M

macromolecular crystal A crys-
talline solid in which the atoms are
all linked together by covalent
bonds. Carbon (in diamond), boron
nitride, and silicon carbide are exam-
ples of substances that have macro-
molecular crystals. In effect, the
crystal is a large molecule (hence
the alternative description giant-
molecular), which accounts for the
hardness and high melting point of
such materials.

macromolecule A very large mol-
ecule. Natural and synthetic poly-
mers have macromolecules, as do
such substances as haemoglobin. See
also colloids.

macroscopic Designating a size
scale very much larger than that of
atoms and molecules. Macroscopic
objects and systems are described by
classical physics although *quantum
mechanics can have macroscopic
consequences. Compare mesoscopic;
microscopic.

Madelung constant A constant
arising in calculations of the cohe-
sion of ionic crystals. The electrosta-
tic interaction per ion pair, U, is
given by U(r) = – αe^2 /r, where αis the
Madelung constant and e^2 /r is the
Coulomb interaction between the
ions, with r being the lattice con-
stant. The value of αdepends on the
type of lattice. For the sodium chlo-
ride lattice, αhas a value of about
1.75. A more realistic calculation of
cohesion is obtained if short-range
repulsions with an inverse power law
are included, i.e.

U(r) = αe^2 /r – C/rn,

where C and n are constants. The

value of αcan be used in calculations

to determine C and n. It wasÜrst cal-

culated by Erwin Madelung in 1918.

Magic acidSee superacid.

magic-angle spinningA tech-

nique used in solid-state *nuclear

magnetic resonance (NMR) for

making the line widths smaller. In

magic-angle spinning, both the

dipole–dipole interaction and the

chemical shift anisotropy have the

angular dependence 1–3cos^2 θ, where

θis the angle between the principal

axis of the molecule and the applied

magneticÜeld. The ‘magic angle’ is

the angle θthat satisÜes 1–3cos^2 θ= 0

and is given by θ= 54.74°. In magic-

angle spinning the material is spun

very rapidly at the magic angle to the

applied magneticÜeld so that the di-

pole–dipole interactions and chemi-

cal shift anisotropies average to zero.

It is necessary for the frequency of

spinning to be at least as large as the

width of the spectrum. This tech-

nique has been extensively used,

with the spinning between 4 and

5 kHz.

MagnadurTradename for a ce-

ramic material used to make perma-

nent magnets. It consists of sintered

iron oxide and barium oxide.

MagnaliumTradename for an alu-

minium-based alloy of high reÛectiv-

ity for light and ultraviolet radiation

that contains 1–2% of copper and be-

tween 5% and 30% of magnesium.

Strong and light, these alloys also

sometimes contain other elements,

such as tin, lead, and nickel.