Manifestation of Fermi resonance in the vibrational spectra of 5-ioduracil, and its deutero-derivatives isolated in low-temperature Ar matrices

Abstract

The FTIR spectra of isolated 5-ioduracil (5IU) molecules, and its deutero-derivatives, were obtained in low-temperature (T = 8 K) argon matrices in the frequency range of 3800–200 cm−1, with a resolution of 0.3 cm−1. The tautomer population is estimated at the evaporation temperature 395 K, using DFT (functionals B3LYP and B2PLYP) and MP2 quantum-mechanical methods. It is found that the main keto tautomer population in 5IU is above 99.8%. 5IU vibrational spectra are calculated using the DFT/B3LYP method and 6-311++G(df,pd) basis, and the calculated frequencies are corrected using a second order polynomial. It is found that in the region of vCO stretching vibrations of 5IU molecules, a triple Fermi resonance is manifested between the fundamental vC4O mode, and the combinational vibrations with frequencies 1745.3 and 1722.5 cm−1. Six combination modes in the region of ring stretching vibrations, and in-plane deformational vibrations, of 5IU are found. This classification of the vibrational spectrum bands of 1,3 dideutero-5-ioduracil (D5IU), demonstrates the presence of six combination D5IU modes, in the entire spectral region. It is established that the Fermi resonance causes a split of vND stretching vibrations in pyrimidine base spectra.