Infrared spectra and quantum chemical calculations of the uranium carbide molecules UC and CUC with triple bonds.

Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm(-1). A new band at 827 cm(-1) becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm(-1) gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C[triple bond]U[triple bond]C ground state as linear 3Sigma(u)+ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U[triple bond]C diatomic molecule has a quintet (Lambda = 5, Omega = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.

Matrix infrared spectra and electronic structure calculations of the first actinide borylene: FB=ThF(2).

Laser-ablated Th atoms react with BF(3) during condensation in excess argon at 6 K to form the first actinide borylene (FB=ThF(2)) and actinide-boron multiple bond. Three new product absorptions in the B-F and Th-F stretching regions of matrix infrared spectra are assigned to FB=ThF(2) from comparison to theoretically predicted vibrational frequencies.

Bonding trends in molecular compounds of lanthanides: the double-bonded carbene cations LnCH(2) (+) (Ln=Sc, Y, La-Lu).

Quasi-relativistic Douglas-Kroll CASPT2 calculations are reported for the title molecules, mainly to provide primary data for a fit of double-bond covalent radii. Indeed, a well-developed sigma(2)pi(2) double bond is identified in all cases. For Eu and Yb, however, it is an excited state. The main valence orbitals of all Ln ions are 6s and 5d. In the sigma bonds, more 5d than 6s character is found at the Ln. The Ln=C bond lengths show a systematic lanthanide contraction of 13 pm from La to Lu. An agostic symmetry breaking is demonstrated for Ce but its effect on the Ln-C length is small.

MOLCAS 7: the next generation.

Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented in this report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two-electron integrals and in the generation of so-called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self-consistent field, density functional theory, 2nd order perturbation theory, complete-active space self-consistent field multiconfigurational reference 2nd order perturbation theory, and coupled-cluster methods. The report further elaborates on the implementation of a restricted-active space self-consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas-Kroll-Hess transformation for one-component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so-called picture-change-free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies.

Covalent vs electrostatic interactions in rare earth systems: a comparative study of U(III), U(IV), and U(V) and Nd(III) bonding properties by DFT and CAS-PT2 approaches.

A description of the electronic structure of F(3)UCO, F(3)NdCO, F(4)UCO, and F(5)UCO has been obtained by Complete Active Space second-order perturbation theory CASPT2 calculations using a relativistic effective core potential. These multiconfigurational calculations have been compared to the DFT description combined with a quasi-relativistic ZORA scalar approach. Geometries have been optimized for both levels of calculations and frequencies computed in the DFT formalism. The bonding properties of U(III) have been compared to those of Nd(III) and of higher oxidation states of U(IV,V). Both methodologies are consistent and show a decrease of the covalent character of the U-CO bonding with a higher oxidation state, U(IV) or U(V), as well as its absence for for the isoelectronic Nd(III) species.

As[triple bond]UF3 molecule with a weak triple bond to uranium.

After reactions of uranium atoms with NF(3) and PF(3) to form the N[triple bond]UF(3) and P[triple bond]UF(3) molecules, the analogous reaction with AsF(3) produced the novel terminal arsenide As[triple bond]UF(3). This first molecule with a uranium-arsenic bond was identified from matrix infrared spectra through comparison with spectra of the uranium nitride and phosphide species, with spectra using other metals, and with frequencies computed by density functional and multiconfigurational wave function methods. The latter calculation describes a weak triple bond to uranium in the As[triple bond]UF(3) molecule, which has slightly less bonding and more antibonding character than the weak triple bond in P[triple bond]UF(3).

Experimental and theoretical investigation of simple terminal group 6 arsenide As[triple bond]MF3 molecules.

