be ELEMENTARY REACTIONs or STEPWISE REACTIONs. (It
should be noted that this definition includes experimen-
tally observable interconversions of conformers.)
Detectable chemical reactions normally involve sets
of MOLECULAR ENTITIES, as indicated by this definition,
but it is often conceptually convenient to use the term
also for changes involving single molecular entities (i.e.,
microscopic chemical events).
See alsoIDENTITY REACTION.
chemical relaxation If the equilibrium mixture of a
CHEMICAL REACTIONis disturbed by a sudden change,
especially of some external parameter (such as tempera-
ture, pressure, or electrical field strength), the system
will readjust itself to a new position of the chemical
equilibrium or return to the original position, if the
perturbation is temporary. The readjustment is known
as chemical relaxation.
In many cases, and in particular when the dis-
placement from equilibrium is slight, the progress of
the system toward equilibrium can be expressed as a
first-order law
[Ct–(Ceq) 2 ] = [(Ceq) 1 –(Ceq) 2 ]e–t/τwhere (Ceq) 1 and (Ceq) 2 are the equilibrium concentra-
tions of one of the chemical species involved in the
reaction before and after the change in the external
parameter, respectively, and Ctis its concentration at
time t.The time parameter t,named relaxation time, is
related to the RATE CONSTANTs of the chemical reaction
involved.
Measurements of the relaxation times by relax-
ation methods (involving a temperature jump [T-jump],
pressure jump, electric field jump, or a periodic distur-
bance of an external parameter, as in ultrasonic tech-
niques) are commonly used to follow the kinetics of
very fast reactions.
See alsoRELAXATION.
chemical shift (NMR), δ(SI unit: 1) The variation
of the resonance frequency of a nucleus in NUCLEAR
MAGNETIC RESONANCE SPECTROSCOPY(NMR) in conse-
quence of its magnetic environment. The chemical shift
of a nucleus, δ, is expressed in ppm by its frequency,
νcpd, relative to a standard, νref, and is defined as
δ= 10^6 (νcpd– νref)/νowhere νois the operating frequency of the spectrometer.
For^1 H and^13 C NMR, the reference signal is usually
that of tetramethylsilane (SiMe 4 ). Other references are
used in the older literature and in other solvents, such
as D 2 O.
If a resonance signal occurs at lower frequency or
higher applied field than an arbitrarily selected refer-
ence signal, it is said to be upfield, and if resonance
occurs at higher frequency or lower applied field, the
signal is downfield. Resonance lines upfield from SiMe 4
have positive δ-values, and resonance lines downfield
from SiMe 4 have negative δ-values.chemical species An ensemble of chemically identi-
cal MOLECULAR ENTITIESthat can explore the same set
of molecular energy levels on the time scale of the
experiment. The term is applied equally to a set of
chemically identical atomic or molecular structural
units in a solid array.
For example, two conformational ISOMERs may be
interconverted sufficiently slowly to be detectable by
separate NMR spectra and hence to be considered as
separate chemical species on a time scale governed by
the radiofrequency of the spectrometer used. On the
other hand, in a slow chemical reaction, the same mix-
ture of conformers may behave as a single chemical
species, i.e., there is virtually complete equilibrium pop-
ulation of the total set of molecular energy levels
belonging to the two conformers.
Except where the context requires otherwise, the
term is taken to refer to a set of molecular entities con-
taining isotopes in their natural abundance.
The wording of the definition given in the first para-
graph is intended to embrace cases such as graphite,
sodium chloride, or a surface oxide, where the basic
structural units may not be capable of isolated existence,
as well as those cases where they are.
In common chemical usage, and in this encyclope-
dia, generic and specific chemical names (such as RADI-
CALor hydroxide ion) or chemical formulae refer either
to a chemical species or to a molecular entity.chemical weight The weight of a molar sample as
determined by the weight of the molecules (the molecu-
lar weight); calculated from the weights of the atoms in
the molecule.48 chemical relaxation