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Szlmmary Homocon jugation in the diphenylmethane system is evidenced by the comparison of the rates of methoxydechlorination of 4-chloro-3,4'-dinitro-and 4chloro-3,3'-dinitro-derivatives.

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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

The molecular ionization potentials of a number of substituted furans, thiophens, selenophens, and pyrroles have been determined by the electron-impact technique. The p value for this reaction is more negative than that for the most selective electrophilic substitutions. The sensitivity to substituent effects appears to vary in the reverse order to that of the ' ground-state aromaticity ' of the rings (furan > pyrrole > thiophen > benzene).THE relative magnitudes of substituent effects in fivemembered heteroaromatic rings vary according to the type of reaction. For reactions in the side-chain, such as the ionization of carboxylic acids 293 and the polarographic reduction of nitro-derivatives+ the order of sensitivity is pyrrole > furan > thiophen > benzene.A different sequence is observed in electrophilic substitutions. Quantitative data which permit a direct comparison among the three heteroaromatic rings are available for trifluoroacetylation by trifluoroacetic anhydride in di~hloroethane.~ In this reaction the furan ring is the most and the pyrrole ring the least sensitive to substituent effects.Other data available seem to confirm that, so far as Preliminary communication on the ionization potentials of 2-substituted thiophens, S.