Laser-ablated group 6 metal atoms react with NF3 and PF3 to form the simple lowest energy N[triple bond]MF3 and P[triple bond]MX3 products, and this investigation has been extended to AsF3. Mo and W atoms react with AsF3 upon excitation by laser ablation or UV irradiation to form stable trigonal As[triple bond]MF3 terminal arsenides. These molecules are identified by comparison of the closely related infrared spectra of the analogous phosphide species and with frequencies calculated by density functional theory and multiconfigurational second order perturbation theory (CASSCF/CASPT2). Computed CASSCF/CASPT2 triple bond lengths for the As[triple bond]MoF3 and As[triple bond]WF3 molecules are 2.240 A and 2.250 A, respectively. The natural bond orders calculated by CASSCF/CASPT2 decrease from 2.67 to 2.60 for P[triple bond]MoF3 to As[triple bond]MoF3 and from 2.74 to 2.70 for P[triple bond]WF3 to As[triple bond]WF3 as the arsenic valence orbitals are less effective than those of phosphorus in bonding to each metal atom and the larger metal orbital size becomes more compatible with the arsenic valence orbitals. The Cr atom reaction gives the arsinidene AsF=CrF2 product instead of the higher energy As[triple bond]CrF3 molecule as the Cr (VI) state is not supported by the softer pnictides.

Bond length and bond order in one of the shortest Cr-Cr bonds.

Multiconfigurational quantum chemical calculations on the R-diimines dichromium compound confirm that the Cr-Cr bond, 1.80 A, is among the shortest Cr(I)-Cr(I) bonds. However, the bond between the two Cr atoms is only a quadruple bond rather than a quintuple bond. The reason why the bond is so short has to be attributed to the strain in the NCCN ligand moieties.

Not innocent: verdict from ab initio multiconfigurational second-order perturbation theory on the electronic structure of chloroiron corrole.

From a suitably broad perspective, transition metal corroles may be viewed as stable, synthetic analogues of high-valent heme protein intermediates such as compounds I and II. Against this backdrop, the electronic structure of chloroiron corrole has provoked a lively debate in recent years. Thus, whereas NMR spectroscopy and DFT calculations suggest an S = 3/2 Fe(III) corrole (*2-) radical description, certain researchers have favored an Fe(IV) formulation. These two descriptions are indistinguishable as far as DFT calculations are concerned. Ab initio CASSCF/CASPT2 calculations provide unambiguous support for the former description. In addition, they rule out any Fe(IV) state, whether high- or low-spin, within 1.5 eV of the ground state.

New relativistic atomic natural orbital basis sets for lanthanide atoms with applications to the Ce diatom and LuF3.

New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are included as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.

A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N[triple bond]MX3 and P[triple bond]MX3 molecules.

Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF 3, PF 3, and PCl 3 to form the simple lowest energy N[triple bond]MF 3, and P[triple bond]MX 3 products following insertion and halogen transfer, with the exception of P[triple bond]CrF3, which is a higher energy species and is not observed. The E[triple bond]MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M[triple bond]N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.

Mapping the d-d excited-state manifolds of transition metal beta-diiminato-imido complexes. Comparison of density functional theory and CASPT2 energetics.

Trigonal-planar, middle transition metal diiminato-imido complexes do not exhibit high-spin states, as might be naively expected on the basis of their low coordination numbers. Instead, the known Fe(III), Co(III), and Ni(III) complexes exhibit S = 3/2, S = 0, and S = 1/2 ground states, respectively. Kohn-Sham DFT calculations have provided a basic molecular orbital picture of these compounds as well as a qualitative rationale for the observed spin states. Reported herein are ab initio multiconfiguration second-order perturbation theory (CASPT2) calculations, which provide a relatively detailed picture of the d-d excited-state manifolds of these complexes. Thus, for a C(2v) Fe(III)(diiminato)(NPh) model complex, two near-degenerate states ((4)B(2) and (4)B(1)) compete as contenders for the ground state. Moreover, the high-spin sextet, two additional quartets and even a low-spin doublet all occur at <0.5 eV, relative to the ground state. For the Co(III) system, although CASPT2 reproduces an S = 0 ground state, as observed experimentally for a related complex, the calculations also predict two exceedingly low-energy triplet states; there are, however, no other particularly low-energy d-d excited states. In contrast to the Fe(III) and Co(III) cases, the Ni(III) complex has a clearly nondegenerate (2)B(2) ground state. The CASPT2 energetics provide benchmarks against which we can evaluate the performance of several common DFT methods. Although none of the functionals examined perform entirely satisfactorily, the B3LYP hybrid functional provides the best overall spin-state energetics.

