Publisher Summary This chapter provides information on the chemistry and pharmacology and physicochemical properties of barbiturates, along with the tautomerism and solvation of the barbituric acid ring. Barbituric acid exists in the solid state in the trioxo structure. NMR investigation of the oxo-hydroxy equilibrium indicates that only the oxo form is present in a solution in anhydrous DMSO. Several spectral properties of the barbituric acid are discussed using UV spectra, infrared and Raman spectroscopy, 1 H-NMR spectroscopy, 13 C-NMR spectroscopy, and mass spectrometry. In the crystalline state, barbiturates show several modes of intermolecular hydrogen bonding, which differ by the number of hydrogen bonds formed between the NH and CO groups. These hydrogen bonds are responsible for layer packing in the crystal lattice. Weak intermolecular van der Waals interactions of carbonyl groups are also responsible for the three-dimensional packing of the molecules in the crystal structure.

Recent Progress in Barbituric Acid Chemistry

13C CP/MAS NMR spectra of nitrogen-containing compounds often show complex signals when recorded at low field strengths. This effect is due to the 13C, 14N residual dipolar coupling which is not averaged to zero by magic-angle spinning. Solid-state 13C NMR spectra at 25 MHz of ribonucleosides are analysed on the basis of the isotropic chemical shifts measured at 50 MHz, together with the asymmetric splittings caused by the presence of 14N calculated using a first-order equation. X-ray diffraction and NQR data, together with several assumptions regarding the quadrupole tensor at each 14N site, are used to obtain a successful spectral simulation.

Analysis of 13C, 14N residual dipolar coupling in the 13C CP/MAS NMR spectra of ribonucleosides

The thiazolylazo compounds and their Co(II), Ni(II) and Cu(II) complexes of barbituric acid, uracil, thiouracil, citrazinic acid, chromotropic acid, gallic acid, pyrogallol and salicylic acid were prepared and characterized by 1H NMR, IR and the effect of pH on the electronic absorption spectra. The mode of ionization, the electronic transitions and the dissociation constants were discussed. The stoichiometries of the complexes were of 1:1, 2:1 and 3:2 (M:L). The copper complexes are of isotropic ESR spectra (except that of gallic acid which showed a complicated one) and are of magnetically diluted behaviour with orbital contribution. Detailed DTA data were obtained and discussed.

Studies on some thiazolylazo compounds and their cobalt, nickel, and copper complexes

14N nuclear quadrupole resonance frequencies have been measured in solid 2-pyridone, 3-hydroxypyridine, and 4-pyridone by 1H–14N nuclear quadrupole double resonance. Two slightly nonequivalent nitr...

Tautomerism and Possible Polymorphism in Solid Hydroxypyridines and Pyridones Studied by 14N NQR

The excited state deactivation of two amino-substituted uracils, 5-aminouracil (5AU) and 6-aminouracil (6AU) in aqueous solution was studied by femtosecond fluorescence upconversion. The fluorescence of 6AU decays as fast as that of uracil with a unique time constant of about 100 femtoseconds. The fluorescence of 5AU exhibits a more complex behavior, fundamentally different from what we found in any other uracils: the decays are globally slower (up to several picoseconds) and depend strongly on the wavelength. This difference is attributed to the particular character of the amino group, affecting the out-of-plane motion of the 5-substituent which has been shown to be crucial for the ultrafast internal conversion occurring in uracils. Our observations indicate instead the formation of a transient fluorescent state which in turn is deactivated by a different relaxation process specific to the amino group.

Effect of amino substitution on the excited state dynamics of uracil.

Nitrogen‐14 nuclear quadrupole resonance (NQR) spectra of several hydroxypyrimidines of biological and pharmacological importance are reported and analyzed in the framework of the semiempirical Townes and Dailey theory. Included among the compounds are thymine, uracil, uridine, barbituric acid, orotic acid, and cyanuric acid. The results obtained suggest that in all cases the molecules in their ground state exist predominantly in the dilactam tautomeric form. Significant differences have been observed in the NQR spectra of the two nitrogens in barbituric acid which cannot be accounted for by the differences in the hydrogen bond lengths alone.

P. J. Bray

Papers

Nitrogen-14 nuclear quadrupole resonance study of several hydroxypyrimidines