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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 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 (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 the best calculation placing the (1)A(2) (a(1) --> a(2)) state lowest. This is consistent with the spectral analysis. The calculations also have 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 of 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).

electronic spectroscopy ; 1, 2, 3-triazine

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