Ionization energy heterocyclic molecules

The He(la) photoelectron spectra of the parent five-membered heteroaromatic molecules have undergone an exhaustive study. In the beginning, the assignment of the ionization energies to appropriate occupied molecular orbitals was confused by the unexpected reversal in the sequence of the two highest MOs in tellurophene relative to the other parent heterocycles. The... 

All the properties that can be calculated by ab initio methods can in principle also be calculated semiempirically, bearing in mind that the more the molecule of interest differs from the training set used to parameterize the SE program, the less reliable the results will be. Eor example, a program parameterized to predict the UV spectra of aromatic hydrocarbons may not give good predictions for the UV spectra of heterocyclic compounds. NMR spectra are usually calculated with ab initio (section 5.5.5) or density functional (chapter ) methods. UV spectra, and ionization energies (ionization potentials) and electron affinities will be discussed here. 

Figure 18. Is EELS spectra of C (top) and N (bottom) in a series of heterocyclic gas phase molecules (modified from Newbury et al. 1986). (a) pyrrole, (b) pyrrolidine, and (c) piperdine. The hatched lines in each figure are the ionization energies, determined using the XPS.

Interestingly, the flavin molecule is significantly easier to excite than the other heterocycles investigated. The first singlet excitation energy is only 3.0 eV (413 nm), and the probability of transition is intermediate. The second band with possible singlet excitation lies at 3.8 eV. Hence, if one intends to construct systems that are more readily excited, substituted flavins seem to be a more appropriate route than the furocoumarins, whereas the latter are easier to ionize. 

---