ESR and proton ENDOR studies in urea oxalic acid (2:1) single crystal:
Radical transformation
M. V. V. S. Reddy, K. V. Lingam, and T. K. Gundu Rao
Citation: J. Chem. Phys. 76, 4398 (1982); doi: 10.1063/1.443554
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ESR and proton ENDOR studies in urea oxalic acid (2:1)
single crystal: Radical transformation
M.
v. V. S. Reddy and K. V. Lingam
Physics Department, Indian Institute o/Technology, Bombay 400 076, India
T. K. Gundu Rao
R. S. I. C, Indian Institute o/Technology, Bombay 400 076, India
(Received 24 February 1981; accepted 14 May 1981)
A radical transformation has been investigated in a single crystal of urea oxalic acid (2: 1) irradiated at room
temperature. ENDOR studies (- - 160 'C) have been performed on both the initial and final radicals. The
initial species has been assigned to a protonated anion radical. The results indicated that a common proton
was involved in the protonation of the anion and in the subsequent transformation to R-CHOH. Two
important differences in relation to the earlier studies on the protonated anions were noted. The maximum
dipolar coupling values (B m ,,) for the O-Hp protons are smaller than normal and the proton transfer is not
across the hydrogen bridge. A qualitative estimation of the spin densities indicated delocalization and the
possible causes for small values of B "''' are discussed. A mechanism proposed earlier is invoked to understand
the radical transformation.
I. INTRODUCTION
H-O
ESR (electron spin resonance) and ENDOR (electronnuclear double resonance) of radicals in systems containing carboxylic groups have been extensively studied
in view of their biological importance. 1,2 The ESR
studies have focused mainly on the identification and on
the structure of the paramagnetic centers. The ENDOR
analysis gives more precise orientation of the radical
and in favorable cases provides valuable information
about the radical formation and transformations. The
interpretation of the superhyperfine tensors in relation
to the molecular structure is the important aspect in
the ENDOR analysis. In carboxylic acid systems, it is
well known that the ejected electron by ionizing radiation
is captured on the carboxyl group to form anions. In
many cases a proton transfer to the anion takes place
and ENDOR can delineate the particular transferred
proton. This protonated species decays subsequently
to give stable secondary radicals. The formation of
stable radicals is also strongly dependent on the environmental factors like molecular packing, hydrogen
bonding, etc.
Oxalates, the simplest of the dicarboxylic acid systems, are comparatively less studied and the radical
mechanisms are not well understood. Among the reported cases, a stable secondary radical of the form
R-CHOH was observed in urea oxalate3 {2[CO(NH2 h]
. C2 H20 4}. In sodium hydrogen oxalate monohydrate, a
CO2 radical was reported4• 5 whereas both these radicals
were observed in potassium hydrogen oxalate. 6 A systematic study of the radicals in irradiated oxalate systems is considered to be of interest. In the present
work, single crystal ESR and ENDOR studies on urea
oxalic acid (2: 1) {2[CO(NH2 )2]' (COOH~} are reported.
According to Harkema et ai. , 7 urea forms an addition
compound with oxalic acid. Their crystal structure
studies showed that the acidic proton is attached to the
oxalic acid molecule as shown below and hence the name
urea oxalic acid.
4398
J. Chern. Phys. 76(9), 1 May 1982
~O
~C-C('
f7
'
0
"O-H
As the chemical formulas indicate, both the crystals
viz., urea oxalate and urea oxalic acid (2: 1) [UOA],
refer to the same compound 8 and the present ESR results are in accordance with this. According to Rao
and Gordy, when irradiated and observed at 77 K, this
system was found to give an ESR spectrum presumably
due to the precursor of the stable secondary radical. 3
However, during the course of the present study (room
temperature irradiation and observation), it was noticed
that the character of the initial ESR Signal changed with
time, and in a few days it finally transformed to the
R-CHOH radical. Present results indicate that the initial radical is the protonated anion:
H-O
O....H
)c-c/
O~
~O-H
(I)
The final secondary radical is confirmed to be
(II)
However, the ESR spectrum of radical I is found to be
different from the reported spectrum3 obtained by low
temperature irradiation.
