Vij D.R. Handbook of Applied Solid State Spectroscopy
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4.5.2 Radicals Trapped in Matrices
1
H ENDOR measurements have been recorded for radicals trapped in xenon,
argon, or freon matrices and some typical examples can be found in
references [40–48]. Typically the radicals are formed by X- or J-irradiated
frozen matrix at 77 K or 4 K. Protons from methyl groups or groups
undergoing tunneling rotation give rise to narrower lines that are easy to
detect, although exo- and endo-protons of a radical cation of s-trans-beta-1,3-
diene has been reported [42].
4.5.3 Triplet-State Radicals in Crystal Polycrystalline
Samples
Triplet-state radicals can be examined from ENDOR measurements. For
example, proton couplings have been measured for the triplet state of
naphthalene [49], and substituents of naphthalene [49] where intramolecular
and intermolecular hyperfine interactions have been deduced for X-traps. The
triplet-state of diphenyl-h
10
in diphenyl-d
10
single crystals indicate that the
triplet-state diphenyl-h
10
molecules have no inversional symmetry [50, 51].
The localized triplet-state (x-trap) at 4.2 K was also examined by ENDOR
measurements in a charge-transfer crystal of naphthalene-tetracyano- benzene
[52]. From the symmetry of the spin density distribution on the donor and
acceptor, it was found that the charge-transfer (CT) sate is distributed over
one donor and two acceptors. Other CT states have also been studied [53, 54].
It was deduced from a study of the triplet-state of benzil in bibenzyl that the
dicarbonyl fragment has to be precisely trans-planar [55] while the lowest
triplet state of trichlorobenzene is an excimer [56] whose structure is dimeric
and located in one of the stacks of the crystal with a mis-orientation of 20º.
by time-resolved ENDOR spectroscopy at 10 K [57]. The cis or trans
configuration was deduced by comparing the A
z
coupling tensor with that
predicted by density functional theory: A time-resolved ENDOR study of the
photoexcited triplet state of the free-base tetraphenyl chlorine has revealed the
nature of the orientation of the solute molecules [58]. Triplet energy transfer
between porphyrin subunits was detected by ENDOR measurements at liquid
helium temperature of the triplet state of several substituted porphyrins [59].
1
H ENDOR analysis of P 700
+
radical cation was reported and the
measurements were consistent with a slight redistribution of spin density from
P
B
chlorophyll to P
A
as well as a slight redistribution of spin within the P
B
chlorophyll [60]. Recent electron spin-cho-ENDOR was used to investigate
the structure of the photosystem II oxygen-evolving complex (OEC) including
the paramagnetic manganese cluster and its surroundings. The measurements
cycle [61].
4. ENDOR Spectroscopy
The photoexcited triplet state of free-based porphycene has been studied
suggested that the substrate waters bind within the OEC early in the S-state
186
4.5.4 Free Radicals in Biological Systems
Numerous examples exist where free radicals that occur in biological media
have been examined by ENDOR measurements. For example, cytochrome
oxidase has been studied by a number of authors [62–67].
1
H and
14
N ENDOR
measurements have identified two different protons with fairly large isotropic
couplings of 12 and 19 MHz [62], the histidine N atoms [65] and
1
H
couplings for protons near a heme site [65], identification of histidine ligands
[64] in cytochrome a and evidence of bisimidazole coordination in
cytochrome a and in cytochrome c oxidase [66].
63
Cu ENDOR spectra have
been observed for cytochrome c oxidase [63, 67]. This established the
presence of Cu in the redox center [63] and the electronic structure of the
Cu
2+
A
center was monitored by hyperfine couplings to clustering protons and
the change correlated with the anti-cooperative redox interaction between
cytochrome a and Cu
A
[67]. High- and low-spin ferric hemoglobins and
myoglobin (aquo derivatives) were studied and from the ENDOR couplings,
couplings to exchangeable heme-bond water protons and the
13
C coupling in
myoglobin and hemoglobin [68] were measured.
