Achiral Crystal Research Articles

Complete band gaps in one-dimensional photonic crystals with negative refraction arising from strong chirality

In this paper, we propose the idea of using chiral negatively refractive media to realize three-dimensional (3D) complete band gaps in the one-dimensional (1D) photonic crystals with a two-layer unit cell. We explain why at least a three-layer unit cell is necessary for realizing the 3D complete band gaps in 1D achiral photonic crystals, and show how we can reduce the number of layers of the unit cell to two if chirality is introduced. Although the media with strong chirality supply negative refraction only for one of the two eigenwaves, the complete band gaps could be still obtained with an even simpler lattice structure compared to the achiral counterpart.

Solubility measurement of a metastable achiral crystal of sodium chlorate in solution growth

The solubility of the metastable achiral monoclinic phase in NaClO3 crystallization from an aqueous solution, which appears prior to the nucleation of chiral crystals, was successfully measured in the range from 10°C to 23°C. Antisolvent crystallization method was used to obtain metastable crystals for the measurement. The solubility was determined to be about 1.6 times higher than that of the chiral stable cubic phase by observing growth or dissolution of the crystal in aqueous solution at the temperature and concentration of which is predetermined.

Entropic chiral symmetry breaking in self-organized two-dimensional colloidal crystals.

Long-range chiral symmetry breaking (CSB) has been recently observed in 2D self-organized rhombic crystals of hard, achiral, 72 degree rhombic microparticles. However, purely entropic selection of a CSB crystal in an idealized system of hard achiral shapes, in which attractions are entirely absent and the shape does not dictate a chiral tiling, has not yet been quantitatively predicted. Overcoming limitations of a purely rotational cage model, we investigate a translational-rotational cage model (TRCM) of dense systems of hard achiral rhombs and quantitatively demonstrate that entropy can spontaneously drive the preferential self-organization of a chiral crystal composed of achiral shapes that also tile into an achiral crystal. At different particle area fractions, ϕA, we calculate the number of accessible translational-rotational microstates, Ω, of a mobile central rhomb in a static cage of neighboring rhombs, which can have different orientation angles, γ, relative to the bisector of the crystalline axes. As we raise ϕA, two maxima emerge in Ω(γ) at non-zero cage orientation angles, ±γmax. These maxima correspond to additional translational microstates that become accessible in the CSB crystalline polymorph through reduced translational tip-tip interference. Thus, entropy, often associated with structural disorder, can drive CSB in condensed phase systems of non-attractive achiral objects that do not tile into chiral structures. The success of the TRCM in explaining the entropic origin of CSB in systems of hard rhombs indicates that the TRCM will have significant utility in predicting the self-organized behavior of dense systems of other hard shapes in 2D.

Achiral Metastable Crystals of Sodium Chlorate Forming Prior to Chiral Crystals in Solution Growth

Chiral symmetry breaking in NaClO3 crystallization from an aqueous solution with perturbations has been of great interest. To understand the mechanism, several models focusing on the early stage of the crystallization have been proposed. However, they are ambiguous because the early stage has been barely explored directly. Here, we investigate the early stages of the crystallization process driven by droplet evaporation using a combination of direct in situ microscopic observations and cryogenic single-crystal XRD experiments. We demonstrate that an achiral crystal having P21/a symmetry, which is newly discovered for a solution growth, first appears in the droplet and then transforms into the chiral crystals. Additionally, determination of the lattice constants by XRD experiments (a = 8.42 A, b = 5.26 A, c = 6.70 A, β = 109.71°) revealed that the achiral phase should be identical to Phase III (a = 8.78 A, b = 5.17 A, c = 6.83 A, β = 110°), which is a high-temperature phase from a melt growth of NaClO3. We...

