SPECTROSCOPY). ESE may be used for measurements of
electron spin relaxation times, as they are influenced by
neighboring paramagnets or molecular motion. It may
also be used to measure anisotropic nuclear HYPERFINE
couplings. The effect is known as electron spin-echo
envelope modulation (ESEEM). The intensity of the
electron spin echo resulting from the application of two
or more microwave pulses is measured as a function of
the temporal spacing between the pulses. The echo
intensity is modulated as a result of interactions with
the nuclear spins. The frequency-domain spectrum cor-
responds to hyperfine transition frequencies.
See also ANISOTROPY; PARAMAGNETIC INTERAC-
TION.
electron spin quantum number The quantum num-
ber representing one of two possible values for the elec-
tron spin; either –1/2 or 1/2.
electron spin-resonance spectroscopy(ESR spec-
troscopy) SeeELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.
electron transfer The transfer of an electron from
one MOLECULAR ENTITYto another, or between two
localized sites in the same molecular entity.
See alsoINNER-SPHERE ELECTRON TRANSFER; MAR-
CUS EQUATION; OUTER-SPHERE ELECTRON TRANSFER.
electron-transfer catalysis The term indicates a
sequence of reactions such as shown in equations
(1)–(3), leading from A to B:
A + e–→ A.–(1)
A.–→ B.–(2)
B.–+ A → B + A.–(3)An analogous sequence involving radical cations (A.+,
B.+) is also observed.
The most notable example of electron-transfer
catalysis is the SRN1 (or T+DN+AN) reaction of aro-
matic halides.
The term has its origin in a suggested analogy to
acid–base catalysis with the electron instead of the pro-
ton. However, there is a difference between the two cat-
alytic mechanisms, since the electron is not a true cata-
lyst, but rather behaves as the initiator of a CHAIN
REACTION. Electron-transfer-induced chain reactionis a
more appropriate term for the situation described by
equations (1)–(3).electron-transfer protein A protein, often contain-
ing a metal ion, that oxidizes and reduces other
molecules by means of electron transfer.electron-transport chain A chain of electron accep-
tors embedded in the inner membrane of the mitochon-
drion. These acceptors separate hydrogen protons from
their electrons. When electrons enter the transport
chain, the electrons lose their energy, and some of it is
used to pump protons across the inner membrane of
the mitochondria, creating an electrochemical gradient94 electron spin quantum number
Demonstration of an electron-transfer reaction in a laboratory. At
left is a test tube containing copper(II) sulfate solution; below this
is a watch glass containing zinc powder. When the zinc is added
to the sulfate, the solution loses its blue color (right). This is due
to the electron transfer between copper(II) ions (blue) and the
zinc to form zinc(II) ions (colorless). The copper is deposited as a
metal.(Courtesy of Jerry Mason/Science Photo Library)