Page 131 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005E-212 June 15, 2001 20:32

342 Electron Spin Resonance

several different kinds of double-resonance experiments the total number of lines is additive for different types
that can be done in ESR. The technique that appears to of coupled nuclei, whereas in ESR the number of lines
be the most useful currently and one that is becoming is multiplicative. A specific example can be considered
more widespread is electron nuclear double resonance, for the triphenylmethyl radical. In the ESR spectrum the
abbreviated ENDOR. Essentially, ENDOR is the obser- total number of lines is 196, spread over a 30-G range.
vation of the effect of applying a second frequency, which By ENDOR there are only 6 lines, which can be easily
induces nuclear spin flips, simultaneously with the mi- resolved because you see only one pair of ENDOR lines
crowave frequency that induces electron spin flips. The for each group of equivalent nuclei. In the triphenylmethyl
observation of ENDOR depends on partial saturation of radical there are three different types of protons—namely,
an ESR transition. In simple terms, the ENDOR effect is meta, ortho, and para protons—each of which have differ-
simply the change of the ESR intensity when a second ent hyperfine couplings. Thus this spectral simplification
radio frequency field is applied to the system. leads to effectively increased resolution for liquid-phase
Experimentally, ENDOR is carried out by fixing the spectra.
external magnetic field at an ESR hyperfine line and then 4. ENDOR is also very useful for studying details of
sweeping the radio frequency from perhaps 1 to 20 MHz. spin relaxation mechanisms. The effect of molecular mo-
ENDOR lines will be observed corresponding to differ- tions on relaxation processes can be studied. Solvation
ently coupled nuclei in the paramagnetic system. For and temperature effects on both electron and nuclear re-
weak hyperfine interactions, the ENDOR lines for each laxation mechanisms can also be investigated.
type of spin- 1 nucleus are given by ν N ± A/2 where
2
ν N = g N β N H/h. For protons, ν H ≈ 14 MHz at 3300 G. Time-domain ESR methods have become quite impor-
Thus a pair of lines is observed in the ENDOR spectrum tant for modern applications of ESR. In a time-domain
for each type of coupled protons. The hyperfine constant experiment one uses microwave pulses rather than steady-
A can be measured more accurately with ENDOR than state microwaves. It is possible to use pulses as short as
by ESR, because one is measuring megahertz instead of a few nanoseconds, which makes fast kinetic processes
gigahertz. Also, the hyperfine nucleus can be identified involving paramagnetic species observable and allows the
from the value of g N , since these values are characteristic direct measurement of relaxation times. In addition, time-
for different nuclei. domain ESR methods have been useful for more direct
The practical advantages of ENDOR experiments can determination and study of relaxation mechanisms and
be summarized as follows: for developing new methods for obtaining structural in-
formation in disordered systems.
1. Increased resolution can be obtained, which is par- Two methods of time-domain ESR will be briefly dis-
ticularly important for inhomogeneously broadened lines cussed. The oldest method is that of saturation recovery,
typically observed in solids where the hyperfine struc- which is a direct method to determine spin–lattice relax-
ture is unresolved. A typical ESR linewidth in solids ation times. The idea is to perturb the steady-state pop-
is 10 G, whereas a typical ENDOR linewidth is about ulation of spins with a partially saturating pulse of mi-
0.03 G. A classic example is that of a trapped electron crowaves and then to observe with a very weak microwave
or of trapped hydrogen atoms in potassium chloride crys- field the recovery of the perturbed spin population to equi-
tals. In these paramagnetic systems, ENDOR couplings librium. In the absence of complications, the recovery pro-
to different shells of magnetic nuclei are resolved, and a cess is exponential and can be related to the time constant
detailed picture of the electron and hydrogen-atom wave forspin–latticerelaxation.Exponentialrecoveriesaregen-
functions can be obtained. By ordinary ESR, only a broad erally observed in liquids and in some cases in solids. If
line is observed, in which the hyperfine information for the spin–lattice relaxation time is not much longer than
the different magnetic nuclei in the solid is not resolved. the spin–spin relaxation time, which is atypical in param-
2. Hyperfine constants can be measured more accu- agnetic systems, the interpretation of saturation recovery
rately by ENDOR than by ESR. This is particularly impor- data becomes more complex.
tant for measuring very small hyperfine couplings. Also, The second type of time-domain method that will be
very small changes in the hyperfine couplings due to tem- discussed is electron spin-echo spectroscopy, abbreviated
perature and so forth are most accurately measured by ESE. Figure 9 gives an illustration of a two-pulse electron
ENDOR. spin-echo response. Two resonant microwave pulses are
3. ENDOR can lead to significant spectral simplifica- applied to the system with typical pulse lengths of 10–
tion, which is particularly important in looking at radicals 100 nsec and pulse powers corresponding to several hun-
in liquids. This is because the total number of spectral dred watts. The first pulse (1) essentially starts a clock and
lines is much less in ENDOR than in ESR. In ENDOR flips the spins from the external magnetic field direction