Relative Stability and Structure of Dihydro-1,2,4-triazines: A Theoretical Study.

A brief survey is given on the synthesis and structure elucidation for dihydro-1,2,4-triazines. The relative stability of nine possible dihydro-1,2,4-triazines and three dihydrotriazinium cations is studied at HF, MP2, generalized gradient approximation DFT, and CBS-4 levels of theory. The structural consequences of the inclusion of the electron correlation are also given. We attempt to rationalize the experimental findings using high-quality theoretical results. The quantum chemical calculations support that the most stable isomer is the 2,5-dihydro-1,2,4-triazine and all the other relatively stable isomers have been experimentally identified correctly. Several experimental papers report structures that have been proved to be nonexistent. These structures have energy that is too high according to the best-quality calculations.