RESUMEN
In order to gain a better understanding of how to improve the quality of small-molecule single-crystal X-ray diffraction data achievable in a finite time, a study was carried out to investigate the effect of varying the multiplicity, acquisition time, detector binning, maximum resolution and completeness. The results suggest that, unless there are strong arguments for a different strategy, a good routine procedure might be to optimize the conditions necessary to get the best data from single scans, and then choose a multiplicity of observations (MoO) to utilize the available time fully. Different strategies may be required if the crystal is highly absorbing, is larger than the incident beam, is enclosed in a capillary tube or is unusual in some other way. The signal-to-noise ratio should be used with care, as collecting data for longer or at higher multiplicity appears to give a systematic underestimate of the intensity uncertainties. Further, the results demonstrate that including poor-quality data in a refinement may degrade the result and, in the general case, the accidental omission of reflections has a very small impact on the refinement as long as they are omitted at random. Systematic omission of reflections needs a convincing procedural justification.
RESUMEN
The resonant-scattering contributions to single-crystal X-ray diffraction data enable the absolute structure of crystalline materials to be determined. Crystal structures can be determined even if they contain considerably disordered regions because a correction is available via a discrete Fourier transform of the residual electron density to approximate the X-ray scattering from the disordered region. However, the corrected model cannot normally account for resonant scattering from atoms in the disordered region. Straightforward determination of absolute structure from crystals where the strongly resonantly scattering atoms are not resolved has therefore not been possible. Using an approximate resonant-scattering correction to the X-ray scattering from the disordered regions, we have developed and tested a procedure (HUG) to recover the absolute structure using conventional Flack x refinement or other post-refinement determination methods. Results show that in favourable cases the HUG method works well and the absolute structure can be correctly determined. It offers no useful improvement in cases where the original correction for the disordered region scattering density is problematic, for example, when a large fraction of the scattering density in the crystal is disordered, or when voids are not occupied equally by the disordered species. Crucially, however, if the approach does not work for a given structure, the statistics for the absolute structure measures are not improved, meaning it is unlikely to lead to misassignment of absolute structure.
RESUMEN
A modern industrial route for the manufacture of methyl methacrylate involves the reaction of methyl propanoate and formaldehyde over a silica-supported Cs catalyst. Although the process has been successfully commercialised, little is known about the surface interactions responsible for the forward chemistry. This work concentrates upon the interaction of methyl propanoate over a representative silica. A combination of infrared spectroscopy, inelastic neutron scattering, DFT calculations, X-ray diffraction and temperature-programmed desorption is used to deduce how the ester interacts with the silica surface.
RESUMEN
The title compound, [Ni(C13H11N4S2)2], was obtained by the reaction of S-2-picolyldi-thio-carbazate and pyridine-2-carbaldehyde with nickel(II) acetate. The Ni(II) atom is located on a twofold rotation axis and is bonded to four N atoms at distances of 2.037â (8) and 2.109â (9)â Å, and to two S atoms at a distance of 2.406â (3)â Å, leading to a distorted octa-hedral coordination. The angle between the mean planes of the coordinating moieties of the two symmetry-related tridentate ligands is 83.3â (2)°. In the crystal, complex mol-ecules are linked by weak C-Hâ¯S hydrogen bonds, π-π inter-actions between the pyridine rings [centroid-centroid distance = 3.775â (9)â Å] and C-Hâ¯π inter-actions. The hydrogen-bonding inter-actions lead to the formation of layers parallel to (010); π-π inter-actions link these layers into a three-dimensional network.
RESUMEN
X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title mol-ecule, C(10)H(16)O(5), in which the 1,5-lactone ring exists in a boat conformation. The absolute stereochemistry was determined by the use of d-ribose in the synthesis. The crystal exists as O-Hâ¯O hydrogen bonded chains of mol-ecules running parallel to the a axis with each mol-ecule acting as a donor and acceptor for one hydrogen bond.
RESUMEN
X-ray crystallography defines the relative configuration at the three-stereogenic centres in the title compound N-benzyl-l-XYLNAc, C(14)H(20)N(2)O(3). The five-membered pyrrolidine ring adopts an envelope conformation with the N atom lying out of the plane of the other four atoms. In the crystal structure, inter-molecular O-Hâ¯O, N-Hâ¯O and O-Hâ¯N hydrogen bonds link the mol-ecules into chains along [100]. The carbonyl group O atom acts as an acceptor for a bifurcated hydrogen bond. The absolute configuration is determined by the use of l-glucuronolactone as the starting material for the synthesis.
