pairing energy Energy required to place two elec-
trons in the same orbital.
See alsoATOMIC ORBITAL; MOLECULAR ORBITAL.

paraffin A natural wax.

parallel reaction SeeCOMPOSITE REACTION.

paramagnetic Substances having a positive MAG-
NETIC SUSCEPTIBILITY are paramagnetic. They are
attracted by a magnetic field.
See alsoDIAMAGNETIC.

parent ion (precursor ion) An ion that dissociates to
a smaller ion fragment, usually from a collision-
induced dissociation in a MS/MS (mass spectro-
metry/mass spectrometry instrument) or tandem mass
spectrometry experiment.

partial agonist An AGONISTthat is unable to induce
maximal activation of a RECEPTORpopulation, regard-
less of the amount of DRUGapplied.
See alsoINTRINSIC ACTIVITY.

partial pressure Each gas in a mixture of gases
exerts a pressure called the partial pressure; the pres-
sure exerted by one gas in a mixture of gases.

partial rate factor The rate of substitution at one spe-

cific site in an AROMATICcompound relative to the rate

of substitution at one position in benzene. For example,

the partial rate factor fpzfor parasubstitution in a mono-

substituted benzene C 6 H 5 Z is related to the rate con-

stants k(C 6 H 5 Z) and k(C 6 H 6 ) for the total reaction (i.e.,

at all positions) of C 6 H 5 Z and benzene, respectively, and

%para (the percentage parasubstitution in the total

product formed from C 6 H 5 Z) by the relation

Similarly for metasubstitution:

(The symbols pfz, mfz, ofzare also in use.) The term

applies equally to the IPSO position, and it can be

extended to other substituted SUBSTRATEs undergoing

parallel reactions at different sites with the same

reagent according to the same RATE LAW.

See alsoSELECTIVITY.

particulate matter A generic term used for a type of

air pollution that contains complex and varying mix-

tures of particles suspended in the air.

pascal The unit of pressure or stress under the Inter-

national System of Units (SI). One pascal is equivalent

f

k

m k

Z=^6

2 100

(C H Z)

(C H )

65 %meta

66

f k

p k

Z=^6

100

(C H Z)

(C H )

65 %para

66

207

P