D.T. Clark

A self consistent field molecular orbital investigation of the importance of d orbital participation in the ground state of thiophene

Abstract The electronic structure of thiophene has been investigated using the Pople-Segal Complete-Neglect of Differential overlap self consistent field theory and including all valence electrons. The effect of including 3d, 4s and 4p orbitals on sulphur is discussed in relation to calculated and experimental dipole moments and spin-spin coupling constants. The evidence is that inclusion of 3d, 4s and 4p orbitals has a very small effect on the calculated total energy of thiophere, but a significant effect on the relative magnitude of proton-proton and directly bonded carbon 13-proton coupling constants, and calculated dipole moments.

A self consistent field molecular orbital treatment of the reactivity of thiophene and the isomeric thienothiophenes

Abstract The reactivity of thiophene and the isomeric thienothiophenes has been investigated using the semi empirical PPP SCF MO method.

A self consistent field molecular orbital treatment of furan including all valence electrons

Abstract The electronic structure of furan has been investigated using a semi empirical self consistent field MO method including all valence electrons. The calculated pi electron distribution has been compared with that obtained using the PPP SCF MO method. The calculation gives satisfactory agreement with experiment for the total dipole moment and first ionization potential of furan.

The electrocyclic transformation of cyclopropyl to allyl cation, an all valence SCF MO treatment

Abstract The transformation of cyclopropyl, to allyl cation has been investigated by the CNDO II SCF MO method. Energy differences for transformations involving planar and non planar cyclopropyl cations, have been considered in the ground state and three lowest energy excited singlet and triplet states. For the ground states the results are compared with those derived from Extended Huckel calculations.

A self consistent field molecular orbital treatment of pyrrole and pyrrole anion including all valence electrons

Abstract The electronic structures of pyrrole and pyrrole anion have been investigated using the Pople-Segal Complete Neglect of Differential overlap self consistent field MO theory, including all valence electrons. For pyrrole the charge distribution and energy levels have been compared with those obtained from the ab initio calculations of Clementi et al. The latter calculation appears to give an excessive amount of charge migration from the H atoms, and the charge distribution obtained by the semi empirical SCF calculation is probably more realistic. In general there is reasonable agreement between the orbital energies calculated by the two methods, energy level differences being particularly well produced. For pyrrole anion comparison of charge distribution, with the obtained using Extended Huckel Theory indicates that the latter is likely to be seriously in error when used in calculations on polar molecules.

All valence electron SCF MO calculations on the electronic structures and reactivities of some 6a-thiathiopthenes

Abstract All valence electron CNDO/2 SCF MO calculations have been performed on 6a-thiathiophthene and some simple derivatives. The electronic structure is discussed and compared with available experimental data. Localization energies for prototype electrophilic (H+) and nucleophilic (H−) substitution of the parent molecule have been calculated. Relative acidities of the methyl protons in 2 and 3 methyl thiathiophthenes have also been investigated.

A non-empirical LCAO-SCF-MO investigation of cross sections through the potential energy surface for the [C2H2Cl]+ systems; comparison with the [C2H5+] and [C2H4F]+ systems

Non-empirical LCAO MO SCF calculations are reported on cross sections through the C2H4Cl+ system and comparisons are drawn with the C2H5+ and C2H4F+ systems. Barriers to rotation in the classical 1- and 2-substituted ethyl cations have been computed and an investigation made of the bridged chloronium and fluoronium ions. The results suggest that the relative stabilities of bridged ions with respect to the corresponding classical 2-substituted ethyl cations increase in the order H < F < Cl. The results are discussed in terms of available experimental data and consideration given to correlation and solvation energy effects.

Model potential energy surfaces for approach of an electrophile to acetylene and fluoroacetylene

Abstract Non-empirical LCAO MO SCF calculations have been made on cross sections through prototype potential energy surfaces for the initial approach of an electrophile to acetylene and fluoroacetylene. The electrophile has been simulated by a unit positive charge and this is shown to give a computationally inexpensive means of providing information on such processes.

The electronic effect of the methyl and methoxyl groups : Partial rate factors for nitration of some substituted naphthalenes

Partial rate factors with respect to benzene have been obtained for the nitration of naphthalene, 2,6-dimethylnaphthalene and 2-methoxy-6-methylnaphthalene. These results together with previously published data for 2-substituted methyl- and methoxy-naphthalene are discussed in terms of cation localization energies, calculated within the Parisher-Parr-Pople framework. For both Me and OMe substituents it is shown that a good account of the reactivity of these molecules, which cover a range of 106 in partial rate factors, can be given by the simple SCF-MO treatment if due allowance is taken of sigma and pi inductive, and mesomeric effects.

The electronic effect of the methyl group: Partial rate factors for nitration of the xylenes

Abstract The electronic effect of the Me group has been investigated using several different models within the PPP formalism. For Me substituted benzenes the optimum agreement between calculated and observed properties is obtained when both, inductive and hyperconjugative mesomeric effects of the substituent are included. The properties investigated are frequency shifts and changes in intensities of electronic transitions, changes in ionization potentials, changes in π charge distribution and positional and relative reactivities in electrophilic substitution. In the case of the latter the nitration (HNO 3 /HOAC media) of the xylenes has been investigated over a wide temperature range, and competitive nitrations at 25° with benzene have been carried out to give partial rate factors.