Theoretical evidence for the resonance-inhibited hydrogen bonding (RIHB) in enol-imine tautomers

The theory of resonance-inhibited hydrogen bonding (RIHB) is tested on some illustrative examples with enol-imine tautomerism. In contrast to the resonance-assisted hydrogen bonding (RAHB), theoretical descriptors show that here, π-resonance weakens the intramolecular hydrogen bond (IMHB) strength in the given structures. The results show that it is possible to tune the strength of RIHB by substitution in a suitable position in the molecule. Once a substituent hiders the original resonance in the RIHB system, it leads to reinforcement of IMHB and vice versa. These effects mostly are enhanced with increasing the magnitude of Hammet resonance constants. Interestingly, the IMHB becomes 0.08 A shorter, and thus quite stronger by using a powerful π-electron donating group like –O− on the basis of the RIHB theory. In general, RIHB and RAHB complement each other well, and provide a very good insight into the interplay between the resonance and hydrogen bonds.