Optical spectra of small rings. I. The n→π^* transition of difluorodiazirine

A rotational analysis of the (0,0) component of the n→π ^*(¹A₁→¹B₂) excitation of difluorodiazirine has been performed and demonstrates that in the upper state, the N=N distance increases by 0.060± 0.005 Å, whereas the F-F distance decreases by 0.034 Å. Comparison of the (0,0) Stark spectrum with the computer simulation of that part of the spectrum which behaves like a symmetric rotor yields an upperstate dipole moment of 1.5±0.2 D, presuming the ground-state dipole moment of difluorodiazirine is zero. The vibrational structure of the n→π^* band is assigned as consisting of a long progression in v₁', the N=N stretch, together with only a few quanta of v₄', the F-C-F angle bend, and hot bands assignable as either v₂→v₂" or v₅→ v₅". An excellent fit to the relative intensities of the eight members of the v₁ progression is obtained using Smith's one-parameter theory of the vibronic band shape. An apparent second origin is also observed, and is tentatively assigned as absorption to the v→π^* triplet state ³B₁. Using Gaussian-type orbitals, the ground- and n→π^* excited-state dipole moments of difluorodiazirine were computed to be 0.082 and 1.964 D, respectively. While the predicted n→π^* excitation energy is in excellent agreement with that observed, the predicted dipole velocity oscillator strength is too large by a factor of 10. Analysis of the MO's involved in the transition shows that n is nearly equally distributed among the C-N-N atoms of the ring, but that π^* is completely localized on the N atoms. Consequently, the n→π^* excitation involves the transfer of about electron from the CF₂ group to the N=N group of difluorodiazirine. The spin-orbit coupling between the n→π ^* singlet and π→π^* triplet states is computed to result in a π→π^* (¹A ₁→³B₁) oscillator strength of the order of magnitude observed for the second origin.