Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 2A / Section B / 23 / c / Solution

Use negative bonding Hückel resonance integrals, ${\beta }_{OO}=-2.0$ and ${\beta }_{CC}=-2.5\mathrm{eV}$. The oxygen HOMO is pi-antibonding, whereas the ethene HOMO is pi-bonding. The frozen-orbital Koopmans estimate therefore givesConsequently ${\alpha }_O\simeq -14.1\mathrm{eV}$ and ${\alpha }_C\simeq -8.0\mathrm{eV}$. These Hückel Coulomb parameters are effective parameters of this spectroscopically calibrated model. Real ionization energies include relaxation and correlation, so they are not exact matrix elements of an isolated-atom Hamiltonian.