FTIR and Raman Spectra Compared with Ab Initio Calculated Frequency Modes for 5-Aminouracil

Infrared (IR) and Raman spectra of 5-aminouracil were recorded in the region 200–4,000 cm^− 1. Assuming under the C_s point group that the distribution of the normal mode of vibrations between the two species are planar (a′) and non-planar (a″), given by 25a′ + 11a″ of which 30 modes (21a′ + 9a″) correspond to the uracil moiety and six modes (4a′ + 2a″) correspond to the NH₂ group, with a comparison of theoretically ab initio calculated frequencies, the results are in reasonably good agreement with the experimental IR and Raman spectra. Consistent assignments have been made for the internal modes of the NH₂ group, especially for the anti-symmetric NH₂ stretching and bending modes. The non-equivalence of the two NH bonds of the NH₂ group suggests a difference in the strength of the two hydrogen bonds on the pyrimidine ring. Symmetry and anti-symmetry NH stretching modes of the NH₂ group show the invalidity of the empirical relationship. These two NH₂ stretching frequencies are distinctly separated from the CH/NH ring stretching frequencies. A strong and sharp IR band that acts at 3,380 cm^− 1 could be identified as the anti-symmetric NH₂ mode whereas the band at 3,290 cm^− 1 smaller density could be identified as the symmetric NH₂ stretching mode.