Journal of Physical Chemistry p. 2636 - 2641 (1984)
Update date:2022-08-31
Topics:
Kasende, O.
Zeegers-Huyskens, Th.
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 pKa value, with the exception of IV.For the same enthalpy of complex formation, the ΔνOH values were markedly higher for V than for the complexes involving the other bases; this fact and the frequency lowering of the νC=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.
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