Dictionary of Chemistry [6th Ed.]

are the two *frontier orbitals of the
molecule.

high frequency (HF)A radio fre-
quency in the range 3–30 megahertz;
i.e. having a wavelength in the range
10–100 metres.

high-performance liquid chro-
matography(HPLC)A sensitive
technique for separating or analysing
mixtures, in which the sample is
forced through the chromatography
column under pressure.

high-resolution electron-loss
spectroscopy (HRELS)A technique
for obtaining information about mol-
ecules absorbed on a surface by in-
elastic scattering of electrons.

high-speed steelA steel that will
remain hard at dull red heat and can
therefore be used in cutting tools for
high-speed lathes. It usually con-
tains 12–22% tungsten, up to 5%
chromium, and 0.4–0.7% carbon. It
may also contain small amounts of
vanadium, molybdenum, and other
metals.

Hilbert space A linear vector space
that can have an inÜnite number of
dimensions. The concept is of inter-
est in physics because the state of a
system in quantum mechanics is
represented by a vector in Hilbert
space. The dimension of the Hilbert
space has nothing to do with the
physical dimension of the system.
The Hilbert space formulation of
quantum mechanics was put forward
by the Hungarian-born US mathe-
matician John von Neumann
(1903–57) in 1927. Other formula-
tions of quantum mechanics, such as
matrix mechanics and *wave me-
chanics, can be deduced from the
Hilbert space formulation. It is
named after the German mathemati-
cian David Hilbert (1862–1943), who
invented the concept early in the
20th century.

Hildebrand ruleIf the density of

the vapour phase above a liquid is

constant, the molar entropy of vapor-

ization is constant. This law does not

hold if molecular association takes

place in the liquid or if the liquid is

subject to quantum-mechanical

effects, e.g. as in superÛuidity. The

rule is named after the US chemist

Joel Henry Hildebrand (1881–1983).

Hill reaction The release of oxygen

from isolated illuminated chloro-

plasts when suitable electron accep-

tors (e.g. potassium ferricyanide) are

added to the surrounding water. The

reaction was discovered by Robert

Hill (1899–1991) in 1939; the electron

acceptors substitute for NADP+, the

natural acceptor for the light-depend-

ent reactions of *photosynthesis.

histidineSee amino acid.

histochemistryThe study of the

distribution of the chemical con-

stituents of tissues by means of their

chemical reactions. It utilizes such

techniques as staining, light and elec-

tron microscopy, autoradiography,

and chromatography.

HMX(octogen; cyclotetramethyl-

enetetranitramine)A colourless crys-

talline compound, C 4 H 8 N 8 O 8 ; r.d. 1.9;

m.p. 276–286°C. It has a structure

similar to that of *cyclonite (RDX),

but with an 8-membered ring rather

than a six-membered ring. An ex-

tremely powerful explosive, it is used

mainly for military purposes and as a

271 HMX

h

CH 2

N

CH 2

N

2 CH

N

2 CH

N

NO 2

NO 2

NO 2

O 2 N

HMX