Photoinduced formation and cleavage of hydrogen bonds

The role of hydrogen bonding in the formation of various types of associates, intermolecular H-bonded networks, the secondary structure of proteins and peptides, stabilization of specific molecular forms, molecular recognition, thedesign of colorimetric anion sensors, etc., cannot be overestimated. In the present review, the formation of intra- and intermolecular hydrogen bonds in the recently synthesized hetarylmethylidene cycloalkanones and their influence on stabilization of otherwise unstable isomers and tautomers is considered on the basis of experimental (NMR, X-ray, FT-IR, UV spectroscopy) and theoretical (DFT and MP2 calculations, QTAIM and NBO analysis). The accent is made on the relationship between the intra- and intermolecular hydrogen bonds, Z, E-isomerization about the exocyclic double bond leading to photoinduced formation of H-bonded isomers (which cannot be realized in their counterparts), self-association leading to stabilization of unusual structures (like, e.g., 2H-indazoles), stabilization of enols by intramolecular hydrogen bonds andring-chain tautomerism. Special focus is made on the bifurcate hydrogen bonds (whose existence in solution was disputable for a long time) with bifurcation on either hydrogen or heteroatom and having either two intramolecular or two intermolecular components, or one intra- and one intermolecular component. The compounds under consideration provide a unique possibility to investigate all these issues both in the solid state (X-ray, IR), in solution (NMR, IR, UV) and in gas phase (theoretical calculations). They also revealed new and unexpected phenomena, like, e.g., stabilization of polymorphs not only in crystal but also in solution due to hydrogen bonding of either intra- or intermolecular type. The cited literature covers mainly studies of the last decade including 2019.