The energetics of electron transfer from an extensive series of alkyl-substituted benzenes are measured both in solution and in the gas phase. The standard oxidation potentials stem from the reversible cyclic voltammograms (CV) in trifluoroacetic acid using the recently developed microvoltammetric electrodes. These values show an excellent correlation with the vertical ionization potentials 7p of the same aromatic hydrocarbons in the gas phase. Thermochemical analysis indicates that the slope of less than unity for the correlation arises mainly from solvation differences, particularly in the highly substituted polyalkylbenzenes. The values of £Ar°a lso correlate extremely well with the anodic peak potentials £p of the irreversible cyclic voltammograms, which can be readily measured in acetonitrile at a standard sweep rate. A slope of unity for the linear correlation is discussed in terms of the maximum error in the CV peak shift (£_ -£°) resulting from the electrochemical kinetics of the EC mechanism for arene oxidation. The correlation of E^0 and £p affords a prediction of the standard oxidation potential of benzene, which otherwise could not be measured directly. The same series of alkylbenzenes form stable x-complexes with chromium tricarbonyl that show reversible electrochemical behavior. The standard oxidation potentials £ArCr°o f the complexes also correlate very well with E^f of the free arene. The diminished slope of the correlation arises from the nonbonding character of the HOMO.The predicted value of E^0 for benzene from this relationship is in reasonable agreement with that obtained from the irreversible peak potentials.

The photoelectron spectra of iodobenzene and 1,2-, 1.3-, and 1.4-di-iodobenzene have been recorded by using He1 excitation. The electronic structure of these compounds is discussed in terms of a simple model of interaction between known orbitals of the composite parts, i.e. the moJecular orbitals of benzene and the 5p atomic orbitals of iodine, taking their energy and symmetry but no spin-orbit coupling into account. The assignment is simplified by the characteristic shapes of several systems, which. holds especially for the lone-pair electrons on iodine. Orbital sequences are given down to ionisation energies of 17 eV. Assignments are supported by comparison with the photoelectron spectra di-iodo-benzene.THE effect of substituents on the x-electron levels in substituted benzenes has been the subject of a number of investigations. Turner and his co-workers have shown the effect of substituents lifting the degeneracy of the elg n-orbitals of benzene and discussed it in terms of x-electron perturbation by inductive and mesomeric effects. They also discussed in general terms the effect of multiple substitution (1,2-, 1,3-, 1,4-, and perfluorosubstitution) and the characteristics of systems associated with lone-pair electrons of substituents. Most of their conclusions are still valid. However, the progress in theoretical (computational) and instrumental methods (instruments with better resolution) made it of interest to study the substituted benzenes systematically in t ?n leave from the Ruder BoSkovi6 Institute, and the Faculty 1 A

The He I and He I1 photoelectron spectra of CBrCI3, CHBrC12, CHBr2CI, CH2BrCl, CHFC12 and CHFzCl have been recorded with low and high resolutions. Correlation with assignments for other halogenated methanes, comparison of He II/He I intensities and vibrational fine-structure analysis allowed all the systems with ionization energies up to 24 eV to be assigned.

The valence-electronic structure of the first three members of the acene series, benzene, naphthalene and anthracene, seems to be confirmed now by their photoelectron (PE) spectra. This especially holds for the r-ionizations which, according to their specific behaviour, can be assigned to different ionization modes. The vibrational fine structure of the observed systems is discussed. The effects of a series of substituents on the electronic structure of the parent molecules were traced in their PE spectra and intercompared. The results for methyl, chloro, methoxy and cyano substitution are described in detail.The effect is most pronounced for the two lowest energy -ionizations (i.e. HOMO and SHOMO in the Koopmans' picture) and can be described by the shift of the mean energy ("center of gravity") of HOMO and SHOMO, and the additional splitting compared to the unsubstituted molecule. Both parameters show good linear dependence for the three series of compounds, thus allowing to formulate general substituent parameters. A linear dependence of the lowest ionization energy with Hammett constants is found to exist as well.

No abstract