The formation of the R-CHOH radical from an oxalate
ion implies that one of the oxygens is replaced by a proton. It was thought that the ENDOR analysis of superhyperfine couplings of all the neighboring protons of both
radicals I and II might help in identifying the proton in
question. Our ENDOR results suggest that in UOA a
proton transfer from a neighboring urea molecule is in-
0021-9606/82/094398-08$02.10
© 1982 American I nstitute of Physics
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Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
H
volved (radical I) and probably the same proton subsequently replaces the oxygen (radical II).
II. EXPERIMENTAL
Single crystalS of UOA were grown by slow evaporation of an aqueous solution of urea and oxalic acid in the
stoichiometric ratio. The number of molecules in the
unit cell are two and the space group of the crystal is
P a Ic' The grown crystals (density 1. 58 gm/cm3 )
1
7
showed pronounced cleavage (102) as reported. They
were irradiated with y rays from a 60Co source (1000
Ci) at room temperature and irradiation time of about
6 h was found to be optimum for the studies on radical
I. The ESR experiments were performed on a Varian
E -line Century Series X-band spectrometer and TCNE
(g= 2.00277) was used as a reference for the g-factor
measurements. The ENDOR experiments were performed with a Varian E -1700 accessory. Nitrogen gas
was passed through the heat exchanger immersed in
liquid nitrogen (without the temperature controller) and
the sample temperature (- -160°C) was found to be
adequate for observing the ENDOR Signals. ENDOR
measurements were made under the following conditions:
The width of the field modulation was 2-6 G; the microwave power was 0.5 to 2 mW; the rf power was set at
about 60% of the maximum output and the rf duty cycle
was 10%.
For mounting the Single crystal, a Teflon tube arrangement9 with a goniometer was used. The crystals
could be preCisely mounted by taking advantage of the
cleavage plane and a 90° wedge made of microscopic
cover glass. For the anisotropy studies, the chosen
three orthogonal axes were labeled as a', b, and c';
the b axis being the crystallographic axis. The c' axis
is perpendicular to the (102) plane and the third mutually
perpendicular axis is a'. This choice of axes is found
to be convenient not only for mounting the crystal but
also for the spectral analysis as the urea and oxalic
acid molecules lie in the a' b plane (102). Thus, all the
radical plane (102) protons can be identified easily as
one of their principal axes will lie apprOXimately along
the c' axis.
III. RESULTS AND PROTON ASSIGNMENTS
The observed radical transformation at room temperature in UOA is shown in Fig. 1. The character of the
ESR spectrum changed with time, and it was confirmed
that the growth of radical II was associated with the decay of the initial radical!. The stable radical II could
be obtained directly with high irradiation doses (- 48 h).
Some weak additional Signals were also noticed, but not
analyzed. For most of the orientations, the ESR spectrum of radical I consisted of a broad unresolved line.
However, a number of well resolved ENOOR signals for
both radicals I and II were observed. At least Six distinguishable proton couplings could be analyzed. The
ENDOR spectra were recorded for every 2° to 5° in
each plane and the angular dependence of the frequencies for both the transitions (v+ and v_ with respect to
the free proton frequency Vn '" 14. 4 MHz) were fitted to
the following expression10
4399
~
a - - -_________
b - - -.....
c
d
e
FIG. 1. ESR spectra of the radical transformation in urea
oxalic acid (2: 1). The spectra were recorded at room temperature with the magnetic field along the a' axis under identical conditions at different stages; (a) soon after irradiation,
(b) after one day, (c) after one week, (d) after one month, and
(e) after two months. The g factor at the pip is 2.00277 (TCNE)
~ =(P * cos 0 + Q * sin 0 + 2R., sinO cosO)
2
2
by a least-square routine. The matrices corresponding
to the transitions for each of the protons were separately
diagonalized to obtain the prinCipal tensors and the respective direction cosines. The differences in the direction cosines of the two transitions were found to be
within the experimental errors and the average values
were taken. Due to the limitations in the frequency
range of the spectrometer, only one of the tranSitions
could be analyzed for some protons.
Figure 2 shows the projection of the crystal structure of UOA on the (102) plane together with the labeling
of the protons of interest. The structure consists of
nearly planar urea and oxalic acid molecules held together by a two-dimensional hydrogen bonding network.
The oxalic acid molecule lies on the center of symmetry
and in Fig. 2 the unpaired electron is associated with
C(2). One of the carboxyl oxygens, viz. 0(2), has two
hydrogen bonds (H" and He) with the nitrogens belonging
to different urea molecules. The hydroxyl proton IIp is
attached to 0(3) which is also hydrogen bonded (H b ) to
a nitrogen. The protons H,., Hd , and Hi are related to
Hp, He, and H b , respectively, by inversion. The proton
He is the nearest among the neighboring layer protons
to C(2).
