Nucleic acid bases studied by matrix isolation vibrational spectroscopy: Uracil and deuterated uracils

Abstract

A vibrational assignment of the isolated uracil molecule is presented, based on i.r. and Raman spectra of uracil trapped in argon and nitrogen matrices and i.r. spectra of seven deuterated derivatives of uracil isolated in argon matrices. The observed spectra agree well with previous quantum mechanical calculations. The relationships observed between the monomer bands, bands due to associated uracil in argon matrices and the spectrum of the pure solid lead to a revised vibrational assignment for uracil in the solid phase. As well as large shifts in the NH stretching and out-of-plane bending modes between the monomer and the hydrogen-bonded solid, many other bands show appreciable shifts as a consequence of the extensive mixing of the NH in-plane bending modes with the CH in-plane bending and ring stretching modes. The spectrum of the associated uracil species in matrices suggests that it is a cyclic dimer, linked by C 2O … HN 1 hydrogen bonds.