aryl

aryl [ORG CHEM] An organic group derived from an aromatic hydrocarbon by removal of one hydrogen. {ar⭈əl }

aryl acid [ORG CHEM] An organic acid that has an aryl group. {ar⭈əl as⭈əd } arylamine [ORG CHEM] An organic compound formed from an aromatic hydrocarbon

that has at least one amine group joined to it, such as aniline. {ar⭈əl⭈əme¯n } arylated alkylSee aralkyl. { ar⭈əla¯d⭈əd al⭈kəl }

aryl compound [ORG CHEM] Molecules with the six-carbon aromatic ring structure characteristic of benzene or compounds derived from aromatics. {ar⭈əl

ka¨mpau˙nd }

aryl diazo compound [ORG CHEM] A diazo compound bonded to the ring structure characteristic of benzene or any other aromatic derivative. {ar⭈əl dı¯a¯z⭈o¯

ka¨mpau˙nd }

arylene [ORG CHEM] A radical that is bivalent and formed by removal of hydrogen from two carbon sites on an aromatic nucleus. {ar⭈əle¯n }

aryl halide [ORG CHEM] An aromatic derivative in which a ring hydrogen has been replaced by a halide atom. {ar⭈əl halı¯d }

arylide [ORG CHEM] A compound formed from a metal and an aryl group, for example, PbR4, where R is the aryl group. {ar⭈əlı¯d }

aryloxy compound [ORG CHEM] One of a group of compounds useful as organic weed killers, such as 2,4-dichlorophenoxyacetic acid (2,4-D). {¦ar⭈əl¦a¨k⭈se¯ ka¨mpau˙nd } aryne [ORG CHEM] An aromatic species in which two adjacent atoms of a ring lack substituents, with two orbitals each missing an electron. Also known as ben-zyne. {arı¯n }

AsSee arsenic.

asarone [ORG CHEM] C12H16O3A crystalline substance with melting point 67⬚C; insolu-ble in water, soluinsolu-ble in alcohol; found in plants of the genus Asarum; used as a constituent in essential oils such as calumus oil. {as⭈əro¯n }

ascaridole [ORG CHEM] C10H16O2A terpene peroxide, explosive when heated; used as an initiator in polymerization. {əskar⭈ədo¯l }

ascending chromatography [ANALY CHEM] A technique for the analysis of mixtures of two or more compounds in which the mobile phase (sample and carrier) rises through the fixed phase. {əsend⭈iŋ kro¯⭈məta¨g⭈rə⭈fe¯ }

ash [CHEM] The incombustible matter remaining after a substance has been inciner-ated. { ash }

ashing [ANALY CHEM] An analytical process in which the chemical material being ana-lyzed is oven-heated to leave only noncombustible ash. {ash⭈iŋ }

aspartame [ORG CHEM] C14H18N2O5A dipeptide ester about 160 times sweeter than sucrose in aqueous solution; used as a low-calorie sweetener. { aspa¨rta¯m } aspirinSee acetylsalicylic acid. {as⭈prən }

assay [ANALY CHEM] Qualitative or quantitative determination of the components of a material, as an ore or a drug. {asa¯ }

association [CHEM] Combination or correlation of substances or functions. {əso¯⭈

se¯a¯⭈shən }

Astage [ORG CHEM] An early stage in a thermosetting resin reaction characterized by linear structure, solubility, and fusibility of the material. {a¯ sta¯j }

astatine [CHEM] A radioactive chemical element, symbol At, atomic number 85, the heaviest of the halogen elements. {as⭈təte¯n }

asterism [SPECT] A star-shaped pattern sometimes seen in x-ray spectrophotographs.

