Electron spin resonance and electron nuclear double resonance studies in
single crystals of Nmethylurea oxalic acid (2:1)
M. Ravi Kumar, K. V. Lingam, and T. K. Gundu Rao
Citation: J. Chem. Phys. 97, 900 (1992); doi: 10.1063/1.463194
View online: http://dx.doi.org/10.1063/1.463194
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Electron spin resonance and electron nuclear double resonance studies
in single crystals of N-methylurea oxalic acid (2:1)
M. Ravi Kumar and K. V. Lingam
Physics Department, Indian Institute a/Technology, Bombay 400 076, India
T. K. Gundu Rao
Regional Sophisticated Instrumentation Centre, Indian Institute a/Technology, Bombay 400 076, India
(Received 9 September 1991; accepted 23 March 1992)
Radicals trapped in a y-irradiated single crystal of N-methylurea oxalic acid have been
investigated using the techniques of electron spin resonance (ESR) and electron nuclear
double resonance (ENDOR). Detailed ENDOR studies have been carried out on the two
initially observed radicals. One of these radicals has been assigned to a protonated anion. Two
protons, belonging to neighboring molecules, have been identified as transferred protons. The
other radical is formed at the urea molecule by hydrogen abstraction from the methyl group.
The observed completely transferred proton exhibits very small isotropic coupling. The third
radical seen, only in aged crystals, after prolonged duration has been assigned to a R-CHOH
type of radical. These results are compared to those from the urea oxalic acid systems.
INTRODUCTION
Extensive studies of radicals in systems containing carboxylic groups have been carried out partly because of their
biological significance. 1.2 In these systems, ~t is well known
than an electron ejected by ionizing radiation is trapped by
molecules to form molecular radical anion. In several cases,
a proton is transferred to the anion through the intermolecular hydrogen bond. The resulting protonated species decays
subsequently to give stable secondary radicals.
Oxalates, the simplest of the dicarboxylic acid systems,
form convenient hosts for the study of carboxyl anion radicals and their transformations. In particular, addition compounds of urea with oxalic acid have some attractive features. According to Harkema et al., 3 urea forms 2: 1 and 1: 1
addition compounds with oxalic acid. Their crystal structure studies showed that the acidic protons are attached to
oxalic acid molecule, and the molecules are held together by
two dimensional hydrogen bonding network.
Urea oxalic acid (UOA) 2:1 (Ref. 4) and 1:1 (Ref. 5)
addition compounds have been investigated earlier. The initial radical formed immediately after irradiation at room
temperature was identified as the protonated carboxyl anion
in both these compounds. This radical subsequently transformed to a stable final radical R-CHOH. In both cases,
protons belonging to neighboring groups located in the molecular plane protonate the anion. However, detailed electron nuclear double resonance (ENDOR) studies showed
some differences in the stereospecific nature of the proton
transfer. As the crystal structures of the compounds are different, it was suggested that environmental factors could
also play an important role in the proton transfer process.
With a view to obtain further information, studies on
the addition compound N-methylurea-oxalic acid (2: 1 )
(MUO) were undertaken. The crystal structure ofMUO is
similar to that ofUOA (2:1).
A radical transformation, similar to the one observed in
urea oxalic acid systems, is also observed in the present
900
J. Chern. Phys. 97 (2). 15 July 1992
study. An additional radical associated with the urea molecule was also analyzed. Arguments leading to the assignment of the above radicals together with the observed proton
transfer processes in these systems will be discussed.
EXPERIMENTAL
Single crystals ofMUO were grown by slow evaporation
of an aqueous solution of N-methylurea and oxalic acid in
the stoichiometric ratio. 6 The monoclinic crystals belonging
to the space group P2 1/C have pronounced micalike cleavage
plane (102). They were irradiated with y rays from a 6OCO
source (-1000 Ci) at room temperature and an irradiation
time of about 8 h was found to be optimum for the present
studies. The electron spin resonance (ESR) experiments
were performed on a Varian E-line Century series X-band
spectrometer. TCNE (g = 2.002 77) was used as a reference
for the g-factor measurements. The ENDOR experiments
were performed with a Varian E-1700 accessory. According
to Harkema et al.,6 MUO undergoes an irreversible phase
transformation at 180 K. During the experiments, the ESR
signal strength suddenly decreased with lowering of temperature to liquid nitrogen temperature. On examination,
the MUO crystals were found to be shattered. Hence, ENDOR experiments were performed at 243 K using a variable
temperature controller. ENDOR measurements were made
under the following conditions. The width of the modulation
was 2-6 G; the microwave power was 2 mW; the rfpower
was set at about 60% of the maximum output, and the rf
duty cycle was 10%.
