Barbituric Acid Dihydrate Research Articles

True Polymorphic Phase Transition or Dynamic Crystal Disorder? An Investigation into the Unusual Phase Behavior of Barbituric Acid Dihydrate

Crystalline barbituric acid dihydrate (BTADH) was previously thought to undergo a subtle temperature-dependent phase transition from a high-temperature orthorhombic Pnma space group to a nonmerohedrally twinned monoclinic P21/n phase below a transition temperature of ∼217 K. Questions remain on the true nature of the unusual transformation and the structure of the high-temperature crystal phase due to difficulties in resolving the Pnma structure from X-ray diffraction data and because of the subtlety of the structural transition. In this study, terahertz (THz) spectroscopy and solid-state density functional theory (DFT) were utilized to explore this suspected phase transition and uncover the principle physical mechanisms contributing to this anomalous transition. Our findings suggest that at temperatures above the previously reported phase transition temperature of 217 K, the crystal does not exist in the Pnma space group configuration. However, two equivalent favorable energetic states with P21/n symmetry related by the twinning plane lie on either side of the proposed Pnma structure. It is these same local potential energy minima that guide the crystal system to a nonmerohedrally twinned configuration identified in the low-temperature crystal structures. At temperatures above 217 K, the system possesses the thermal energy necessary to readily transition between these degenerate states separated by the low-lying Pnma structural barrier. The evidence acquired by THz spectroscopy and DFT indicate the absence of a true temperature-dependent phase change, and rather a system that exists in a tenuous thermally disordered state above the previously alleged transition temperature.

Noncentrosymmetric organic crystals of barbiturates as potential nonlinear optical phores: experimental and theoretical analyses

This paper details with design, synthesis, isolation, spectroscopic and structural elucidation of seven crystals of organic barbiturates (a red phase of bis(5-(hydroxyimino)pyrimidine-2,4,6(1H,3H,5H)-trione) 4H-1,2,4-triazol-4-amine co-crystal (1y), 2-methylamino-1-phenyl-propan-1-ol 2-thioxo-dihydro-pyrimidine-4,6-dione (2), 4-phenylpyridine thiobarbituric acid co-crystal (3), new monoclinic polymorph of violuric acid monohydrate (4), 2-pyrrolidin-1-yl-ethylammonium bis(violurate) dihydrate (5) and redetermination of barbituric acid dihydrate (6) and noncentrosymmetric orthorhombic violuric acid monohydrate (7)). The noncentrosymmetric crystals (1r) (2) and (7) can efficiently be employed in the field of nonlinear optical (NLO) materials research. We aim at studying relationship between molecular, respectively, crystal structure and NLO properties within a relatively large range of temperatures (T = 273–500 K). The single crystal X-ray diffraction, electronic absorption, conventional and linear polarized infrared spectroscopies have been used to provide experimental structural and spectroscopic data. The deuterated derivatives of (4) and (7) have been analysed, as well. The theoretical description of linear and NLO properties of crystals (1)–(7) is based on quantum chemical ab initio methods and density functional theory (DFT). The work contributes, importantly, to the fields of nonlinear optics, crystal engineering or materials research, reflecting in mind frequency conversion of lasers, electro-optics, modulators, waveguides and more.

Temperature- and moisture-dependent phase changes in crystal forms of barbituric acid

The dihydrate of barbituric acid (BAc) and its dehydration product, form II were investigated by means of moisture sorption analysis, hot-stage microscopy, differential scanning calorimetry, thermogravimetry, solution calorimetry, IR- and Raman-spectroscopy as well as powder X-ray diffraction. The dihydrate desolvates already at and below 50% relative humidity (RH) at 25 °C whereas form II is stable up to 80% RH, where it transforms back to the dihydrate. The thermal dehydration of barbituric acid dihydrate (BAc-H2) is a single step, nucleation controlled process. The peritectic reaction of the hydrate was measured at 77 °C and a transformation enthalpy of Δ trs H H2-II = 17.3 kJ mol −1 was calculated for the interconversion between the hydrate and form II. An almost identical value of 17.0 kJ mol −1 was obtained from solution calorimetry in water as solvent (Δ sol H H2 = 41.5, Δ sol H II = 24.5 kJ mol −1). Additionally a high-temperature form (HT-form) of BAc, which is enantiotropically related to form II and unstable at ambient conditions has been characterized. Furthermore, we observed that grinding of BAc with potassium bromide (KBr) induces a tautomeric change. Therefore, IR-spectra recorded with KBr-discs usually display a mixture of tautomers, whereas the IR-spectra of the pure trioxo-form of BAc are obtained if alternative preparation techniques are used.

A temperature-induced phase transition in barbituric acid dihydrate

A temperature-induced phase transition in barbituric acid dihydrate

A variable-temperature study of a phase transition in barbituric acid dihydrate

The crystal structure of barbituric acid dihydrate (C4H4N2O3*2H2O) has twice been reported as orthorhombic, space group Pnma, with all atoms (except for CH(2) H atoms) lying on the mirror plane [Al-Karaghouli et al. (1977). Acta Cryst. B33, 1655-1660; Jeffrey et al. (1961). Acta Cryst. 14, 881-887]. The present study has found that at low temperatures, below 200 K, the crystal structure is no longer orthorhombic but is non-merohedrally twinned monoclinic, space group P2(1)/n. This phase is stable down to 100 K. Above 220 K the crystal structure is orthorhombic, and between 200 and 220 K the structure undergoes a phase change, with the monoclinic-to-orthorhombic phase transition itself taking place at around 216-217 K. The size of the beta angle in the monoclinic structure is temperature dependent; at 100 K beta is around 94 degrees and it decreases in magnitude towards 90 degrees as the temperature increases. Although the hydrogen-bonding motifs are the same for both crystal systems, there are significant differences in the crystal packing, in particular the out-of-plane displacement of the two water molecules and the sp3-hybridized C atom of barbituric acid.

