Steric and aromatic impact on intramolecular hydrogen bonds in o ‐hydroxyaryl ketones and ketimines

X‐ray measurements at 100 K and quantum‐mechanical calculations showed a domination of the enol (OH) form in o ‐hydroxyaryl ketones [6‐methoxy‐2‐hydroxyacetophenone ( 6OMeK ), 4‐methoxy‐2‐hydroxyacetophenone ( 4OMeK ), 5‐chloro‐4‐methyl‐2‐hydroxybenzophenone ( 5Cl4MeK ) and 2‐hydroxyacetonaphthone ( o ‐HAN )], whereas a prevailing proton‐transfer (NH) form was found in o ‐hydroxyacetonaphthylimine [2‐( N ‐methyl‐α‐iminoethyl)naphthol ( o ‐HIN )]. The effective mechanism of the reduction in hydrogen bridge length due to steric repulsion in the o ‐hydroxyaryl ketones is presented. The following phenomena were found: a decrease in phenol ring aromaticity caused by the proton‐transfer process, dependence of the HOMA index of aromaticity on the HOSE index defining the destabilization of the chelate ring and a dependence of the HOMA index on the lengths of the carbonyl bonds. Moreover, a correlation between phenol ring aromaticity [HOMA(phenol)] and the pseudo‐aromatic ring [HOSE(chelate)] is shown. Correlations between structural parameters of the chelate ring [ d (C—O), d (C aryl —C alkyl )] and aromatic rings [HOMA(phenol)] of the o ‐hydroxyaryl ketimines and ketones are presented. Copyright © 2005 John Wiley & Sons, Ltd.