The electronic spectroscopy of 1,2,3-triazine Original Research Article

The electronic absorption spectrum of 1,2,3-triazine has been measured in the vapour and solution. Excited state geometries and vibrational frequencies have been calculated at CIS/6-31G(d) and CASSCF(6,6)/6-31G(d) levels of ab initio theory. Adiabatic and vertical transition energies have been calculated at CASSCF(6,6)/DZP, CASSCF(6,12)/DZP and CASPT2/DZP levels for optimised geometries determined at the CASSCF/6-31G(d) level, and at the MP2/6-311G(d,p) ground-state respectively. The calculations all show the first three excited singlet states, comprising 1A2 (largely a1 → a2 electron promotion), 1A2 (largely b2 → b1), and 1B1 states, to be energetically close, with the best calculation placing the 1A2 (a1 → a2) state lowest. This is consistent with the spectral analysis. The calculations also have the lowest π-π∗ state as 1B2. The very weak system with origin near 400 nm is assigned to a 1A2 state. It lies on the red edge of a stronger allowed system, assigned 1B1, with absorption maximum at 290 nm. A third system seen at 230 nm is assigned 1B2.