Crystals of hexa‐tert‐butyldisilane, C24H54Si2, undergo a reversible phase transition at 179 (2) K. The space group changes from Ibca (high temperature) to Pbca (low temperature), but the lattice constants a, b and c do not change significantly during the phase transition. The crystallographic twofold axis of the molecule in the high‐temperature phase is replaced by a noncrystallographic twofold axis in the low‐temperature phase. The angle between the two axes is 2.36 (4)°. The centre of the molecule undergoes a translation of 0.123 (1) Å during the phase transition, but the conformation angles of the molecule remain unchanged. Between the two tri‐tert‐butylsilyl subunits there are six short repulsive intramolecular C—H...H—C contacts, with H...H distances between 2.02 and 2.04 Å, resulting in a significant lengthening of the Si—Si and Si—C bonds. The Si—Si bond length is 2.6863 (5) Å and the Si—C bond lengths are between 1.9860 (14) and 1.9933 (14) Å. Torsion angles about the Si—Si and Si—C bonds deviate by approximately 15° from the values expected for staggered conformations due to intramolecular steric H...H repulsions. A new polymorph is reported for the crystal structure of 1,1,2,2‐tetra‐tert‐butyl‐1,2‐diphenyldisilane, C28H46Si2. It has two independent molecules with rather similar conformations. The Si—Si bond lengths are 2.4869 (8) and 2.4944 (8) Å. The C—Si—Si—C torsion angles deviate by between −3.4 (1) and −18.5 (1)° from the values expected for a staggered conformation. These deviations result from steric interactions. Four Si—C(t‐Bu) bonds are almost staggered, while the other four Si—C(t‐Bu) bonds are intermediate between a staggered and an eclipsed conformation. The latter Si—C(t‐Bu) bonds are about 0.019 (2) Å longer than the staggered Si—C(t‐Bu) bonds. 相似文献
The structures of 4‐chloro‐3‐nitroaniline, C6H5ClN2O2, (I), and 4‐iodo‐3‐nitroaniline, C6H5IN2O2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single‐crystal X‐ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high‐temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene‐ring planes at two different orientations. In the low‐temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room‐temperature cell. Each of the low‐temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low‐temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low‐temperature phases. It seems that competition between the primary amine–nitro N—H...O hydrogen bonds which form three‐centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N—H...N hydrogen bonding of moderate strength which results in the graph‐set motif C(3). This graph‐set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high‐ and the low‐temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high‐temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C 60 , o328–o330]. 相似文献
Novel dual molecular‐ and ion‐recognition responsive poly(N‐isopropylacrylamide‐co‐benzo‐12‐crown‐4‐acrylamide) (PNB12C4) linear copolymers with benzo‐12‐crown‐4 (B12C4) as both guest and host units are prepared. The copolymers exhibit highly selective sensitivities toward γ‐cyclodextrin (γ‐CD) and Na+. The presence of γ‐CD induces the lower critical solution temperature (LCST) of PNB12C4 copolymer to shift to a higher value due to the formation of 1:1 γ‐CD/B12C4 host‐guest inclusion complexes, while Na+ causes a negative shift in LCST due to the formation of 2:1 “sandwich” B12C4/Na+ host‐guest complexes. Regardless of the complexation order, when γ‐CD and Na+ coexist with PNB12C4, competitive complexation actions of B12C4 as both guest and host units toward γ‐CD and Na+ finally form equilibrium 2:2:1 γ‐CD/B12C4/Na+ composite complexes, and the final LCST values of PNB12C4 copolymer reach almost the same level. The results provide valuable guidance for designing and applying PNB12C4‐based smart materials in various applications.