The ENDOR spectra together with the labeling of lines
of radicals I and II for the magnetic field along the b
axis are shown in Figs. 3 and 4, respectively. Capital
letters are used to label the lines of radical I. None
of the observed lines (for both the radicals) could be
assigned to the proton H" in the crystal structure (Fig.
2). On the other hand, for the lines T (radical I) and
Q (radical II), unlike the rest of the lines, there are no
associated protons in the undamaged crystal.
The aSSignments of the lines to the corresponding
protons are implied in the labeling. For example, line
B of radical I in Fig. 3 is aSSigned to the proton 14 in
Fig. 2. The results of radical II will be discussed
first for the sake of convenience.
J. Chem. Phys., Vol. 76, No.9, 1 May 1982
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4400
Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
L
"M}-
E5~
b
\IV "m-G-
ENDOR
a'
10
20MHz
15
p
Ci
~""-L,.",,, . . . . ~~
FIG. 4. ESR and ENDOR spectra (at - -160°C) of the radical
II (R-CHOH) in urea oxalic acid (2: 1) with the magnetic field
along the b axis.
FIG. 2. The projection of the crystal structure of urea oxalic
acid (2: 1) in the 102 plane (a 'b) with the labeling used in the
present study. The unpaired electron is assumed to be centered on C(2) for both radicals I and II. The numbers with signs
in the brackets are the coordinates along the c ' axis for the
protons belonging to upper and lower layers indicated by dotted
lines.
A. Radical II
The ESR results of radical II including the spinHamiltonian parameters are essentially the same as reported earlier. S The triplet seen in the ESR spectrum
of Fig. 4 (1: 2 : 1 intensity) was understood in terms of
two equivalent protons, viz., C-H", and C-OH/I protons.
However, its ENDOR spectrum shows that these proton
couplings are not quite equal (lines a and p). The splitting of the line a is due to slight misorientation and, in
general, this line is found to be broader (~150 KHz).
The complete ENDOR analysis of this proton could not
be made because of the limitations mentioned earlier.
However, it was confirmed that in the a' b plane, the
C-H", direction makes an angle of about 9° with respect
to the b axis, in agreement with the ESR results. This
direction corresponds to C(2)-0(2) in the crystal struc-
ture (Table I) and is consistent with the conclusion that
0(2) is replaced by a proton.
The complete ENDOR analysis was made for the line
p mentioned above and its precise direction cosines were
determined (Table II). A number of weak couplings, not
resolved in the ESR, were also analyzed. The angular
variation of the ENDOR lines in the three planes are
shown in Figs. 5 and 6. Figure 5 illustrates a typical
anisotropic behavior of the hydroxyl protons in UOA.
The assignments of the 14 and Hq protons could unambiguously be made from their expected behavior mainly
in the be' plane (Figs. 5 and 6). The proton having
larger coupling is associated with H, on the basiS of the
spin density anticipated on C(2) and 0(3). The sign for
its isotropic coupling value is chosen to be negative so
that the observed direction of the maximum dipolar
coupling (Bmu) is in agreement with the crystal structure. However, the following pOints, which will be discussed later, may be noted. The dipolar tensor is not
quite axially symmetric and the Bmu direction deviates
slightly from the direction connecting the radical car-
TABLE 1. The distances and direction cosines (with respect
to the a '. b. and c' axes) of the vectors calculated from the
crystallographic data.
Direction cosines
Distance
Vector
C(2)-O(2)
C(2)-H..
O(3)-H,
C(2)-H q
O'(3)-Hq
C(2)-H a
11
13
115
17
19 MHz
FIG. 3. ESR and ENDOR spectra (at - - 160 ·C) of the radical
I (protonated anion) in urea oxalic acid (2 : 1) with the magnetic
field along the b axis. (In labeling. the ENDOR lines of radical
I capital letters are used).
