| Abstract
| - The characteristics of the intramolecular hydrogen bond (IMHB) for a series of 40 different enols of β-diketonesand their nitrogen counterparts have been systematically analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In some cases, two tautomers may exist which are interconnected by a hydrogenshift through the IMHB. In tautomer a the HB donor group (YH) is attached to the six-membered ring, whilein tautomer b the HB acceptor (X) is the one that is attached to the six-membered ring. We found thatchanging an O to a N favors the a tautomer when the atom is endo and the contrary when it is exo, while thepresence of a double bond favors the a tautomers. As expected, the OH group behaves as a better HB donorthan the NH2 group and the CNH group as a better HB acceptor than the CO group, although the firsteffect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH,CNH] > [OH, CO] > [NH2, CNH] > [NH2, CO]. For all those compounds in which the functionalityexhibiting the IMHB is unsaturated (I-type), the IMHB is much stronger than in their saturated counterparts(II-type). However, when the systems of the II-type subset, which are saturated, are constrained to have theHB donor and the HB acceptor lying in the same plane and at the same distance as in the correspondingunsaturated analogue, the IMHB is of similar or even larger strength. Hence, we conclude that, at least forthis series of unsaturated compounds, the resonance-assisted hydrogen bond effect is not the primary reasonbehind the strength of their IMHBs, which is simply a consequence of the structure of the σ-skeleton of thesystem that keeps the HB donor and the HB acceptor coplanar and closer to each other.
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