Substituent effects on two-center three-electron bonds and hydrogen bonds involving unsaturated organic functional groups and an ammonia radical cation - The resonance contribution

A theoretical investigation of the substituent effects on the two-center, t hree-electron (2c-3e) bond involved between unsaturated functional groups a nd an amine nitrogen is presented. The competitive hydrogen-bonded complexe s are also studied. In both cases, the bond energies are found to be in the range of 20 - 30 kcal mol(-1). The variation of these energies is discusse d with respect to the electron-donating effect of the substitutents, as wel l as with respect to the Jc-bonded atom of the organic functional group (O, S, NH). For the 2c - 3e bonds, the results are discussed on the basis of t he differences of the ionization potential (Delta IP) of the separated frag ments and can be rationalized through the valence bond theory. For the hydr ogen bonds, the substituent influence is discussed by using the differences of the proton affinities (Delta PA) of the substrates. The resonating natu re of the hydrogen bond in these cationic systems is investigated and is fo und to account for most of the binding energy. Marcus theory is compared wi th the proposed resonating model.