THE ELECTRONIC SPECTROSCOPY OF 1,2,3-TRIAZINE

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 ener gies have been calculated at CASSCF(6,6)/DZP, CASSCF(6,12)/DZP and CAS PT2/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 c alculations all show the first three excited singlet states, comprisin g (1)A(2) (largely a(1) --> a(2) electron promotion), (1)A(2) (largely b(2) --> b(1)), and B-1(1) states, to be energetically close, with th e best calculation placing the (1)A(2) (a(1) --> a(2)) state lowest. T his is consistent with the spectral analysis. The calculations also ha ve the lowest pi-pi state as B-1(2). The very weak system with origin near 400 nm is assigned to a (1)A(2) state. It lies on the red edge o f a stronger allowed system, assigned B-1(1), with absorption maximum at 290 nm. A third system seen at 230 nm is assigned B-1(2). (C) 1997 Elsevier Science B.V.