Open-shell Electronic Configurations Research Articles

Phosphine substitution in (.eta.5-cyclopentadienyl)bis(triphenylphosphine)cobalt(I): evidence for a dissociative mechanism

The substitution of trimethylphosphine for triphenylphosphine in {eta}{sup 5}-cyclopentadienyl-bis-triphenylphosphinecobalt(I), 1, to form {eta}{sup 5}-cyclopentadienyltrimethylphosphinetriphenylphosphinecobalt(I) was studied at -60°C in an NMR spectrometer. Kinetic measurements show the process to be first order in 1 and zero order in PMe{sub 3}; added PPh{sub 3} strongly inhibits the reaction rate. This information indicates the reaction proceeds by rapid reversible phosphine dissociation through the unsaturated CpCo(PPh{sub 3}) intermediate. The rate for generation of that intermediate, k{sub 1}, is 1.15 x 10{sup -3} sec{sup -1} while the ratio of rate constant k{sub 2} (for conversion of intermediate to products) to k{sub -1} (return to starting materials) is 4 at -60°C. Possible structures for CpCo(L) are discussed in light of recent indications that the linear structure has an open-shell electronic configuration.

Local approach to polarizabilities and trends in the Raman spectra of semiconductors

The Raman spectra of several II-VI and III–V compounds are well described with one parameter, namely the ratio of non-linear intra-ionic polarizabilities of the individual ions. This approach is justified by the open-shell electronic configuration of the compounds and is based on a simple overlap shell model which reproduces the phonons in many binary compounds. The relations to bond polarizabilities and the anomalous case of oxides are discussed.

Configuration Interaction in the Simple Valence-Bond Wavefunction for the Potential-Energy Surfaces of Sigma-Bonded Four-Center Exchange-Reaction Complexes

The two closed-shell and nine open-shell symmetry-adapted MO electron configurations contained in the simple nonionic VB wavefunction for H4 in the limit of 2H2 with D2h symmetry and in a D4h complex are identified and their coefficients are obtained in terms of overlap integrals. Numerical comparison of the configuration coefficients obtained from the nonionic VB wavefunction with those obtained by the full CI calculation of Wilson and Goddard show that the ionic contribution to the D4h H4 complex of side 2.54a0 is small in the ground state but significant in the upper state. The implications of a VB or CI treatment of four-center potential-energy surfaces regarding a recent extension of the Woodward–Hoffmann rules (conservation of orbital symmetry) to the reactions H2 + D2→2HD and H2 + I2→2HI are discussed.

Lambda-Doubling Effect on Molecular Electric-Field Gradient and Nuclear-Quadrupole Coupling

Physical reasons and mathematical arguments are given to show that for a rotating linear molecule or radical, between the degenerate [open phi]+Ω and [open phi]−Ω electronic states of opposite senses of angular momentum, there exists a ``cross-term'' matrix element connected by a nonaxial field-gradient-tensor operator (∇E)2Ω=T2,2Ω/re5. The resulting effect is that the electric-field gradients eq for the two Λ-doubling components are different, giving rise to an additional nuclear-quadruple contribution to the Λ-doubling energy. When the nuclear spin is quantized with respect to the space z axis as in a molecule belonging to Case aβ (Sec. III), it is shown that because the nuclear quadrupole tensor operator transforms with a rotation matrix of the second rank, such Λ-doubling effect on field gradient vanishes for the 2Π½(a) state. For a Case bβ2Π state and a 2Π state intermediate between Cases a and b, the dependence of this effect on J is derived using wavefunctions with a consistent phase and inversion symmetry not otherwise obtainable from the solution of a secular equation. For states with Ω=0 (Table I) the effect is shown to arise from electron spin and orbital interaction in a stationary molecule. For 3Π1(a) and 1Π states, the relative sizes of the ``nonaxial field gradient'' eq2, and the axial field gradient, eq0, are estimated using open-shell electron configurations constructed from one-electron molecular orbitals. The general selection rule based on Ω, the total spin and orbital angular momentum, for the matrix elements over the spinless, one-electron field-gradient operator is justified for various open-shell electron configurations by invoking electron spin—spin interaction in addition to the customary electron spin—orbit interaction. A general method is devised for obtaining diagonal and off-diagonal spin—spin interaction matrix elements over Slater determinantal wavefunctions. In terms of these interactions the relative importance of these configurations of equivalent as well as nonequivalent electrons is given. When the nuclear spin is quantized with respect to the molecular figure axis as in a molecule belonging to Case aα (Sec. V), the additional nuclear quadrupole splitting of the Λ-doubling levels is shown to be independent of J and to be present in states with ΩF=0 only, when the molecule is rotating.