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Photoelectron spectroscopic data for amino-and oxy-benzenes are discussed in terms of a simple p-7r interaction scheme. Steric hindrance is shown only to be important in ortho substituted NNdialkylanilines and in 2,6-dimethylanisole. In the nitrogen series there is indication that the N atom is not planar.* AG values k 80 cm-'.

Soinc aspects of the He1 photoelectron spectroscopy of substituted phenylethylenes and related molecules are presented. The interpretation of the variation of the photoelectron T ionization energies with substituents allows the separation of steric from electronic effects. Interaction of semilocalised T molecular orbitals incorporated within a Huckel type theory provides a means of estimation of the dihedral angles.

The helium I photoelectron spectroscopy of 30 biphenyls is reported. An empirical correlation between certain bands in the spectra and dihedral angle is noted and its use in conformational studies emphasized. An interpretation of the higher energy bands is given in terms of a Hiickel-type theory based on the interaction between semi-localized T molecular orbitals. Substituent effects are divided into electronic and steric and means for separating these two factors are outlined.Nearly every experimental technique of molecular structure determination has been applied with varying success to the question of '' steric hindrance " in chemistry ; * X-ray diffraction indicates that biphenyl is planar in the solid phase l4 ; electron diffraction finds an angle of 45" (and 42") ; in solution the value is intermediate depending on the solvent -19- 26O.'We find from our data that a dihedral angle of 45" fits appreciably better the regression than 42". P = (2PiIH12P,) B,. =sr Bl.38 s1.38 where S, = {2p1(2p,), i, j separated by r (A) we obtain a value sf 2,O eV at -1.50 A

The helium-(I) photoelectron spectra of the species (CH&A, where A = C, Si, Ge, Sn and Pb, are reported. The observed bands are assigned using a simple molecular orbital model which eniploys localized bond orbitals as the basis set.* The sample of (CH3)4Pb gave the correct mass spectrometric cracking pattern, and was apparently free of (CH3)6Pbz.