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

Abstract

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.

COSMO Implementation in TURBOMOLE: Extension of an efficient quantum chemical code towards liquid systems

“…Regarding the cations, 4,5-dihydro-1,2,4-triazin-2-ium 13 is found to be the most stable isomer in vacuo, in agreement with the results of Nagy et al [37] and experimental findings [34,36]. Upon the application of COSMO, the relative energies do not change much compared to the gas phase results.…”

“…If a substituent is present at the N-4 atom, the most stable species is the 4,5-dihydro isomer [34], whereas the 1,6-dihydro isomer is preferred if a substituent is present at N-1 [36]. Thus, for the unsubstituted dihydro-1,2,4-triazine a tautomeric equilibrium between mainly the 2,5-dihydro-(8), 4,5-dihydro (9) and 1,6-dihydro (4) isomers can be expected [37]. Only the unsubstituted dihydro-1,2,4-triazines and a selection of dihydro-1,2,4-triazinium cations are considered in our calculations, in order to allow for an unambiguous comparison with the gas phase stabilities of these molecules calculated by Nagy et al [37].…”

“…However, the experimental work on these compounds left some uncertainty about their detailed geometric and electronic structure. Recently, a theoretical study on the relative stabilities of the dihydro-1,2,4-triazines and the dihydro-1,2,4-triazinium cations based on density functional theory and high level ab-initio quantum chemical calculations, has been published [37]. So far, all of the theoretical calculations done on these molecules have been restricted to the gas phase, although the dihydro isomer 8 [34,36].…”

“…Thus, for the unsubstituted dihydro-1,2,4-triazine a tautomeric equilibrium between mainly the 2,5-dihydro-(8), 4,5-dihydro (9) and 1,6-dihydro (4) isomers can be expected [37]. Only the unsubstituted dihydro-1,2,4-triazines and a selection of dihydro-1,2,4-triazinium cations are considered in our calculations, in order to allow for an unambiguous comparison with the gas phase stabilities of these molecules calculated by Nagy et al [37]. Table 2 shows the optimized gas phase and COSMO energies for the nine dihydro- From the COSMO calculations, 9 is second lowest in energy, which is consistent with the experimental UV spectroscopic results mentioned above.…”

Six‐Membered Heterocycles: Triazines, Tetrazines and Other Polyaza Systems

“…The quantum chemical calculations support that the most stable isomer is the 2,5-dihydro-1,2,4-triazine [148].…”