Polarized Single-Crystal Absorption Spectrum of 1-Methyluracil

Abstract

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 $\mathrm{n\hspace{0.17em}\to \hspace{0.17em}\pi *}$ transition at 2640 Å. The longest wavelength $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ 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 $\mathrm{n,\; \pi *}$ state. The transition moment for the lowest $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ 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 $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ transition is in qualitative agreement with dipole–dipole calculations for weakly coupled chromophores. The angular dependence of absorption of plane‐polarized light is investigated.