Molecular dynamics and Rotational Isomer State/Monte Carlo techniques with a Dreiding 1.01 Force Field are employed to study the excimer formation of isolated 1,3‐di(1‐pyrenyl)propane and the probe adsorbed into a low‐density polyethylene (LDPE) matrix model. The probability of formation of each molecular conformer at several temperatures was calculated using these theoretical techniques. Conformational statistical analysis of the four torsion angles (ϕ1, ϕ2, θ1, θ2) of Py3MPy showed that the angles —C—Car— (ϕ1, ϕ2) present two states c ± = ±90°; and the angles —C—C— (θ1, θ2), the three trans states = 180°, g ± = ±60°. The correlation of θ1–θ2 torsion angles showed that the most probable pairs were g+g– and g–g+ for the excimer‐like specimens, although these angles are distorted because of interactions with the polymer matrix. The temperature dependence of the excimer‐formation probability revealed that this process was thermodynamically controlled in the isolated case. When the probe was adsorbed into the LDPE matrix, the excimer formation process was reversed at T = 375 K. At T > 375 K, the behavior was similar to the isolated case but, at T < 375 K, excimer formation probability increased with temperature as found experimentally by steady‐state fluorescence spectroscopy. This temperature was coincident with the onset of the LDPE melting process, determined experimentally by thermal analysis. 相似文献
(1RS,3RS,4RS,10SR)‐2,2,3,10‐Tetrabromo‐1,2,3,4‐tetrahydro‐1,4‐ethanonaphthalene, C12H10Br4, (I), is the first structure to be reported with four Br atoms bound to a 1,4‐ethanonaphthalene framework and also the first which possesses three Br atoms in exo positions. Interactions between the Br atoms [three short intramolecular Br...Br distances of 3.1094 (4), 3.2669 (4) and 3.4415 (5) Å] have little effect on the C—C bond lengths but lead to significant twisting of the cage structure compared with the parent hydrocarbon, which is expected to be fully eclipsed at the two saturated C2H4 bridge positions. Chemically related (1SR,4RS)‐2,3‐dibromo‐1,4‐ethenonaphthalene, C12H8Br2, (II), obtained by double dehydrobromination of (I), represents the first structure of any halogen‐substituted benzobarrelene. This cis‐dibromide shows little evidence of steric congestion at the double bond [Br...Br = 3.5276 (8) Å] as a consequence of the large C—C—Br angles [average C=C—Br angle = 126.15 (10)°]. 相似文献
Methyl (2E,4R)‐4‐hydroxydec‐2‐enoate, methyl (2E,4S)‐4‐hydroxydec‐2‐enoate, and ethyl (±)‐(2E)‐4‐hydroxy[4‐2H]dec‐2‐enoate were chemically synthesized and incubated in the yeast Saccharomyces cerevisiae. Initial C‐chain elongation of these substrates to C12 and, to a lesser extent, C14 fatty acids was observed, followed by γ‐decanolactone formation. Metabolic conversion of methyl (2E,4R)‐4‐hydroxydec‐2‐enoate and methyl (2E,4S)‐4‐hydroxydec‐2‐enoate both led to (4R)‐γ‐decanolactone with >99% ee and 80% ee, respectively. Biotransformation of ethyl (±)‐(2E)‐4‐hydroxy(4‐2H)dec‐2‐enoate yielded (4R)‐γ‐[2H]decanolactone with 61% of the 2H label maintained and in 90% ee indicating a stereoinversion pathway. Electron‐impact mass spectrometry analysis (Fig. 4) of 4‐hydroxydecanoic acid indicated a partial C(4)→C(2) 2H shift. The formation of erythro‐3,4‐dihydroxydecanoic acid and erythro‐3‐hydroxy‐γ‐decanolactone from methyl (2E,4S)‐4‐hydroxydec‐2‐enoate supports a net inversion to (4R)‐γ‐decanolactone via 4‐oxodecanoic acid. As postulated in a previous work, (2E,4S)‐4‐hydroxydec‐2‐enoic acid was shown to be a key intermediate during (4R)‐γ‐decanolactone formation via degradation of (3S,4S)‐dihydroxy fatty acids and precursors by Saccharomyces cerevisiae.相似文献
A new three‐dimensional open‐framework cobalt‐zinc phosphite [Co(H2O)4Zn(HPO3)2]·H2O ( 1 ), has been prepared under hydrothermal conditions and characterized by single‐crystal X‐ray diffraction, XRD, IR and SQUID magnetometer. The compound crystallizes in the triclinic space group with a = 7.552(5), b = 7.671(5), c = 9.443(5) Å, α = 88.538(5)°, β = 89.109(5)°, γ = 87.056(5)°, V = 546.1(6) Å3, Z = 2. The structure of 1 consists of corner‐shared (cs) four‐membered‐ring chains formed by alternating ZnO4 tetrahedra and HPO3 pseudopyramids, which are further linked through CoO2(H2O)4 octahedra giving rise to a three‐dimensional (3‐D) neutral open‐framework with intersecting 12‐ and 16‐MR channels. The synthesis of system required the presence of L‐histidine which is not incorporated into the structure of the product. It is noteworthy that compound 1 represents the only known example of purely inorganic open‐framework cobalt‐zinc phosphite. 相似文献
