Spacer conformation in biologically active molecules. Part 1. Structure and conformational preferences of 2-substituted benzoxazoles

In the crystal of the 2:2 complex of 1,5,7-triazabicyclo[4.4.0]dec-5-ene with 5,5ʹ-dibromo2,2ʹ-biphenol a cooperative inter-intra-molecular hydrogen-bonded system is formed. The intermolecular hydrogen bonds are relatively long [2.708(6) and 2.895(7)] and do not show large proton polarizability. The intramolecular OHO- hydrogen bond is relatively short [2.489 (6)]. The structure of the complex is very well reflected in its FT-IR spectrum in the solid. On the basis of the FT-IR and 1H NMR studies of the complex in chloroform and in acetonitrile a structure different than in the solid is proposed. DFT structures of both isolated ions in their ground electronic states at the B3LYP/6-31++G(d,p) level are employed to follow the effects of the hydrogen bonding on the intrinsic ionic structures.