China
Africa
Explore chapters and articles related to this topic
Clay Mineral Catalysis of Isomerization, Dimerization, Oligomerization, and Polymerization Reactions
Published in Benny K.G. Theng, Clay Mineral Catalysis of Organic Reactions, 2018
The isomerization of an organic molecule/compound refers to its rearrangement into a new molecule/compound with the same number and type of atoms but differing in bonding arrangement. As the term suggests, dimerization is the process of combining two structurally identical compounds (monomers) through covalent bonding or other bonding modes. For our purposes, oligomerization denotes the process of combining a few (3–50) monomers into a molecular entity, while polymerization refers to the process of converting a monomer, or a mixture of monomers, into a large chain-like or network molecule. Since organic reactions over acid-activated clay minerals, such as K10 montmorillonite, take place in the proton-rich mesopores and interlayers, there is scope for shape-selective and dimensionally-confined transformations.
Laser Ionization Techniques
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
Despite the high resolution achieved in ZEKE spectroscopy, the technique is not free of certain drawbacks because electron detection, in the absence of mass information, can potentially give ambiguous or uninterpretable results. In particular, such ambiguity can occur in situations when more than one molecular entity may be ionized simultaneously, such as, e.g., in cluster, radical, and vdW complex spectroscopy, or when the investigated sample is a product of a complex reaction chain. Different techniques have been developed to overcome this limitation; the two most common ones are (1) photoelectron–photoion coincidence measurements, which simultaneously record both charged particles, albeit with very low acquisition rates, and (2) MATI spectroscopy, in which only ions produced at ionization thresholds are measured (MATI was originally developed by Zhu and Johnson 1991).
Halogen Bond Catalysis
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
According to the definition proposed by IUPAC, “A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity.”1 Halogen bond is depicted by “⋯” in the halogen-bonded complex R-X⋯Y; X is a halogen, and Y is an electron donor or Lewis base (Figure 4.1). The basic features of halogen bond are similar to that of hydrogen bond and they are: (a) interatomic distance between X and Y is usually less than the sum of the van der Waals radii, (b) analysis of the electron density topology usually shows a bond path and a (3,−1) bond critical point, (c) nuclear magnetic resonance (NMR) chemical shifts are usually affected by formation of halogen bond, (d) the IR and Raman scattering frequencies of both R-X and Y are affected by halogen bond formation; new vibration modes associated with X⋯Y are also observed, and (e) UV–vis bands of the halogen bond donor usually shift to shorter wavelengths.1
Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base complexes and their iodomethane oxidative addition kinetics
Published in Journal of Coordination Chemistry, 2020
Pennie P. Mokolokolo, Alice Brink, Andreas Roodt, Marietjie Schutte-Smith
The neutral rhodium(I) complex 1a contains a complete molecular entity in the asymmetric unit, while 2a crystallizes with an acetone molecule (see Figure 1 for coordination compounds). In both 1a and 2a, the rhodium centers are coordinated by the carbonyl and the phosphorus ligands, and the square planar geometry is completed by the bidentate chelating Schiff base ligands 2-(cyclopentyliminomethyl)-5-methylphenolate and 2-(cyclohexyliminomethyl)phenolate for 1a and 2a, respectively, forming six-membered chelate rings. The square planar geometry around the metal center in both complexes is distorted as illustrated by the N1-Rh-O1, O1-Rh-C01 and C01-Rh-P angles of 88.16(19), 173.6(2) and 91.7(2)°, respectively, for 1a, compared with N1-Rh-O1, O1-Rh-C01 and CO1-Rh-P angles of 89.35(17), 175.7(2) and 86.68(19)°, respectively, for 2a.
The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules
Published in Molecular Physics, 2019
Robert A. Skutnik, Louis Lehmann, Sergej Püschel-Schlotthauer, George Jackson, Martin Schoen
In fact, the experimental confirmation of biaxial nematics has remained quite murky and to some extent controversial. For example, to realise thermotropic liquid crystals of biaxial symmetry experimentally, one can ‘glue’ together rod- and disc-like molecules as demonstrated by Hunt et al. [11]. At the end of their paper, these authors claim that ‘[…] the combination of rod and disc units in a single molecular entity has created a highly biaxial molecule. This approach could well provide an example of a low-molecular mass, thermotropic biaxial nematic'. Referring to the paper by Hunt et al. [11], Madsen et al. [10] emphasise that ‘Even extravagant molecular architectures […] have failed to exibit the [biaxial nematic] phase '. Madsen et al. [10] used 2H NMR to provide evidence for a biaxial phase in a liquid crystal in which the mesogens have a boomerang-shaped oxadiazole unit. In other cases smectic phases of biaxial symmetry have also been observed [12].