Influence of hydroxyl substitution on benzyne properties. Quantum chemicalcharacterization of the didehydrophenols

Geometries and singlet-triplet splittings for the 10 geometrical isomers of didehydrophenol are characterized at a variety of levels of electronic str ucture theory. The influence of the hydroxyl group is primarily to increase /decrease the weight of zwitterionic singlet mesomers that place positive/n egative charge adjacent to oxygen in valence bond descriptions of the aryne s. For some of the meta isomers, this interaction stabilizes distortion in the direction of a bicyclic geometry. The net effect, relative to the unsub stituted benzynes, is to increase the singlet-triplet splittings in 2,3-, 2 ,6-, and 3,5-didehydrophenol and to decrease that splitting in 2,4- and 2,5 -didehydrophenol (3,4-didehydrophenol is essentially unaffected). As shown for other arynes, the singlet-triplet splittings can also be accurately est imated by correlation with proton hyperfine coupling constants in anteceden t monoradicals, these values being accessible from very economical calculat ions.