RESUMEN
The relative stereochemistry of the title compound, C(10)H(15)N(3)O(5), was confirmed by the crystal structure determin-ation. The absolute configuration was determined from the use of d-lyxonolactone as the starting material. The six-membered ring adopts a boat conformation with the larger azide group, rather than the methyl group, in the bowsprit position. In the crystal structure, a bifurcated inter-molecular O-Hâ¯O/O-Hâ¯N hydrogen bond links mol-ecules into chains running parallel to the b axis.
RESUMEN
In the title compound, C(13)H(21)N(3)O(6), the six-membered ring adopts a twist-boat conformation with the azide group in the bowsprit position. The azide group is disordered over two sets of sites in a 0.642â (10):0.358â (10) ratio. The crystal structure consists of O-Hâ¯O hydrogen-bonded trimer units. The absolute configuration was determined from the use of d-mannose as the starting material.
RESUMEN
The title compound, C(22)H(25)F(5)N(4)O(9), is a stable penta-fluoro-phenyl ester inter-mediate in the synthesis of novel homo-oligomeric structures containing branched carbon chains. The structure is epimeric to the previously characterized dimeric penta-fluoro-phenyl ester with stereochemistry (3R,4R,5R), which was synthesized using d-ribose as starting material. The crystal structure of the title mol-ecule removes any ambiguities arising from the relative stereochemistries of the six chiral centres. Two hydrogen bonds, bifurcating from the NH group, stabilize the crystal: one intra-molecular and one inter-molecular, both involving O atoms of the meth-oxy groups. The asymmetric unit contains two independent mol-ecules not related by any pseudo-symmetry operators. The major conformational differences are localized, leading to one mol-ecule being extended compared to the other. The collected crystal was twinned (twin ratio is 0.939:0.061), and the azide group is positionally disordered over two positions in one mol-ecule [occupancy ratio 0.511â (18):0.489â (18)].
RESUMEN
X-ray crystallographic analysis with Cu Kα radiation established the relative configurations of the stereogenic centers in the title compound, C(15)H(20)N(2)O(5), and clarified mechanistic ambiguities in the synthesis. The conformation of the five-membered ring approximates twisted, about a C-O bond. The absolute configuration of this carbon-branched dipeptide isostere was known based on the use of d-ribose as the starting material. Refinement of the Flack parameter gave an ambiguous result but the refined Hooft parameter is in agreement with the assumed (d-ribose) absolute structure. The crystal structure consists of N-Hâ¯O and O-Hâ¯O hydrogen-bonded bi-layers, with the terminal methyl and phenyl groups forming a hydro-phobic inter-layer inter-face. Some weak C-Hâ¯O inter-actions are also present.
RESUMEN
The name of one of the authors in the paper by Newton et al. [Acta Cryst. (2004), E60, o909-o910] is corrected.[This corrects the article on p. o909 in vol. 60.].
RESUMEN
The unusual methylene aziridine 6-tert-butyl-3-oxa-2-thia-1-azabicyclo[5.1.0]oct-6-ene 2,2-dioxide, C(9)H(15)NO(3)S, was found to crystallize with two molecules in the asymmetric unit. The structure was solved in both the approximately orthogonal and the oblique settings of space group No. 14, viz. P2(1)/n and P2(1)/c, respectively. A comparison of these results clearly displayed an increase in the correlation between coordinates in the ac plane for the oblique cell. The increase in the corresponding covariances makes a significant contribution to the standard uncertainties of derived parameters, e.g. bond lengths. Since there is yet no CIF definition for the full variance-covariance matrix, there are clear advantages to reporting the structure in the nonstandard space-group setting.
