moment is a measure of the polar na-
ture of the bond (see polar mol-
ecule); i.e. the extent to which the
average electron charge is displaced
towards one atom (in the case of HCl,
the electrons are attracted towards
the more electronegative chlorine
atom). In a polyatomic molecule, the
dipole moment is the vector sum of
the dipole moments of the individual
bonds. In a symmetrical molecule,
such as tetrachloromethane (CCl 4 ),
there is no overall dipole moment, al-
though the individual C–Cl bonds are
polar.dipole–dipole interactionThe
interaction of two systems, such as
atoms or molecules, by their *dipole
moments. The energy of dipole–
dipole interaction depends on the
relative orientation and the strength
of the dipoles and how far apart they
are. A water molecule has a perma-
nent dipole moment, thus causing a
dipole–dipole interaction if two
water molecules are near each other.
Although isolated atoms do not have
permanent dipole moments, a dipole
moment can be induced by the pres-
ence of another atom near it, thus
leading to induced dipole–dipole in-
teractions. Dipole–dipole interactions
are responsible for *van der Waals’
forces and *surface tension in liq-
uids.
dipole radiationSee forbidden
transitions.dipyridyl(bipyridyl)A compound
formed by linking two pyridine rings
by a single C–C bond, (C 5 H 4 N) 2. Vari-
ous isomers are possible depending
on the relative positions of the nitro-
gen atoms. A mixture of isomers can
be made by reacting pyridine with
sodium metal and oxidizing the re-
sulting sodium salt. The 22′-isomer is
a powerful chelating agent denoted
bipyin chemical formulae. Both the
22 ′- and 44′-isomers can form quater-nary compounds that are used in her-
bicides such as Paraquat(44′) and Di-
quat(22′).dipole–dipole interaction 180d N N
N NDipyridylDiquatTradename for a herbicide.
See dipyridyl.Dirac, Paul Adrien Maurice
(1902–84) British physicist, who
shared the 1933 Nobel Prize for
physics with Erwin *Schrödinger
for developing Schrödinger’s non-
relativistic wave equations to take
account of relativity. Dirac also in-
vented, independently of Enrico
Fermi, the form of *quantum statis-
tics known as Fermi–Dirac statistics.
Dirac constantSee planck con-
stant.
Dirac equationA version of the
nonrelativistic *Schrödinger equa-
tion taking special relativity theory
into account. The Dirac equation is
needed to discuss the quantum me-
chanics of electrons in heavy atoms
and, more generally, to discussÜne-
structure features of atomic spectra,
such as *spin–orbit coupling. The
equation was put forward by Paul
*Dirac in 1928. It can be solved ex-
actly in the case of the hydrogen
atom but can only be solved using
approximation techniques for more
complicated atoms.diradicalSee biradical.
direct dye See dyes.disaccharideA sugar consisting of