C(2)-H b
C(2)-H c
C(2)-H d
C(2)-He
C(2)-H,
(A)
1. 20
1. 87
0.96
3.21
0.96
2.90
3.35
3.23
4.02
3.08
3.53
a'
0.069
0.984
0.858
-0.992
-0.858
0.684
0.373
-0.133
-0.228
0.278
-0.734
b
0.987
-0.175
0.501
- 0.117
- O. 501
0.729
- O. 926
0.991
- O. 973
- 0.122
0.679
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c'
- 0.144
- O. 007
- 0.118
0.043
0.118
- O. 002
0.057
- O. 002
0.033
- O. 952
-0.018
4401
Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
TABLE II. The principal values (A) and direction cosines of the hyperfine coupling tensors of
the radical II. al 80 and B are the isotropic and dipolar components, respectively. The direction
cosines are given for one of the two sitesa •
(MHz)
Direction cosines
B
al 80
(MHz)
A
(MHz)
a'
b
c'
P
- 21. 7
-16.3
10.2
-9.3
-12.4
-7.0
19.5
- O. 372
0.021
0.928
0.927
0.043
0.371
-0.032
0.999
- O. 036
q
-7.6
-6.4
3.0
-3.7
-3.9
-2.7
6.7
- O. 211
- 0.077
- O. 974
0.976
0.031
-0.213
0.047
- O. 997
0.069
b
5.7
-2.9
-2.8
0
5.7
-2.9
-2.8
0.289
0.888
0.358
- 0.955
0.239
0.177
0.072
- O. 393
0.917
c
4.1
-2.2
-1.6
0.1
4.0
-2.3
-1.7
0
0
1
d
3.5
-1.7
-1.6
0.1
3.4
-1.8
-1.7
- 0.103
0.632
0.768
-0.992
-0.126
-0.029
0.078
- O. 765
0.640
e
4.1
-2.6
-1.6
0
4. 1
-2.6
-1. 6
0
0
-1
0
-1
0
-1
0
0
1
0
0
0
1
0
~he
hyperfine coupling tensors for the lines a <CH", proton) and f could not be determined. For
the line p only one of the transitions could be analyzed.
bon atom C(2) and the hydroxyl proton HI>' Similar behavior is also noted for the proton lIq.
The aSSignments of the rest of the lines were mainly
based on the comparison of the Bmu directions with the
expected proton directions from the crystal structure
(Table I). The tensors are given in Table II. The tensors for the ENDOR lines b, c, d, and e show approximate cylindrical symmetry with negligible isotropic
couplings. The anisotropy of the line f could not be fol-
lowed in all the planes and hence the tensor was not determined. However, an examination in the db plane
showed that its maximum coupling (3.2 MHz) direction
makes an angle of about 40° with respect to the b axis.
Hence, the line f is likely to be due to the HI proton.
For all these protons, except He, the Bmu direction lies
in the a' b plane as expected from the crystal structure.
As mentioned earlier, the proton labeled as Ha, which
is the nearest among the protons of the neighboring
molecules to C(2), remains unassigned as no ENDOR
lines with the expected couplings and directions are observed.
al
60·
30·
b~
60·
30·l
b
60·r:....
30·
"60·
Cl
cl
0-
30-
30·
0'"
60-
60·
I-
ag
q
I
13
15
"
FIG. 5. Angular variation for one of the ENDOR transitions of
17
19
21
23
25 MHz
the hydroxyl proton lip (radical II) in the three orthogonal planes.
The solid lines represent the theoretical curves and the dots
represent the experimental values.
aIlO
19 MHz
FIG. 6. Angular variations of the ENDOR frequencies in the
three orthogonal planes for the lines q, b, c, d, and e of radical II. The solid lines represent the theoretical curves, and
the dots represent the experimental values.
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4402
b
c
,
Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
~.
\
30'
60'
a'L_
...
---;i~~l.U>.*......~y;.~........ok10
11
FIG. 7. The angular variations of the ENOOR frequencies in
the three orthogonal planes for the lines T, P, Q, B, C,
D, and E of radical I. The solid lines represent the theoretical curves and the dots represent the experimental values.
B. Radical I
The ESR spectra of radical I can be seen in Figs.
l(a) and 3. From the general anisotropic behavior of
radical I, it appeared to have interaction mainly with
two protons of comparable hyperfine couplings. The g
factor was found to be minimum along the c' axis [gl (a')
=2.0051, gz(b) = 2.0048, and &(c') =2. 0026].
The ENDOR anisotropy curves are shown in Fig. 7
and the proton tensors are given in Table III. The as-
signments for the protons Hp and H. could easily be
made from their characteristic anisotropic behavior
(Figs. 7 and 5). The assignments of the remaining lines
require more detailed considerations based on the expected changes in the spin densities of radicals I and II.
