Ionization potential trimethylamine

As expected, DABCO is a much stronger base than triethylamine, has two ionization potentials separated by several electron-volts, and is more easily oxidized than trimethylamine. What is unusual about DABCO is that the in-phase combination of the nonbonded pairs is the HOMO, the lowest IP is lower than that of trimethylamine, and the next higher IP is higher than that of trimethylamine. Explain. 

Ionization potential of Continued) butenone, 123 cyclic diacetylenes, 305 cyclohexene, 48, 102 cis-cyclooctene, 102 Zraus -cyclooctene, 102 DABCO, 81 dimethyl ether, 123 ethylene, 80, 319 formaldehyde, 123, 319 hydrogen atom, 55, 75 methanol, 123 methyl acetate, 123 methyl acrylate, 123 nitrous oxide (N2O), 172 norbornadiene, 48 norbornene, 48 oxetane, 123 tetrahydrofuran, 123 trimethylamine, 81 water, 123... 

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