Neutron electric dipole moment

2013-10-21T13:40:10Z

163.1.243.225: Modify statement such that it can be supported by the paper referenced.

'''Dextrorotation''' and '''levorotation''' (also spelled '''laevorotation''')<ref name="word">The first word component ''[[wikt:dextro-|dextro-]]'' comes from [[Latin language|Latin]] word for [[wiktionary:dexter|dexter]] "right (as opposed to left)". [[wiktionary:laevo-|Laevo-]] or [[wiktionary:levo-|levo-]] comes from the Latin for [[wiktionary:laevus|laevus]], "left side."</ref> refer to the properties of rotating [[plane polarized light]]. If the light rotates clockwise as it approaches an observer, this is known as dextrorotation, light with a rotation to the right. If the light rotates counterclockwise as it approaches the observer, then the light exhibits levorotation, rotation to the left.

A compound with dextrorotation is called '''dextrorotatory''' or '''dextrorotary''',<ref name="Solomons, T.W. Graham 2004">{{Cite book | author = Solomons, T.W. Graham, and Graig B. Fryhle | title = Organic Chemistry | edition = 8th | location = Hoboken | publisher = John Wiley & Sons, Inc. | year = 2004}}</ref> while a compound with levorotation is called '''levorotatory''' or '''levorotary'''.<ref name="Solomons, T.W. Graham 2004"/> Compounds with these properties are said to have [[optical activity]] and consist of [[Chirality (chemistry)|chiral]] molecules. If a chiral molecule is dextrorotary, its [[enantiomer]] (geometric mirror image) will be levorotary, and vice-versa. In fact, the enantiomers will rotate plane polarized light the same number of degrees, but in opposite directions.

Apart from direct measurement of the [[optical rotation]] of an actual sample, it is only possible to determine whether a given chiral molecule will be levorotatory or dextrorotatory, directly from its [[absolute configuration]], via detailed computer modeling.<ref name=Stephens>See, for example,{{cite journal | doi = 10.1002/chir.10270 | title = Determination of absolute configuration using ab initio calculation of optical rotation | year = 2003 | last1 = Stephens | first1 = P.J. | last2 = Devlin | first2 = F.J. | last3 = Cheeseman | first3 = J.R. | last4 = Frisch | first4 = M.J. | last5 = Bortolini | first5 = O. | last6 = Besse | first6 = P. | journal = Chirality | volume = 15 | pages = S57–64 | pmid = 12884375}}</ref> That is to say, both [[Cahn–Ingold–Prelog priority rules|"R" and "S"]] [[stereocenter]]s have the ability to be dextrorotatory or levorotatory.

==Chirality prefixes==
{{Main|Chirality (chemistry)}}

===(+)-, (–)-, d-, l-, D-, and L-===
A dextrorotary compound is often prefixed "(+)-" or "d-". Likewise, a levorotary compound is often prefixed "(–)-" or "l-". These "d-" and "l-" prefixes are distinct from the uppercase (though [[Small caps|SMALL CAPS]]) "D-" and "L-" prefixes, which are based on the actual configuration of each enantiomer, with the version synthesized from naturally occurring (+)-glyceraldehyde being considered the D-form. For example, nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and D-fructose is also referred to as levulose because it is levorotatory.

===(''R'')- and (''S'')-===
The (''R'')- and (''S'')- prefixes are different from the preceding ones in that the labels ''R'' and ''S'' characterize the [[absolute configuration]] of a specific [[stereocenter]], not a whole molecule. A molecule with just one stereocenter can be labeled ''R'' or ''S'', but a molecule with multiple stereocenters needs more than one label, for example [[diastereomer|(2''R'',3''S'')]].

If there is a pair of enantiomers, each with one stereocenter, then one enantiomer is ''R'' and the other is ''S'', and likewise one enantiomer is levorotary and the other is dextrorotary. However, there is no general correlation between these two labels. In some cases the (''R'')-enantiomer is the dextrorotary enantiomer, and in other cases the (''R'')-enantiomer is the levorotary enantiomer. The relationship can only be determined on a case-by-case basis with experimental measurements or detailed computer modeling.<ref name=Stephens/>

==Specific rotation==
{{Main|Specific rotation}}
A standard measure of the degree to which a compound is dextrorotary or levorotary is the quantity called the [[specific rotation]] [α]. Dextrorotary compounds have a positive specific rotation, while levorotary compounds have negative. Two [[enantiomers]] have equal and opposite specific rotations.

The formula for specific rotation, [α], is:

<math>[\alpha] = \frac{\alpha}{c \cdot l}</math>

where:
:α = observed rotation
:c = concentration of the solution of an enantiomer
:l = length of the tube ([[polarimeter]] tube) in decimeters

The degree of rotation of plane-polarized light depends on the number of chiral molecules that it encounters on its way through the tube of polarimeter (thus, the length of the tube and concentration of the enantiomer). In many cases, it also depends on the temperature and the wavelength of light that is employed.

==Other terminology==

The equivalent French terms are '''dextrogyre''' and '''levogyre'''. These are occasionally (but very infrequently) used in English.<ref>For example: {{Cite book | url = http://books.google.com/books?id=fOyAvZ08nvAC&pg=PA126 | title = Farnesyltransferase inhibitors in cancer therapy | editor = Sebti and Hamilton | page = 126 | isbn = 9780896036291 | year = 2001}}</ref>

== See also ==
* [[Isomer]]

==References==
{{Wiktionary|levorotatory}}
{{Wiktionary|dextrorotatory}}
{{Reflist}}

{{DEFAULTSORT:Levorotation And Dextrorotation}}
[[Category:Polarization (waves)]]
[[Category:Stereochemistry]]

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Neutron electric dipole moment