Peter G. Perkins

A molecular-orbital study of the ground and excited-state properties of metallocenes of the first-row transitionmetal ions. I

Abstract The CNDO-MO formalism has been used to study the ground- and excited-state properties of unsubstituted metallocenes of the first-row transition-metal ions. The multi-electron configuration interaction (MECI) method has been applied to the calculation of both photo-electron and absorption spectra and the agreement with experiment is satisfactory. The electronic properties of the metalloceneseries are described and the variations in the bonding schemes within the series are rationalised.

On the electronic structure of some metal hexaborides

An LCAO technique has been used for calculating the band structures of the hexaborides of Ca, Sr, Ba and La. The first three of these are shown to have small band gaps. The stability of the metal hexaborides and the B6 sublattice is discussed and it is shown that electron density accumulates inside each cage. The metallic conductivity of LaB6 is explained and the Hall coefficient for this substance is calculated.

A molecular orbital study of the trans effect

Abstract The trans-effect in Pt II complexes has been studied by a self-consistent molecular orbital method based on the CNDO approach. Systems investigated included the ions [Pt(X)Cl 3 ] − and the neutral species trans-[Pt(X) NH 3 Cl 2 in which X = C 2 H 4 , PH 3 , CO, H 2 S, NH 3 ,H 2 O, H − , CN − , CH 3 − , Br − , Cl − , OH − ,NO 2 − . It is concluded that the order of ability of ligands to exert the trans-effect depends on two factors; the trans-influence and the stabilisation of the trigonal bipyramidal transition state. If π-bonding is possible in the transition state this dictates the course of reaction. For a σ-bonding ligand both its trans-influence and its ability to act as a donor in the transition state are more important but the trans-influence is more important when the two effects do not reinforce each other.

The role of stereoelectronic factors in the oxidation of phenols

Abstract The oxidative coupling of both 3,5-dimethylphenol and phenol leads to the ortho-ortho and ortho-para coupled products as the predominant C-C dimers: stereoelectronic factors determine the preferred mode of approach of the phenoxyl radicals.

An ab initio molecular orbital study of some coordination compounds of boron trifluoride

The electronic structures and component energies of the compounds BF3, BF2OH, BF(OH)2, and B(OH)3 are reported. Replacement of fluorine atoms in BF3 by hydroxyl groups is found to be energetically unfavoured and the results are discussed in terms of reactions bringing about the slow hydrolysis of BF3 in aqueous solution. The hydrolysis of BF4− affording BF3OH− was also investigated. The rotational energies about the BN bond for NH3·BH3, NH3·BH2F, NH3·BHF2, and NH3·BF3 are calculated. In this series, fluorine substitution causes the rotational energy barrier to decrease.

Molecular orbital theory for the excited states of transition metal complexes

A general theoretical approach to the calculation of electronic excited states for molecules is described. A good ground state is not required and the method allows for arbitrary multiplicity of the states.

An improved LCAO SCF method for three-dimensional solids and its application to polyethylene, graphite, diamond, and boron nitride

An improved semi-empirical self-consistent scheme is described for calculating the band structures of three-dimensional solids. The basic level is that of CNDO theory. The non-orthogonality of the Bloch functions is recognised and allowance is made for all degrees of involvement of the overlap matrix. The calculation of the electron-repulsion integrals is formulated in a way suitable for solid-state problems. The method is tested on the standard systems; polyethylene, graphite, diamond, and hexagonal and cubic boron nitride. It is found that the valence band properties are satisfactorily reproduced. For optical spectra a configurational interaction scheme is required.

Oxidative coupling of phenols. Part 10. The role of steric effects in the formation of C–O coupled products

The formation of C–O coupled products in the oxidative coupling of phenols is hindered by bulky ortho-substituents in the aryloxyl radical intermediates as shown by the predominant formation of the ortho-ortho-C–C coupling products in the oxidation of o-t-butyl- and 2,4-di-t-butyl-phenol. 3,5-Di-t-butyl-phenol gives exclusively the ortho-O-dimeric product as steric effects prevent the sandwich approach of two aryloxyl radicals necessary for C–C coupling.

A new rapid method for orbital localisation

A new method is described which allows the rapid construction of localised molecular orbitals from eigenvector matrices and for a single-determinant wavefunction stemming from any quantum-chemical calculation. The essential modification over present methods is the application of the ZDO (zero differential overlap) approximation. A practical scheme is presented which facilitates applications of the method to complex molecular systems. As an illustration of the method, it is applied to a series of systems where the bonding is of the “classical” type (nitrogen, carbon dioxide, ethane, benzene, pyridine, neo-pentane and ethylene), of the “non-classical” type (diborane, B4Cl4, B12H2–12 and cis-C2B4H6) and a solid (polyethylene). The results give an informative picture of the bonding in these molecules. Two new quantities, the bond number and the lone-pair character, are defined quantitatively.

Cluster model for solids

A formulation of the cluster approach to the electronic structure of solids is proposed. This allows exact matching of cluster eigenvalues to points in the Brillouin zone without interference by edge effects. The scheme can be made self-consistent with respect to charge distribution.