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The possible occurrence of static/dynamic disorder at the Mg site in pyrope (Mg3Al2Si3O12), with or without anharmonic contribution to the thermal vibrations even at low temperatures, has been largely debated but conclusions were contrasting. Here a report is given on the experimental charge density distribution, ρEXP, of synthetic pyrope at T = 30 K, built through a Stewart multipolar expansion up to l = 5 and based on a very precise and accurate set of in-home measured single-crystal X-ray diffraction amplitudes with a maximum resolution of 0.44 Å. Local and integral topological properties of ρEXP are in substantial agreement with those of ρTHEO, the corresponding DFT-grade quantum charge density of an ideal pyrope crystal, and those derived from synchrotron investigations of chemical bonding in olivines. Relevant thermal atomic displacements, probably anharmonic in nature, clearly affect the whole structure down to 30 K. No significant (> 2.5σ) residual Fourier peaks are detectable from the ρEXP distribution around Mg, after least-squares refinement of a multipole model with anharmonic thermal motion at the Mg site. Experimental findings were confirmed by a full analysis of normal vibration modes of the DFT-optimized structure of the perfect pyrope crystal. Mg undergoes wide displacements from its equilibrium position even at very low temperatures, as it is allocated in a ∼ 4.5 Å large dodecahedral cavity and involved in several soft phonon modes. Implications on the interplay among static/dynamic disorder of Mg and lattice vibrational degrees of freedom are discussed. [ABSTRACT FROM AUTHOR]

A very efficient synthesis of 2,3-diaryl-β-amino acid derivatives is realized by a TiCl4/triethylamine-catalyzed Mannich-like reaction of arylacetic or arylthioacetic esters with arylimines. The presence on the arylacetic moiety of an ortho-heteroatom group able to coordinate to the titanium center is of dramatic importance for the outcome of the reaction and to tune the syn/ anti diastereoselection. In particular, with o-F-, o-Cl-, and o-Br-arylacetates, the syn adduct was isolated as the largely prevalent isomer. When (-)-8-phenylmenthyl (2-fluorophenyl)acetate was used, the condensation with imines resulted in high diastereo- and enantioselectivity. In agreement with the stereochemical results and NMR studies, a conceivable reaction mechanism was proposed. [ABSTRACT FROM AUTHOR]

The title 1,4-naphthoquinone, 2-dichloromethyl-3-methyl-1,4-dihydronaphthalene-1,4-dione, C12H8Cl2O2, is a chlorinated derivative of vitamin K3, which is a synthetic compound also known as menadione. Molecules of (I) are planar and lie on a crystallographic mirror plane ( Z′ = 0.5) in the space group Pnma. They are connected to each other by C-H...O hydrogen bonds, forming two-dimensional layers parallel to the ac plane. In addition, Cl...Cl and π-π interactions link adjacent molecules in different layers, thus forming zigzag ribbons along the b axis, such that a three-dimensional architecture is generated. [ABSTRACT FROM AUTHOR]

The title 1,4-naphthoquinone, 2-dichloromethyl-3-methyl-1,4-dihydronaphthalene-1,4-dione, C12H8Cl2O2, is a chlorinated derivative of vitamin K3, which is a synthetic compound also known as menadione. Molecules of (I) are planar and lie on a crystallographic mirror plane ( Z′ = 0.5) in the space group Pnma. They are connected to each other by C-H...O hydrogen bonds, forming two-dimensional layers parallel to the ac plane. In addition, Cl...Cl and π-π interactions link adjacent molecules in different layers, thus forming zigzag ribbons along the b axis, such that a three-dimensional architecture is generated. [ABSTRACT FROM AUTHOR]

