Infrared Study of Hydrogen-Bonded Complexes Involving Phenol Derivatives and Polyfunctional Bases. 2. 3-Methyl-4-pyrimidone, 1-Methyl-2-pyrimidone, 1,4,4-Trimethylcytosine, and 1,3-Dimethyluracil

Article abstract of DOI:10.1021/j150656a042

In this work, the electron-donating properties of 3-methyl-4-pyramidone (I), 1-methyl-2-pyramidone (II), 1,4,4-trimethylcytosine (III), 1,3-dimethyluracil (IV), and pyrimidine (V) have been studied by infrared spectrometry, using the phenol derivatives as reference acids.The thermodinamic parameters (K, -ΔH, ΔS) determined in 1,2-dichloethane (1,2-DCE) are ordered according to III > II > IV > V, the same order as that predicted from the pK_a value, with the exception of IV.For the same enthalpy of complex formation, the Δν_OH values were markedly higher for V than for the complexes involving the other bases; this fact and the frequency lowering of the ν_C=O vibration clearly show that for compounds I-IV the carbonyl group is the preferred H-bond site.For IV, the infrared data suggest that complex formation takes place on the O₄ atom in agreement with the lower ionization potential of the n(O₄) electrons.As shown by the X-ray diffraction method and the infrared spectra available in the literature, protonation takes place on the nitrogen atom for I-III and on the O₄ atom for IV.This shows that, with the exception of IV, the preferred H-bonding site is not the preferred site of protonation.The intermolecular streetching vibrations ν_? are observed between 130 and 110 cm^-1 and the corresponding force constants k_? are calculated by the Lippincott-Schroeder potential function.The k_? values are compared with those of hydrogen bonds involving nitrogen and sulfur atoms.It is shown that, for the same enthalpy of complexation, the k_? values are related to the extension of the lone-pair electrons of the base.