Q

QSARSee quantitative
structure–activity relationship.

QSMRSee quantitative
structure–metabolism relation-
ship.

quadrivalentHaving a valency of
four.

quadrupoleA set of four point
charges that has zero net charge and
dipole moment. An example of a
quadrupole is a carbon dioxide (CO 2 )
molecule. Quadrupole interactions
are much smaller than dipole inter-
actions. Transitions involving
quadrupole moments are much
weaker than transitions involving di-
pole moments, but can allow transi-
tions forbidden in dipole moment
transitions.

quadrupole mass spectrometer
See mass spectroscopy.

qualitative analysis See analysis.

quantitative analysisSee analy-
sis.

quantitative structure–activity
relationship (QSAR)A statistical al-
gorithm that quantitatively deÜnes
the relationship between the chemi-
cal structure of a drug and its effect
on an organism. QSAR studies are
often used to predict the activity or
toxicity of new drugs. Similar meth-
ods can be used to predict the metab-
olism of new drugs (quantitative
structure–metabolism relationships).

quantitative structure–metabo-
lism relationship (QSMR)See
quantitative structure–activity
relationship.

quantum(pl. quanta) The mini-

mum amount by which certain prop-

erties, such as energy or angular

momentum, of a system can change.

Such properties do not, therefore,

vary continuously, but in integral

multiples of the relevant quantum,

and are described as quantized. This

concept forms the basis of the *quan-

tum theory. In waves andÜelds the

quantum can be regarded as an exci-

tation, giving a particle-like interpre-

tation to the wave orÜeld. Thus, the

quantum of the electromagneticÜeld

is the *photon and the graviton is

the quantum of the gravitational

Üeld.

quantum chaosThe *quantum

mechanics of systems for which the

corresponding classical system can

exhibit *chaos. This subject was initi-

ated by Einstein in 1917, who

showed that the quantization condi-

tions associated with the *Bohr

theory need to be modiÜed for sys-

tems that show chaos in classical me-

chanics. The subject of quantum

chaos is an activeÜeld of research in

which many basic issues still require

clariÜcation. It appears that systems

exhibiting chaos in classical mechan-

ics do not necessarily exhibit chaos

in quantum mechanics.

quantum chemistryThe applica-

tion of quantum mechanics to chem-

istry. Quantum chemistry mostly

used empirical methods atÜrst but as

computing power has developed it

has been increasingly based on the

Ürst principles of quantum mechan-

ics.

quantum entanglementA phe-