For the anisotropy studies, the chosen three orthogonal
axes are 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 for mounting the crystals
and also in identifying the interacting protons lying in the
molecular (102) plane.
0021-9606/92/140900-08$06.00
@ 1992 American Institute of Physics
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Kumar, Lingam, and Rao: N-methylurea oxalic acid
901
RESULTS
Figure I shows the ESR spectrum ofMUO immediately
after irradiation. Initially, two radicals labeled I and II were
observed. For most of the orientations, the ESR spectrum of
radical I consists of a broad single line (linewidth -0.55
mT). Radical II is characterized by four broad lines of equal
intensity. These four broad lines showed additional complex
hyperfine pattern for the magnetic field along the b axis (Fig.
I). As the site splittings for the radicals could not be followed in the three orthogonal planes, the ESR tensors of
radicals I and II could not be determined. However, it was
established that their gmin directions lie approximately along
the c' axis.
A number of well resolved ENDOR lines were observed
for both radicals. For radicals I and II, the ENDOR spectra
are shown in Figs. 2 and 3, respectively. The angular variations of the ENDOR lines of radical I in the three orthogonal
planes are shown in Fig. 4. Figures 5 (lines a and b), 6 (lines
c, d, e, f, g, and 3), and 7 (line I ) show the angular variations
of the ENDOR lines of radical II in the three orthogonal
planes. The experimental ENDOR tensors of radical I and II
are given in Tables I and II, respectively.
Figure 8 shows the ESR spectrum recorded for the magnetic field along the b axis of irradiated MUO after a period
of 9 months. This radical, labeled III, was observed only in
aged crystals. Due to poor signal strength, no ENDOR studies on radical III could be carried out. g and hyperfine tensors were determined from the ESR analysis and are given in
Table III. Figures 9 and 10 show the anisotropy of g and
hyperfine tensors of radical III in the three orthogonal
planes. Figure II shows the projection of the crystal structure of MUO on the (102) plane. The structure consists of
planar N-methylurea and oxalic acid molecules held together by two dimensional hydrogen bonding network. The unit
cell contains four N-methylurea and two oxalic acid molecules. This corresponds to two molecules of the addition
compound viz. N-methylurea oxalic acid (2: I). According
to the x-ray crystallographic data,6 oxalic acid molecules lie
on centers of inversion and N-methylurea molecules are 10-
9
14
15
Frequency/MHz
FIG. 2. ENDOR spectrum of radical I (-243 K) in MUO with the magnetic field along the c' axis.
b
a
~
39
40
41
19 38
FIG. 3. ENDOR spectrum of radical II (-243 K) in MUO with the magnetic field along the c' axis.
a'
30'
b
··
·
30
60
,
c
30
60
a'
8
9
20 21
FIG. 4. The angular variation of the ENDOR frequencies in the three orthogonal planes for radical!.
o·
3d
6d
b
I 1.0mT
30
60
C
3
6
a'
25
FIG. 1. Room temperature ESR spectrum of radical I and II in MUO for
the magnetic field along the b-axis.
a
30
35
40
45
50
Frequency/MHz
55
60
FIG. 5. The angular variations in the three orthogonal planes for the
ENDOR lines a and b (Fig. 3) of radical II.
J. Chern. Phys., Vol. 97, No.2, 15 July 1992
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Kumar, Lingam, and Rao: N-methylurea oxalic acid
902
dr-------------------------~~_,
0'
30
3
60
6
b
b~~--~~=-------------------~
30
60'
30
c'
60
30
c'l----------'I;,---------------------i
60
3
0'
8
9
10
Frl'qul'ncy I MHz
Qli---'F-------i~--__;';;,__--.,,;:;_-----;>E_~___;;:
1
5
10
15
20
25
30
Frequency/MHz
FIG. 6. The angular variations in the three orthogonal planes for the
ENDOR lines c, d, e,f, g, and 3 (Fig. 3) of radical II.