Asunto(s)
Compuestos Aza/química , Aziridinas/química , Óxidos/química , Cristalografía por Rayos X , Modelos Moleculares , EstereoisomerismoRESUMEN
Homobimetallic complexes of nickel, palladium and platinum, [(L2M)2(S2CNC4H8NCS2)]2+, are formed on reaction of the piperazine bis(dithiocarbamate) linker, KS2CNC4H8NCS2K, with [MCl2L2] (M=Ni, L2=dppe, dppf; M=Pd, L2=dppf; M=Pt, L=PEt3, PMePh2, PPh3, L2=dppf). [{Pd(C,N-C6H4CH2NMe2)}2(S2CNC4H8NCS2)] can be obtained in the same way. On reaction of [MCl2L2] (M=Pd, Pt) with the zwitterion S2CNC4H8NH2, a symmetrisation process occurs to yield a mixture of the complexes [M(S2CNC4H8NH2)L2]2+ and [(L2M)2(S2CNC4H8NCS2)]2+. However, the monometallic complexes [L2Ni(S2CNC4H8NH2)]2+ (L2=dppe, dppf) and [(L2Ni)2(S2CNC4H8NCS2)]2+ can be prepared without ready symmetrisation. Starting from the previously reported [(dppm)Ru(S2CNC4H8NH2)]2+, the heterotrimetallic products [(dppm)Ru(S2CNC4H8NCS2)M(dppf)]2+ (M=Pd, Pt) can be prepared without symmetrisation occurring. The crystal structures of five complexes are reported. The metalla-dithiocarbamate complexes [L2Ni(S2CNC4H8NCS2)] (L2=dppe, dppf) were used to functionalise the surface of gold nanoparticles by the displacement of a citrate shell to yield NiAu and FeNiAu materials.
RESUMEN
The chemical name of the title compound in the paper by Jenkinson, Fenton, Booth, Fleet & Watkin [Acta Cryst. (2009), E65, o610-o611] is corrected.[This corrects the article DOI: 10.1107/S1600536809006357.].
RESUMEN
The crystal structure of the title compound, C(13)H(14)O(5), establishes (i) the (S) - rather than (R) - configuration at the acetal carbon and (ii) that both the acetal and the lactone form five- rather than six-membered rings; the absolute configuration is determined by the use of 2-C-methyl-d-ribono-1,4-lactone as the starting material. The compound consists of hydrogen-bonded chains of mol-ecules running along the a axis; there are no unusual packing features. Only classical hydrogen bonding has been considered.
RESUMEN
The crystal structure of the title compound, C(6)H(11)NO(5), establishes the relative configuration at the four stereogenic centres; the absolute configuration is determined by the use of d-glucuronolactone as the starting material for the synthesis. Mol-ecules are linked by inter-molecular O-Hâ¯O and N-Hâ¯O hydrogen bonds into a three-dimensional network, with each mol-ecule acting as a donor and acceptor for five hydrogen bonds.
RESUMEN
X-ray crystallographic analysis of the title hydro-bromide salt, C(10)H(20)N(+)·Br(-), of (1R,2S,3R,5R,8aR)-3-hydroxy-meth-yl-5-methyl-octa-hydro-indolizine-1,2-diol defines the absolute and relative stereochemistry at the five chiral centres in steviamine, a new class of polyhydroxy-lated indolizidine alkaloid isolated from Stevia rebaudiana (Asteraceae) leaves. In the crystal structure, mol-ecules are linked by inter-molecular O-Hâ¯Br and N-Hâ¯Br hydrogen bonds, forming double chains around the twofold screw axes along the b-axis direction. Intra-molecular O-Hâ¯O inter-actions occur.
RESUMEN
The reaction of 5-azido-5-de-oxy-2,3-O-isopropyl-idene-2-C-methyl-d-ribose with N,N-diethyl-2-(dimethyl-sulfuranyl-idene)acetamide gave the title compound, C(15)H(26)N(4)O(5), as the major product arising from initial formation of an epoxide which was subsequently opened by intra-molecular attack of the free 4-hydroxyl group. X-ray crystallography confirmed the relative stereochemistry of the title compound and the absolute configuration was determined by the use of d-ribose as the starting material. The crystal structure contains chains of mol-ecules running parallel to the a axis, being linked by weak bifurcated O-Hâ¯(N,N) hydrogen bonds.
RESUMEN
X-ray crystallographic analysis has established that the major product from the protection of d-glucoronolactone with benzaldehyde is (1S)-1,2-O-benzyl-idene-α-d-glucurono-6,3-lactone, C(13)H(12)O(6), rather than the R epimer. The crystal structure exists as O-Hâ¯O hydrogen-bonded chains of mol-ecules lying parallel to the a axis. The absolute configuration was determined by the use of d-glucuronolactone as the starting material.
RESUMEN
X-ray crystallography unequivocally confirmed the stereochemistry of the C atom at position 2 in the carbon scaffold of the title mol-ecule, C(10)H(18)O(4). The pyran-ose ring exists in a chair conformation with the methyl group on the C atom in the 2 position in an equatorial configuration. The absolute stereochemistry was determined from the starting material. The crystal structure consists of O-Hâ¯O hydrogen-bonded chains of mol-ecules running parallel to the b axis.
