Photoelectron Spectroscopic Study of Skew Compounds. III. N,N′-Dimethylhydrazine, Dimethyl Peroxide, and Dimethyl Disulfide

He I photoelectron spectra of CH3HNNHCH3, (CH3)2O2, and (CH3)2S2 have been measured and the spectra below 18 eV have been resolved into nine ionization bands corresponding to p-type localized molecular orbitals. Orbital assignments of the resulting vertical ionization energies have been carried out with CNDO/2 calculations in which orbital energies were calculated as a function of the dihedral angle in the region 0–180°. The separation of the nonbonding photoelectron bands suggests that CH3HNNHCH3 exists in outer-inner skew form, whereas (CH3)2O2 is in trans form. The sum rule consideration previously proposed by the present authors has been applied to the compounds studied here. The total sum of the vertical ionization energies in the p-type region for each compound has been well reproduced in terms of appropriate empirical parameters based on the photoelectron spectra of simple molecules. The effects of methyl substitution on the ionization-energy total sums for N2H4, H2O2, and H2S2 have been found to be very similar to those for ethane and ethylene.