Polarized Absorption Spectra of Purines and Pyrimidines

Abstract

Polarized absorption spectra in the molecular planes for thin sections (0.1 μ and up) of single crystals of 1‐methylthymine, 9‐methyladenine, and their 1:1 hydrogen‐bonded complex (the AT dimer) have been obtained. For 1‐methylthymine, the first absorption band (λmax≈275 mμ) is polarized close to the N1–C4 axis, and the second uv band (λmax<230 mμ) is polarized approximately perpendicular to the first. For 9‐methyladenine, the crystal structure is not known but the data for the AT dimer can be interpreted by assuming that the intense long‐wavelength band [λmax≈275 is short‐axis (C6–C5) polarized]. There is a weak band with λmax≈255 which is then long‐axis polarized, and probably the second strong band (λmax<230) is also long‐axis polarized. All of the crystal spectra, and especially the AT dimer, are hypochromic with respect to the solution spectra. The 9‐methyladenine spectrum is markedly red shifted in the crystals. The intensity changes and spectra shifts probably cannot be explained by a dipole—dipole excitoninteraction. The spectrum of the AT dimer perpendicular to the molecular planes sets in at about 300 mμ and is still rising at 230 mμ; it is attributed to n—π* transitions.