FIG. 7. The angular variations in the three orthogonal planes for the
ENDOR line 1 (Fig. 3) of radical II.
cated in a general position of the space group P2,IC' The positional parameters of hydrogen atoms were also determined,
except the ones belonging to the methyl group. The methyl
hydrogens' parameters could not be determined accurately
due to disorder in their positions. The final parameters given
by Harkema et al. 6 were used in the present work assuming
one possible orientation for the methyl group.
The structure ofMUO and UOA (2: I) have many similarities. In UOA (2: 1), the primary radical was identified as
a protonated carboxyl anion. In MUO, the possibility of radical I to be such a protonated anion was initially considered. Since the acidic protons HI and H2 were known to be
attached to oxalic acid molecule, it was convenient to identify their tensors. Tensors 1 and 2 could be readily assigned to
HI and H 2 , respectively, following the normal procedure of
locating the interacting protons by the direction cosines of
Bmax. The magnitudes of the principal components of these
tensors indicated that the major part of the spin density resides in one of the carboxyl group of the oxalic acid molecule.
This carboxyl group is labeled as C 3 0 3 O 2 in Fig. 11. The
DISCUSSION
The assignment of radical I will be considered first. As
stated earlier, the only information that could be obtained
from ESR experiments on radical I was that the direction of
gmin (2.0022) to be along the c' axis. Hence, its identification
was based on the interpretation of the proton ENDOR tensors. Seven distinguishable proton coupling tensors (Table
I) were analyzed.
TABLE I. Experimental ENDOR tensors of radical I in MUO.
Tensor
Assigned
to
H,
2
H2
3
H3
4
H.
5
H,
6
H.
7
H7
A(MHz)
10.84
- 8.59
- 11.61
3.58
- 5.72
-6.32
14.71
-0.24
- 2.75
7.75
-7.33
- 8.46
5.25
- 2.01
- 3.43
4.31
- 1.15
- 1.20
1.50
- 3.62
- 5.17
a(MHz)
- 3.12
- 2.82
3.91
- 2.68
-0.06
0.65
- 2.43
B(MHz)
13.96
- 5.47
- 8.49
6.40
-2.90
- 3.50
10.80
-4.15
- 6.66
10.43
- 4.65
- 5.78
5.31
- 1.95
- 3.37
3.66
- 1.80
- 1.85
3.93
- 1.19
-2.74
a'
Direction cosines
b
c'
0.3592
0.3495
0.8654
- 0.1710
0.2653
- 0.9489
0.6658
0.6251
0.4073
- 0.9913
- 0.0961
0.0896
-0.8960
-0.4441
- 0.0011
- 0.1474
0.0239
- 0.9888
0.9286
- 0.0892
- 0.3601
0.1526
- 0.9368
0.3149
- 0.0979
- 0.9629
-0.2515
- 0.3394
- 0.2324
0.9115
0.0930
- 0.9949
- 0.0382
- 0.0269
0.0519
0.9983
0.9888
-0.0204
-0.1479
0.3518
- 0.0967
0.9331
0.9207
0.0190
- 0.3893
- 0.9804
0.0499
0.1906
0.6645
- 0.7451
0.0574
0.0928
- 0.0295
0.9952
- 0.4433
0.8945
- 0.0585
0.0237
0.9995
0.0206
-0.1178
- 0.9913
- 0.0584
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Kumar, Lingam, and Rao: N-methylurea oxalic acid
903
TABLE II. Experimental ENDOR tensors of radical II in MUO.
Tensor
Assigned
to
a
H.
b
Hb
c
He
3
H,
d
H"
e
H,
f
HI
g
H.