{as⭈təriz⭈əm }

astigmatic mounting [SPECT] A mounting designed to minimize the astigmatism of a concave diffraction grating. {¦a⭈stigmad⭈ik mo˙unt⭈iŋ }

astronomical spectrograph [SPECT] An instrument used to photograph spectra of stars. {as⭈trəna¨m⭈ə⭈kəl spek⭈trəgraf }

astronomical spectroscopy [SPECT] The use of spectrographs in conjunction with tele-scopes to obtain observational data on the velocities and physical conditions of astronomical objects. {as⭈trəna¨m⭈ə⭈kəl spektra¨s⭈kə⭈pe¯ }

asymmetric carbon atom [ORG CHEM] A carbon atom with four different atoms or

atomic spectroscopy

groups of atoms bonded to it. Also known as chiral carbon atom; stereogenic center. {¦a¯⭈sə¦me⭈trik ¦ka¨r⭈bən ad⭈əm }

asymmetric synthesis [ORG CHEM] Chemical synthesis of a pure enantiomer, or of an enantiomorphic mixture in which one enantiomer predominates, without the use of resolution. {¦a¯⭈sə¦me⭈trik sin⭈thə⭈səs }

asymmetry [PHYS CHEM] The geometrical design of a molecule, atom, or ion that cannot be divided into like portions by one or more hypothetical planes. Also known as molecular asymmetry. {¦a¯sim⭈ə⭈trə }

asymmetry effect [PHYS CHEM] The asymmetrical distribution of the ion cloud around an ion that results from the finite relaxation time for the ion cloud when a voltage is applied; leads to a reduction in ion mobility. { a¯sim⭈ə⭈tre¯ ifekt }

AtSee astatine.

ATASee aminotriazole.

atactic [ORG CHEM] Of the configuration for a polymer, having the opposite steric configurations for the carbon atoms of the polymer chain occur in equal frequency and more or less at random. { a¯tak⭈tik }

atom [CHEM] The individual structure which constitutes the basic unit of any chemical element. {ad⭈əm }

atom cluster [PHYS CHEM] An assembly of between three and a few thousand atoms or molecules that are weakly bound together and have properties intermediate between those of the isolated atom or molecule and the bulk or solid-state material.

{ad⭈əm kləs⭈tər }

atomic absorption spectroscopy [SPECT] An instrumental technique for detecting con-centrations of atoms to parts per million by measuring the amount of light absorbed by atoms or ions vaporized in a flame or an electrical furnace. {ə¦ta¨⭈mik əb¦so¨rp⭈

shən spektra¨s⭈kə⭈pe¯ }

atomic connectivity [PHYS CHEM] The specific pattern of chemical bonds between atoms in a molecule. {ə¦ta¨m⭈ik kənektiv⭈əd⭈e¯ }

atomic emission spectroscopy [SPECT] A form of atomic spectroscopy in which one observes the emission of light at discrete wavelengths by atoms which have been electronically excited by collisions with other atoms and molecules in a hot gas.

{ə¦ta¨m⭈ik ə¦mish⭈ən spektra¨s⭈kə⭈pe¯ }

atomic fluorescence spectroscopy [SPECT] A form of atomic spectroscopy in which the sample atoms are first excited by absorbing radiation from an external source containing the element to be detected, and the intensity of radiation emitted at characteristic wavelengths during transitions of these atoms back to the ground state is observed. {ə¦ta¨m⭈ik flu˙¦res⭈əns spektra¨s⭈kə⭈pe¯ }

atomic heat capacity [PHYS CHEM] The heat capacity of a gram-atomic weight of an element. {əta¨m⭈ik he¯t kəpas⭈əd⭈e¯ }

atomic hydrogen [CHEM] Gaseous hydrogen whose molecules are dissociated into atoms. {əta¨m⭈ik hı¯⭈drə⭈jən }

atomicity [CHEM] The number of atoms in a molecule of a compound. {ad⭈əmis⭈

əd⭈e¯ }

atomic percent [CHEM] The number of atoms of an element in 100 atoms representative of a substance. {əta¨m⭈ik pərsent }

atomic photoelectric effectSee photoionization. {əta¨m⭈ik fo¯d⭈o¯⭈ilek⭈trik ifekt } atomic polarization [PHYS CHEM] Polarization of a material arising from the change in

dipole moment accompanying the stretching of chemical bonds between unlike atoms in molecules. {əta¨m⭈ik po¯l⭈ə⭈rəza¯⭈shən }

atomic radius [PHYS CHEM] Also known as covalent radius.1.Half the distance between the nuclei of two like atoms that are covalently bonded. 2.The experimentally determined radius of an atom in a covalently bonded compound. {əta¨m⭈ik ra¯d⭈

e¯⭈əs }

atomic spectroscopy [SPECT] The branch of physics concerned with the production, measurement, and interpretation of spectra arising from either emission or absorp-tion of electromagnetic radiaabsorp-tion by atoms. {əta¨m⭈ik spektra¨s⭈kə⭈pe¯ }

