Polarized Single-Crystal Absorption Spectrum of 1-Methyluracil

The single-crystal absorption spectrum of 1-methyluracil, polarized along three mutually perpendicular axes, is investigated from 3200 to 2400 Å at room temperature. The spectral measurements were made with a microspectrophotometer, and crystal extinction coefficients were obtained from Lambert's law plots. The spectrum polarized normal to the molecular planes uncovers an n → π* transition at 2640 Å. The longest wavelength π → π* absorption band at 2755 Å exhibits well-resolved vibrational structure which broadens considerably on the high-energy side of the band. It is suggested that this broadening results from a perturbation by the underlying n, π* state. The transition moment for the lowest π → π* transition is found to be almost parallel to the N1–C4 molecular direction. The difference of about 23° between the transition moment directions for 1-methyluracil and 1-methylthymine agrees well with π-electron calculations. The exciton splitting of the 0, 0 band for the lowest π → π* transition is in qualitative agreement with dipole–dipole calculations for weakly coupled chromophores. The angular dependence of absorption of plane-polarized light is investigated.