57
Fe ENDOR was recorded
for azotobacter vinelandii Mo-Fe proteins in frozen solution [69],
bidirectional hydrogenase [70], oxidized hydrogenase [71], costridium
pasteurianum [72], escherichia coli sulfite reductase [73], hydrogenase-I [74],
hydrogenase-II [71], and nitrogenase molybdenum [75].
17
O ENDOR has been recorded in beef heart aconetase [76] and in
horseradish peroxidase [77]. In horseradish peroxidase, the O atom comes
from the oxidant, the Fe portion of the system is an oxy-ferryl (Fe
IV
= O)
moiety. The [4Fe-4S] cluster was studied using
17
O ENDOR in beef heart
aconetase.
95,97
Mo ENDOR was recorded for MoFe proteins [72], and
95
Mo
protein [79], where
95
Mo hyperfine and quadrupole coupling constants
indicated that a single Mo atom was integrated into the MoFe spin system.
33
S ENDOR has been recorded for MoFe protein [69] and
19
F ENDOR has
been detected for flavin radicals [80] in flavins [81] and flavo-proteins [81] in
different protonation sites. The influence of different substituents on the spin
distribution within the isocelloxazine ring was deduced [81]. Proton ENDOR
was recorded for photosystem-II [82], P
865
+
[83], P
960
[84], and numerous other
systems.
The effects of ionizing radiation on single crystals of quanine
hydrobromide monohydrate were studied by ENDOR and field-swept-
ENDOR techniques. Five different radicals were identified and comparison
to other systems containing the N
7
-protonated quonine bases demonstrated the
effect of the environment on the primary radical formation [85]. Time-
resolved ENDOR measurements were carried out on the spin-correlated
radical pair P700
+
… A
–1
in frozen solution samples of PSI [86]. It was found
that the mixed electronic eigenstates give rise to transitions not usually found
in ENDOR and that absorptive/emissive TR-ENDOR spectra are observed.
187
4.5 Applications
Pulsed-ENDOR spectroscopy at X- and W-band were employed to probe
the strongly coupled
14
N nuclei and the E-protons of the CuA center perturb
by the axial methionine ligand. The high-field data resolved the Tz and CuA
signals in the g perpendicular region and gave
1
H ENDOR spectra free of
overlapping
14
N signals [87]. ENDOR measurements were also used to
characterize the oxygen-evolving center Mn cluster [88]. Examples of Q-band
ENDOR have been reported where measurements were made on the protein-
bound, high-affinity ubisemiquione radical to determine its electronic spin
distribution [89].
1
H pulsed-ENDOR and ESEEM measurements have shown
that the bis-imidazole complexes of iron(III) tetraphenylchlorine and
tetraphenylporphyrin have the same g values and the same electronic ground
state [90]. Several reviews are available that can provide additional
information. The ENDOR crystallography of disordered systems of some
Cu(II) S-containing chelate complexes and oxovanadium(II) chelate
complexes in the solid state and in frozen solution has been reviewed [91]. A
review has been published on the Q-band-ENDOR studies of the nitrogenase
MoFe protein in which it was demonstrated how to analyze orientational
selective
13
C and
57
Fe ENDOR spectra. From such analysis, the binding
modes and the charge of the cofactor can be derived [92]. A briefer review
details how ENDOR techniques can be used to understand metal ion functions
in proteins and enzymes [93].
4.5.5 Polymeric Systems
1
H and
13
C ENDOR studies have been reported for polymeric systems
[94–120]. For example studies of pristine cis(I) and trans-polyacetylene (II)
have shown an isotropic coupling of 1.8 MHz for poly(methyl methacrylate)
and an activation energy of 34 K. On the other hand for poly(methyl-D-
chloro-acrylate, an isotropic hyperfine coupling of 1.4 MHz and 400 K was
observed [95]. In stretched films of undoped polyacetylene [97], the
anisotropy of the hyperfine coupling showed the S-electron character
consistent with the assumption of the bond alternation kink in the S-electron
system in undoped (CH-)
x
. For polyacetylene, the soliton model ENDOR
data measured the size of the chain segment over which a soliton diffuses.
