The characteristics of the intramolecular h
ydrogen bond (IMHB) for a series of 40 different enols of
![](/images/gifchars/beta2.gif)
-diketonesand their nitrogen counterparts have been s
ystematicall
y anal
yzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theor
y. In some cases, two tautomers ma
y exist which are interconnected b
y a h
ydrogenshift 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 contrar
y 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 NH
2 group and the C=NH group as a better HB acceptor than the C=O group, although the firsteffect clearl
y dominates. Accordingl
y, the expected IMHB strength follows the [donor, acceptor] trend: [OH,C=NH] > [OH, C=O] > [NH
2, C=NH] > [NH
2, C=O]. For all those compounds in which the functionalit
yexhibiting the IMHB is unsaturated (
I-t
ype), the IMHB is much stronger than in their saturated counterparts(
II-t
ype). However, when the s
ystems of the
II-t
ype subset, which are saturated, are constrained to have theHB donor and the HB acceptor l
ying 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 h
ydrogen bond effect is not the primar
y reasonbehind the strength of their IMHBs, which is simpl
y a consequence of the structure of the
![](/images/gifchars/sigma.gif)
-skeleton of thes
ystem that keeps the HB donor and the HB acceptor coplanar and closer to each other.