Optical Spectra of Small Rings. I. The n → π Transition of Difluorodiazirine

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Abstract
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 ν1′, the N=N stretch, together with only a few quanta of ν4′, the F–C–F angle bend, and hot bands assignable as either ν3′ − ν3″ or ν5′ − ν5″. An excellent fit to the relative intensities of the eight members of the ν1′ 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 π → π* triplet state B13. 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 13 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* (1A1 → 3B1) oscillator strength of the order of magnitude observed for the second origin.

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