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Topic Review
Basis Set
A basis set in theoretical and computational chemistry is a set of functions (called basis functions) that is used to represent the electronic wave function in the Hartree–Fock method or density-functional theory in order to turn the partial differential equations of the model into algebraic equations suitable for efficient implementation on a computer. The use of basis sets is equivalent to the use of an approximate resolution of the identity: the orbitals [math]\displaystyle{ |\psi_i\rangle }[/math] are expanded within the basis set as a linear combination of the basis functions [math]\displaystyle{ |\psi_i\rangle \approx \sum_\mu c_{\mu i} |\mu\rangle }[/math], where the expansion coefficients [math]\displaystyle{ c_{\mu i} }[/math] are given by [math]\displaystyle{ c_{\mu i} = \sum_{\nu} \langle \mu|\nu \rangle^{-1} \langle \nu |\psi_i \rangle }[/math]. The basis set can either be composed of atomic orbitals (yielding the linear combination of atomic orbitals approach), which is the usual choice within the quantum chemistry community; plane waves which are typically used within the solid state community, or real-space approaches. Several types of atomic orbitals can be used: Gaussian-type orbitals, Slater-type orbitals, or numerical atomic orbitals. Out of the three, Gaussian-type orbitals are by far the most often used, as they allow efficient implementations of Post-Hartree–Fock methods.
  • 933
  • 17 Oct 2022
Topic Review
Symbol
In chemistry, a symbol is an abbreviation for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised. Earlier symbols for chemical elements stem from classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, Pb is the symbol for lead (plumbum in Latin); Hg is the symbol for mercury (hydrargyrum in Greek); and He is the symbol for helium (a new Latin name) because helium was not known in ancient Roman times. Some symbols come from other sources, like W for tungsten (Wolfram in German) which was not known in Roman times. A 3-letter temporary symbol may be assigned to a newly synthesized (or not-yet synthesized) element. For example, "Uno" was the temporary symbol for hassium (element 108) which had the temporary name of unniloctium, based on its atomic number being 8 greater than 100. There are also some historical symbols that are no longer officially used. In addition to the letters for the element itself, additional details may be added to the symbol as superscripts or subscripts a particular isotope, ionization, or oxidation state, or other atomic detail. A few isotopes have their own specific symbols rather than just an isotopic detail added to their element symbol. Attached subscripts or superscripts specifying a nuclide or molecule have the following meanings and positions: In Chinese, each chemical element has a dedicated character, usually created for the purpose (see Chemical elements in East Asian languages). However, Latin symbols are also used, especially in formulas. Many functional groups also have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group. A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight, or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol. Hazard pictographs are another type of symbols used in chemistry.
  • 932
  • 14 Nov 2022
Topic Review
Structure of Piezoelectric Accelerometers
Compared with other types of sensors, piezoelectric accelerometers have the advantages of a large range, a wide-frequency band, a simple structure, stable performance, good output linearity, etc. The principle of a piezoelectric accelerometer is based on the property of the active element, and its structure is mainly composed of a mass block, a piezoelectric sensitive element, and a base.
  • 932
  • 28 Sep 2023
Topic Review
Acrylic Carrier
Wood used outdoor is subjected to different sources of degradation and should be protected properly. In this study, acrylic resins were added to a wood impregnation system using amine oxides and propiconazole, an organic fungicide, to create a two-part wood protection preservation treatment. Since amine oxides can diffuse readily into wood, this treatment protected both the surface and inner structure of the treated wood following a simple dipping. Many aspects of the treatment were studied: the adhesion of the acrylic coatings, their permeability to water, and the impregnation depth of the propiconazole. In each case, a particular attention was accorded to the interactions between the resins and the impregnation system. Adhesion and permeability tests were coupled with an artificial aging process simulating severely wet conditions. Amine oxides reduced the adhesion of the coatings, but did not impair their aging properties. Because of their hydrophilic nature, they also increased the permeability to liquid water, although they did not affect the air moisture permeability. The penetration of the propiconazole, estimated with a dye, decreased with the resin. Overall, the two parts of the treatment lightly impaired each other, but the practical aspect of this treatment may overcome these disadvantages.
  • 930
  • 30 Oct 2020
Topic Review
Screening of Medicinal Plants
Secondary metabolites, used as a single compound or as a mixture, are medicines that can be effective and safe even when synthetic drugs fail. They may even potentiate or synergize the effects of other compounds in the medicine. To meet the positive demands, heterologous expression of the genes involved in the biosynthesis of the potent compound will help in the production on the required scale. Further modifications to enhance the ADME properties of the compounds will have a significant advantage in the drug’s biological activities and bioavailability.
