an insoluble precipitate, then B may
be carried down with the precipitate
of A, even though it is soluble under
the conditions. This can occur by oc-
clusion or absorption.
cordite An explosive mixture of cel-
lulose nitrate and nitroglycerin, with
added plasticizers and stabilizers,
used as a propellant for guns.
core orbital An atomic orbital that
is part of the inner closed shell of an
atom. Unlike the valence orbitals,
which are the atomic orbitals of the
valence shell of an atom, core or-
bitals on one atom have a small over-
lap with core orbitals on another
atom. They are therefore usually ig-
nored in simple calculations of chem-
ical bonding.
Corey–Pauling rules A set of
rules, formulated by Robert Corey
and Linus Pauling, that govern the
secondary nature of proteins. The
Corey–Pauling rules are concerned
with the stability of structures pro-
vided by hydrogen bonds associated
with the –CO–NH– peptide link. The
Corey–Pauling rules state that:
(1) All the atoms in the peptide link
lie in the same plane.
(2) The N, H, and O atoms in a hydro-
gen bond are approximately on a
straight line.
(3) All the CO and NH groups are in-
volved in bonding.
Two important structures in which
the Corey–Pauling rules are obeyed
are the alpha helix and the *beta
sheet.
CORN ruleSee absolute configu-
ration.
correlation diagram A diagram
that relates the energy levels of sepa-
rate atoms to the energy levels of di-
atomic molecules and united atoms,
the two limiting states being corre-
lated with each other. Using this dia-
gram, the types of molecular orbital
possible for the molecules and their
order as a function of increasing en-
ergy can be seen as a function of in-
teratomic distance. Lines are drawn
linking each united-atom orbital to a
separated atom orbital. An important
rule for determining which energy
levels correlate is the noncrossing
rule. This states that two curves of
energy plotted against interatomic
distance never cross if the invariant
properties (e.g. parity, spin, etc.) are
the same.correlation functions Quantities
used in condensed-matter physics
that are *ensemble averages of prod-
ucts of such quantities as *density at
different points in space. Each parti-
cle affects the behaviour of its neigh-
bouring particles, so creating
correlations, the range of which is at
least as long as the range of the inter-
molecular forces. Thus, correlation
functions contain a great deal of in-
formation about systems in con-
densed-matter physics. Correlation
functions can be measured in consid-
erable detail, frequently by experi-
ments involving scattering of such
particles as neutrons or of *electro-
magnetic radiation (e.g. light or X-
rays) and can be calculated
theoretically using *statistical me-
chanics.
correlation spectroscopy (COSY)
A type of spectroscopy used in *nu-
clear magnetic resonance (NMR) in
which the pulse sequence used is:
90°x– t 1 – 90°x– acquire (t 2 ). The
delay t 1 is variable and a series of ac-
quisitions is made. Fourier trans-
forms are then performed on both
the delay t 1 and the real time t 2. The
information thus gained can be
shown on a diagram that plots con-
tours of signal intensity. This repre-
sentation of the information enables
NMR spectra to be interpreted more145 correlation spectroscopy
c