M
macromolecular crystal A crystalline solid in which the atoms are
all linked together by covalent
bonds. Carbon (in diamond), boron
nitride, and silicon carbide are examples of substances that have macromolecular crystals. In effect, the
crystal is a large molecule (hence
the alternative description giantmolecular), which accounts for the
hardness and high melting point of
such materials.
macromolecule A very large molecule. Natural and synthetic polymers 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 cohesion of ionic crystals. The electrostatic interaction per ion pair, U, is
given by U(r) = – αe
2
/r, where α is the
Madelung constant and e
2 /r is the
Coulomb interaction between the
ions, with r being the lattice constant. The value of α depends on the
type of lattice. For the sodium chloride lattice, α has 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/r
n
,
where C and n are constants. The
value of α can be used in calculations
to determine C and n. It was Ürst calculated by Erwin Madelung in 1918.
Magic acid See superacid.
magic-angle spinning A technique 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
θ is the angle between the principal
axis of the molecule and the applied
magnetic Üeld. The ‘magic angle’ is
the angle θ that satisÜes 1–3cos
2
θ = 0
and is given by θ = 54.74°. In magicangle spinning the material is spun
very rapidly at the magic angle to the
applied magnetic Üeld so that the dipole–dipole interactions and chemical 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 technique has been extensively used,
with the spinning between 4 and
5 kHz.
Magnadur Tradename for a ceramic material used to make permanent magnets. It consists of sintered
iron oxide and barium oxide.
Magnalium Tradename for an aluminium-based alloy of high reÛectivity for light and ultraviolet radiation
that contains 1–2% of copper and between 5% and 30% of magnesium.
Strong and light, these alloys also
sometimes contain other elements,
such as tin, lead, and nickel.
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macromolecular crystal A crystalline solid in which the atoms are
all linked together by covalent
bonds. Carbon (in diamond), boron
nitride, and silicon carbide are examples of substances that have macromolecular crystals. In effect, the
crystal is a large molecule (hence
the alternative description giantmolecular), which accounts for the
hardness and high melting point of
such materials.
macromolecule A very large molecule. Natural and synthetic polymers 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 cohesion of ionic crystals. The electrostatic interaction per ion pair, U, is
given by U(r) = – αe
2
/r, where α is the
Madelung constant and e
2 /r is the
Coulomb interaction between the
ions, with r being the lattice constant. The value of α depends on the
type of lattice. For the sodium chloride lattice, α has 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/r
n
,
where C and n are constants. The
value of α can be used in calculations
to determine C and n. It was Ürst calculated by Erwin Madelung in 1918.
Magic acid See superacid.
magic-angle spinning A technique 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
θ is the angle between the principal
axis of the molecule and the applied
magnetic Üeld. The ‘magic angle’ is
the angle θ that satisÜes 1–3cos
2
θ = 0
and is given by θ = 54.74°. In magicangle spinning the material is spun
very rapidly at the magic angle to the
applied magnetic Üeld so that the dipole–dipole interactions and chemical 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 technique has been extensively used,
with the spinning between 4 and
5 kHz.
Magnadur Tradename for a ceramic material used to make permanent magnets. It consists of sintered
iron oxide and barium oxide.
Magnalium Tradename for an aluminium-based alloy of high reÛectivity for light and ultraviolet radiation
that contains 1–2% of copper and between 5% and 30% of magnesium.
Strong and light, these alloys also
sometimes contain other elements,
such as tin, lead, and nickel.
www.AzShimi.ir www.AzShimi.com