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Analysis of vibrational spectra of methyl-substituted uracils in the anharmonic approximation

“…The assignment suggested in [12] for in-plane vibrations (symmetry type A ′ ) of the uracil molecule match the results of the direct quantum calculation (Table 3). In this case, good agreement with experiment is achieved only when the problem is solved in the anharmonic approximation.…”

“…Such a situation also occurs for substituted uracils. The limited amount of reliable experimental data on the vibrational spectra of isotopically substituted uracils and the presence of close vibrational levels in the low-frequency range narrow the possibilities even more for application of the conventional scheme (see, for example, [5][6][7][8][9][10][11][12]). …”

“…We will use the most complete spectra, obtained in the gas phase [10], by the matrix isolation method [6][7][8], in solutions [12], and in the polycrystalline state [5,9]. In the last two states, the vibrational spectra carry information about intermolecular bonds.…”

“…In the last two states, the vibrational spectra carry information about intermolecular bonds. According to [12], in the polycrystalline state, uracil forms hydrogen bonds of the type C 4 =O ... HN 1 , C 4 = O ... HN 3 , C 2 =O ... HC 5 . The conclusions were based on shifts of the bands for stretching vibrations of the NH and CH bonds.…”

“…We only studied in-plane vibrations of uracil (symmetry type A ′ ). The changes in the frequencies of the ring vibrations proved to be comparable in magnitude with the effect of anharmonicity of the vibration, which was not analyzed in [12], although now such estimates have already been made using various scaling schemes [16,17]. When we constructed the structural dynamic models, the initial geometric parameters for the six-membered skeleton, the bonds, and the methyl group are assumed to be like those for toluene and xylene [1].…”