The relative changes of the spin densities are assessed
from the observed isotropic couplings of the hydroxyl
protons. The spin densities on C(2), 0(3), and 0'(3) for
radical II are estimated to be O. 74, 0.13, and 0.05,
respectively, by uSing McConnell's relation. Similarly,
for radical I, the spin densities on 0(3) and 0'(3) are
found to be 0.07 and 0.04. The values used for ifcH and
ifoH are 22.5 and 25 G, respectively. The spin density
on 0(2) for radical I is estimated to be 0.06 and will be
discussed later. This implies that the spin density on
C(2) is about 0.77. As the couplings of the proton H.
are relatively unaffected, the overall spin denSity in
the C'(2) 0'(2) 0'(3) fragment is assumed to be unaltered.
From Fig. 2 it is seen that the protons Hb and He are
almost symmetrically disposed with respect to the carboxyl group C(2) 0(2) 0(3). From the estimated spin
densities together with the proton distances (Table I),
the expected Bmu. value for these protons is calculated
to be about 5 MHz by USing point dipole approximation. 11
Hence the lines Band C are ascribed to the protons Hb
and He, respectively. The lines D and E are aSSigned
to the protons ~ and He mainly on the baSis of the direction cosines. The line F, which showed similar behavior to that of the line f in radical II, was also ob-
TABLE Ill. The principal values (A) and direction cosines of the hyperfine coupling tensors of
the radical I (protonated anion). alao and B are the isotropic and dipolar components, respectively. The direction cosines are given for one of the two sites ...
A
alao
B
(MHz)
(MHz)
(MHz)
Direction cosines
a'
b
c'
T
17.3
5. 1
0.6
7.7
9.6
-2.6
-7.1
0.657
- O. 751
-0.061
0.624
0.588
- O. 515
0.423
0.300
0.855
P
-14.1
-11. 0
9.5
-5.2
-8.9
-5.8
14.7
- O. 378
0.074
0.923
0.904
0.246
0.350
- 0.201
0.966
-0.160
Q
-7.1
-6.1
3.9
-3.1
-4.0
-3.0
7.0
-0.162
0.082
- 0.983
0.986
- 0.018
-0.164
0.031
-0.997
-0.079
B
4.9
-3.9
-1.9
-0.3
5.2
-3.6
-1. 6
0.345
0.928
0.142
- O. 927
0.313
0.205
0.146
- O. 202
0.969
C
5.3
-2.4
-2.9
0
5.3
-2.4
-2.9
-0.157
0.987
0.031
0.988
0.156
0.021
-0.016
- 0.033
0.999
D
3.8
-2.5
-1. 7
-0.1
3.9
-2.4
-1.8
-0.115
0.918
0.380
- O. 985
-0.154
0.075
0.128
- O. 366
0.922
E
3.6
-2.0
-2.3
-0.2
3.8
-1.8
-2.1
0.389
0.0
- 0.921
0.0
-1.0
0.0
-0.921
0.0
- O. 389
"Only one of the transitions could be analyzed for the lines T and P of the radical 1. Complete
tensor for the line F could not be determined.
J. Chern. Phys., Vol. 76, No.9, 1 May 1982
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Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
served, particularly in the a' b plane. The complete
tensor was not determined, but the maximum coupling
was found to be 3.5 MHz.
Regarding line T, the direction of its Bmu was found
to make an angle of about 65° with the c' axis (verified
by direct mounting). This line showed larger line width
and is assigned to the "transferred proton H.," as will
be discussed later. It is to be noted that the position of
the transferred proton H.. is different from that of Ha in
Fig. 2.
IV. DISCUSSION
The radical products in carboxylic acid and amino acid
derivatives are known to transform as secondaries
either by decarboxylization or by deamination. In oxaiate systems, decarboxylization would lead to the formation of a CO2 radical, and in fact is observed. 4,5 It
is interesting to note that UOA, a hydrogen-bonded system, behaves differently.
The present ENDOR studies enable a systematic comparison of the intramolecular and intermolecular proton couplings of radicals I and II, whose assignments
should be considered as satisfactory from the results
presented. The results of both the radicals indicate no
reorientation with respect to the undamaged molecule
in the crystals. However, some differences were noted
(in UOA) in comparison with the known behavior of the
anion radicals; mainly with respect to (1) the isotropic
and anisotropic parts of the O-He couplings, and (2) the
proton transfer. These will be discussed presently.