13
C-enriched undoped cis-rich polyacetylene was measured down to 4 K and
ENDOR measurements showed that the structures observed at higher
temperature [110], which were previously assigned to proton or
13
C hyperfine
tensor components, grew into sharp single resonance lines as the temperature
was lowered.
4. ENDOR Spectroscopy
188
Examples of ENDOR measurements for small inorganic radicals formed by
ionizing radiation of inorganic crystals have been reported [121–129]. For
example, K
2
SO
4
single crystals doped with CrO
4
2–
ions could be characterized
by the location of the nearby protons [122]. In a J-irradiated BaClO
4
:3H
2
O
single crystal at 120 K, the hyperfine coupling tensors for 24 protons were
detected with small isotropic components and their dipolar components were
found to be close to axial [124]. In J-irradiated CsH
2
AsO
4
and CsD
2
AsO
4
ferroelectric crystals,
133
Cs and
1
H superhyperfine couplings for the AsO
4
–
center were studied [125]. The behavior at the phase transition and in the
ferroelectric phase were deduced by ENDOR measurements of the free
radical in J-irradiated single crystals of KH
2
AsO
4
[126]. In X-irradiated
K
2
SO
4
:BF
4
–
at 300 K, the FSO
2
radical was identified as trapped at the
sulphate site [129].
Numerous examples exist [130–152] where ENDOR measurements have
deduced
1
H,
1
N,
19
F,
107,109
Ag,
63
Cu,
59
Co,
13
C,
31
P couplings for inorganic
complexes in a single crystal host. For example, paramagnetic Cu(II) and
Ag(II) tetraphenylporphyrin (TPP) were oriented in a (H
2
O) Zn(TPP) single
crystal. Measurements from 20–70 K, yielded the
1
H,
63
Cu,
14
N,
107
Ag,
109
Ag
anisotropic tensors and
14
N quadrupole tensors from which the electron
distribution in the Cu(TPP) and Ag(TPP) could be deduced [134]. Typically a
suitable diamagnetic host is selected into which paramagnetic complexes can
be grown at a dopant < 1%. The effect of the axial anionic ligands on the
51
V
nuclear quadrupolar coupling constant was deduced by ENDOR
measurements [153], Q-band-ENDOR was used to detect the cation vacancies
near the cis [RbCl
16
]
4–
complex in NaCl [154].
4.5.8 F and H Centers in Inorganic Host Crystals
F and H color centers can be formed in inorganic host crystals by ionizing
radiation [155–172]. The centers formed have been characterized by ENDOR
measurements. For examples, F
+
centers [156] characterized by the
parameters g = 1.9849(2), A = ± 49.19(2) MHz, B = ± 36.40(2) MHz, and
Q = ±1.00(2) MHz can be formed by neutron irradiation of SRO while two
types of F centers are formed in BaFCl [158]. They are F(CL
–
) and F(F
–
). In
an irradiated KCl:Li
+
host, a new paramagnetic H center was detected [161]
with a Li
+
to H
o
separation of 2.17 Å. In irradiated NaCl, a A
o
g
–
A
+
g
pair center
Single Crystals
4.5.6 Inorganic Radicals in Irradiated Inorganic
Single Crystals
4.5.7 Inorganic Paramagnetic Complexes in Organic
189
4.5 Applications
was detected [163]. In irradiated KCl and RbCl [167, 168], the superhyperfine
and quadrupole interactions of the nearest Cl neighbors of atom D or cation
sites in KCl and RbCl and of first- and second-shell neighbors of interstitial D
in RbCl were measured. The O
–
center was identified by ENDOR
measurements as a self-trapped hole in X-ray irradiated ZnWO
4
[173].
