb band origin. The results also revealed that the 1La state is sensitive to the polarity of the solvent.

A theoretical study of the electronic spectra of uracil and thymine using the complete active space SCF method combined with multi-configurational second-order perturbation theory find that the computed excitation energies have errors which are less than 0.3 eV. Research enabled the prediction of the presence of four pi to pi* singlet excited states with transition energies of less than 7.0 eV. Considering the solvent shifts, the calculated transition energies are in agreement with measured energies.

The theoretical study of P-aminobenzonitrile (ABD) and (p-dimethylamino)benzonitrile (DMABN) molecules at the CASPT2 and CASSCF level reveals the dual fluorescence process occurring in these systems. The molecules exhibit two fluorescence bands in the polar solvents. The fluorescence occurs primarily due to the small energy gap between the interacting states.

Theoretical and experimental analyses of the lower energy region of the norbornadiene spectrum reveal the associated symmetries, characteristics and vertical energies. Theoretical calculations assess the experimental ability to treat problems involving indirect conjugation and pi, sigma interactions associated with a strained molecule. A comparison of the energy levels of related diene series reveals the accuracy of calculations performed using other methods.

A newly suggested quantum chemical approach for ab initio computations of dynamic electron correlation influences in molecular systems aimed to theoretically examine the lowest valence and Rydberg excited states of cyclopentadiene, furan and pyrrole. These valence excited singlet states and the 1a2 leads to 3s, 3p and 3d Rydberg states for cyclopentadiene and furan were included. The valence excited states 1A (sub 1) (super +) (7.89, 7.46, 7.74 eV) and 1B2 (5.27, 5.92, 6.04 eV) induced the two primary characteristics in the spectra. The results are consistent with experimental findings but vary dramatically from earlier theoretical predictions.

Calculations for the inner-sphere reorganization energies for some small-nolecule inorganic redox couples are presented. Most of the calculations were done using the coupled pair functional approach except for O3- which was done using the complete active space SCF method with dynamic correlation using second-order perturbation theory. A comparison of values showed that the ab initio calculations were accurate to within +/- 2 kcal mol-1 or better than classical values. This showed that earlier conclusions obtained using the latter method are still valid.

The carbonate system of dioxouranyl(VI) and -(V), [UO(sub)2(CO(sub)3)(sub)3](sup)4- and 5- is discussed with respect to structure, chemical bonding, and reactivity. A theoretical analysis reveals only minor geometrical rearrangements in a reversible one-electron reduction of [UO(sub)2(CO(sub)3)(sub)3](sup)4- to 5- form.

Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K has provided weak infrared absorptions for CUO. CASPT2 calculations with dynamical correlation find the C[triple bond]U[triple bond]C ground state as linear and similar calculations with spin-orbit coupling has shown that the U[triple bond]C diatomic molecule has a quintet ground state and a fully developed triple bond of 2.82 effective bond order.

Analogous combined experimental and theoretical studies of the group 6 E[triple bond]M[X.sub.3] molecules and a comparison of the triple bonds formed with different pnictides, metals and halogens are reported. The group 6 metal phosphide bond orders are found to be less than the corresponding nitrides as expected for the larger phosphorus valence orbitals.

A series of quantum chemical calculations used to demonstrate the agostic interaction between the metal atom and H(C) in methylidene metal dihydride complexes [H.sub.2]MC[H.sub.2] suggest that the interaction varies with the character of the metal.

The near-infrared electronic spectrum of AuO was re-examined in light of the new microwave data on [X.sup.2][PI.sub.3/2] state of AuO. Discrepancies in the ground-state rotational constants between the near-infrared analysis and the microwave analysis clearly showed that the rotational assignments of the near-infrared spectrum were incorrect.

Two diuranium chlorides, U(sub 2)CI(sub 6) and U(sub 2)CI(sub 8)(super 2-), and three different carboxylates, U(sub 2)(OCHO)(sub 4), U(sub 2)(OCHO)(sub 6) and U(sub 2)(OCHO)(sub 4)CI(sub 2) are studied. All species are found to be stable with a multiply bonded U(sub 2) unit.

New basis sets of the atomic natural orbital (ANO) type have been developed for the main group and rare gas atoms. Scalar relativistic effects are included through the use of a Douglas-Kroll Hamiltonian and correlation of semicore electrons.

Studies the electronic spectrum of magnesium-porphyrin using multiconfigurational self-consistent field and second order perturbation methods. ANO-type basis set used in the study; Computation of singlet states of E[sub 1u] symmetry; Interpretation of the electronic spectrum; Computed transition energies.

Research has been conducted on Re (sub)2 Cl (sub)8 (super)2-, a prototype compound for metal-metal multiple bonding. The study of this compound has been performed theoretically via the use of multiconfigurational quantum chemical methods, and the details are reported.

Theoretical evidence for the octahedral structure of CrF6 is presented. The methodology employed considers the near-degeneracies in the electronic structure, with the ANOs type of extended basis set utilized. ANOs were chosen from a set of 17s12p9d4f GTOs for Cr 5s4p3d2f and from 10s6p3d, 4s3pld for F. Complete active space SCF calculations were also performed with 10 electrons in 10 active orbitals.

The electronic spectra of three rhombic type 1 blue copper proteins, namely, pseudoazurin, nitrite reductase and cucumber basic protein were investigated using ab initio multiconfigurational second-order perturbation theory. The six lowest excitations were calculated and assigned with an error of less than 1800 centimeters(super -). Experimental results showed that small variations in the structure and electrostatics of different proteins may result in the stabilization of one or the other of the structures.

The electronic spectrum of the azurin Met121Gln variant used as a model of the blue copper protein stellacyanin was investigated by ab initio multiconfigurational second-order perturbation theory. The six lowest electronic transitions were calculated and assigned with an error of less than 2400 cm(super -1). Experimental results showed that the ground-state singly occupied orbital is a predominantly pi antibonding orbital involving Cu3d and S(sub cys)3p(sub pi).

Results are presented from a theoretical study of the X[sup 6]σ[sup +], A [sup 6]σ[sup +] and [sup 6]Δ electronic states of CrH using multiconfiguration second-order perturbation theory in the multi-state formalism (MS-CASPT2). It is shown that the results for the spectroscopic constants and radiative lifetimes for the A state are in agreement with experiment and an earlier multireference configuration interaction study only if the two close lying X and A states are allowed to interact at a level of approximation that includes dynamic correlation. [ABSTRACT FROM AUTHOR]

The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U-O(H20) distance is 2.40 A, which is close to the experimental estimates. A second coordination shell starts at about 4.7 A from the uranium atom. No hydrogen bonding is found between the uranyl oxygens and water. Exchange of waters between the first and second solvation shell is found to occur through a path intermediate between association and interchange. This is the first fully ab initio determination of the solvation of the uranyl ion in water. [ABSTRACT FROM AUTHOR]