= 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 gives
$$
\alpha_O-\beta_{OO}\simeq-12.1,\qquad
\alpha_C+\beta_{CC}\simeq-10.5.
$$
Consequently \b[$\alpha_O\simeq-14.1\ \mathrm{eV}$ 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>.
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