As a corollary, it follows that a factor stabilizing a
reaction intermediate will also stabilize the transition
state leading to that intermediate.
The acronym Bemahapothle(Bell, Marcus, Ham-
mond, Polanyi, Thornton, Leffler) is sometimes used in
recognition of the principal contributors toward expan-
sion of the original idea of the Hammond postulate.
Hansch analysis The investigation of the quantita-
tive relationship between the biological activity of a
series of compounds and their physicochemical sub-
stituent or global parameters representing hydropho-
bic, electronic, steric, and other effects using
multiple-regression correlation methodology.
Hansch constant A measure of the capability of a
solute for HYDROPHOBIC(LIPOPHILIC) interaction based
on the partition coefficient P for distribution of the
solute between octan-1-ol and water. The most general
way of applying P in CORRELATION ANALYSIS, QSAR,
etc., is as log P, but the behavior of substituted benzene
derivatives may be quantified by a substituent constant
scale, π, which is defined in a way analogous to the
Hammett σscale. There are various πscales, depending
on the substrate series used as reference.
hapten A molecule (usually a small organic molecule)
that can be bound to an ANTIGENICdeterminant/epi-
tope. Usually they are too small to give a response of
their own. They become antigenic if they are coupled to
a suitable macromolecule, such as a protein.
hapto The hapto symbol, η(Greek eta) with numeri-
cal superscript, provides a topological description for the
bonding of hydrocarbons or other π-electron systems to
metals, by indicating the connectivity between the LIG-
ANDand the CENTRAL ATOM. The symbol is prefixed to
the ligand name, or to that portion of the ligand name
most appropriate. The right superscript numerical index
indicates the number of COORDINATINGatoms in the li-
gand that bind to the metal. Examples:
[PtCl 2 (C 2 H 4 )(NH 3 )] amminedichloro(η^2 -ethene)platinum
[Fe(η^5 -C 5 H 5 ) 2 ] bis(η^5 -cyclopentadienyl)iron (ferrocene)hard acid A LEWIS ACIDwith an acceptor center of
low polarizability. It preferentially associates with
HARD BASEs rather than with soft bases, in a qualitative
sense (sometimes called the HSAB [hard and soft acid
and base] rule). Conversely, a soft acid possesses an
acceptor center of high polarizability and exhibits the
reverse preference for a partner for COORDINATION.hard base A LEWIS BASEwith a donor center of low
polarizability; the converse applies to soft bases.
See alsoHARD ACID.hard drug A nonmetabolizable compound, charac-
terized either by high lipid solubility and accumulation
in adipose tissues and organelles, or by high water sol-
ubility. In the lay press, the term hard drugrefers to a
powerful DRUGof abuse such as cocaine or heroin.hard water A property of water that causes the for-
mation of an insoluble residue when used with soap or
scales in vessels in which water has been allowed to
evaporate. Properties are due primarily to the presence
of ions of calcium, magnesium, and iron.HCP(hexagonal close packing) A type of crystal lat-
tice structure found in zinc, titanium, and cobalt, for
example.heat Kinetic energy in the process of being trans-
ferred from one object to another due to a temperature
difference. It moves in one of three ways: radiation,
conduction, or convection.heat capacity The amount of heat required to
change a unit mass (or unit quantity, such as a mole) of
a substance by one degree in temperature.heat capacity of activation,∆‡Cpo(SI unit: J mol–1
K–1) A quantity related to the temperature coefficient
of ∆‡H (ENTHALPY OF ACTIVATION) and ∆‡S (ENTROPY
OF ACTIVATION) according to the equations:124 Hansch analysis