The crystal and molecular structure of seven oxaphosphinin-4-one (OxP) and three chroman (ChD) derivatives have been analyzed by single X-ray diffraction analysis. Benzo-g-pyrone chromone derivatives are another class of biologically important compounds. On the other hand chromone and coumarin derivatives containing phosphorus create a novel group of compounds with biological and alkylating properties. Their in vitro alkylating properties were confirmed in Preussmann test with 4-(4-nitrobenzyl)pyridine (NBP). What is more these compounds exhibit strong cytotoxic effects and also some anticancer activity in vivo. For these reasons X-ray studies were undertaken.

The intramolecular N-H…O resonance assisted hydrogen bond closing extra six-membered ring is observed in all investigated compound. The strongest p-delocalization effect influencing the strength of resonance assisted hydrogen bond seems to exist in ChD1 structure. This observation was confirmed by SP calculations performed at B3LYP/6-311++G(d,p) level. The topological 'atoms in molecules' (AIM) analysis was also applied. The oxaphosphinane rings adopt half-chair or twist-boat conformations except of OxP2 structure. The P atom connections possess tetrahedral geometry slightly distorted towards trigonal pyramidal with elongated P-C bond. This is typical for such derivatives with one double bond.

The tautomerism of the studied compounds was also examined. The DFT calculations have been performed at B3LYP/6-311++G** level of theory for similar model species. The proton transfer process N-H…O↔N…H-O for some model systems have been studied. The geometrical, energetic and topological parameters were analyzed. Numerous interrelations between them were found. The substituent effect for this class of compounds was also studied.

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