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1 Polish Journal of Chemistry

1 1998

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Matrix isolation and DFT quantum mechanical studies of vibrational spectra of uracil and its methylated derivatives

Szczepaniak K., Person W.B., Leszczynski J., Kwiatkowski J. S.

Polish Journal of Chemistry

1998

Vol. 72, nr 2

402-420

The infrared spectra of uracil and its derivatives in the carbonyl region are strikingly complex, and extremely sensitive to substitution, particularly at ring nitrogen. In this work the infrared spectra of uracil, 1,3-di-deuterouracil, 1-methyluracil, 3-methyluracil and 1,3-dimethyluracil are studied experimentally by low temperature (12 K) matrix isolation infrared spectroscopic techniques, and theoretically by DFT/B3LYP/6-31G(d,p) quantum mechanical methods. Particular attention is focused on the carbonyl region and on the mechanical coupling of the C2=O and C4=O stretching vibrations with each other, with the N1H and N3H bending motions, and with other motions. It is shown that this coupling plays a crucial role in determining the frequencies and intersities of the normal modes that determine the spectral patterns in the infrared spectrum in the carbonyl region. The extreme sensitivity of the frequencies, intersities and spectral pattern in the carbonyl region to isotopic substitution and methylation (and to the intermolecular interactions, particularly hydrogen bonding) is attributed, to a large extent, to changes in this coupling. The DFT calculation appear to give quite accurate values for the force constants and allow separation of simple mass effects and chemical substituent effects on the coupling. Fermi resonance in the carbonyl region is an important factor contributing to the observed complexity of this spectral region. Examination of this effect making use of the visualization of normal modes of vibration provides rules for when it may be expected to be important.