Stephen Marriott

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.

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.

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.

Electron densities in simple organic molecules

Abstract Dipole moments and atomic electron populations have been calculated for a series of simple organic molecules by ab initio molecular orbital theory using various basis sets. The effect of geometry optimization on these properties is examined. It is shown that values obtained from STO-3G and, particularly, 4-31G basis sets using standard geometries correlate well with those from molecules optimized at the 6-31G* level. Atomic electron populations derived by the normal Mulliken analysis are shown to follow those obtained by topological procedures within restricted series.

Calculated dipole moments for monosubstituted benzenes

Abstract Ab-initio molecular orbital calculations are reported for the dipole moments of a series of monosubstituted benzenes at the STO-3G, 3–21G, 4–31G, 6–31G and 6–31G* (5D) bases both with and without geometry optimization.

A scale of variable π-electron transfer

Abstract The response of ten common substituents to varying π-electron demand has been examined by calculations at the ab initio STO-3G basis. The system used was XCHCH-CH 2 + or XCHCH-CH 2 − , with variation in the length of the C-C single bond. The results allow the quantification of resonance effects for the substituents.

Theoretical studies of the structure of some mono- and di-substituted ethylenes

Abstract The structures of some monosubstituted ethylenes have been optimized at the ab-initio 4-31G level. The geometries found are in reasonable agreement with experimental gas phase values. Experimental and calculated results show that the carbon—carbon double bond length is almost constant, the small variations not apparently being related to the electronic effects of the substituent. The HCC angle at the carbon to which the substituent is attached, does vary considerably and analogously to the changes previously found at the ipso position in monosubstituted benzenes. The carbon—carbon length in 2-nitro-1-aminoethylene is only lengthened by some 0.02 A as a result of through conjugation.

π-Electron transfer as calculated at various levels of basis set. A redefinition of the theoretical scale of σR° values

Ab initio molecular orbital calculations are reported for the π-electron transfer to or from the substituent for 20 monosubstituted benzenes, both at the minimal STO-3G basis set and at the split-valence 4-31 G basis set. Evidence from i.r. intensities, experimental σR° values, rotational barriers, dipole moments, the protonation of cyanides XCN, and stabilisation energies all indicate that it is the STO-3G basis that better reproduces the experimental data for systems where π-electron transfer is important. The theoretical σR° scale is redefined using π-electron transfers for monosubstituted ethylenes at the STO-3G//STO-3G basis.

Bond angles at the ipso position of monosubstituted ethylenes

Abstract The structure of a series of monosubstituted ethylenes have been optimized at the ab-initio 4-31G//4-31G level. The geometries are in good agreement with experimental gas-phase determinations which are tabulated. The HCC angle at the ipso carbon is shown to follow the field effect of the substituent in a manner analogous to that found for the CCC angle at the ipso position in monosubstituted benzenes.