H,
A(MHz)
- 24.20
- 52.38
- 86.46
- 23.94
-49.14
- 84.76
6.82
-7.29
- 8.49
1.44
-4.22
- 6.33
1.19
- 3.65
-4.34
1.34
- 1.42
- 2.32
3.84
- 1.21
- 1.53
3.93
- 1.24
- 2.10
26.90
- 8.86
- 21.06
a(MHz)
- 54.35
- 52.61
-2.99
-3.04
-2.27
-0.80
0.37
0.20
-1.01
B(MHz)
30.15
1.97
- 32.11
28.67
3.47
- 32.15
9.81
-4.30
- 5.50
4.48
- 1.18
- 3.29
3.46
- 1.38
-2.07
2.14
-0.62
- 1.52
3.47
- 1.58
- 1.90
3.73
-1.44
-2.30
27.91
-7.85
- 20.05
a'
-
-
-
-
Direction cosines
b
c'
0.3549
- 0.0338
0.9433
- 0.9509
0.1263
- 0.2827
0.9947
0.0845
- 0.0586
0.0492
0.1843
0.9816
0.9667
- 0.1080
0.2320
- 0.1352
0.2152
- 0.9672
- 0.5715
0.8135
- 0.1077
- 0.2783
- 0.8902
0.3607
- 0.3031
- 0.0473
- 0.9518
- 0.0608
- 0.9981
- 0.0130
0.1121
0.9915
0.0658
- 0.0969
0.9607
- 0.2602
-0.0297
- 0.9827
0.1830
-0.2070
0.2027
0.9571
- 0.0434
0.9739
0.2228
- 0.0232
- 0.1472
- 0.9888
0.9598
- 0.2723
0.0678
- 0.0420
- 0.9971
0.0629
0.9329
0.0522
0.3563
0.2886
0.0308
0.9569
0.0343
0.2645
0.9638
0.9983
0.0201
0.0533
0.1504
0.9733
0.1736
0.9899
0.0721
0.1223
0.8203
0.5625
0.1030
0.0370
0.3654
0.9301
0.9520
0.0591
0.3002
other group is denoted by C3 0 3O 2, The Bmu values of the
OHp protons usually lie in the range of 17 to 21 MHz.7 The
observed lower values, in the present case (Table I), could
arise from delocalization of the unpaired electron density.
The above arguments lead to the conclusion that major part
of the unpaired electron density is in 2pz orbital ofC3 atom.
The observed direction of gmin is in accordance with the
above conclusion.
The dipolar tensors of H3 and H4 were calculated for
various 0 3H3 and O 2 H4 distances assuming these protons
to be transferred along the hydrogen bridge. Assuming the
undamaged crystal structure, the relevant proton unit vectors were calculated and are given in Table IV. The rest of
the tensors were assigned to various matrix protons belonging to neighboring symmetry related molecules on the basis
of the direction cosines. The final assignments are given in
Table I. Tensors 5 and 6 are dipolar protons and have been
assigned to protons Hs and H6 (upper layer), respectively.
Tensor 7 is assigned to one of the methyl protons H7 from
the neighboring molecule. To confirm these assignments, the
dipolar tensors were calculated based on the methods developed by McConnell and Strathdee. 8 Corrected formulas involving Slater orbitals were utilized for these calcula-
TABLE III. g and hyperfine tensors of radical III in MUO.
Direction cosines
Proton
I
1.0mT
a proton
OH
ton
FIG. 8. Room temperature ESR spectrum of radical III with the magnetic
field along the b axis.
A(mT)
-0.78
- 1.64
-2.70
0.21
a(mT)
-1.71
B(mT)
a'
b
0.93
0.07
-0.99
0.57
0.1000
0.0953
- 0.9904
0.9915 -
- 0.19
- 0.37
- 0.0788 - 0.4090
0.1039
0.9036
c'
0.9947
0.0235
0.0139 - 0.9953
0.1018 - 0.0935
0.1272
0.0287
pro- 0.55
-0.36
- 0.73
gvalues
2.00167
2.00390
2.00538
0.3100
0.4618
- 0.8311
J. Chem. Phys., Vol. 97, No.2, 15 July 1992
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0.9091
0.4155
0.1934 - 0.9309
0.8252
0.3252
0.5307 - 0.1665
904
Kumar, Lingam, and Rao: N-methylurea oxalic acid
a",------------""7"r---,
60
b~---------~---~
d.~~~=-~~~~=-~~~~=-~.
2.000 2.001 2.002 2.003 2.004 2.005 2.006
FIG. 11. Projection of the crystal structure of MUO onto the 102 plane.
