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Topic Review
Asymmetric Recognition by Chiral Porphyrinoids
Over the years, chiral discrimination of chiral molecules such as amino acids, alcohols, amines, hydroxy-carboxylic acids, etc. has aroused the interest of the scientific community. Thus, numerous studies have reported the possibility to discriminate several organic guests by using both symmetric and asymmetric porphyrin derivatives. Nevertheless, this entry exclusively focuses on chiral porphyrinoids as probes for asymmetric recognition and sensing, illustrating the main aspects concerning the chiral recognition phenomena of a multitude of chiral organic guests through several chiral mono- and bis-porphyrins via different spectroscopic techniques.
  • 731
  • 17 Nov 2021
Topic Review
Asymmetric Synthesis of BINOL Derivatives
The chirality resulting from restricted rotation around a single bond is called atropisomerism (axial chirality). This phenomenon was first described by Christie and Kenner in 1922 when investigating the biaryl 6,6’-dinitro-2,2’-diphenic acid, and the term “atropisomer”, derived from the Greek where “a” means “not” and “tropes” means “turn”, was created by Kuhn. Atropisomers belong to the class of axially chiral compounds; however, here, the enantiomers exist due to restricted rotation around a single bond. Axial chirality has also been considered as an important structural element of many natural products and bioactive compounds, whose enantiomers generally exhibit different pharmacological activities and metabolic processes in vivo and in vitro.
  • 937
  • 10 Jan 2023
Topic Review
Atmospheric Pressure Plasma Surface Treatment
Atmospheric plasma treatment is an effective and economical surface treatment technique. The main advantage of this technique is that the bulk properties of the material remain unchanged while the surface properties and biocompatibility are enhanced.
  • 1.7K
  • 09 Apr 2021
Topic Review
Atomic and Molecular Layer Deposition's Mechanisms and Characteristics
Alkali metals (lithium, sodium, and potassium) are promising as anodes in emerging rechargeable batteries, ascribed to their high capacity or abundance. They are currently inhibited from public use by issues with interfacial reactivity, volume change, and dendrite growth. Surface coatings are among the most promising of techniques to address all of these issues. Atomic and molecular layer deposition (ALD and MLD) enable a variety of inorganic, organic, and even inorganic-organic hybrid materials, featuring accurate nanoscale controllability, low process temperature, and extremely uniform and conformal coverage. Coatings applied to alkali metals via ALD and MLD may offer some of the most scalable, tailorable, and effective surface coatings for safe and stable rechargeable anodes.
  • 425
  • 08 Oct 2022
Topic Review
Atomic Carbon
Atomic carbon, systematically named carbon and λ0-methane, also called monocarbon, is a colourless gaseous inorganic chemical with the chemical formula C (also written [C]). It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation. Atomic carbon is the simplest form of carbon, and is also the progenitor of carbon clusters. In addition, it may be considered to be the monomer of all (condensed) carbon allotropes like graphite and diamond.
  • 500
  • 28 Sep 2022
Topic Review
Atomic Force Microscopy
Atomic force microscopy (AFM) is one of the microscopic techniques with the highest lateral resolution. It can usually be applied in air or even in liquids, enabling the investigation of a broader range of samples than scanning electron microscopy (SEM), which is mostly performed in vacuum. Since it works by following the sample surface based on the force between the scanning tip and the sample, interactions have to be taken into account, making the AFM of irregular samples complicated, but on the other hand it allows measurements of more physical parameters than pure topography.
  • 925
  • 06 Apr 2022
Topic Review
Atomic Force Microscopy in Biological Systems
Biological interactions often involve the transport of molecules, ions, or other species across biological membranes or between interacting proteins. The understanding of these transport phenomena is crucial for the development of therapies for various diseases. Atomic force microscopy is a powerful tool that has been increasingly used to study biological systems at the nano scale.
  • 245
  • 23 Aug 2023
Topic Review
Atomic Layer Deposition on 2D Materials
Atomic layer deposition (ALD) of high-κ dielectrics on two-dimensional (2D) materials (including graphene and transition metal dichalcogenides) still represents a challenge due to the lack of out-of-plane bonds on the pristine surfaces of 2D materials, thus making the nucleation process highly disadvantaged. The typical methods to promote the nucleation (i.e., the predeposition of seed layers or the surface activation via chemical treatments) certainly improve the ALD growth but can affect, to some extent, the electronic properties of 2D materials and the interface with high-κ dielectrics. Hence, direct ALD on 2D materials without seed and functionalization layers remains highly desirable. In this context, a crucial role can be played by the interaction with the substrate supporting the 2D membrane. In particular, metallic substrates such as copper or gold have been found to enhance the ALD nucleation of Al2O3 and HfO2 both on monolayer (1 L) graphene and MoS2. Similarly, uniform ALD growth of Al2O3 on the surface of 1 L epitaxial graphene (EG) on SiC (0001) has been ascribed to the peculiar EG/SiC interface properties.
  • 881
  • 15 Dec 2021
Topic Review
Atomic Radii of the Elements (Data Page)
The atomic radius of a chemical element is the distance from the center of the nucleus to the outermost shell of an electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Depending on the definition, the term may apply only to isolated atoms, or also to atoms in condensed matter, covalently bound in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical models. Under some definitions, the value of the radius may depend on the atom's state and context. Atomic radii vary in a predictable and explicable manner across the periodic table. For instance, the radii generally decrease rightward along each period (row) of the table, from the alkali metals to the noble gases; and increase down each group (column). The radius increases sharply between the noble gas at the end of each period and the alkali metal at the beginning of the next period. These trends of the atomic radii (and of various other chemical and physical properties of the elements) can be explained by the electron shell theory of the atom; they provided important evidence for the development and confirmation of quantum theory.
  • 758
  • 02 Dec 2022
Topic Review
Atomic Radius
The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Four widely used definitions of atomic radius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Typically, because of the difficulty to isolate atoms in order to measure their radii separately, atomic radius is measured in a chemically bonded state; however theoretical calculations are simpler when considering atoms in isolation. The dependencies on environment, probe, and state lead to a multiplicity of definitions. Depending on the definition, the term may apply to atoms in condensed matter, covalently bonding in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical models. The value of the radius may depend on the atom's state and context. Electrons do not have definite orbits nor sharply defined ranges. Rather, their positions must be described as probability distributions that taper off gradually as one moves away from the nucleus, without a sharp cutoff; these are referred to as atomic orbitals or electron clouds. Moreover, in condensed matter and molecules, the electron clouds of the atoms usually overlap to some extent, and some of the electrons may roam over a large region encompassing two or more atoms. Under most definitions the radii of isolated neutral atoms range between 30 and 300 pm (trillionths of a meter), or between 0.3 and 3 ångströms. Therefore, the radius of an atom is more than 10,000 times the radius of its nucleus (1–10 fm), and less than 1/1000 of the wavelength of visible light (400–700 nm). For many purposes, atoms can be modeled as spheres. This is only a crude approximation, but it can provide quantitative explanations and predictions for many phenomena, such as the density of liquids and solids, the diffusion of fluids through molecular sieves, the arrangement of atoms and ions in crystals, and the size and shape of molecules.
  • 1.5K
  • 18 Oct 2022
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