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

John R. Lombardi, W. Rlemperer, M. B. Robin, Harold Basch, N. A. Kuebler

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46 Scopus citations


A rotational analysis of the (0,0) component of the n→π *(1A11B2) 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 13B1) oscillator strength of the order of magnitude observed for the second origin.

Original languageEnglish
Pages (from-to)33-44
Number of pages12
JournalJournal of Chemical Physics
Issue number50
StatePublished - 1969
Externally publishedYes


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