FIG. 9. Angular variation in the three orthogonal planes of the g tensor of
radical III.
tions. 9.!O Initially, computations were done to determine an
acceptable spin density distribution that gave good agreement with tensors 1 and 2. However, the calculated Bmax
values for tensors 3 and 4 were found to be much smaller
compared to the experimental values. Hence, it was concluded that the interacting protons associated with tensors 3 and
4 are not located in their crystallographic positions. The dipolar tensors ofH 3 and H4 were calculated for various OH
distances. It was assumed that these protons are transferred
along the hydrogen bridge. The results showed that OH{3
distances in the range of 1.2 to 1.3 A (from 0 3 and O 2
atoms) could account for the observed Bmax for both these
protons. The OH{3 distances for acidic protons (HI and H2 )
are of the order of 1 A in this crystal. Hence, it is concluded
that the transfer ofH3 and H4 protons should be considered
as partial. The calculated tensors together with the assumed
spin densities are given in Table V. Comparison with the
TABLE IV. The relevant unit vectors along with the direction cosines with
respect to Q', b, and c' axes.
Vector
Q'
Direction cosines
b
c'
For radical I
C3H,
02 H,
C3H2
°i H2
C;H2
C3H3
03 H3
C3H.
02 H•
C,H 7
03 H7
C3H,
O;H,
C;H,
C3H.
0.9890
0.8803
0.9873
0.8803
0.9890
0.5575
0.8980
0.5491
0.1859
0.2962
0.4335
0.7455
0.8980
0.6707
0.2319
- 0.1439
0.4610
-0.1436
- 0.4610
0.1439
0.8299
0.4331
- 0.8356
- 0.9825
0.9538
0.9010
- 0.6658
-0.4331
- 0.7417
- 0.1511
0.9605
- 0.2410
- 0.3485
0.3073
- 0.9407
- 0.9749
- 0.7043
0.3123
0.8303
0.5998
-0.0191
- 0.9752
- 0.9257
0.2636
- 0.9645
0.9374
0.9500
0.3390
- 0.2224
0.7088
0.9353
- 0.4829
- 0.7944
- 0.2518
- 0.2169
- 0.3659
- 0.0890
0.1084
- 0.0402
- 0.0553
0.0102
0.0081
- 0.0404
- 0.1664
- 0.2785
- 0.0959
0.9676
0.0449
-0.0961
0.0172
0.0412
- 0.9989
0.9890
0.9925
0.1200
0.0220
- 0.1439
0.1209
- 0.9919
- 0.0388
0.0339
-
-
-
-
0.0340
0.1121
0.0680
0.1121
0.0340
0.0216
0.0777
0.0141
0.0130
0.0507
0.0156
0.0325
0.0777
0.0060
0.9609
For radical II
C,H
C,H b
C,Hc
N,Hc
C,H,
N2H,
C,H d
N2 Hd
C,H.
C,HJ
C,Hg
C,H,
O,H,
Q
0',--,,--------------:>\"--,
cit Proton
d~~~~-~-~~-~~-~~-~
0.5
1.0
1.5
2.0
2.5
Hyperfine Coupl!ni I mT
For radical III
C3O,
02C3 XC,03
(02 C3XC30 3) XC30 3
C3H,
FIG. 10. Angular variation of the hyperfine couplings of radical III.
J. Chem. Phys., Vol. 97, No.2, 15 July 1992
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Kumar, Lingam, and Rao: N-methylurea oxalic acid
TABLE V. Calculated dipolar components of the proton tensors of radical I
in MUO. The assumed spin densities on C 3 , O2 , 0 3 are 0.6, 0.06, 0.05, and
on q, Oi, 0;, are 0.2,0.02,0.05, respectively.
Proton
H,
H,
H3
H4
Hs
H.
H7
B(MHz)
13.64
-4.18
- 9.46
6.87
- 2.33
-4.53
10.73
- 4.15
- 6.58
9.94
- 3.21
- 6.73
3.14
- 1.42
-1.72
4.62
- 2.27
- 2.34
2.66
- 1.20
- 1.46
a'
Direction cosines
b
c'
0.9981
0.0518
- 0.0325
- 0.9967
0.0561
0.0595
0.6041
0.7936
0.0733
0.4674
-0.0181
0.8839
- 0.7637
0.6409
-0.0776
0.2543
0.7900
0.5573
- 0.2536
- 0.9665
-0.0403
0.0291
0.0643
0.9975
- 0.0558
0.0644
- 0.9964
0.7969
- 0.6020
- 0.0498
- 0.8825
0.0504
0.4676
- 0.6454
- 0.7554
0.1133
-0.1272
- 0.5445
0.8291
0.9660
- 0.2509
- 0.0619
0.0537
- 0.9966
0.0627
- 0.0597
- 0.9963
- 0.0610
-0.0046
-0.0885
0.9961
0.0530
0.9986
-0.0076
- 0.0140
- 0.1366
- 0.9905
0.9587
- 0.2818
- 0.0380
0.0497
- 0.0546
0.9973
corresponding experimental tensors (Table I) shows that
the assignments of the tensors should be considered as reasonable justifying the identification of the radical as a protonated anion.