Agostic interaction in the methylidene metal dihydride complexes H2MCH2 (M=Y, Zr, Nb, Mo, Ru, Th, or U).

Multiconfigurational quantum chemical methods (complete active space self-consistent field (CASSCF)/second-order perturbation theory (CASPT2)) have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U. The geometry of some of these complexes is highly irregular due to the formation of a three center bond CH...M, where the electrons in the CH bond are delocalized onto empty or half empty orbitals of d- or f-type on the metal. No agostic interaction is expected when M=Y, where only a single bond with methylene can be formed, or when M=Ru, because of the lack of empty electron accepting metal valence orbitals. The largest agostic interaction is found in the Zr and U complexes.

Multiconfigurational quantum chemical methods for molecular systems containing actinides.

Recent advances in computational actinide chemistry are reported in this tutorial review. Muticonfigurational quantum chemical methods have been employed to study the gas phase spectroscopy of small actinide molecules. Examples of actinide compounds studied in solution are also presented. Finally the multiple bond in the diuranium molecule and other diactinide compounds is described.

Infrared spectrum and bonding in uranium methylidene dihydride, CH2=UH2.

Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U+CH4 reaction products are identified through isotopic substitution (13CH4, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C1 ground-state structure for the triplet CH2=UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2=UH2 complex is distorted more than CH2=ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes.

Performance of density functionals for first row transition metal systems.

This article investigates the performance of five commonly used density functionals, B3LYP, BP86, PBE0, PBE, and BLYP, for studying diatomic molecules consisting of a first row transition metal bonded to H, F, Cl, Br, N, C, O, or S. Results have been compared with experiment wherever possible. Open-shell configurations are found more often in the order PBE0>B3LYP>PBE approximately BP86>BLYP. However, on average, 58 of 63 spins are correctly predicted by any functional, with only small differences. BP86 and PBE are slightly better for obtaining geometries, with errors of only 0.020 A. Hybrid functionals tend to overestimate bond lengths by a few picometers and underestimate bond strengths by favoring open shells. Nonhybrid functionals usually overestimate bond energies. All functionals exhibit similar errors in bond energies, between 42 and 53 kJmol. Late transition metals are found to be better modeled by hybrid functionals, whereas nonhybrid functionals tend to have less of a preference. There are systematic errors in predicting certain properties that could be remedied. BLYP performs the best for ionization potentials studied here, PBE0 the worst. In other cases, errors are similar. Finally, there is a clear tendency for hybrid functionals to give larger dipole moments than nonhybrid functionals. These observations may be helpful in choosing and improving existing functionals for tasks involving transition metals, and for designing new, improved functionals.

Exploring the actinide-actinide bond: theoretical studies of the chemical bond in Ac2, Th2, Pa2, and U2.

Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac2, Th2, Pa2, and U2. Scalar relativistic effects and spin-orbit coupling have been included in the calculations. In the Ac2 and Th2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac2 is characterized by a double bond with a 3Sigmag-(0g+) ground state, a bond distance of 3.64. A, and a bond energy of 1.19 eV. Th2 has quadruple bond character with a 3Dg(1g) ground state. The bond distance is 2.76 A and the bond energy (D0) 3.28 eV. Pa2 is characterized by a quintuple bond with a 3Sigmag-(0g+) ground state. The bond distance is 2.37 A and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a 7Og (8g) ground state, a bond distance of 2.43 A, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa2, characterized by a well-developed quintuple bond.