  • 930
  • 30 Oct 2020
Topic Review
Porphyrins/Chlorins for Viruses Inactivation
The problem of treating viral infections is extremely important both in connection with the emergence of new viral diseases and in connection with the low efficiency of existing approaches to the treatment of known viral infections. This entry is devoted to the use of porphyrins, chlorins, and phthalocyanines for the fight against viral infections using chemical and photochemical inactivation methods. The purpose of this work is to summarize the main approaches developed to date to chemical and photodynamic inactivation of human and animal viruses using porphyrins and their analogs, as well as to analyze and discuss information on viral targets and antiviral activity of porphyrins, chlorins and their derivatives obtained in the last 10-15 years, in order to identify the most promising areas.
  • 929
  • 23 Jun 2021
Topic Review
Leucoagaricus gongylophorus and Leaf-Cutting Ants
Leaf-cutting ants are eusocial insects, as they show a highly developed social structure, manifesting ecological relationships. Their complex structure is characterized by an organized social behavior, the cultivation of a fungus garden and high levels of hygiene, which hinders the management of leaf-cutting ants compared to other insects. Leaf-cutting ants cause damage in agricultural and silviculture areas, mainly in monocultures.
  • 929
  • 21 Apr 2022
Topic Review
Opioid Epidemic
The opioid epidemic or opioids crisis is the rapid increase in the use of prescription and non-prescription opioid drugs in the United States and Canada beginning in the late 1990s and continuing throughout the next two decades. The increase in opioid overdose deaths has been dramatic, and opioids are now responsible for 49,000 of the 72,000 drug overdose deaths overall in the US in 2017. A recent report indicated that the rate of prolong opioid use is increasing globally. Opioids are a diverse class of moderately strong painkillers, including oxycodone (commonly sold under the trade names OxyContin and Percocet), hydrocodone (Vicodin, Norco), and a very strong painkiller, fentanyl, which is synthesized to resemble other opiates such as opium-derived morphine and heroin. The potency and availability of these substances, despite their high risk of addiction and overdose, have made them popular both as medical treatments and as recreational drugs. Due to their sedative effects on the part of the brain which regulates breathing, the respiratory center of the medulla oblongata, opioids in high doses present the potential for respiratory depression, and may cause respiratory failure and death. In a 2015 report, the U.S. Drug Enforcement Administration stated that "overdose deaths, particularly from prescription drugs and heroin, have reached epidemic levels.":iii Nearly half of all opioid overdose deaths in 2016 involved prescription opioids. From 1999 to 2008, overdose death rates, sales, and substance abuse treatment admissions related to opioid pain relievers all increased substantially. By 2015, there were more than 50,000 annual deaths from drug overdose, causing more deaths than either car accidents or guns. Drug overdoses have since become the leading cause of death of Americans under 50, with two-thirds of those deaths from opioids. In 2016, the crisis decreased overall life expectancy of Americans for the second consecutive year. Overall life expectancy fell from 78.7 to 78.6 years. Men were disproportionately more affected due to higher overdose death rates, with life expectancy declining from 76.3 to 76.1 years. Women's life expectancy remained stable at 81.1 years. In 2016, over 64,000 Americans died from overdoses, 21 percent more than the almost 53,000 in 2015. By comparison, the figure was 16,000 in 2010, and 4,000 in 1999. While death rates varied by state, public health experts estimate that nationwide over 500,000 people could die from the epidemic over the next 10 years. In Canada , half of the overdoses were accidental, while a third were intentional. The remainder were unknown. Many of the deaths are from an extremely potent opioid, fentanyl, which is trafficked from Mexico. The epidemic cost the United States an estimated $504 billion in 2015. CDC former director Thomas Frieden said that "America is awash in opioids; urgent action is critical." The crisis has changed moral, social, and cultural resistance to street drug alternatives such as heroin. Many state governors have declared a "state of emergency" to combat the opioid epidemic or undertook other major efforts against it. In July 2017, opioid addiction was cited as the "FDA's biggest crisis". In October 2017, President Donald Trump concurred with his Commission's report and declared the country's opioid crisis a "public health emergency". Federal and state interventions are working on employing health information technology in order to expand the impact of existing drug monitoring programs. Recent research shows promising results in mortality and morbidity reductions when a state integrates drug monitoring programs with health information technologies and shares data through centralized platform.
  • 928
  • 29 Sep 2022
Topic Review
Thermoresponsive Polypeptoids
Thermoresponsive polypeptoids exhibiting a reversible phase transition in a controlled manner to temperature are a promising class of smart polymers that have drawn growing interest because of its excellent biocompatibility, biodegradability, and bioactivity. The phase transition behavior of these polymers can be tuned by polymer architectures, chain-end, and various functional groups. 
  • 927
  • 29 Dec 2020
Topic Review
Cholinergic Signaling and Epilepsy
Epilepsy is a common brain disorder characterized by recurrent epileptic seizures with neuronal hyperexcitability. Apart from the classical imbalance between excitatory glutamatergic transmission and inhibitory γ-aminobutyric acidergic transmission, cumulative evidence suggest that cholinergic signaling is crucially involved in the modulation of neural excitability and epilepsy.
  • 927
  • 07 May 2021
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