A. O-Hil couplings
In general, the characterization of the O-Hs hyperfine couplings in the anion radicals is known to be less
satisfactory as compared to the C-He couplings. Both
the C-Hs and O-H8 protons show that the overall anisotropy of the hyperfine couplings is fairly constant, a
feature which has been pointed out to be useful in identifying the anion radical products in radiation research. 12
The Brou, is found to lie in the range of 17.4 to 21. 0 MHz
and is approximately independent of the dihedral angle
which is a measure of the isotropic coupling. 13 This observation for the O-HI! protons apparently refers to the
protons that are transferred to the anion. The characterization of the O-Hs couplings is particularly useful
as deuteration is often used to aid the aSSignments.
The isotropic coupling also depends on whether the radical is planar or pyramidal. A cosze relation (8 is the
dihedral angle) has been used to describe the isotropic
couplings of both the exchangeable and nonexchangeable
Ha protons with appropriate proportionality constants.
The observed O-Hs couplings in UOA are of significance in the above context. In the anion radical I, the
Bmu values are found to be low for both the transferred
proton H.. and the hydroxyl proton HI' which was originally present in the molecule. A comparison of the
O-He couplings in radicals I and II is worth considering. The Brou, value for the Hp coupling of radical II is
in the above-mentioned range. It may be noted here that
low Bmu values of the O-He couplings were observed for
the corresponding R-CHOH radical in the glycollate
15
systems 14- 17 ; the case of lithium glycollate anhydrous
is an exception where the proton is not in the molecular
plane due to nonlinearity of the hydrogen bond. 11l A
cursory examination of the dipolar components of the
hydroxyl protons in radicals I and II (Tables II and III)
show a proportionate decrease with the isotropic values.
It is well known that for a R -eH type of radical, as the
spin density on the carbon decreases, both the isotropic
and anisotropic components of the a proton decrease
proportionately. For example, in UOA the isotropic and
Bmu values for the a proton are - 47 and + 24 MHz as
compared to the corresponding values of - 59 and + 29
MHz in malonic aCid. 19 Hence, the decrease in the dipolar components of the proton HI' in radical I could be
attributed to the differences in the spin density distribution in the carboxylic group. These arguments would
imply a de localization of the spin density in the 71 system which is not surpriSing from the planarity of the
molecule. In fact this is evident from the observed isotropic couplings for the protons Hp and Ha, and the Slight
deviations of their Bmu directions are also in accordance
with the above. Thus, the small values of Bmu for the
O-H8 couplings in the anion product I of UOA could be
due to the redistribution of the spin density. The limited
range of variation of the Bnuu values for the O-H/l proton
coupling in the reported anion radicals of different systems appears to be due to the similarities in the spin
denSities in the carboxylic group.
B. Proton transfer
For the transfered protons in the anion products of
glycine and a-amino isobutyric acid, 20 anomalous hyperfine couplings were also noticed in the proton transfer studies by Iwasaki et al. These studies clearly
showed that the proton transfer is stereospecific. 13,20,21
The observed small values for both the isotropic and
anisotropic couplings were interpreted by assuming that
the proton might be jumping between the close and far
positions of the hydrogen bond. Two important factors
were found to control the proton transfer: (1) the hydrogen bonded proton to be in the direction of the unpaired
spin orbital and (2) the shorter hydrogen bond distances.
Before we discuss the present observations in UOA, the
following may be noted: (1) none of the observed ENDOR
lines of radicals I and II could be associated with the H"
proton in the crystal structure (Fig. 2), and (2) the
width of the lines T and a (Figs, 3 and 4) are somewhat
larger.
Regarding the first point, unfavorable relaxation
mechanisms could also be the reason for the absence of
a particular ENDOR line. However, the proton in question (Ha) is similar to the other NH protons for which
strong ENDOR lines were seen. In view of the fact that
the proton transfer is a common reaction for the anion
products, it is reasonable to assume that the proton H,.
which was originally in the COO plane is transferred to
the anion. As pointed out earlier, the direction of its
Bmu is not in the COO plane and the positive sign for
the isotropic coupling is consistent with such a kind of
proton. Hence it is concluded that the proton transfer
is not along the hydrogen bond.
J. Chern. Phys., Vol. 76, No.9, 1 May 1982
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4404
Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
To estimate the isotropic coupling constant, it is
necessary to have a knowledge of the dihedral angle.
