ديسمبر 19th, 2013
A sequence of pulses used for spin-echo experiments in nuclear magnetic resonance (NMR) in which the initial pulse is 90° followed by a series of 180° pulses. A CPMG sequence is designed so that the spin echoes die away exponentially with time. Spin-spin relaxation occurs characterized by a time constant T2, which can be determined from the decay signal.
كلمات دليلية: CPMG sequence (Carr-PurcellMeiboom-Gill sequence)
A chemical reaction in which two molecular beams are crossed; one beam is regarded as the incident beam of gas and the other as the target gas. This technique enables a great deal of information to be gained about the chemical reaction since the states of both the target and projectile molecules can be controlled. The incident beam is characterized by its incident beam flux, I, which is…
كلمات دليلية: crossed-beam reaction
Compounds with large three-dimensional molecular structures containing ether chains linked by three-coordinate nitrogen atoms. Thus cryptands are macropolycyclic polyaza-polyethers. For example, the compound (2,2,2)-cryptand has three chains of the form -CH2CH2OCH2CH2OCH2CH2-.
These chains are linked at each end by a nitrogen atom. Cryptands, like the *crown ethers, can form coordination complexes with ions that can fit into the cavity formed by the open three-dimensional structure, i.e. they can…
كلمات دليلية: cryptands, three-dimensional
A type of presumptive test in which a substance is identified by the formation of characteristic crystals when a certain reagent is added. Usually, such tests are conducted using a microscope (microcrystal test). An example is the acetone-chlor-haemin test for blood.
كلمات دليلية: crystal test, microcrystal test
A resin obtained from the bark of South American trees of the genera Strychnos and Chondrodendron that causes paralysis of voluntary muscle. It acts by blocking the action of the neurotransmitter acetylcholine at neuromuscular junctions. Curare is used as an arrow poison by South American Indians and was formerly used as a muscle relaxant in surgery.
كلمات دليلية: curare
The susceptibility (X) of a paramagnetic substance is proportional to the thermodynamic temperature (T), i.e. x = C/T, where C is the Curie constant. A modification of this law, the Curie-Weiss law, is more generally applicable. It states that X = C/(T- θ), where θ is the Weiss constant, a characteristic of the material. The law was first proposed by Pierre Curie and modified by another French physicist,…
كلمات دليلية: Curie's law, paramagnetic, Weiss
A derivative of ATP that is widespread in animal cells as a second messenger in many biochemical reactions induced by hormones. Upon reaching their target cells, the hormones activate adenylate cyclase, the enzyme that catalyses cyclic AMP production. Cyclic AMP ultimately activates the enzymes of the reaction induced by the hormone concerned. Cyclic AMP is also involved in controlling gene expression and cell division, in immune responses, and…
كلمات دليلية: ATP, biochemical, cyclic AMP, hormones
A reaction in which two or more unsaturated compounds form a cyclic adduct or in which a cyclic compound is formed by addition between unsaturated parts of the same molecule. In cycloaddition, there is no net reduction in bond multiplicity. The Diels-Alder reaction is an example. Cycloadditions may be stepwise reactions or may be peri-cyclic reactions.
كلمات دليلية: cyclic, cycloaddition, unsaturated
A compound consisting of one or more aromatic rings forming part of a larger ring system in which aliphatic chains of the CH2 groups link the aromatic rings. Compounds of this type have the suffix phane in their names. Depending on the sizes of the (CH2)11 chains, the aromatic rings may not be planar.
كلمات دليلية: Aromatic, cyclophane
طريقة ديباي-شيرر Debye-Scherrer method :
تقنية تستخدم في إنحراف اشعة إكس ، بحيث يتم تثبيت البلورة على شكل مسحوق على شعيرة أو أنبوب دقيق جدا من السيليكات و الذي يتم تدويره في مسار أشعة أكس أحادي اللون ، حيث يتشكل حلقة إنحراف دائرية و التي تعرف باسم حلقة ديباي شيرر. يتم تسجيل مسار الإنحرافات على فيلم أسطواني و الذي محاوره تتوازي مع محور التدوير للمادة. تستخدم طريقة ديباي-شيرر…
كلمات دليلية: أشعة إكس, بلورات, بلورة, طريقة ديباي-شيرر, مسحوق