The Effects of Lattice Water on Free Radical Yields in X-Irradiated Crystalline Pyrimidines and Purines: A Low-Temperature Electron Paramagnetic Resonance Investigation

The hydration layer of DNA increases the target size of DNA with respect to the formation of direct-type damage by ionizing radiation. The mechanisms that give rise to this increase are being investigated by EPR spectroscopy. To determine these mechanisms, it is necessary to distinguish between the change in sample mass and changes in packing/conformation brought about by the change in the level of hydration. Certain model compounds that crystallize as hydrates provide a system where the effects of mass and packing can be discerned. Three such hydrate crystals were used in this work: barbituric acid dihydrate (BA:2H2O), inosine dihydrate (IR:2H2O) and thymine monohydrate (T:H2O). The free radical yields (+/-25%) in the native crystals at 7-11 K are 0.08, 0.03 and 0.02, respectively. Removal of the lattice water leaves behind an ordered lattice and results in free radical yields of 0.08, 0.03 and < 0.004, respectively. Thus removal of the lattice water does not affect the free radical yield in BA:2H2O or IR:2H2O but decreases the free radical yield in T:H2O by an order of magnitude. Based on these observations and the known crystal packing, we conclude that the hydrogen bonding network is a major factor in determining the distribution and yield of trapped free radicals. We ascribe this to the importance of proton transfer processes which act to reduce the probability of radical combination. Consistent with this conclusion are the types of free radicals trapped in these crystalline materials before and after dehydration. From these results, we argue that a major determinant of free radical yields in solidstate samples of DNA constituents is molecular packing. In addition, the absence of HO. radicals trapped in single crystals of BA:2H2O provides an upper limit for the yield of trapped HO. of less than 10(-4) mumol/J. This supports the thesis that at < 77 K direct ionization of those waters associated directly with a pyrimidine or purine results in hole transfer to that molecule. Hydroxyl radical formation on a water adjacent to a DNA base is predicted to be negligible.

Chemical Reaction, Diffusion, and Ionization Model for Dissolution of Oxygen Acids and Carbon Acids

A chemical reaction, diffusion, and ionization model, which assumes the establishment of ionization equilibrium at an equilibrium plane located within the diffusion layer, is tested by use of experimental dissolution rates for some oxygen acids and carbon acids. For oxygen acids both two- and three-zone models predict observed dissolution rates; however, for barbituric acid dihydrate and phenylbutazone neither model successfully depicts the dissolution behavior. Although the three-zone model approximately predicts dissolution rates of oxygen acids, its complexity does not add to the accuracy of the calculated rate. The three-zone model does support the frequent assumption that ionization is instantaneous and can be lgnored in modeling.

A neutron diffraction study of barbituric acid dihydrate

A neutron diffraction study of barbituric acid dihydrate

Oxidation and reduction of barbituric acid dihydrate by ionizing radiation at 4.2 °K

The primary oxidation and reduction products produced by ionizing radiation in single crystals of barbituric acid irradiated at 4.2 °K have been identified from their ESR and ENDOR spectra. The primary reduction product is an anion formed by electron addition. The primary oxidation product is a neutral radical resulting from the loss of hydrogen from an /\ NH group.

Electron Spin Resonance Transitions involving Simultaneous Changes in Spin States of up to Three Neighbouring Protons

OCCASIONALLY an electron spin resonance absorption line is accompanied by satellite lines of weak intensity. These satellite lines, arising from a change in spin state of a nearby nucleus concurrent with the change in spin state of the electron, were first observed in irradiated frozen acids1. A theoretical explanation for these transitions has been given by Trammell et al.2, who also predicted a second set of satellites corresponding to two neighbouring protons concurrently changing their spin state. Recently Snipes and Bernhard3 observed such satellite lines in γ-irradiated single crystals of barbituric acid dihydrate. We report here experimental evidence of a second and even a third set of satellites corresponding to two and three neighbouring protons simultaneously changing spin state.

Electron Spin Resonance of a Gamma-Irradiated Single Crystal of Barbituric Acid Dihydrate

Free radicals produced in single crystals of barbituric acid dihydrate by ionizing radiation have been identified by ESR spectroscopy and the parameters of the Hamiltonian describing the free radicals have been determined. The stable radical observed at room temperature is formed by loss of a hydrogen atom from C(5) of the original molecule, thus leaving an unpaired electron mainly in a 2pπ orbital on the carbon atom. Within experimental error, the free radicals have the same orientation in the crystal lattice as did the original molecules. Principal values of the g tensor characterizing the resonance were found to be gu=2.0044, gv=2.0056, gw=2.0022. Hyperfine interaction of the unpaired electron with the single hydrogen atom bonded to C(5) of the free radical splits the absorption into a doublet. Principal values of the hyperfine tensor were found to be Ax=11.2 G, Ay=30.0 G, Az=20.4 G. The principal axis systems for the g tensor and the hyperfine coupling tensor coincide. Two sets of satellite lines corresponding to changes in the spin states of one and two neighboring protons concurrent with the change in spin state of the unpaired electron were observed.

The crystal structure of barbituric acid dihydrate

The crystal structure of barbituric acid dihydrate