Making a reasonable assumption that the unit vector
along the proton direction has the same direction cosines
to that of its B mu , the estimated angle is about 510. The
direction of the carbon 2P.. orbital is taken to be along
the c' axis as indicated by the minimum g factor. The
following relation given by Muto et al. 13 for a planar
molecule is used to estimate the spin density on the
oxygen atom 0(2):
p~(2)=a+bsinee ,
where a + b =O. 08 and a =O. 02. The spin density on 0(2)
is calculated to be 0.06. Similarly the isotropic coupling is estimated from the following relation13
OaH
=130 + Bl cos 2e ,
where Bo =- 2 G and Bl =16. 4 G. Hence OaH for the
transferred proton is calculated to be 12.6 MHz and the
observed value is 7.6 MHz. It must be mentioned that
the tensor associated with the transferred proton is
nonaxial and in view of the approximations made this
agreement should be considered as satisfactory.
Regarding B max , if the low value is caused by any dynamic motions (resulting in incomplete transfer), one
would expect line width effects with temperature. As
mentioned earlier both the lines a and T showed increased line widths. No dynamic motions are expected
for the CHo: proton in radical II. As such, there is no
valid reason for attributing the increased line width of
the transferred proton (radical I) to dynamic effects.
To our knowledge, there appears to be no direct experimental confirmation of the predicted dynamic nature
by the line width effects. An attempt was made to study
the temperature dependence of radical 1. But, detailed
ESR analysis could not be made due to poor resolution.
Unfortunately, in the ENDOR spectra, the signal to
noise ratio was deteriorating with the increase in temperature. However, the line T showed marked temperature dependence in the observed small range of
temperature (-160 to -100°C). This aspect requires a careful investigation in favorable cases.
In this regard, an alternative explanation is worth
considering. The fact that the proton Ha is displaced
from the crystallographic position suggests that the excess negative charge on 0(2) (double-bonded oxygen of
the oxalic acid molecule) might be the cause. In such a
case one would expect changes in the potential minima
associated particularly with the nearest neighboring Hbonded protons. In favorable cases a complete transfer
of a proton to the anion can take place along the hydrogen bond. In general, it is not necessary that the proton transfer should be across the bridge as the environmental effects can be complex. Hence, the dihedral
angle depends upon the transfered position and need not
correspond to the crystallographic value. Under such
circumstances the isotropic coupling can show deviations and low values can arise from large dihedral
angles. Moreover, the delocalization of the unpaired
spin on to the oxygens not only reduces the positive isotropic contribution via spin polarization but also affects the dipolar contributions. In the present system,
the observed change in the direction of the transferred
proton from the direction of O(2)-!f" could be caused by
the asymmetric disposition of the atoms from the upper
and lower layers (Fig. 2). The results of UOA suggest
that the proton transfer mainly depends on the shorter
H-bond distance and also on the environmental factors.
Among the special features of UOA, the stability of
the protonated anion radical and the observation of its
transformation at room temperature may be mentioned.
Such transformations are usually observed at low temperatures. The results of the analysis of radical I indicated that the unpaired electron is somewhat delocalized and this might be the cause for its stability.
The formation of radical II is presumably triggered subsequent to the protonation. One expects the reaction to
occur when a sufficient overlap between the unpaired
electron orbital and the electron cloud of the transferred
proton is achieved. When such a situation is not favorable the reaction could proceed slow enough to observe
the transformation at room temperature.
The actual fragmentation path leading to the formation of radical II can be understood within the framework of the mechanism proposed earlier by McCalley
et al. ee It was suggested that the scission of the CO
bond might be the main path for the anions to form
R-CH kind of radicals. In UOA, the breakage of the
C(2)-0(2) bond and the subsequent replacement of 0(2)
by a proton account for the observed features. In fact,
the present findings appear to illustrate directly the
above envisaged mechanism in which the hydroxide ion
left in the cavity can donate its hydrogen atom among
the various sites about the cavity. This raises the
question as to whether a common proton could be involved in the transformation process. Even though there
is no direct evidence in this regard, two observations
favor such a possibility. First, the ESR studies indicated that the transformation is direct but not through
any intermediate species. Second, the conditions for
the ready formation of radical II are favorable in UOA.
