EPR evidence for the restricted mobility of NO2 in gamma irradiated thorium nitrate pentahydrate Th(NO3)4· 5H2O

Article abstract of DOI:10.1016/j.saa.2011.02.031

Electron paramagnetic resonance (EPR) studies were conducted on gamma irradiated polycrystalline sample of thorium nitrate pentahydrate, Th(NO ₃)₄·5H₂O, in the temperature range of 100-300 K. The most prominent species with triplet hyperfine structure in the EPR spectrum was identified as NO₂. The EPR spectrum gave evidence for the stabilization of NO₂ in at least three different sites slightly differing in spin Hamiltonian parameters (Site₁: g _x = 2.0042, g_y = 1.9911, g_z = 2.0020, A _x = 54.20 G, A_y = 48.50 G and A_z = 65.25 G; Site₂: g_x = 2.0042, g_y = 1.9911, g_z = 2.0020, A_x = 54.20 G, A_y = 48.50 G and A_z = 67.85 G; Site₃: g_x = 2.0045, g_y = 1.9911, g_z = 2.0015, A_x = 54.20 G, A_y = 49.05 G and A_z = 72.45 G). The EPR spectra for Site₁ revealed molecular dynamics of NO₂ from a slow motion region to fast motion region as the sample temperature was varied from 100 to 300 K. This led to a change in EPR spectrum from orthorhombic to axial, indicating preferred rotation of NO₂ molecule about the O-O bond direction. However, the NO ₂ molecule at Site₂ was found to be rigid throughout the entire temperature range. The differences in the mobility of NO₂ molecules occupying the two sites could be attributed to the fact that in one case NO₂ was bonded to thorium or water and in the other case it was weakly bound. The NO₂ bound to thorium through two oxygen atoms or bound to thorium on one side through one oxygen atom and hydrogen bonded to water on the other side remains rigid throughout the entire temperature range, while NO₂ situated at interstitial sites or adsorbed on the surface exhibits mobility with increase in temperature above 100 K.