Enantiodivergent formation of a chiral cytosine crystal by removal of crystal water from an achiral monohydrate crystal under reduced pressure

An achiral nucleobase cytosine forms an achiral monohydrate crystal (space group: P21/c) by crystallization from a water solution. It was found that the removal of crystal water under reduced pressure at room temperature afforded a chiral crystal of anhydrous cytosine (P212121). The crystal chirality of anhydrous cytosine corresponds to the enantiotopic crystal face of the achiral monohydrate; therefore, when the enantiotopic b1-face is exposed to the reduced pressure, dehydration occurred in the direction from the b1-face to provide [CD(+)310KBr]-cytosine crystal. In contrast, dehydration from the b2-face gave the opposite enantiomorphous [CD(−)310KBr]-cytosine crystal. The correlation between enantiotopic faces and the formed crystal chirality is opposite to that from dehydration by heating. The formed chiral cytosine crystals act as a chiral trigger for asymmetric autocatalysis with enantioenrichment amplification of pyrimidylalkanol.

Light-driven Bending of Polymer Films in Which Salicylidenephenylethylamine Crystals are Aligned Magnetically

Abstract Silicone polymer films in which plate-like chiral and achiral crystals of salicylidenephenylethylamines were aligned by a magnetic field were prepared. The polymer films exhibited reversible photomechanical bending upon alternate irradiation with UV and visible light.

Light-Driven Bending Crystals of Salicylidenephenylethylamines in Enantiomeric and Racemate Forms

The photomechanical bending behavior of chiral crystals composed of S- and R-enantiomers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in enol form [enol-(S)-1 and enol-(R)-1] has been compared with that of achiral crystals of the racemic compound [enol-(rac)-1] of S- and R-enantiomers. Both platelike chiral and achiral crystals, a few millimeters in length, exhibited similar reversible bending upon alternate irradiation with ultraviolet (UV) and visible light. The reversible bending of the achiral enol-(rac)-1 crystal could be repeated over 100 times, while the tip displacement angles of the chiral enol-(S)-1 and enol-(R)-1 crystals gradually decreased with repeated cycles. In situ X-ray measurements revealed that the bending motion of the chiral and achiral crystals was caused by a slight shrinkage of the a and b axes of the unit cell, respectively, corresponding to the long axis of the platelike crystals; shrinkage was induced by the shape change of component salicylidenephenylethy...

Synthesis and characterization of Cu(II), Ni(II), and Fe(III) complexes of 5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin

We have synthesized three complexes of 5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin (ENTPP), [Cu(ENTPP)], [Ni(ENTPP), and [Fe(ENTPP)Cl]. X-ray crystallography reveals that copper and nickel species are four-coordinate, while the iron complex is five-coordinate. The porphyrin in all the three species maintains similar conformations, in which rotational hindrance makes the ester group at the 8-position of naphthyl lie above the porphyrin plane leading to axial chirality in the single molecule. However, only chiral crystals of [Cu(ENTPP)] were obtained by spontaneous resolution upon crystallization, while [Ni(ENTPP) and [Fe(ENTPP)Cl] were obtained as achiral crystals of racemic compounds. CD spectra have also confirmed the chirality of the crystalline samples. The origin of CD signals has also been discussed.

Achiral Organic, Inorganic, and Metal Crystals as Auxiliaries for Asymmetric Transformations

AbstractThe use of achiral crystalline architectures as intermediate auxiliaries for the performance of “absolute” asymmetric transformations is reviewed. Such architectures are delineated, in some cases, by pairs of homochiral surfaces of opposite handedness. This phenomenon is more common among organic crystals that frequently appear in triclinic, monoclinic, orthorhombic, or tetragonal space groups. Consequently, the chiral surfaces of such crystals have been shown to display enantiomeric recognition for molecules of the environment, a process that has been instrumental in the conversion of achiral host crystals into enantiomorphous solid‐solutions, for a successful performance of “absolute” asymmetric transformations and for the control of crystal polymorphism. Mixed crystals of reduced symmetry display properties such as second harmonic generation or pyroelectricity. On the other hand, achiral faces delineate metals, which crystallize in cubic space‐groups of high symmetry. However, by slicing such crystals in particular directions, they might express homochiral high Miller index faces that contain homochiral kink sites, which have been successfully exploited in electrochemical separations of sugars and for the resolution of enantiomers by enantioselective desorption. Representative examples of each class of materials are described.

