Protonation Thermochemistry of Gaseous 2 , 2 ' - , 4 , 4 ' - and 2 , 4 ' - Bipyridines and 1 , 10 - phenanthroline †

Quantum chemical composite methods G3MP2B3, G3B3, G4MP2 and G4 have been used to calculate enthalpies of formation and gas phase basicities of the title compounds. Comparison of the results with the available experimental heats of formation values reveals correct agreement (to within ≈ 2 kJ/mol) with G3B3 and G4 methods. Systematic errors on the heats of formation of these aromatic molecules are detected when calculated using the G3MP2B3 and G4MP2 methods. Using G3B3 and G4 atomization energies, ΔfH°298 of 2,2’-bipyridine, 1, 1,10-phenanthroline, 2, 4,4’-bipyridine, 3, and 2,4’bipyridine, 4, equal to 269, 316, 287 and 282 kJ/mol, respectively were obtained. Homodesmotic reactions confirm these ΔfH° estimates for the three isomeric bipyridines 1, 3 and 4. G3MP2B3, G3B3, G4MP2 and G4 methods lead to comparable proton affinities (PA) values for the four molecules 1−4, in particular because of error compensation in the case of G3MP2B3 and G4MP2 results. Excellent agreement is found with experimental PA values of reference nitrogen bases (within less than 1.2 kJ/mol) allowing us to safely predict PA values of 974, 999, 933 and 958 kJ/mol for 2,2’-bipyridine, 1, 1,10-phenanthroline, 2, 4,4’bipyridine, 3, and 2,4’-bipyridine (protonated at the most basic site N(4')), 4, respectively. Estimate of the corresponding gas phase basicities is also proposed after consideration of the entropy of hindered rotations: GB(1) = 943, GB(2) = 966, GB(3) = 900, GB(4) = 927 kJ/mol.