Full text of DOI:10.1016/j.saa.2011.02.031

Spectrochimica Acta Part A 79 (2011) 405–411
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
EPR evidence for the restricted mobility of NO in gamma irradiated thorium
2
nitrate pentahydrate Th(NO ) ·5H O
3
4
2
∗
B. Rajeswari, R.M. Kadam , B.A. Dhawale, Y. Babu, V. Natarajan, S.V. Godbole
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Electron paramagnetic resonance (EPR) studies were conducted on gamma irradiated polycrystalline
sample of thorium nitrate pentahydrate, Th(NO3)4·5H2O, in the temperature range of 100–300 K. The
most prominent species with triplet hyperfine structure in the EPR spectrum was identified as NO2. The
EPR spectrum gave evidence for the stabilization of NO2 in at least three different sites slightly differing
in spin Hamiltonian parameters (Site1: gx = 2.0042, gy = 1.9911, gz = 2.0020, Ax = 54.20 G, Ay = 48.50 G and
Az = 65.25 G; Site2: gx = 2.0042, gy = 1.9911, gz = 2.0020, Ax = 54.20 G, Ay = 48.50 G and Az = 67.85 G; Site3:
gx = 2.0045, gy = 1.9911, gz = 2.0015, Ax = 54.20 G, Ay = 49.05 G and Az = 72.45 G). The EPR spectra for Site1
revealed molecular dynamics of NO2 from a slow motion region to fast motion region as the sample
temperature was varied from 100 to 300 K. This led to a change in EPR spectrum from orthorhombic to
axial, indicating preferred rotation of NO2 molecule about the O–O bond direction. However, the NO2
molecule at Site2 was found to be rigid throughout the entire temperature range. The differences in the
mobility of NO2 molecules occupying the two sites could be attributed to the fact that in one case NO2
was bonded to thorium or water and in the other case it was weakly bound. The NO2 bound to thorium
through two oxygen atoms or bound to thorium on one side through one oxygen atom and hydrogen
bonded to water on the other side remains rigid throughout the entire temperature range, while NO2
situated at interstitial sites or adsorbed on the surface exhibits mobility with increase in temperature
above 100 K.
Received 17 September 2010
Received in revised form 1 February 2011
Accepted 16 February 2011
Keywords:
EPR
Hydrogen bonding
Molecular dynamics
Th(NO3)4·5H2O
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
lations. The EPR spectrum of NO in rigid state is highly anisotropic
2
(orthorhombic g and A tensors), whereas in dynamic state the
Electron paramagnetic resonance (EPR) has been widely used
anisotropy of g and A gets averaged out yielding a simple triplet
in the EPR spectrum. Therefore, NO2 molecule is used as an effec-
tive spin probe for the study of molecular dynamics at the gas–solid
interface by EPR. The temperature dependent EPR spectra of NO2
adsorbed on crushed Vycor glass and on zeolite have been the
subject of extensive investigations for understanding molecular
motion at microscopic level [14–17]; however, most of these inves-
tigations were restricted to motion of NO2 on solid surfaces (Vycor
glass, zeolites, silica gel, MgO, ZnO [18]).
to study paramagnetic defects and radicals formed in solids by
irradiation with gamma rays [1], in situ irradiation caused by dop-
ing actinide ions in inorganic matrices [2–5] or auto radiolysis
of actinide compounds [6,7]. These studies are most informative
when hyperfine effects are seen in the EPR spectra. Most extensive
EPR studies of irradiation damage in inorganic nitrates were car-
2
−
2−
2+
ried out on potassium nitrate (NO3 , NO , NO , NO
, NO) [8],
2
3
2
−
−
sodium nitrite (NO , N O ) [9,10], silver nitrate (Ag , Ag(NO ) ,
2
2
4
3
NO ) [11], lead nitrate (NO , NO ) [12] and strontium nitrate (NO ,
NO2 produced by alpha particle emission (auto-radiolysis
effects) were investigated on cesium plutonium (IV) nitrate
Cs Pu(NO ) by EPR technique, wherein the author observed
2
2
3
3
2
−
NO3 , NO ) [13]. Irradiation of nitrates lead to the formation of
2
−
several nitrogen centered free radicals, viz., NO , NO , NO , NO
and NO3 , of which nitrogen dioxide, NO , a stable paramagnetic
2
2
3
2
3 6
2
−
build up in the intensity of triplet signal of NO2 (giso = 2.0016;
Aiso = 58.6 G) upon in situ ␣ irradiation [7]. ␥-ray induced decom-
position of zirconium and thorium nitrates was studied in binary
mixtures containing different oxides at varying compositions and
absorbed doses [19,20]. The observation of an increase or decrease
in the decomposition of binary nitrates in the presence of var-
ious oxide additives was explained in terms of their electron
donor/acceptor properties and the formation of paramagnetic cen-
2
molecule exhibits simple EPR spectrum. The EPR parameters of
NO2 and their relation to the electronic structure have been well
established from single crystal studies and molecular orbital calcu-
∗ Corresponding author at: Radiochemistry Division, BARC, Trombay, Mumbai 400
0
94, India. Tel.: +91 22 25592000; fax: +91 22 25505151.
ters (NO2, NO3, NO22 , O3 and O2 ). EPR studies of NO3 radical in
−
−
−
E-mail address: rmkadam2003@yahoo.co.in (R.M. Kadam).
1
386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.02.031

