Polarization Assignments of the Electronic Spectrum of Purine

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.