Ab initio calculations of molecular orbital energies

As already mentioned, by the use of Koopmans theorem319 the negative of the Hartree-Fock MO energies may be equated to IP values. We present MO orbital 

Structural formulae and names of compounds for which ionization potentials are presented in Table 18 

The 7T-HOMO of ethene lies at —0.379 hartree (—10.32 eV) and that of propene at — 0.357 hartree ( — 9.71 eV), which are 0.2 to 0.3 eV less than the experimental values collected in Table 18. For planar NH3 the lone-pair HOMO lies at —0.380 hartree, which is lowered by pyramidalization to —0.418 hartree (—11.38 eV). 

The interaction of ethene with planar ammonia leads to the MOs of coplanar vinylamine (115) in the form 116. The HOMO n2 (with N lone-pair and C=C -character) is now at —0.298 hartree ( —8.10eV) and the splitting between nx and n2 has a maximum value of 5.20 eV. On torsion of the planar amino group to the perpendicular conformation 119 this splitting decreases to 0.18 eV. All three values differ strongly from experimental data presented in Table 18, but they refer to variously substituted tertiary enamines, so that from calculations of 115 we only may detect trends. 

FIGURE 7. HF-6-31G calculated MO energies for vinylamine (115) in several conformations 116 to 121 and those of related molecules (in hartree = 27.2117 eV) 

---