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Vertical ionization energies of the two highest molecular orbitals and of orbitals mainly localized on the substituent of 31 a-substituted derivatives of furan, thiophen, selenophen, and tellurophen are discussed. The substituent effects confirm the reversal in the energy sequence of the t w o highest occupied x-MOs of tellurophen with respect to other five-membered heteroaromatic congeners and permit the assignment of the ionization energy values of the x , and xg MOs in selenophen. Assignments for some of the bands other than the first two in the photoelectron spectra of tellurophen and selenophen are proposed. The effect of the ring on the orbitals mainly localized on the substituent is briefly discussed.SEVERAL authors 1-4 have discussed in some detail the photoelectron spectra of furan and thiophen and to a lesser extent those of some of their derivatives. Recently 5*6 the photoelectron study has been extended to selenophen and tellurophen and the comparison of the spectra has made it possible to obtain some interesting information. In particular it has been suggested that there is a reversal in the sequence of ionization energies ( I ) of the two highest occupied orbitals of tellurophen jI(x3) > I(x,)] with respect to the other congeners [I(T,) > I ( x 3 ) ] .In order to confirm this hypothesis and contribute to the solution of some other problems raised in the preliminary study, we have undertaken a systematic photoelectron spectral investigation of the substituent

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The U.V. photoelectron spectra of a number of push-pull substituted ethylenes with two donor groups and with planar or twisted double bonds have been interpreted by the aid of perturbational arguments, CN D0/2 calculations, and comparison with published spectra of similar compounds. The highest bonding molecular orbital in all cases is of x-type, and its energy is raised with increasing twist about the double bond, whereas the orbital formed by antisymmetric combination of the donor atom pz orbitals is unchanged or lowered by increased twist. Through-bond interaction splits the ' non-bonding ' oxygen orbitals by 0 . 6 4 . 7 eV in 1.1 -diacetyl-2,2-diaminoethylenes, in which the carbonyl groups are €,Z oriented with respect to the C=C double bond. When Z,Z orientation is enforced by ring closure, the splitting is only 0.38 eV, in agreement with calculations. In the 2.2-dithio-analogues, interaction with the C-S bond orbitals leads to a more complex splitting pattern.

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