The title compound {systematic name: 4‐amino‐5‐cyclopropyl‐7‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl)‐7H‐pyrrolo[2,3‐d]pyrimidine}, C14H18N4O3, exhibits an anti glycosylic bond conformation, with the torsion angle χ = −108.7 (2)°. The furanose group shows a twisted C1′‐exo sugar pucker (S‐type), with P = 120.0 (2)° and τm = 40.4 (1)°. The orientation of the exocyclic C4′—C5′ bond is ‐ap (trans), with the torsion angle γ = −167.1 (2)°. The cyclopropyl substituent points away from the nucleobase (anti orientation). Within the three‐dimensional extended crystal structure, the individual molecules are stacked and arranged into layers, which are highly ordered and stabilized by hydrogen bonding. The O atom of the exocyclic 5′‐hydroxy group of the sugar residue acts as an acceptor, forming a bifurcated hydrogen bond to the amino groups of two different neighbouring molecules. By this means, four neighbouring molecules form a rhomboidal arrangement of two bifurcated hydrogen bonds involving two amino groups and two O5′ atoms of the sugar residues. 相似文献
The title compound, C11H15NO2, crystallized in the centrosymmetric space group P21/n with one molecule in the asymmetric unit. There is a single intermolecular hydrogen bond, in which the Ndonor?Oacceptor distance is 3.0374 (11) Å and the N—H?O angle is 171.0 (12)°. The single intramolecular hydrogen bond has an Odonor?Oacceptor distance of 2.6279 (11) Å and an O—H?O angle of 161.8 (14)°. The four leading intermolecular C—H?O interactions have H?O distances ranging from 2.52 to 2.65 (2) Å and C—H?O angles ranging from 125.2 (9) to 143°. Chains of interactions form two‐dimensional networks. 相似文献
The title complex, C17H9N5·C6H4S4, contains π‐deficient bis(dinitrile) and TTF molecules stacked alternately in columns along the a‐axis direction; the interplanar angle between the TTF molecule and the isoindolinyl C4N[C(CN)2]2 moiety is 1.21 (4)°. The N‐allyl moiety in the TCPI molecule is oriented at an angle of 87.10 (10)° with respect to the five‐membered C4N ring, and the four C[triple‐bond]N bond lengths range from 1.134 (3) to 1.142 (3) Å, with C—C[triple‐bond]N angles in the range 174.3 (3)–176.9 (2)°. In the TTF system, the S—C bond lengths are 1.726 (3)–1.740 (3) and 1.751 (2)–1.763 (2) Å for the external S—C(H) and internal S—C(S) bonds, respectively. 相似文献
The crystal structure of methyl α‐d ‐mannopyranosyl‐(1→3)‐2‐O‐acetyl‐β‐d ‐mannopyranoside monohydrate, C15H26O12·H2O, ( II ), has been determined and the structural parameters for its constituent α‐d ‐mannopyranosyl residue compared with those for methyl α‐d ‐mannopyranoside. Mono‐O‐acetylation appears to promote the crystallization of ( II ), inferred from the difficulty in crystallizing methyl α‐d ‐mannopyranosyl‐(1→3)‐β‐d ‐mannopyranoside despite repeated attempts. The conformational properties of the O‐acetyl side chain in ( II ) are similar to those observed in recent studies of peracetylated mannose‐containing oligosaccharides, having a preferred geometry in which the C2—H2 bond eclipses the C=O bond of the acetyl group. The C2—O2 bond in ( II ) elongates by ~0.02 Å upon O‐acetylation. The phi (?) and psi (ψ) torsion angles that dictate the conformation of the internal O‐glycosidic linkage in ( II ) are similar to those determined recently in aqueous solution by NMR spectroscopy for unacetylated ( II ) using the statistical program MA′AT, with a greater disparity found for ψ (Δ = ~16°) than for ? (Δ = ~6°). 相似文献
The crystal structure of the title mixed azine, C17H17ClN2O, contains four independent molecules, A–D, and molecule B is disordered. All four molecules have an N—N gauche conformation, with C—N—N—C torsion angles of 136.5 (4), 137.0 (4), ?134.7 (4) and ?134.7 (4)°, respectively. The phenyl rings are also somewhat twisted with respect to the plane defined by Cipso and the imine bond. On average, the combined effect of these twists results in an angle of 64.7° between the best planes of the two phenyl rings. Arene–arene double T‐contacts are the dominant intermolecular interaction. The methoxy‐substituted phenyl ring of one azine molecule interacts to form a T‐contact with the methoxy‐substituted phenyl ring of an adjacent molecule and, similarly, two chloro‐substituted phenyl rings of neighboring molecules interact to form another T‐contact. The only exception is for molecule B, for which the disorder leads to the formation of T‐contacts between methoxy‐ and chloro‐substituted phenyl rings. The prevailing structural motif of T‐contact formation between like‐substituted arene rings results in a highly dipole‐parallel‐aligned crystal structure. 相似文献
