from Oxford and Cambridge universities. He died in
Stockholm on October 2, 1927, after a brief illness,
and is buried at Uppsala.
Arrhenius equation SeeENERGY OF ACTIVATION.
artificial transmutation Artificially induced nuclear
reaction made by bombarding a nucleus with sub-
atomic particles or small nuclei.
aryl group Group of atoms remaining after a hydro-
gen atom is removed from the aromatic system.
aryne A hydrocarbon derived from an arene by
abstraction of two hydrogen atoms from adjacent car-
bon atoms, e.g., 1,2-didehydroarene. Arynes are com-
monly represented with a formal triple bond. The
analogous heterocyclic compounds are called het-
eroarynes or hetarynes. For example:
assimilation To transform food and other nutrients
into a part of the living organism.
assimilative SeeASSIMILATION.
assimilator SeeASSIMILATION.
associated ions Short-lived ions formed by the colli-
sion of dissolved ions of opposite charges.
association The assembling of separate MOLECULAR
ENTITIESinto any aggregate, especially of oppositely
charged free ions into ION PAIRs or larger, not necessar-
ily well-defined, clusters of ions held together by elec-
trostatic attraction. The term signifies the reverse of
DISSOCIATION, but it is not commonly used for the for-
mation of definite ADDUCTs by COLLIGATIONor COOR-
DINATION.asymmetric carbon A carbon atom covalently
bonded to four different atoms or groups of atoms.asymmetric induction The traditional term describ-
ing the preferential formation in a CHEMICAL REACTION
of one ENANTIOMER or DIASTEREOISOMER over the
other as a result of the influence of a chiral feature in
the substrate, reagent, CATALYST, or environment. The
term also refers to the formation of a new chiral feature
preferentially in one CONFIGURATIONunder such influ-
ence.asymmetric synthesis A traditional term for stereo-
selective synthesis. A chemical reaction or reaction
sequence in which one or more new elements of CHI-
RALITYare formed in a SUBSTRATEmolecule to produce
the STEREOISOMERIC(ENANTIOMERic or DIASTEREOISO-
MERic) products in unequal amounts.asymmetry parameter In nuclear quadrupole reso-
nance spectroscopy, the parameter, η, is used to
describe nonsymmetric fields. It is defined as η= (qxx–
qyy)/qzz, in which qxx, qyy, and qzzare the components
of the field gradient q(which is the second derivative of
the time-averaged electric potential) along the x-, y-,
and z-axes. By convention, qzzrefers to the largest field
gradient, qyyto the next largest, and qxxto the smallest
when all three values are different.atmosphere A unit of pressure; the pressure that will
support a column of mercury 760 mm high at 0°C.atom The smallest particle of an element.atom 19