Alastair F. Cuthbertson

MINDO/3 study of some tetra-atomic clusters

Abstract MINDO/3 calculations have been made of the energies of tetra-atomic clusters M+n4 (M = Si, P, S; ¦n¦ = 0, 2, 4) for a wide range of point groups, as well as for unconstrained geometries, in both singlet and triplet electronic configurations. The results are discussed in terms of the Jahn—Teller effect, in the first and second orders.

Electrostatic potentials of tumour promoters

Abstract Molecular-modelling studies and quantum-chemical calculations have been undertaken on a variety of structurally unrelated tumour promoters. The modelling studies reveal some broad areas of agreement in the associated electrostatic potentials for the molecules TPA, teleocidin, aplysiatoxin and ingenol. Azulene-type derivatives were used as models for inactive and active tumour promoters in the quantum chemical studies. The results obtained from these calculations of the electrostatic potentials show very little difference between active and inactive compounds, thus suggesting that other factors are of importance in these systems.

The structure of silyl cyclopentadienes

Calculations have been made in the MNDO approximation of the structures and energies of three isomeric forms of (η1-C5H5)SiR3 (R = H, Me) and of (η5-C5H5)SiR3, together with potential energy curves for the folding of the C5 ring in 5-R3SiC5H5; all the η1 species have planar C5rings at equilibrium. In addition (1,2) and (1,3) shift mechanisms have been investigated for migration of both SiH3 and hydrogen in (η1-C5H5)SiH3: the lowest energy pathway in each case is the (1,2) shift mechanism; structures and energies of the transition states are reported.

Intermolecular potentials using partial charges in quantum mechanical calculations

Abstract Pair interaction energies of model systems are reproduced using an approximation where one of the molecules is treated as a set of point partial charges which act as effective nuclei in the ab initio calculation of the other partner. The attractive portions of full-ab initio calculations are reproduced well enough to support the use of this type of approximation in modelling enzyme—substrate or enzyme—inhibitor interactions.

Binding of cyanoferrates to cobaloximes and cobalamins: an EHMO study

Abstract The binding of nitroprusside, [Fe(CN) 5 NO] 2− , and hexacyanoferrate(II) to methylcobaloxime, and to a cobalt corrin, has been analysed by means of extended Huckel calculations. The binding of [Fe(CN) 5 NO] 2− to the cobalt atom in methylcobaloxime can occur in three isomeric configurations, of which the most stable involves the axial cyano ligand in a FeCNCo bridge, and the least stable involves the nitrosyl ligand in a FeNOCo bridge. The binding of both cyanoferrates involves primarily a σ orbital largely localised as a lone pair on a cyano nitrogen and the vacant d z 2 , orbital of cobalt: π interactions involving cobalt d-orbitals and the π orbitals of the cyano ligands are of negligible importance.

The effect of fluorination on the binding energies of drugs

Abstract Ab initio calculations using partial point charges to represent enzymic atoms in the methotrexate-dihydrofolate reductase binary complex indicate that the influence of fluorination on binding free energy is probably an entropic effect involving solvent rather than of enthalpic origin.

Ligands of low electronegativity in the VSEPR model: The structures of singlet carbenes

Abstract Equilibrium structures and force constants for skeletal bending from linearity have been calculated, in the MNDO approximation, for twenty five singlet carbenes CX 2 . When the substituent X bears neither vacant orbitals nor lone pairs, the force constant becomes steadily more negative as the electronegativity of X increases; when X bears vacant orbitals, the C→ X π bond order and the force constant both increase with the electronegativity of X. When X bears lone pairs, the force constant parallels the HOMO—LUMO gap at linearity. Previous discussions of the structures of singlet carbenes are shown to be inadequate: the reported results support the interpretation in terms of the second-order Jahn-Teller effect of the observed stereochemical inactivity of lone pairs in the presence of ligands of low electronegativity.

Ligands of low electronegativity in the vsepr model: Isoelectronic molecules and ions (MX3)2 YE2n+

Abstract Equilibrium geometries, inversion barriers and force constants at linearity for the skeletal bending vibration have been calculated by the MNDO method for species of general type (X 3 M) 2 YE 2 n + ; in particular for twenty-eight hydrido species (H 3 M) 2 Y n + ; isoelectronic with H 3 COCH 3 in which M = Be, B, C, N, O, Si, P or S, and Y = B, C, N, O or F; and for eight fluoro analogues (F 3 M) 2 Y n + . For a given Y the force constant moves towards more negative values as the electronegativity of MX 3 increases: when M is held constant and Y is varied, the interplay of electronegativity and sp energy gap in Y causes a complex variation in observed behaviour. A structural description of MX 3 groups having less than C 3V symmetry is outlined.

Isomerism and adduct formation in the hector's base series: A MNDO study of model compounds

Abstract MNDO calculations on a series of a model compounds show that the observed structures for Hectors base, Dosts base and Dosts keto compound are the thermodynamically most stable tautomers and that the bond-switched structure observed for the 1:1 adduct of Hectors base with carbon disulphide and the non-bond-switched structure observed for the corresponding adducts with isocyanates and isothiocyanates are both the thermodynamically most favoured isomers, so that the occurrence or otherwise of a bond switch in these compounds is determined by thermodynamic rather than by mechanistic factors. Proposed mechanisms for the formation of the carbon disulphide adduct of Hectors base, and for its desulphurisation are supported by MNDO calculations.

The structure and conformation of tetraoxaquaterene as an isolated molecule

Recent interest [l-4] in the macrocyclic tetraoxaquaterenes (I) prompted us to attempt a determination of the structure and conformation of the parent molecule having R = H (systematic name 21,22,23,24tetraoxapentacyclo-[16.2.1.13~6.l~111,l13~16]-tetraco~-3,5,8,10,13,15,18,2O~ctaene) by means of semi-empirical SCF-MO calculations using the MNDO technique [5,6]. All computations were carried out on a VAX 11/780 computer.