Earlier proton transfer studies revealed that the selective proton transfer to the molecular radical anion in carboxylic and amino acid crystals is stereospecific. 7 ,11-13 The following factors were found to control the proton transfer: (1)
the proton located in the direction of the unpaired electron
orbital ofthe anion is more easily transferred than the others,
and (2) the hydrogen bond distance which is shorter than
the others is favorable for transfer.
In this context, some observed differences in MUO in
relation to the earlier studies may be pointed out. It was
assumed that the location of the proton after transfer is indicated by its Bma. axis.
( 1) In MUO, protons H3 and H4 located in the molecular plane (Fig. 11) are partially transferred.
(2) For these protons, the transfer is not taking place
across the intermolecular hydrogen bond.
(3) The final location of proton H3 is found to be out of
the molecular plane. Its Bma. axis makes an angle
of 70° with c' axis. Similar out-of-plane proton
transfer was also observed in UOA (2:1)4 and the
corresponding angle was found to be 65°.
( 4) In the case of proton H 4, the final location after
transfer was found to be in the molecular plane.
Similar in-plane transfer was observed in UOA
(1:1).5
From the above, the following general observations may be
made with the three addition compounds viz. MUO, UOA
905
(2:1), and (1:1).
( 1) Hydrogen bonded proton which is nearest to the
major spin density center is invariably involved in
proton transfer as observed in previous studies.
(2) Contrary to earlier observations, protons in the direction of the unpaired spin orbital of the anion are
not transferred.
(3) The proton transfer is not necessarily through the
intermolecular hydrogen bond.
In UOA (2:1)4 and UOA (1:1)5 crystals, it was observed that the protonated anion radicals decay giving rise to
secondary radicals of the form R--CHOH. This would mean
that one of the oxygen atoms of the oxalic acid molecule is
replaced by a hydrogen atom. Similar transformation is also
noted in MUO as discussed below. The general behavior of
radical III (Table III) indicates that it is a R-CHOH type of
radical. The components of the large hyperfine tensor (Table III) are characteristic of an a proton. This implies that
the unpaired electron resides mainly in the 2pz orbital on
carbon perpendicular to the molecular plane. The observed
gmin direction is in accordance with the above. Assuming
negative isotropic coupling, the a-proton direction was
found to be approximately along the C 3-0 3 bond. The angle
between these two directions is 7.3°. Hence, it is concluded
that 0 3 atom is replaced by an hydrogen atom.
In MUO the protonated anion (radical I) is, presumably, transforming to radical III as observed in UOA (2: 1)
crystal. The fragmentation path, which leads to the formation of radical III, can be understood within the framework
of the mechanism proposed in the earlier studies. It was suggested 14•15 that the scission of the C-O bond might be the
main path for the molecular radical anions to form RCH0 H kind of radicals. The observed features of radical III
are accounted for by the breakage of the C-O bond and the
subsequent replacement of oxygen by a proton. Thus it appears that radical III is a R-CHOH type of radical formed
by the scission of the C 3-0 3 bond. In UOA (2:1), it was
established that a common proton is involved in the protonation of the anion and the subsequent transformation to RCHOH radical. The absence of END OR signals in radical
III makes it difficult to conclude the involvement of a common proton in the transformation process in MUO.
The ESR spectrum of radical II consists of four broad
lines of approximately equal intensities for most of the orientations (Fig. 1). This spectrum could result from the interaction of the unpaired electron with two inequivalent strongly coupled protons. Additional small couplings were seen for
the magnetic field along the b axis (Fig. 1). The superhyperfine pattern was found to be complex and the site splittings
could not be followed in the three planes. Hence, the ESR
tensors could not be determined. However, the minimum gfactor (2.0018) direction was found to be along the c' axis.