4.5.9 Paramagnetic Inorganic Ions in Organic Host Crystals
In some systems under study by ENDOR techniques inorganic metal ions are
doped into organic single crystals. Examples are Cu
2+
[174–176], Ce
3+
[177],
Fe
3+
[178], Nd
3+
[179], Mn
2+
[180, 181], VO
2+
[182], Gd
3+
[183, 184]. In such
studies, the oriented hyperfine coupling tensor is deduced for these ions and
the location of the crystal site that has been substituted is established. For
instance, Cu
2+
substituted in D-glycine crystals [174], Mn
2+
in tris(sarcosine)
calcium chloride [180] crystal, VO
2+
in triglycine sulfate [182] crystal, Gd
3+
in lanthanium nicotinate dihydrate crystal [183], Cl
3+
in Cs
2
NaYCl
6
[177]
crystal, Fe
3+
in RbAl(SO
4
)
2
.
12H
2
O [178] crystal, Gd
3+
in Cs
2
NaYCl
6
[184]
crystal, and Nd
3+
in YCl
3
crystal [179]. Metal-quinone high spin complexes
were studied at X- and W-band by ENDOR measurements [185]. The VO
2+
complex was examined by ENDOR techniques [186] as well as Fe
3+
in a
heme protein [187].
4.5.10 Transition Metal Ion Complexes in Frozen Solutions
and Powders
Often it is not possible to grow single host crystals, so the transition metal ion
complex is incorporated in a powder or a frozen solution. Although it may
not always be possible to obtain a complete ENDOR hyperfine tensor from a
powder spectrum, numerous examples exist [188–205] where
1
H [188–205],
14
N [192, 205]
47,49
Ti [201],
31
P [195],
13
C [202] tensors have been deduced
from ENDOR measurements for VO
2+
[188, 191, 192, 197, 198, 202, 204],
Cu
2+
[190, 193, 195], Co
2+
[199, 200], Gd
3+
[196], Ti
+3
[201], and Mo
+5
[203].
More recently
61
Ni hyperfine and nuclear quadrupole tensors, the
77
Si
hyperfine tensor for four nearest neighbors, and the
69,71
Ga hyperfine and
nuclear quadrupole matrixes for four neighbors, have been detected [206].
The water coordination to Gd
3+
has been deduced by pulsed-ENDOR
measurements [201].
4.5.11 Defects and Complexes on Surfaces
Radicals can be formed by ionizing irradiation of surfaces [208] such as silica,
alumina, zeolite, and nafion. For example, F
+
centers are formed at O(1),
O(2), O(3), O(4) and O(5) sites [209] in irradiated sodium E-alumina single
4. ENDOR Spectroscopy
190
crystals at room temperatures.
27
Al and
23
Na ENDOR measurements at 60 K
provided the Al and Na hyperfine coupling tensors that, when combined with
the g and quadrupole tensors, made it possible to identify the site. F
+
were
also identified as being found in D-alumina upon fast-neutron irradiation
[210], and in Na, K, and Li E-alumina single crystals upon irradiation with
1.5 meV electrons [211]. ENDOR measurements were carried out at 2 K.
From
1
H and
23
Na ENDOR measurements at 120 K, the VO
2+
species was
ionomer nafion [213] and from
27
Al ENDOR at 8 K as being adsorbed on
G-alumina [214].
1
H ENDOR measurements at 110 K [215] showed that
perylene radicals could be adsorbed on alumina and silica-alumina powders.
Copper(II) complexes were identified on G-alumina from
1
H ENDOR
measurements at 6 K [216]. Examples have been reported where the
carotenoid radical cation can be formed and stabilized on a silica-alumina
matrix at room temperature as shown by
1
H ENDOR measurements [217].
The methyl groups on the carotenoid radical cation rapidly rotate from 5 K up
to 300 K, which results in the appearance of narrow detectable ENDOR lines.
Radical cations of benzene-d
6
and benzene-d
1
were generated by ionizing
radiation and stabilized on silica gel and HY molecular sieves [218]. The
electronic structure and dynamic features were different from those stabilized
on a freon matrix (CFCl
3
) as deduced from ENDOR and ESEEM
measurements between 3.5 and 105 K.
