Intramolecular H-bonds in LLM-105 and its derivatives: a DFT study

DFT calculations on molecules with intramolecular hydrogen bonds have been performed at B3LYP/6-31G** level of theory. The investigated 2,6-dinitro-3,5-diaminopyrazine-1-oxide (LLM-105) and its derivatives contain two types of intramolecular H-bonds and thus two types of intramolecular H-bonded rings. The results of calculations show that the N-H⋯O (to the ring) contacts are the classical, red shift H-bonds, whereas the N-H⋯O (in nitro group) H-bonds are the improper, blue shift ones. The electron density transfer mechanisms on the basis of natural bond orbital (NBO) analysis have been suggested.