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Stereochemistry
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
Enantiomers are defined as stereoisomers that have non-superimposable mirror images. When a mirror is held up to a molecule possessing a stereogenic center, its mirror image is observed. If one tries to superimpose (match every atom in both molecules by laying one on the other), a chiral molecule and its mirror image are not superimposable. They are, therefore, different molecules. A more precise definition is that two stereoisomers that are non-superimposable mirror images are enantiomers. It is important to understand that recognizing enantiomers as stereoisomers is an important part of the definition. How can enantiomers of 2-chlorobutane be compared?
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Published in Luis Liz-Marzán, Colloidal Synthesis of Plasmonic Nanometals, 2020
Andrés Guerrero-Martinez, José Lorenzo Alonso-Gómez, Baptiste Auguie, M. Magdalena Cid, Luis M. Liz-Marzán
Within the generic class of individual chirality, three main types can be identified (Fig. 11.3) [15]: (a) the presence of chiral ligands can favor the growth of an intrinsically chiral core; (b) for achiral cores, optical activity can be induced by a chiral shell through vicinal effects or through a chiral electrostatic field; and (c) in an originally achiral core the relaxation of the surface atoms involved in the adsorption of the chiral ligand may create a chiral footprint. Most of the chiral nanoparticles reported to date with individual dissymmetry have been mainly sub-nanometer clusters (see the next section, Nanoparticles with Individual Dissymmetry); their chirality originating from combinations of (a), (b), and/or (c) mechanisms. The corresponding CD responses have been restricted to the UV region of the spectrum (outside the usual regime of LSPRs), with relatively low optical activity efficiencies (Fig. 11.2).
Asymmetric Centers, Functional Groups, and Characterization
Published in Armen S. Casparian, Gergely Sirokman, Ann O. Omollo, Rapid Review of Chemistry for the Life Sciences and Engineering, 2021
Armen S. Casparian, Gergely Sirokman, Ann O. Omollo
Enantiomers are a pair of stereoisomers that are nonsuperimposable mirror images. A molecule with one asymmetric center will exist in two enantiomeric forms. An asymmetric center is an atom in a molecule that is bonded to four different atoms/groups of atoms. Enantiomeric molecules have no plane of symmetry and are said to be chiral (Figure 9.1). Conversely, molecules with a plane of symmetry are said to be achiral. Achiral molecules are superimposable onto their mirror images (Figure 9.2).
The method of fundamental solutions for scattering of electromagnetic waves by a chiral object
Published in Applicable Analysis, 2023
E. S. Athanasiadou, I. Arkoudis
The interaction of electromagnetic fields with chiral materials has been of interest since such materials can be met in various natural and artificial objects. A material is considered to be chiral if it is non-superimposable with its mirror image. The interest of chiral materials can be met in many areas of science. In physics, optical activity occurs in chiral materials and has various applications in optics [10]. In chemistry, a characteristic example of chiral structures are the enantiomers which have applications in the pharmaceutical industry [11]. In geology, chiral crystal structures (like quartz crystals) are used in industry such as GPS and cell phone equipment [12]. Another interesting example is the chiral artificial materials met in the engineering industry [13,14]. This type of material can have a lot of interesting properties like auxetic effects or twisting in response to a linear force [15,16]. Chiral media are categorized under the wider class of bianisotropic media and they are characterized by a set of constitutive relations such as the Drude–Born–Fedorov used in the present work [17]. The scattering of time-harmonic electromagnetic waves by a chiral scatterer has been studied in [18]. The well-posedness of these problems using Beltrami fields together with their associated vector potentials and the boundary integral equations has been studied in [19–21]. Recently, inverse scattering problems in chiral media using the linear sampling method, Herglotz functions and the reciprocity gap functional method have been studied in [22,23].
Mirror symmetry breaking in liquids and liquid crystals
Published in Liquid Crystals, 2018
Molecules are usually divided into chiral and achiral, where chiral is commonly used for permanently chiral molecules (lacking mirror symmetry) and all others are considered as achiral. However, strictly speaking, all molecules involving at least four atoms can assume chiral conformations. Common organic molecules, including those forming LC phases, are significantly above this limit and thus can be divided into permanently chiral, transiently chiral and prochiral, depending on the relative energies of chiral and achiral conformations/configurations [7,8]. Molecules are considered as transiently chiral if enantiomorphic conformations/configuration represent the energy minima in an equilibrium, and the enantiomerisation barrier is low with respect to kT that they coexist in a thermodynamic equilibrium. If the enantiomerisation barrier becomes sufficiently high (≫ kT) that no enantiomerisation takes place at the given temperature and in the considered time frame, then these molecules are permanently chiral. Prochiral molecules have non-chiral lowest energy conformations, but can adopt chiral conformations after application of an external twisting force [7,8].
The effect of chiral dopant on the rewriting speed of optically driving liquid crystal display
Published in Liquid Crystals, 2020
J. T. Sun, K. L. Deng, J. X. Sang, X. H. Gong, Y. Liu, J. H. Shang, H. Liu, Y. H. Zhang, Q. Guo, V.G. Chigrinov
The LCs consisting of molecules without reflectional symmetry are considered here. The molecules are different from their mirror images and are called chiral molecules. Such an example is S811, a left-hand chiral dopant. It can be regarded as a screw, instead of a rod, in considering its physical properties. After considering the symmetry, the generalised elastic energy density is