ديسمبر 19th, 2013

 

CPMG sequence – Carr-Purcell­Meiboom-Gill sequence

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.

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crossed-beam reaction

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…

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cryptands

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…

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crystal test

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.

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curare

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.

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Curie’s law

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,…

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cyclic AMP

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…

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cycloaddition

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.

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cyclophane

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.

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طريقة ديباي-شيرر

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…

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