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

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Infrared (IR) and Raman spectra of 5-aminouracil were recorded in the region 200–4,000 cm−1. Assuming under the Cs 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.

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