The identification of radical II is also based on the interpretation of proton ENDOR analysis.
This additional radical was absent in UOA (2: 1) and
( 1: 1) systems. Earlier studies on irradiated urea compounds 16 showed no detectable radicals associated with the
urea molecules. However, in methyl urea, the observed radical was identified as
J. Chem. Phys., Vol. 97. No. 2.15 July 1992
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906
Kumar, Lingam, and Rao: N-methylurea oxalic acid
It ~as assumed that the above radical was formed by
abstractIOn of one of the methyl protons. It is expected to
have hyperfine interaction with the other two strongly coupled methyl protons. Additional weaker interactions are also
anticipated from adjacent NH groups giving rise to broadening .of the resonance lines. In view of the broadening, this
radIcal was not clearly characterized in the earlier studies.
The observed ESR spectrum of radical II in MUO is in qualitative agreement with the above expectation.
The multiplet structure for the magnetic field along the
b axis could not be analyzed. Even though interaction with
three protons was suspected, the relative intensities could
not be accounted for. An examination of the ENDOR spectrum for the magnetic field along the b axis showed that the
hyperfine couplings associated with lines 1,3, and c are approximately in the expected range of the multiplet splittings.
Using the ENDOR couplings, the multiplet pattern was simulated for two of the four lines of radical II. The simulated
spectrum is shown in Fig. 12. It is seen that the agreement is
reasonable. Hence, it was concluded that the complex multiplet pattern was due to a single radical II with three distinct
proton couplings. Further justification for the assumed
model for the radical II is provided by the interpretation of
its ENDOR tensors. Nine distinguishable proton tensors
were analyzed (Table II).
Tensors a and b could readily be identified as the strongly coupled protons (Ha and Hb ) giving rise to the prominent
four line pattern of the ESR spectrum. Both of them have
characteristics of a protons. The radical was assumed to be
formed by abstraction of one of the three methyl protons.
There is some uncertainty regarding which of the three protons was abstracted. As mentioned earlier, the position parameters of hydrogen atoms belonging to methyl group were
not uniquely determined due to disorder. From one of the
possible orientation of the methyl group given by the x-ray
crystallographic data,6 it was found that none of the methyl
protons lie in the molecular plane. Examination of the direction cosines of Bmax of tensors a and b shows that the associated a protons must lie approximately in the radical plane.
These results can be understood if we make a reasonable
assumption that after the abstraction of one of the methyl
protons hybridization of C I atom changes from Sp3 to Sp2.
The final assignments of tensors a and b are given in Table II.
Assuming least possible rearrangement, the abstracted hydrogen atom is identified as H* (Fig. 11). The rest of the
tensors were assigned to various protons in the crystal as
indicated by the direction cosines of their respective Bmax.
Tensors c, 3, and d with small but finite negative isotropic
couplings are assigned to intramolecular NH protons H
H 3 , and Hd and the tensors e,J, and g to the matrix proto~~
He' HI' and Hg (upper layer), respectively. The assignment
of tensor 1 requires explanation. This tensor shows small
isotropic but large dipolar components. It may be mentioned
that the associated line in one of the planes (a' b) could not be
followed due to poor signal strength. Hence, the tensor was
determined from limited available data. From the direction
cosines of B max , the tensor has been assigned to HI.
To confirm the above assignments, the dipolar tensors
were calculated as described earlier. The position coordinates of Ha and Hb were transformed by an appropriate
rotation about N I C I bond direction such that they lie in the
radical plane. The results showed that the calculated values
of the dipolar components for proton HI were too low compared with those of tensor 1. Hence, it is concluded that HI
proton must be considered as a transferred proton. Assuming the proton transfer to be along the hydrogen bridge, the
calculated OH distance for good a&reement with the experimental tensor was found to be 1 A. This corresponds to a
complete transfer. The small isotropic coupling ( - 1.01
MHz) for such a proton has not been understood. The assumed spin densities and the calculated tensors are given in
Table VI. It may be noted that appreciable spin density (0.2)
is assumed on the oxygen atom (0 1 ). Comparison of the
calculated results with the experimental values shows that
the assignment and hence the radical identification should
be considered as reasonable.