Novel ternary phases, (Pd1?xZnx)18(Zn1?yAly)86?δ (0≤x≤0.162, 0.056≤y≤0.088, 0≤δ≤4), which adopt a superstructure of the γ‐brass type (called γ′‐brass), have been synthesized from the elements at 1120 K. Single‐crystal X‐ray structural analysis reveals a phase width (F$\bar 4$ 3m, a=18.0700(3)–18.1600(2) Å, Pearson symbols cF400–cF416), which is associated with structural disorder based on both vacancies as well as mixed site occupancies. These structures are constructed of four independent 26‐atom γ‐clusters per primitive unit cells and centered at the four special positions A (0, 0, 0), B (1/4, 1/4, 1/4), C (1/2, 1/2, 1/2) and D (3/4, 3/4, 3/4). Two of these, centered at B and C , are completely ordered Pd4Zn22 clusters, whereas the other two, centered at A and D , contain all structural disorder in the system. According to our single‐crystal X‐ray results, Al substitutions are restricted to the A ‐ and D ‐centered clusters. Moreover, the outer tetrahedron (OT) site of the 26‐atom cluster at D is completely vacant at the Al‐rich boundary of these phases. Electronic structure calculations, using the tight‐binding linear muffin‐tin orbital atomic‐spheres approximation (TB‐LMTO‐ASA) method, on models of these new, ternary γ′‐brass phases indicate that the observed chemical compositions and atomic distributions lead to the presence of a pseudogap at the Fermi level in the electronic density of states curves, which is consistent with the Hume‐Rothery interpretation of γ‐brasses, in general. 相似文献
The crystal structures of three products of the reaction of 2‐phenylphenol and BCl3 have been determined. The structures show intriguing packing patterns and an interesting case of pseudosymmetry. In addition, one of the two polymorphs has a primitive monoclinic crystal system, but it is twinned and emulates an orthorhombic C‐centred structure. Tris(biphenyl‐2‐yl) borate, C36H27BO3, ( III ), crystallizes with only one molecule in the asymmetric unit. The dihedral angles between the planes of the aromatic rings in the biphenyl moieties are 50.47 (13), 44.95 (13) and 42.60 (13)°. The boron centre is in a trigonal planar coordination with two of the biphenyl residues on one side of the BO3 plane and the remaining biphenyl residue on the other side. One polymorph of 10‐oxa‐9‐boraphenanthren‐9‐ol, C12H9BO2, ( V a ), crystallizes with two almost identical molecules (r.m.s. deviation of all non‐H atoms = 0.039 Å) in the asymmetric unit. All non‐H atoms lie in a common plane (r.m.s. deviation = 0.015 Å for both molecules in the asymmetric unit). The two molecules in the asymmetric unit are connected into dimers via O—H...O hydrogen bonds. A second polymorph of 10‐oxa‐9‐boraphenanthren‐9‐ol, ( V b ), crystallizes as a pseudo‐merohedral twin with two almost identical molecules (r.m.s. deviation of all non‐H atoms = 0.035 Å) in the asymmetric unit. All non‐H atoms lie in a common plane (r.m.s. deviation = 0.012 Å for molecule 1 and 0.014 Å for molecule A). Each of the two molecules in the asymmetric unit is connected into a centrosymmetric dimer via O—H...O hydrogen bonds. The main difference between the two polymorphic structures is that in ( V a ) the two molecules in the asymmetric unit are hydrogen bonded to each other, whereas in ( V b ), each molecule in the asymmetric unit forms a hydrogen‐bonded dimer with its centrosymmetric equivalent. 9‐[(Biphenyl‐2‐yl)oxy]‐10‐oxa‐9‐boraphenanthrene, C24H17BO2, ( VI ), crystallizes with four molecules in the asymmetric unit. The main differences between them are the dihedral angles between the ring planes. Apart from the biphenyl moiety, all non‐H atoms lie in a common plane (r.m.s. deviations = 0.026, 0.0231, 0.019 and 0.033 Å for molecules 1, A, B and C, respectively). This structure shows pseudosymmetry; molecules 1 and A, as well as molecules B and C, are related by a pseudo‐translation of about in the direction of the b axis. Molecules 1 and B, as well as molecules A and C, are related by a pseudo‐inversion centre at ,,. Neither between molecules 1 and C nor between molecules A and B can pseudosymmetry be found. 相似文献