Bonding in tropolone, 2-aminotropone, and aminotroponimine: no evidence of resonance-assisted hydrogen-bond effects.

The properties of the intramolecular hydrogen bond (IMHB) in tropolone, aminotropone, and aminotroponimine have been compared with those in the corresponding saturated analogues at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In general, all those compounds in which the seven-membered ring is unsaturated exhibit a stronger IMHB than their saturated counterparts. Nevertheless, this enhanced strength is not primarily due to resonance-assisted hydrogen-bond effects, but to the much higher intrinsic basicity and acidity of the hydrogen-bond acceptor and donor groups, respectively, in the unsaturated compounds. These acidity and basicity enhancements have a double origin: 1) the unsaturated nature of the moiety to which the hydrogen-bond donor and acceptor are attached and 2) the cyclic nature of the compounds under scrutiny. As has been found for hydroxymethylene and aminomethylene cyclobutanones, and cyclobutenones and their nitrogen-containing analogues, the IMHB strength follows the [donor, acceptor] trend: [OH, C=NH]>[OH, C=O]>[NH(2), C=NH]>[NH(2), C=O] and fulfills a Steiner-Limbach correlation similar to that followed by intermolecular hydrogen bonds.