CONCLUSIONS
,
1mT
. ~ ~ A.-...
~
v
v
_ ~erimentally
~
recorded
FIG. 12. Comparison of the computer simulated spectrum of radical III
(only h~lfth: spec.trum) with the experimentally recorded spectrum along
the b aXIs. A lme Width of 0.15 mT and four proton couplings with the values
1.13, 0.42, 0.24, and 0.22 mT have been used for simulation of the ESR
spectrum.
The present study illustrates that the protonated anion
radical has a strong possibility of being trapped in hydrogen
bonded oxalic acid systems. The precursor of the protonated
anion viz. anion resulting from an electron capture by the
carboxyl group is not observed in the present study where
t~e ~rradiation is being carried out at room temperature.
SImIlar results were observed in (2: 1) and (1: 1) urea oxalic
acid crystals. In situ low temperature irradiation and ENDOR studies are required to establish the proton transfer
mechanism. Unfortunately, such studies are not possible
with a Varian E-1700 ENDOR accessory due to experimental limitations. Moreover, these studies are difficult in the
present case as MUO crystals are shattering with lowering of
temperature.
The excess negative charge on the carboxyl oxygen of
J. Chem. Phys., Vol. 97, No.2, 15 July 1992
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Kumar, Lingam, and Rao: N-methylurea oxalic acid
TABLE VI. Calculated dipolar components of the proton tensors of radical
II. The assumed spin densities on C I , C2 , N I' N 2 , and 01 are 0.65, 0.05,
0.05,0.05, and 0.2, respectively.
Proton
H.
H.
H,
H,
Hd
H,
HI
H.
HI
B(MHz)
a'
Direction cosines
b
c'
31.19
-0.03
- 31.16
29.35
2.22
- 31.57
10.30
- 1.55
- 8.74
4.92
-0.17
- 4.75
4.25
- 0.83
- 3.42
3.78
- 1.85
- 1.93
3.95
- 1.91
-2.04
3.12
- 1.40
-1.72
25.64
-7.90
- 17.74
0.9661
-0.0844
- 0.2441
- 0.2205
0.0313
0.9749
- 0.1130
-0.0264
- 0.9935
- 0.9763
- 0.0225
0.2154
- 0.1743
- 0.9832
0.0547
0.8672
- 0.0331
0.4969
0.6335
0.7648
-0.1172
0.0656
- 0.0764
0.9949
- 0.8157
0.2048
0.5410
0.2429
- 0.0242
0.9697
- 0.9694
0.1037
- 0.2226
0.9935
- 0.0316
- 0.1094
0.2164
- 0.0584
0.9746
0.9727
- 0.1633
0.1646
- 0.4537
0.3591
0.8156
- 0.7546
0.6442
0.1249
- 0.2559
- 0.9651
- 0.0572
- 0.5452
0.0404
- 0.8373
- 0.0877
- 0.9961
- 0.0029
0.1081
0.9941
- 0.0074
- 0.0285
- 0.9992
0.0297
0.0094
- 0.9980
- 0.0618
- 0.1529
0.0819
0.9848
- 0.2054
- 0.9327
0.2964
- 0.1710
- 0.0093
- 0.9852
0.9645
- 0.2607
- 0.0828
- 0.1934
- 0.9780
0.0787
the anion disturbs the potential minima associated with the
nearest neighbor hydrogen bonded protons. In favorable
cases, a complete transfer of a proton to the molecular anion
can take place through the intermolecular hydrogen bond.
Present results in MUO together with the previous observa-
907
tions show that the environmental factors also play a key
role in deciding the nature of proton transfer. The urea centered radical II has not been observed in previous studies of
similar systems. The interesting feature of this radical is the
observation of a complete proton transfer. However, the reasons for the very small isotropic value for this transferred
proton are not clear. Radical III is a R-CHOH type ofradical. Unlike in (2: 1) and (1: 1) urea oxalic acid systems, this
radical is observed in MUO only after about 9 months. ENDOR signals are not observed for radical III due to poor
signal strength. Consequently, it has not been possible to
conclude definitely that a common proton is involved in protonating the anion and in the subsequent replacement of the
oxygen atom to form R-CHOH radical.
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J. Chem. Phys., Vol. 97, No.2, 15 July 1992
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