atomic spectrum

atomic spectrum [SPECT] The spectrum of radiations due to transitions between energy levels in an atom, either absorption or emission. {əta¨m⭈ik spek⭈trəm } atomic theory [CHEM] The assumption that matter is composed of particles called

atoms and that these are the limit to which matter can be subdivided. {əta¨m⭈ik

the¯⭈ə⭈re¯ }

atomic volume [PHYS CHEM] The volume occupied by 1 gram-atom of an element in the solid state. {əta¨m⭈ik va¨l⭈yəm }

atomic weight [CHEM] The relative mass of an atom based on a scale in which a specific carbon atom (carbon-12) is assigned a mass value of 12. Abbreviated at.

wt. Also known as relative atomic mass. {əta¨m⭈ik wa¯t }

atomization [ANALY CHEM] In flame spectrometry, conversion of a volatilized sample into free atoms. [CHEM] A process in which the chemical bonds in a molecule are broken to yield separated (free) atoms. {ad⭈ə⭈məza¯⭈shən }

atoms-in-molecules method [PHYS CHEM] The description of the electronic structure of a molecule as a perturbation of the isolated states of its constituent atoms. {¦ad⭈

əmz in ma¨l⭈əkyu¨lz meth⭈əd }

atom trap trace analysis [ANALY CHEM] An atom-counting method in which individual atoms of a chosen isotope are captured and detected with a laser trap. {¦ad⭈əm

¦trap tra¯s ə¦nal⭈ə⭈səs }

ATRSee attenuated total reflectance.

atrazine [CHEM] C8H14ClN5A white crystalline compound widely used as a photosyn-thesis-inhibiting herbicide for weeds. {a⭈trəze¯n }

atropisomer [ORG CHEM] One of two conformations of a molecule whose interconver-sion is slow enough to allow separation and isolation under predetermined condi-tions. {¦a⭈tro¯¦pı¯z⭈ə⭈mər }

attachment [ORG CHEM] The conversion of a molecular entity into another molecular structure solely by formation of a single two-center bond with another molecular entity and no other changes in bonding. {ətach⭈mənt }

attenuated total reflectance [SPECT] A method of spectrophotometric analysis based on the reflection of energy at the interface of two media which have different refractive indices and are in optical contact with each other. Abbreviated ATR. Also known as frustrated internal reflectance; internal reflectance spectroscopy. {əten⭈yəwa¯d⭈

əd to¯d⭈əl riflek⭈təns } at. wtSee atomic weight.

AuSee gold.

Aufbau principle [CHEM] A description of the building up of the elements in which the structure of each in sequence is obtained by simultaneously adding one positive charge (proton) to the nucleus of the atom and one negative charge (electron) to an atomic orbital. {au˙fbau˙ ¦prin⭈sə⭈pəl }

Auger electron spectroscopy [SPECT] The energy analysis of Auger electrons produced when an excited atom relaxes by a radiationless process after ionization by a high-energy electron, ion, or x-ray beam. Abbreviated AES. { o¯zha¯ ilektra¨n spektra¨s⭈

kə⭈pe¯ }

auramine hydrochloride [ORG CHEM] C17H22ClN3⭈H2O A compound melting at 267⬚C;

very soluble in water, soluble in ethanol; used as a dye and an antiseptic. Also known as yellow pyoktanin. {o˙r⭈əme¯n hı¯⭈drəklo˙rı¯d }

aurantia [ORG CHEM] C12H8N8O12An orange aniline dye, used in stains in biology and in some photographic filters. { o˙ranch⭈ə }

aurantiinSee naringin. { o˙ran⭈te¯⭈ən } auric oxideSee gold oxide. {o˙r⭈ik a¨ksı¯d }

aurin [ORG CHEM] C19H14O3A derivative of triphenylmethane; solid with red-brown color with green luster; melting point about 220⬚C; insoluble in water; used as a dye intermediate. {o˙r⭈ən }

auroral line [SPECT] A prominent green line in the spectrum of the aurora at a wave-length of 5577 angstroms, resulting from a certain forbidden transition of oxygen.

{əro˙r⭈əl lı¯n }