On the electronic spectrum of protonated adenine: A single crystal study of adenine hydrochloride

The polarized reflection spectra of four faces of adenine·HCl single crystals have been measured between 6000 and 1830 Å. The faces which were examined included the naturally occurring (100), (001), and (102̄) planes plus the artificially prepared (104̄) face. Absorption spectra are derived from the corresponding reflectance curves via the Kramers-Kronig transformation. Clear evidence for the presence of four transitions which are polarized in the molecular plane and are presumably π → π* in origin is observed. The effects of intermolecular interactions are considered within the dipole approximation, and a procedure is given for extracting free molecule transition moments from the crystal spectra. This treatment leads to a satisfactory accounting of the observed dichroic ratios and the observed k̂ dependence of the factor group splittings and band shapes. Transition moment directions (measured contraclockwise from the C4–C5 axis) and oscillator strengths are evaluated to be as follows: I (λ ≃ 2730 Å)−28°, f=0.08; II (λ ≃ 2570 Å) + 100°, f=0.20; III (λ ≃ 2060 Å)+15°, f=0.16; IV (λ ≃ 1960 Å)+120°, f=0.51. No evidence for perpendicularly polarized n→ π* transitions was found.