4.5.12 Impurity Centers in Semiconductor Host Crystals
Inorganic ions of
57
Fe [219]
55
Mn [220, 221], Te [221],
53
Cr [219, 223], Ge
[224], Ni [225], Co [226], Zn [226], were identified by ENDOR
measurements of these nuclei as being impurity centers in semiconductor GaP
host crystals. The V
3+
ion was detected in the GaAs semiconductor host [227]
where hyperfine interactions with the V nucleus and the first As and Ga
neighbor shells were detected. The V resides in a Ga substitutional or in a
high symmetry interstitial site tetrahedrally surrounded by four nearest As
neighbors. ENDOR measurements made it possible to determine the
symmetry and interaction with typically three or four neighboring shells by
detecting the
31
P nucleus [224, 228]. The use of advanced ENDOR
techniques such as pulsed- and CW-ENDOR optically detected ENDOR and
field-stepped ENDOR techniques in the study of boron-vacancy and thermal
donors in silicon and the gallium vacancy in gallium phosphide has been
reviewed [229]. A general review can be found in reference [230].
identified as undergoing adsorption on Y zeolite [211] and on the perfluorinated
191
4.5 Applications
4.5.13 Spin Centers in Silicon and Borate Systems
Silicon has been shown to be a convenient host for trapping various interstitial
impurities. Ions of
47,49
Ti,
61
Ni, Si,
105
Pb,
29
Si,
31
P,
31
Al,
10
B,
17
O,
33
S have been
detected by ENDOR measurements, usually at low temperatures < 20 K.
Some general review papers discussing the various vacancies that can be
formed upon ionizing irradiation at low temperature and subsequent annealing
have been published [231]. More recently, pulsed-ENDOR measurements
have been carried out at 20 K on room temperature J-irradiated glasses
consisting of (100 – x) mol % B
2
O
3
·x mol % Li
2
O. CW-EPR, ID-ESEEM,
2D-HYSCORE results, and SCF-HF calculations have shown the formation
of various boroxal ring structures, boron oxygen hole centers, pentaborate,
diborate, tetraborate and borate networks with nonbridging atoms [232].
4.5.14 Paramagnetic Centers in Cubic Host Crystals
Cubic crystal hosts of PbF
2
[233], CdF
2
[233], SrCl
2
[234],
CdF
2
[235], CaF
2
,
CaF
2
[236–346], BaF
2
[243, 248],
PbF
2
[244, 245, 247]
and SrF
2
[238, 243,
246] have been used to trap the ions of Gd
3
[233, 234, 236, 240, 244, 245,
247], Eu
2+
[233, 241, 244, 245, 248], Cu
2+
[235], Nb
3+
[237], Tm
2+
[238],
Yb
3+
[243], Cr
+
[246]. ENDOR measurements provide the site symmetry of
the ion and the nearest neighbor interactions up to four shells away from the
trapped ion.
4.5.15 Perovskite-Type Materials
Inorganic ions can also be doped in perovskite-type materials. A perovskite
crystal structure is cubic with ions at the cube corners, face centers and the
body center. A typical example is barium titanate where Ba
2+
is located at the
cube corners, O
2–
ions at the face center, and a Ti
4+
ion at the body center.
ENDOR measurements permit a measure of the site symmetry and the
interaction with the nearest neighbors surrounding the paramagnetic impurity
that are located in the first or second or third nearest neighbors. The
19
F
nuclei have been often detected surrounding the ion. ENDOR spectra due to
the doped inorganic ion have been recorded for
181
Ta [264],
57
Fe [265, 266],
7
Li [250],
51
V [267], Mn [268] in various lattices, and
133
Cs [269] in hosts as
varied as KMgF
3
[266, 253], LiBaF
3
[250], KmgF
3
[251, 260–262], CsCdF
3
[252, 254, 268, 258, 259], RbCdF
3
[255–257], KZnF
3
[263], KTaO
3
[264,
265] and LiTaO
3
[266], and PbTiO
3
[268]. The magnetic properties of double
perovskites of A
2
B'NbO
6
where A = Ba, Sr, or BaSr, and B' = Fe or Mn have
been examined [270].
4. ENDOR Spectroscopy
192
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