Enantioselective C-C bond formation as a result of the oriented prochirality of an achiral aldehyde at the single-crystal face upon treatment with a dialkyl zinc vapor.

Enantioselective C-C bond formation as a result of the oriented prochirality of an achiral aldehyde at the single-crystal face upon treatment with a dialkyl zinc vapor.

Enantioselective CC Bond Formation as a Result of the Oriented Prochirality of an Achiral Aldehyde at the Single‐Crystal Face upon Treatment with a Dialkyl Zinc Vapor

Enantioselective CC Bond Formation as a Result of the Oriented Prochirality of an Achiral Aldehyde at the Single‐Crystal Face upon Treatment with a Dialkyl Zinc Vapor

Controlling Mesoscale Crystal Helicity with Additives, Again

Various researchers recently reported that they had first controlled the mesoscale helicity of achiral crystals with the use of chiral additives. In fact, Wallerant showed in 1907 that resorcinol crystals, among others, grew from the melt as helical fibers whose sense of twist was determined by resolved additives. Wallerant’s demonstration is reproduced herein and his observations are given interpretation via computation of adsorption energetics through simulated annealing.

Molecular modelling of the incorporation of habit modifying additives: α-glycine in the presence of l-alanine

Molecular modelling has been applied to study the mechanism through which L-alanine, acting as an impurity, disrupts the growth and modifies the morphology of α-glycine crystals. L-alanine was selected for study because of its structural similarity to glycine differing only by the substitution of a methyl group for a hydrogen atom. An approach employing an atom-atom formalism has been extended to model the effects of the chiral molecule, L-alanine, when it becomes incorporated as an impurity in α-glycine (achiral) crystals. A number of cases are treated which provide a quantitative measure of the changes in lattice-site energy within the host α-glycine lattice accompanying incorporation of an L-alanine molecule. Both the conformation of the L-alanine molecule and the possibility of creating a site vacancy in the host lattice are taken into account. Slice and attachment energies are calculated for the significant growth forms {020}, {011} and {110} in the presence of L-alanine. A new approach is described which employs a weighted average of attachment energies calculated for the pure host, and host modified by additive, to predict crystal morphology and its dependence on the proportion of lattice sites at which host molecules are replaced by additive molecules. The results show that whereas L-alanine is likely to incorporate both at the growing (020) (which has the pro-chiral R hydrogen atom, bonded to the α carbon, orientated normal to the surface) and (00) crystal surface of α-glycine (which has the pro-chiral S hydrogen, atom bonded to the α carbon, orientated normal to the surface), the reduction in the rate of growth perpendicular to the (00) surface, ascribed from the reduction in the associated relative attachment-energy, is greater. The crystal morphologies predicted are found to be in general agreement with experimental observations reported in the literature.

Irreversible transformation of chiral to achiral polymorph of K[Co(HCOO)3]: synthesis, structures, and magnetic properties

We report the rare observation of an irreversible transformation from a chiral to an achiral crystal through the syntheses of two polymorphs of K[Co(HCOO)(3)], and their characterization using single and powder crystal X-ray diffractions, infrared spectroscopy, thermal analyses, and the study of their magnetic properties. The kinetic phase (1), obtained under a high concentration of reactants, has a chiral (4(9)·6(6)) framework and adopts the hexagonal space-group P 6(3)22. The thermodynamic phase (2), obtained from dilute solution, has an achiral (4(12)·6(3)) framework and adopts a monoclinic space group C 2/c. Once 1 has crystallized out from the concentrated solution, it dissolves slowly at the expense of the formation of 2. In the solid state 1 is irreversibly transformed to 2 very slowly at ambient conditions. The transformation is accompanied by an increase of the crystal density, a change of coordination mode of some formates from anti-anti to syn-anti and a change in the coordination number of K from 6 to 8. 1 is a spin-canted antiferromagnet (T(N) = 8.3 K) displaying two steps in the isothermal magnetization at 2 K, a spin-flop at 1.2 kOe and a flip to paramagnetic phase at 42.0 kOe. 2 is an antiferromagnet (T(N) = 2.0 K). Thermal properties and IR spectra of the two polymorphs have been studied, showing some differences.

