Ionization energies of triazines and tetrazines. Application of Green's function method coupled with semiempirical molecular orbital calculations

Abstract

The ionization energies of the valence electrons of eighteen 1,2,4,5-tetrazines, six 1,2,3-triazines and seven 1,2,4-triazines derivatives have been calculated with the Outer Valence Green Function (OVGF) technique, using as the zeroth order approximation the wavefunctions obtained with semiempirical AM1 and PM3 approximations. In many cases the SCF(AM1), SCF(PM3), HAM/3 and OVGF(AM1) methods fail to reproduce correctly the ordering of the ionization energies for the azines under consideration. On the other hand, the OVGF(PM3) method gives quite satisfactory quantitative agreement with the experimental ionization potentials and for dimethoxy-s-tetrazine, it is of comparable accuracy to ADC(3)ab initio calculations with a polarized basis set. OVGF(PM3) calculations predict reliably the effect of substituents on the ionization potentials of azines, confirming previous suggestions that strong electron-donating groups lead to inversion of the ordering of the two highest orbitals, relative to their ordering in the parent s-tetrazine. In 1,2,3-triazine and 1,2,4-triazine methyl substitution does not change the ordering of the ionization energies relative to the corresponding parent molecules.