Photoinduced Intramolecular Bifurcate Hydrogen Bond: Unusual Mutual Influence of the Components

A series of 7-hydroxy-2-methylidene-2,3-dihydro-1H-inden-1-ones with 2-pyrrolyl (3), 4-dimethylaminophenyl
(4), 4-nitrophenyl (5), and carboxyl group
(6) as substituents at the exocyclic double bond was
synthesized in the form of the E-isomers (4–6) or predominantly as the Z-isomer (3) which in solution is converted to the E-isomer. The synthesized
compounds and their model analogues were studied by NMR spectroscopy,
X-ray analysis, and MP2 theoretical calculations. The E-isomers having
intramolecular O–H···OC hydrogen bond
are converted by UV irradiation to the Z-isomers having bifurcated
O–H···O···H–X hydrogen
bond. Unexpected shortening (and, thus, strengthening) of the O–H···OC
component of the bifurcated hydrogen bond upon the formation of the
CO···H–X hydrogen bond was found experimentally,
proved theoretically (MP2), and explained by a roundabout interaction
of the H-donor (HX) and H-acceptor (CO) via the system of
conjugated bonds.