The accurate, experimental charge density distribution, ρ( r), of the potent antimalarial drug dihydroartemisinin (DHA) has been derived for the first time from single-crystal X-ray diffraction data at T=100(2) K. Gas-phase and solid-state DFT simulations have also been performed to provide a firm basis of comparison with experimental results. The quantum theory of atoms in molecules (QTAIM) has been employed to analyse the ρ( r) scalar field, with the aim of classifying and quantifying the key real-space elements responsible for the known pharmacophoric features of DHA. From the conformational perspective, the bicyclo[3.2.2]nonane system fixes the three-dimensional arrangement of the 1,2,4-trioxane bearing the active OO redox centre. This is the most nucleophilic function in DHA and acts as an important CH⋅⋅⋅O acceptor. On the contrary, the rest of the molecular backbone is almost neutral, in accordance with the lipophilic character of the compound. Another remarkable feature is the CO bond length alternation along the O-C-O-C polyether chain, due to correlations between pairs of adjacent CO bonds. These bonding features have been related with possible reactivity routes of the α- and β-DHA epimers, namely 1) the base-catalysed hemiacetal breakdown and 2) the peroxide reduction. As a general conclusion, the base-driven proton transfer has significant non-local effects on the whole polyether chain, whereas DHA reduction is thermodynamically favourable and invariably leads to a significant weakening (or even breaking) of the OO bond. The influence of the hemiacetal stereochemistry on the electronic properties of the system has also been considered. Such findings are discussed in the context of the known chemical reactivity of this class of important antimalarial drugs. [ABSTRACT FROM AUTHOR]

The article presents an experimental and theoretical study of polymorphism in a highly flexible organic Schiff-base macrocycle ligand. A single-crystal X-ray diffraction and both gas- and solid-phase density function theory (DFT) calculations were employed to study polymorphism. Two polymorphs and a new solvated macrocycle ligand were presented for the first time. DFT calculations observed a potential interconversion path in the gas phase between exo and endo conformers.

High-resolution X-ray diffraction data were collected at T = 19 K from two similar spherical crystals of the fungal metabolite citrinin, C13H14O5. The two data sets were of markedly different quality, one data set (MQ, medium quality) comprising a single octant of the reciprocal lattice and including reflections with obviously peculiar intensity profiles while the second (HQ, high quality) comprised a hemisphere of reflections and showed no flawed profiles. Parallel multipolar refinements were carried out for both. While most of the resulting geometric parameters, including bond lengths and angles, were in close agreement (the standard uncertainties were approximately twice as large for the MQ data, reflecting the smaller number of observations), the agreement is noticeably worse for electronic properties such as electron densities and their Laplacians at the bond and ring critical points. These latter features are especially sensitive to the quality of the low-angle (and strong) intensities, which was not high for the MQ data. By contrast, the magnitudes of the molecular dipole moment from the two experiments are the same within 1 standard uncertainty, with an angle of about 13° between the two vectors. It is concluded that only true high-quality data allow a fully significant and quantitative analysis of the details of the experimental electron density ρexp, while high-resolution medium-quality data, measured at very low temperature and adequately processed, can still be used for a qualitative analysis, or for the derivation of overall electronic properties. [ABSTRACT FROM AUTHOR]

A new monoclinic form of the title compound, C(14)H(17)N(3)O(3)S, has been found upon slow crystallization from water. Another monoclinic form of the compound was obtained previously from a mixture of dichloro-methane and diethyl ether [Clerici et al. (2002 ▶). Tetra-hedron, 58, 5173-5178]. Both phases crystallize in space group P2(1)/n with one mol-ecule in the asymmetric unit. The formally single exocyclic C-N bond that connects the -NEt(2) unit with the thia-zete ring is considerably shorter than the adjacent, formally double, endocyclic C=N bond. This is likely to be due to the extended conjugated system between the electron-donor diethyl-ammine fragment and the electron-withdrawing sulfonyl group. In the newly discovered polymorph, the meth-oxy group is rotated by almost 180° around the phen-yl-OCH(3) bond, resulting in a different mol-ecular conformation.

The title compound, (7R,8S)-7,8-dihydroxy-3,7-dimethyl-6-oxo-7,8-dihydro-6H-isochromene-5-carbaldehyde, C(12)H(12)O(5), is a trans-vicinal diol. Of the two fused rings, which lie approximately in the same plane, the pyran ring is almost perfectly planar, while the cyclohexenone ring adopts a slightly distorted half-chair conformation. The crystal packing is dictated by two strong intermolecular O-H.O interactions, one involving hydroxy and keto groups, the other involving two hydroxy groups. Molecules are linked together through twofold axes, forming zigzag ribbons extended along the a axis.

The asymmetric unit of the title compound, C(22)H(31)N(3)O(4).H(2)O, incorporates one water molecule, which is hydrogen bonded to the 3-oxo O atom of the indolizidinone system. The two rings of the peptidomimetic molecule are trans-fused, with the six-membered ring having a slightly distorted half-chair conformation and the five-membered ring having a perfect envelope conformation. The structure is stabilized by intermolecular O-H...O interactions between the water and adjacent peptide molecules, and by N-H...O interactions between the peptide molecules, which link the molecules into infinite chains.