Solid-to-solid formation at the solid–liquid interface leading to a chiral coordination polymer from an achiral monomer

An achiral crystal of a simple mononuclear copper complex [Cu(II)(C(6)H(8)N(2))(2)SO(4)]·H(2)O (1), on dipping into an aqueous azide solution, transforms into a chiral crystal of a coordination polymer [Cu(II)(C(6)H(8)N(2))(N(3))(2)](n) (2) in a solid-liquid interface reaction demonstrating replacement of a sulfate anion by an azide anion from an aqueous solution.

Generation of absolute controlled crystal chirality by the removal of crystal water from achiral crystal of nucleobase cytosine.

The enantioselective formation of chiral crystal of achiral nucleobase cytosine was achieved mediated by the crystal direction selective dehydration of crystal water in the achiral crystal of cytosine monohydrate (P21/c). Heat transfer from the enantiotopic face of the single crystal of cytosine monohydrate afforded the enantiomorphous crystal of anhydrous cytosine.

Solid-state Transformation of Achiral Crystals to a Chiral Crystal by Guest Release from Host–Guest Inclusion Crystals of Achiral Host Compound

Abstract Solid-state transformation of achiral crystals into a chiral crystal was induced by thermal guest release from the host–guest inclusion complexes of 2,5-bis(hydroxydiphenylmethyl)thieno[3,2-b]thiophene as a host component. The chilarity of the resulting guest-free crystals, also obtained by recrystallization from dichloromethane, is attributed to conformational freezing of the achiral molecule into a chiral molecular structure in the solid state.

Crystal structures of (2-substituted-5- N-tosyl)bicyclo[3.3.0]-5-azacyclooct-2-enone: a pseudo achiral crystal from enantiopure compound and a counter-example of Wallach’s rule

An unusual crystallization affording an apparently pseudoachiral crystal from single enantiomers 1a and 1b was observed. In cases of enantiopure molecules such as 1a and 1b with structural flexibility, the tendency toward a centrosymmetric arrangement was so overwhelming that apparently achiral crystals were formed even by homochiral compounds via the formation of a pseudo-centrosymmetric asymmetric unit accompanying structural adjustment. In addition, these crystals of 1a constitute as a counter-example of Wallach’s rule.

Optical Rotation of Achiral Compounds

Oriented achiral molecules and crystals with D(2d) symmetry or one of its non-enantiomorphous subgroups, S(4), C(2v), or C(s), can rotate the plane of transmitted polarized light incident in a general direction. This well-established fact of crystal optics is contrary to the teaching of optical activity to students of organic chemistry. This Minireview gives an overview of the measurement and calculation of the chiroptical properties of some achiral compounds and crystals. Methane derivatives with four identical ligands related by reflection symmetry are quintessential optically inactive compounds according to the logic of van't Hoff. Analysis of the optical activity of simple achiral compounds such as H(2)O and NH(3) provides general aspects of chiroptics that are not readily broached when considering chiral compounds exclusively. We show here, through the use of group theoretical arguments, the transformation properties of tensors, and diagrams, why some achiral, acentric compounds are optically active while others are not.

Asymmetric crystallization of bisguanidinobenzene derivatives

We show that the mono-N-methylated and -ethylated derivatives of the achiral compound bisguanidinobenzene undergo spontaneous asymmetric crystallization into a chiral form with chiral space group P2 12 12 1. The absolute configurations of the chiral crystals were determined by X-ray crystallography and correlated with circular dichroism (CD) spectra recorded in the solid state. The corresponding protonated and isopropylated derivatives, by contrast, afforded achiral crystals.