Bjoern O. Roos

Quantum chemical calculations show that the uranium molecule U2 has a quintuple bond

Covalent bonding is commonly described by Lewiss theory, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for actinide–actinide bonds is restricted to the matrix-isolation of uranium hydrides, including H2U–UH2, and the gas-phase detection and preliminary theoretical study of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each—so all known covalent bonding types are contributing.

The Cr2 potential energy curve studied with multiconfigurational second-order perturbation theory

The Cr2 potential energy curve studied with multiconfigurational second-order perturbation theory

Vibrational frequencies of ozone: A multiconfigurational approach

The electronic ground state of ozone and, in particular, its equilibrium geometry and harmonic vibration frequencies was studied by a variety of multiconfiguration and single‐configuration methods. It is well known that the antisymmetric stretch frequency cannot be correctly obtained by single‐reference methods unless at least triple excitations are included. Extensive comparison with other work in the literature shows that basis‐set effects must be taken into account since the ω3 frequency is very sensitive to computational details. The multiconfiguration methods are shown to give good results provided that an adequate configuration space is used. In particular, the second‐order complete active space perturbation method performs very satisfactorily. Traditional multireference configuration interaction (MRCI) methods, using a few reference functions, do not perform so well. A two‐reference CI is able to give reasonable results, but only when the orbitals have been prepared by some properly correlated meth...

Electric properties of the ozone molecule

Abstract The dipole moment, quadrupole moment and dipole polarisability of O 3 are calculated by the self-consistent field (SCF), many-body perturbation theory (MBPT), complete active space SCF (CASSCF), restricted active space SCF (RASSCF), multi-reference CI (MRCI) and CASSCF-perturbation theory (CASPT) methods, using a 5s3p2d polarised basis set. At the CASPT-2 level, the dipole moment is μ b =0.2136 au. The quadrupole moments are Θ aa =−1.158, Θ bb =−0.260, Θ cc =+1.418; and the polarisability anisotropies are α aa −α bb =16.2 and α aa −α cc =18.6, all in atomic units. Experimental data for these quantities are 0.2100(1), −1.03(12), −0.52(16), +1.55(19), and 19.2(7), 17.9(7), respectively. Similar results are obtained by CASSCF, RASSCF and MRCI, and by MBPT when taken to fourth order. The transverse in-plane polarisability obtained by the single-reference methods shows an erratic behaviour, which is similar to known variations in computed antisymmetric stretching frequencies.

The structure of dichromium tetraformate

Abstract Second-order perturbation theory (CASPT2) calculations using large basis sets have been used to study the CrCr bond in dichromium tetraformate, Cr 2 (O 2 CH) 4 The CrCr potential was found to have a double minimum with one short equilibrium at 1.94 A, exhibiting considerable 3d-3d bonding, and a long equilibrium at 2.50 A, which is essentially determined by the structure of the bridging ligands. The energy difference between the two minima is small (of the order of 0.1 eV) and the relative order depends on the level of theory used. Inclusion of relativistic effects and 3s,3p correlation (estimated from the corresponding effect in the 3 Σ u + state of Cr 2 locates the inner minimum 0.07 eV below the outer. The measured distance in Cr 2 (O 2 CCH 3 ) 4 is 1.966 A. It is likely that addition of axial ligands like water will make the outer minimum more stable, thus explaining the variations in Cr{inCr} distances in dichromium complexes. The DFr approach is also applied and long, 2.296 and 2.487 \rA, CrCr bond distances are found for Cr{in2}(O{in2}CH){in4} and Cr{in2}(O{in2}CCH{in3}){in4}, respectively. Addition of axial water ligands increases the CrCr distance by about 0.19 and 0.07 \rA, respectively, for these two systems.

Ab initio characterization of C5

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X1Sigmag+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu7(piu) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope.

Multiconfigurational second order perturbation theory applied to the calculation of electronic spectra of conjugated systems

A newly proposed method for the calculation of complex electronic structures in molecules is applied to the calculation of excited states. The method is a two step procedure: The Complete Active Space (CAS) SCF method is used to calculate the molecular orbitals and a reference function. This step thus takes care of the basic interactions including the coupling of nearly degenerate configurations, which is very common for excited states. Dynamic electron correlation effects are added in the second step, where second order perturbation theory is used with the CASSCF wave function constituting the reference function. The procedure has been successfully applied to a number of electronic spectra, yielding results of much higher accuracy than has been possible to obtain with traditional CI based methods. The lectures will give examples from studies of benzene, the azabenzenes, short polyenes, and three hetero-cyclic pentadienes. In several of these studies have the theoretical results provided a new understanding of the spectra and suggested a number of new assignments.

As≡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).

Matrix infrared spectra and electronic structure calculations of the first actinide borylene: FB=ThF2

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.