Polarization Assignments of the Electronic Spectrum of Purine

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Abstract
Polarized reflection spectra of the (100) face of purine crystals have been measured to 185 mμ. The corresponding absorption spectra which are computed by a Kramers–Kronig transformation of the reflection spectra are analyzed so as to assign the transition moment directions of the electronic absorption bands. The analysis is made assuming the oriented-gas model and again for a case which treats intermolecular interactions. The polarization of the first absorption band (λmax = 294mμ) is out-of-plane as expected and is consistent with an n → π* assignment. The next band at 263 mμ is polarized in-plane (π → π*) at + 48° from the C4 − C5 axis (toward C6). The weak band at 250 mμ in the crystal is another perpendicular n → π* transition. The strong absorption in the 200-mμ region and below can be plausibly assigned as two transitions. One (at 200 mμ) is polarized in nearly the same direction as the 263-mμ band. The other band, at 190 mμ, can have reasonable intensity relative to the solution spectrum only if it is polarized out-of-plane. Some evidence for another weak, in-plane, long-axis band at about 230 mμ is found.

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