Energies and Potential Surfaces of the Excited Electronic States of the Triazines

Triazines are aromatic heterocycles with three
adjacent nitrogens in the one ring. In addition to
the typical electronic transitions of the parent hydrocarbon benzene, there are also electronic transitions arising from the presence of the nitrogens. All these electronic transitions occur in a narrow wavelength range giving rise to the possibility of interactions among the various transitions. The electronic spectroscopy of these azines is studied both experimentally and by ab initio molecular orbital calculations. The theoretical calculations allow the vibrational frequencies and molecular structures in the excited states to be determined. In turn the calculations can be used to predict the vibrational structure in the electronic spectrum. The work on the triazines has been extended to other members of the aza-aromatics, particularly to those molecules that have biological and environmental importance. Thus, calculations are currently being undertaken on some of the symmetrically substituted diaza-phenanthrenes.
   

Principal Investigator

Gad Fischer

Department of Chemistry

The Faculties

     

Co-Investigators

     

Xiaolin Cao

Department of Chemistry,

The Faculties

     

Projects

u01 - VPP, PC

     
         
               

     
               

What are the results to date and the future of this work?

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 pi-pi* 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. Calculations are currently being undertaken on related aza-aromatic molecules, in particular the diaza substituted symmetrical phenanthrenes. Work that has been completed on the the diaza substituted benzenes, pyrazine, pyridazine and pyrimidine is being prepared for publication.

               
- Appendix A

 
               
       

What computational techniques are used?

Both the GAUSSIAN and MOLPRO packages have been used. The CIS/6-31G* calculations were performed using Gaussian 94, and most of the CASSCF calculations with MOLPRO. The CIS computations give excited state optimized geometries and potential surfaces (vibrational frequencies), while the CASSCF can give optimized geometries, and in some cases vibrational frequencies.

Publications

G. Fischer, D. M. Smith, A.U. Nwankwoala, The electronic spectroscopy of 1,2,3-triazine, Chemical Physics, 221, 11-21 (1997).

       
Appendix A -