Ronald D. Topsom

Electron densities and resonance interactions in substituted benzenes, ethylenes and acetylenes

Abstract Molecular orbital calculations at the ab initio STO-3G, 4-31G and 6-31G* level have been used to investigate electron populations in monosubstituted benzenes, ethylenes and acetylenes. It is shown that the π-electron transfers to or from the substituent in ethylenes are accurately proportional to those in the corresponding substituted benzenes using either the STO-3G or 4-31G basis sets. However, figures at the split-valence basis levels are not proportional, in either series, to those at the minimal basis STO-3G level. The values for a series of substituted ethylenes and acetylenes are used to investigate the validity of the σR0 and σR0 (twist) series of substituent constants.

Theoretical studies of the structures of some mono- and di-substituted benzenes

Abstract The structures of some monosubstituted benzenes have been optimized at the ab initio STO—3G level, and, in some cases, also using split valence base sets. It is found that the geometries obtained are in reasonable agreement with experimental gas phase values, particularly as far as substituent induced trends are concerned. The work has been extended to para-disubstituted benzenes to examine the contribution of quinonoid forms. The geometrical evidence and interaction energies obtained indicate that such forms are of minor significance in the neutral molecules, but can be quite significant in ions such as the para-nitrophenoxide anion.

Infrared intensities as a measure of intramolecular interactions XIX. Electronic interactions in substituted phenyl- and benzyltrimethylsilanes

Abstract An infrared method has been used to establish that the SiMe 3 group is a weak resonance acceptor in the ground state when joined to a benzene ring. This acceptance is considerably exalted when resonance donors are attached to the para position. The relatively strong resonance donation of a CH 2 SiMe 3 group is explicable in terms of hyperconjugation.

Acidities of substituted acetic acids

Abstract Experimental gas phase acidities of a series of substituted acetic XCH 2 CO 2 H, and related acids are reported. These relative acidities seem mainly determined by substituent field effects. Ab initio molecular orbital calculations for these acidities at the STO-3G and 3-21G basis levels are also discussed.

Standard bond lengths for use in ab initio molecular orbital calculations

Abstract Standard bond lengths are proposed for a wide variety of bond lengths involving first row elements. These were obtained as average values from a large number of calculations made at the ab initio molecular orbital 4-31G level with geometry optimization. It is shown that these are generally in good agreement with accurate experimental values, where available.

Infrared intensities as a quantitative measure of intramolecular interactions. Part XXXIV. Quantitative relations between conjugation and strain energies and σ°R values: rotational barriers in monosubstituted benzenes

Known rotational barriers for substituents in monosubstituted benzenes are correlated with ring–substituent resonance interactions and strain energies. Rotational barriers are estimated for a variety of other substituents. σ Constants are related directly to the energy scale.

Proton affinities of substituted cyanides

Abstract Molecular orbital calculations at the ab initio STO-3G, 3–21G, 4–31G, and 6–31G* bases have been made for the relative proton affinities of an extended series of substituted cyanides, XCN. It is shown that geometry optimization at the 3–21G level is adequate and that 3–21G//3–21G calculations give very similar results to those obtained at the 6–31G*//3–21G level, except for X = Cl. Gas phase proton affinities are reported for many of the XCN compounds and these values are compared with the calculated results. Agreement between theory and experiment is excellent, except for some substituents which exert large field inductive effects.

A theoretical scale of substituent resonance parameters (σR

It is shown that molecular orbital calculations at the ab-initio level can provide a scale of resonance effects in good agreement with experimental values appropriate for the gas-phase or non-polar solvents. Using monosubstituted ethylenes as models allows calculations for a wide range of substituents on a split-valence basis. Values of σR° have been calculated for more than 40 common substituents, including some for which experimental values are not well established. Conformation effects on resonance interactions are discussed.

The nature of substituent electronic effects; the existence of the π-inductive effect

Measurements of 13C n.m.r. shifts and of u.v and i.r. intensities for ω-substituted alkylbenzenes show the presence of a π-inductive effect. Various possible mechanisms contributing to such effects are analysed and the discussion is extended to related benzyl systems.

Reproducibility of Integrated Infrared Intensity Measurements

Careful control of instrument parameters and the use of a computer program for integration gives a reproducibility of <±2% between intensity measurements of reasonably isolated absorptions on different grating spectrometers in different laboratories.