Furan vertical ionization energies

Vertical ionization energies of the two highest molecular orbitals and of orbitals mainly localized on the substituent of a-substituted derivatives of furans, thiophenes, selenophenes, and tellurophenes have been determined.25 Assignments of some of the bands other than the first two in the photoelectron spectra of tellurophene and selenophene are proposed, and the effect of the ring on the orbitals mainly localized on the substituent is briefly discussed. 

Support for the reverse sequence hypothesis is derived from a comparative study on the photoelectron spectra of numerous a-substituted derivatives of furan, thiophene, selenophene, and tellurophene.33 The vertical ionization energies (I, and I2) of the two highest MOs (n2 and n ) are reported in Table XIV. An examination of the correlations obtained between the ionization energies of molecular orbitals of substituted furans, selenophenes, and tellurophenes vs. those of substituted thiophenes makes clear the diverse nature of the upper orbitals in tellurophenes. Slopes close to I, very high correlation coefficients and low standard deviations are obtained when I, and I2 values for substituted furans and selenophenes are plotted against the corresponding values for substituted thiophenes. On the contrary, slopes differing... 

The heteroaromatic radical cations furan, pyrrole, thiophene represent a beautiful series of molecules to illustrate this dependence [42,43]. Their two lowest electronic states exhibit a characteristic change in their vertical ionization potentials such that their difference substantially decreases in the series. As a consequence also the minimum energy of the resulting Coin decreases, not only absolutely but also in relation to the upper adiabatic PES, see Fig. 6.5 for a schematic drawing. For furan the Coin occurs well above ( 0.5 eV) the upper state minimum, for pyrrole and thiophene it is very close (within 0.01 and 0.04 eV). Thus the IC process in the latter cases is expected to compete with the vibrational motion, while in the former it should be slower. This is nicely confirmed by the electronic populations obtained from ab initio-based WP propagations and displayed in Fig. 6.6. A typical stretching... 

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