MATRIX-ISOLATION AND DFT QUANTUM-MECHANICAL STUDIES OF VIBRATIONAL-SPECTRA OF URACIL AND ITS METHYLATED DERIVATIVES

The infrared spectra of uracil and its derivatives in the carbonyl reg ion are strikingly complex, and extremely sensitive to substitution, p articularly at ring nitrogens. Tn this work the infrared spectra of ur acil, 1,3-di-deuterouracil, 1-methyluracil, 5-methyluracil and 1,3-dim ethyluracil are studied experimentally by low temperature (12 K) matri x isolation infrared spectroscopic techniques, and theoretically by DF T/B3LYP/B31G(d,p) quantum mechanical methods. Particular attention is focused on the carbonyl region and on the mechanical coupling of the C 2=O and C4=O stretching vibrations with each other, with the N1H and N 3H bending motions, and with other motions. It is shown that this coup ling plays a crucial role in determining the frequencies and intensiti es of the normal modes that determine the spectral patterns in the inf rared spectrum in the carbonyl region. The extreme sensitivity of the frequencies, intensities and spectral pattern in the carbonyl region t o isotopic substitution and methylation (and to the intermolecular int eractions, particularly hydrogen bonding) is attributed, to a large ex tent, to changes in this coupling. The DFT calculations 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 contrib uting to the observed complexity of this spectral region. Examination of this effect making use of the visualization of normal modes of vibr ation provides rules for when it may be expected to be important.