Simplified Chemistry

 

Flory temperature -theta tempera­ture

 Symbol θ. The unique tempera­ture at which the attractions and repulsions of a polymer in a solution cancel each other. It is analogous to the Boyle temperature of a nonideal gas. A polymer solution at the Flory temperature is called a theta (θ) solu­tion. At the Flory temperature the virial coefficient B, associated with the excuded volume of the polymer, is zero, which results in the polymer chain…

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fluorite structure

 A type of ionic crystal structure in which the cations have an expanded face-centered cubic arrangement with the anions occupy­ing both types of tetrahedral hole. The cations have a coordination number of 8 and the anions have a coordination number of 4. Examples of compounds with this structure are CaF2, BaCl2, and PbO2.

The antifluorite structure is the op­posite arrangement, with anions in the fcc array with coordination…

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flunitrazepam

A benzodiazepine used medically in some countries as a powerful hypnotic, sedative, and muscle relaxant. It was marketed in the US under the trade name Rohyp­nol. Flunitrazepam has become notorious as a so-called ‘date rape’ drug. It is quickly eliminated from the body, difficult to detect, and causes amne­sia, so that victims cannot remember events that occur when under the influence of the drug.

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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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