Journal of Molecular Structure (Theochem) , 219 (1993) 249-266 Elsevier Science Publishers B.V., Amsterdam 249 Multiconfiguration response theory calculations of singlet and triplet spectra of the azabenzenes S&en KnuW, Olav Vahtrasb and Hans Wgrenb aDepartment of Quantum Chemistry, University of Uppsala, P.O.B. 518, S-75120 Uppsala (Sweden) bInstitute of Physics and Measurement Technology, University of Linkiiping, S-58183 Linkiiping (Sweden) (Received 21 May 1992) Abstract Multiconfigurational linear response theory calculations of singlet and triplet excitations of the azabenzenes are presented and used for assignment of their electron energy loss and UV/optical spectra up to the region of the first ionization potentials at approximately 10eV. Particular attention is given to the role of n-n* versus X-Z* transitions, and to the onset of triplet versus singlet states in the low energy part of the spectra. Calculations test dependences on geometry, one-particle basis set, reference wavefunction with Hartree-Fock, complete and restricted active spaces, and the fulfilment of gauge invariance and sum rules. Introduction The proper characterization of optical and UV excitation spectra of aromatic compounds is an important goal for quantum chemical calculations. Like other substituted benzenes the azabenzenes form a bridge between small and large systems that can be subject to accurate quantum chemical cal- culations and at the same time can be used as subunits in molecular modelling of extended species, polymers or species of interest for mole- cular electronic devices. The character of the ring 7~ - X* and n - X* transitions are little per- turbed by substitution and can possibly be used as fingerprints in larger compounds, with aza- benzenes as building blocks. Azabenzenes are all isoelectronic with benzene and have similar elec- tronic and conformational structures. The replace- ment of CH groups with nitrogen atoms, the intro- duction of lone pair to X* transitions and the Correspondence to: H. Agren, Institute of Physics and Measurement Technology, University of LinkZjping, S-58183 Linkiiping, Sweden. successive lowering of benzene symmetry should thus cause small but systematic trends in the spectra. Apart from the general considerations given above the choice of azabenzene spectra as the objects of the present investigation is motivated by our earlier study of solvatochromatic shifts in these compounds. In that work the effect of solvation on different types of excitation like n - n* (blue shift) and 7~ - X* transitions (weakly shifted red) was studied by separate state calculations of the spectral transitions. However, although there has been considerable interest in azabenzenes, reflected in the fairly large number of calculations mostly of the semiempirical type, the assignment of their spectra in terms of symmetries and molecular orbitals is in some cases still uncertain. Of par- ticular interest is the positioning of low lying triplets and the number of singlet-triplet tran- sitions preceding the first intense singlet-singlet transition [ 11. Mostly state-specific methods have been applied to the azabenzene spectra when considering 0166-1280/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.