Photochemistry of N-heterocycles. Part 3. Synthesis and photochemistry of some new dihydro-1,2,4-triazines and their quaternary salts

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.…”

“…Dihydro-1,2,4-triazines are well known experimentally and their derivatives have found a broad range of applications in organic chemistry and life sciences [30][31][32]. Their structures have been studied and characterized by 1 H-NMR- [33] and UV-spectroscopy [34] as well as via preparative considerations [33][34][35][36]. 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]. 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].…”

“…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]. 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].…”

“…increased Boltzmann factor WB) in the equilibrium of the tautomers. Based on UV spectroscopic measurements in ethanol and acetic acid, Nagy et al [36,37] have assumed a tautomeric equilibrium between predominantly isomer 8 and a small amount of isomer 9. This result is…”