The latter observation is justified by the efficient formation of the R-CHOH radical in UOA and glycollate
systems 14 - 17 where the molecular geometries are similar. Hence it is suggested that radical II is formed almost concurrent with the scission of the CO bond and a
common proton is involved in the process. In addition,
the absence of ENDOR lines associated with Ha (Fig. 2)
leads to the possibility that the common proton might
be Ha itself. Finally, it must be stated that the credibility of the above suggestion lies in the internal consistency achieved from the mechanistic view point.
V. CONCLUSION
The present system UOA has many advantages for
the study of radical transformations. The presence of
both the hydroxyl protons and less interference from
additional radicals permit a systematic study. 13C enrichment studies will be quite useful in determining the
accurate spin denSities, etc. The observed low temperature radical by Rao and Gordy 3 is presumably the
unprotonated anion product, which could be the precursor of radical I. In situ, low temperature irradiation
and ENDOR studies are deSirable to confirm the same.
J. Chem. Phys., Vol. 76, No.9, 1 May 1982
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Reddy, Lingam, and Rao: ESR and ENDOR of urea oxalic acid
ACKNOWLEDGMENTS
We would like to thank Professor B. N. Bhattacharya
for his encouragement. We would also like to thank
the referee for his valuable suggestions.
tH. C. Box, Radiation Effects, ESR and ENDOR Analysis
(Academic, New York, 1977l.
2L . Kevan, and L. D. Kispert, Electron Spin Double Resonance Spectroscopy (Wiley-Interscience, New York, 1976).
3D. V. G. L. N. Rao, and W. Gordy, J. Chern. Phys. 35, 362
(1961).
40. Edlund, A. Lund, and R. Tellgren. Colloq. Ampere IS,
293 (1969); O. Edlund, A. Lund, and A. Grasslund, J. Magn.
Reson. 10, 7 (1973).
5M. V. V. S. Reddy, K. V. Lingam, and T. K. Gundu Rao,
Mol. Phys. (in press).
sM. V. V. S. Reddy, K. V. Lingam, and T. K. Gundu Rao,
Mol. Phys. 41, 1493 (1980).
1S. Harkema, J. W. Bats, A. M. Weyenberg, and D. Feil,
Acta Crystallogr. B 28, 1646 (1972); B 29, 143 (1973).
8A. F. Schuch, L. L. Merritt, and J. H. Sturdivant, American
Crystallography Assoc., Abstracts of Meeting, 10-12 April,
1950, p. 21; [Struct. Rep. 13, 477 (1950)].
sM. V. V. S. Reddy, K. V. Lingam, and T. K. Gundu Rao,
4405
J. Mol. Struct. 73, 105 (1981).
10H. A. Farach and C. P. Poole Jr., Adv. Magn. Reson.
5, 229 (1971).
l1J. Huttermann, G. Schmidt, and D. Weymann, J. Magn.
Reson. 21, 221 (1976).
12 C hing-Lung Ko and H. C. Box, J. Chern. Phys. 68, 5357
(1978).
13H. Muto, K. Nunome, and M. Iwasaki, J. Chern. Phys. 61,
5311 (1974).
t4N. M. Atherton and D. H. Whiffen, MoL Phys. 3, 1 (1960);
ibid. 103 (1960).
15D• Pooley and D. H. Whiffen, Trans. Faraday Soc. 57,
1445 (1961).
16W. Derbyshire, Mol. Phys. 5, 225 (1962),
17 D . E. Henn and D. H. Whiffen, Mol. Phys. 8, 407 (1964).
18E . J. Gabe and M. R. Taylor, Acta Crystallogr. 21, 418
(1966).
ISH. M. McConnell, C. Heller, T. Cole, and R. W. Fessenden,
J. Am. Chern. Soc. 82, 766 (1960); A. Horsfield, J. R. Morton, and D. H. Whiffen, Mol. Phys. 4, 327 (1961).
20M. Iwasaki and H. Muto, J. Chern. Phys. 61, 5315 (1974).
21H. Muto and M. Iwasaki, J. Chern. Phys. 59, 4821 (1973);
H. Muto, K. Nunome, and M. Iwasaki, J. Chern. Phys. 61,
1075 (1974).
22R • C. McCalley and A. L. Kwiram, J. Chern. Phys. 53,
2541 (1970); M. B. Yim, and R. E. Klinck, J. Chern. Phys.
60, 538 (1974).
J. Chem. Phys., Vol. 76, No.9, 1 May 1982
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