Three-dimensional refinement of urea

A conventional and chemometric analytical approach to solving urea determination with accuracy and precision

Urea is an essential molecule usually detected using spectroscopy, particularly ultraviolet and visible spectroscopy (UV-vis). However, its detection represents a not always fully acknowledged issue. Its concentration dependency has raised questions about the reliability of the UV-vis results. Derivatization reactions, common alternatives to achieve accuracy and precision with UV-vis measurements, still represent an additional step in the measurement process. Besides the problems mentioned earlier, urea forms complex mixtures in aqueous mediums. Therefore, this work proposes to investigate the accuracy and precision of urea determination by UV-vis spectroscopy in the pure form and derivatized with para-dimethylaminobenzaldehyde. The results show that UV-vis spectroscopy could not quantify urea in both forms with precision and accuracy. On the other hand, when applying multivariate curve resolution with alternating least squares (MCR-ALS) to the UV-vis data, the pure urea analytical signal is mathematically separated. Then, those parameters of merit were successfully achieved.

Lattice thermal expansion and anisotropic displacements in urea, bromomalonic aldehyde, pentachloropyridine, and naphthalene

Anisotropic displacement parameters (ADPs) are commonly used in crystallography, chemistry, and related fields to describe and quantify thermal motion of atoms. Within the very recent years, these ADPs have become predictable by lattice dynamics in combination with first-principles theory. Here, we study four very different molecular crystals, namely, urea, bromomalonic aldehyde, pentachloropyridine, and naphthalene, by first-principles theory to assess the quality of ADPs calculated in the quasi-harmonic approximation. In addition, we predict both the thermal expansion and thermal motion within the quasi-harmonic approximation and compare the predictions with the experimental data. Very reliable ADPs are calculated within the quasi-harmonic approximation for all four cases up to at least 200 K, and they turn out to be in better agreement with the experiment than those calculated within the harmonic approximation. In one particular case, ADPs can even reliably be predicted up to room temperature. Our results also hint at the importance of normal-mode anharmonicity in the calculation of ADPs.

Synthesis and Conformational Analysis of Sterically Congested (4R)-(−)-1-(2,4,6-Trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone: X-Ray Crystallography and Semiempirical Calculations

The crystal structure of (4R)-(−)-1-(2,4,6-trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone(3)was determined by single-crystal X-ray diffraction. Compound3crystallizes in triclinic system in space groupP1 (≠1). The crystal data area=10.62165 Å,b=16.5321 Å,c=8.95729 Å,∝=91.1936∘,β=93.8496∘,γ=88.0974∘,V=1568.22 Å3,Z=3,Dcalc=1.253 g/cm3,μCuKα=15.98 cm−1,F000=636.00,T=20.0°C, andR=0.037. The crystal structure confirmed the occurrence of three molecules of3A,3B, and3Cin which then-butyryl moiety adopted thes-transoidconformation. Crystal structure also revealed that the conformation of 2,4,6-trimethylbenzenesulfonyl groups was inanti-position relative totert-butyl group. The crystal packing showed that three molecules of compound3are stacked as a result of intermolecularπ-πinteractions between the phenyl ring of one molecule and the phenyl ring of the other molecule by approaching each other to an interplanar separation of 5.034 Å. Interestingly, these stacked molecules are also connected by intermolecularCH-πinteraction. The conformational analysis of thes-transoid  3A,3B, and3Cwas separately performed by molecular mechanic MM+ force field. Additionally, computational investigation using semiempirical AM1 and PM3 methods was performed to find a correlation between experimental and calculated geometrical parameters. The data obtained suggest that the structural data furnished by the AM1 method is in better agreement with those experimentally determined for the above compound. It has been found that the lowest energetic conformer computed gives approximate correspondence with experimental solid state data.

Molecular structure of thiourea

The molecular structure of thiourea has been investigated under C(s), C(2), and C(2v) symmetry constraints. At the coupled-cluster level in conjunction with a triple-ζ basis set, only the C(2) conformer has been found to be a real minimum on the potential energy surface. Its equilibrium structure has therefore been accurately evaluated using both theoretical and experimental data. With respect to the former, high-level quantum-chemical calculations at the coupled-cluster level in conjunction with correlation-consistent basis sets ranging in size from triple- to quintuple-zeta have been carried out. Extrapolation to the complete basis-set limit as well as core-correlation effects and inclusion of full treatment of triple excitations in the cluster operator have been considered. On the basis of the vibrational ground-state rotational constants available for five isotopic species and the corresponding computed vibrational corrections, the semiexperimental equilibrium geometry of thiourea has also been determined for the first time.

Design, synthesis, single-crystal and preliminary antitumor activity of novel arenesulfonylimidazolidin-2-ones

Novel series of 1-(arenesulfonyl)imidazolidin-2-one (3a-i) and 1,3-bis(arenesulfonyl)imidazolidin-2-one (5a-i) have been synthesized and tested for their antitumor activity against 60 tumor cell lines taken from nine different organs. A significant inhibition for cancer cells was observed with series 5a-i compounds compared with the series 3a-i which showed a weak inhibition. Compounds 5a-i showed good inhibitory effect at the lung cancer HOP-92 and renal cancer CAKI-1 and UO-31 cell lines. Compound 5e showed remarkable broad-spectrum antitumor activity.

A rotational study of laser ablated thiourea

A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of solid thiourea for the first time. The sensitivity reached in the experiment allowed the observation of the isotopomers (34)S, (13)C, and (15)N in their natural abundance. The spectrum of D(4)-thiourea was also analyzed from an enriched sample. The complicated hyperfine structure arising from the presence of two (14)N quadrupolar nuclei has been fully resolved and analyzed. The substitution r(s) structure has been derived from the experimental moments of inertia. Thiourea in gas phase presents a planar heavy atom skeleton. Experimental inertial defect values and high-level ab initio calculations reveal that the amino groups hydrogen atoms lie out-of-plane with a C(2) symmetry configuration and are involved in large amplitude inversion motions.

A structural basis for the interaction of urea with lysozyme

The effect of urea on the crystal structure of hen egg-white lysozyme has been investigated using X-ray crystallography. High resolution structures have been determined from crystals grown in the presence of 0, 0.7, 2, 3, 4, and 5 M urea and from crystals soaked in 9 M urea. All the forms are essentially isomorphous with the native type II crystals, and the derived structures exhibit excellent geometry and RMS differences from ideality in bond distances and angles. Comparison of the urea complex structures with the native enzyme (type II form, at 1.5 A resolution) indicates that the effect of urea is minimal over the concentration range studied. The mean difference in backbone conformation between the native enzyme and its urea complexes varies from 0.18 to 0.49 A. Conformational changes are limited to flexible surface loops (Thr 69-Asn 74, Ser 100-Asn 103), the active site loop (Asn 59-Cys 80), and the C-terminus (Cys 127-Leu 129). Urea molecules are bound to distinct sites on the surface of the protein. One molecule is bound to the active site cleft's C subsite, at all concentrations, in a fashion analogous to that of the N-acetyl substituent of substrate and inhibitor sugars normally bound to this site. Occupation of this subsite by urea alone does not appear to induce the conformational changes associated with inhibitor binding.