4
06
B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
irradiated M(UO )(NO ) (M = K), M (UO )(NO ) (M = K and NH4)
2
3
3
2
2
3 4
gave evidence for the NO3 radical trapped at the bidentate and
unidentate nitrate sites [21].
In the present work, we have investigated radiation induced
defects, mainly NO , formed in gamma irradiated polycrystalline
2
sample of Th(NO ) ·5H O. We have shown that NO exhibits tem-
3
4
2
2
perature dependent EPR spectra, which can be explained by the
restricted rotation of the NO2 molecule, where the preferred axis
of rotation is the O–O bond direction. Based on the reported crystal
structure and the present temperature dependent EPR studies, it
is suggested that NO2 molecule is situated in environments hav-
ing different bond strengths due to which it exhibits either rigidity
or mobility. If the NO2 is bound to thorium through two oxygen
atoms or bound to thorium on one side through one oxygen atom
and hydrogen bonded to water on the other side, it remains rigid
throughout the entire temperature range. On the other hand, if NO2
is formed from NO3 trapped at interstitial sites (non coordinated
NO group) or NO adsorbed on the surface, it will be weakly bound
Fig. 1. Oxygen arrangement around the thorium atom as viewed in the b direction.
Reproduced from Ref. [22] with the IUCr’s permission; http://journal.iucr.org/.
3
3
in the lattice and will be amenable to molecular dynamics.
. Experimental
Polycrystalline sample of Th(NO ) ·5H O was prepared by dis-
3
. Results and discussion
2
3.1. EPR spectrum of NO2 at liquid nitrogen temperature and
3
4
2
determination of g and A tensor
solving 1 g of pure ThO2 (99.99%) in 0.05 M HF-HNO3 mixture.
The sample was heated repeatedly with concentrated supra pure
HNO3 to remove traces of HF present in the sample. The tho-
rium nitrate solution thus obtained was evaporated to obtain dry
powder. The sample thus prepared was characterized by X-ray
diffraction method. As the sample was hygroscopic in nature, the
dry polycrystalline sample was immediately transferred to a quartz
tube and sealed at room temperature. The sample was gamma irra-
diated at room temperature at a dose rate of 1.5 kGy/h for 5 h.
EPR spectra were recorded with a Bruker ESP-300 spectrometer
The EPR spectrum of gamma irradiated powder sample of Th
(NO ) ·5H O at liquid nitrogen temperature showed a prominent
3
4
2
signal with triplet hyperfine structure (Fig. 3). The number and rel-
ative intensities of the hyperfine lines indicated that they were
associated with a single atom of nitrogen. It has been shown that
the EPR spectrum of NO2 molecule observed at low temperature,
as in the present investigations, can be interpreted in terms of
anisotropic g and A factors arising due to the interaction of unpaired
14
electron with a nitrogen nucleus (7N , I = 1, 99.63% abundance).
(
X-band frequency ∼9.5 GHz) using 100 kHz frequency modulation
Both g and A factors had orthorhombic symmetry.
with low enough modulation amplitude such that no line shape
distortion was observed. g-values were measured with reference
to DPPH sample having g = 2.0036. Temperature dependence of the
spectra was studied with a Bruker B-VT 2000 variable temperature
unit. The temperature of the sample was regulated within ± 0.5 K
between 100 and 300 K. Spectra at 77 K were obtained by direct
immersion of sample tube in a quartz dewar containing liquid nitro-
gen. EPR spectra were recorded in first and second derivative modes
at different temperatures.
The assignment of the other EPR signal at g = 2.0092 as due to the
presence of other nitrogen centered radicals is completely ruled out
mainly because of the absence of any hyperfine structure on this
radical signal. Similarly, based on the g value and the thermal sta-
bility of this radical we rule out the possibility of this signal being
due to formation of F center (electron trap center) or VK center (hole
trap center). The g value of F center is slightly less than 2.00, while
that of VK center is greater than 2.00. In addition to this, VK cen-
ter which is formed on low temperature irradiation is reported to
be thermally unstable at room temperature. The EPR signal with-
out hyperfine structure observed in the present investigations may,
2.1. Crystal structure
The crystal structure of thorium nitrate pentahydrate was
determined by X-ray diffraction and neutron diffraction studies
22,23]. The crystal structure of thorium nitrate pentahydrate is
[
isomorphous with its analogous rare earth counterpart cerium
tetranitrate pentahydrate; Ce(NO ) ·5H O and plutonium tetrani-
3
4
2
trate pentahydrate; Pu(NO ) ·5H O [24]. It is orthorhombic, with
3
4
2
space group Fdd2 with a = 11.191 A˚ , b = 22.889 A˚ and c = 10.597 A˚ .
Eleven oxygen atoms are bound to the thorium atom, eight of
which are from four bidentate nitrate groups (two crystallograph-
ically inequivalent NO3 units differing in N· · ·O bond distances and
O–N–O bond angles) and the other three are from water molecules.
The nitrate groups are planar and the N· · ·O distances are reported
to be significantly different within the group. Average bond dis-
tances are Th· · ·O (water) 2.46 A˚ , Th· · ·O (nitrate) 2.57 A˚ , N· · ·O (next
to Th) 1.27 A˚ and N· · ·O (terminal) 1.22 A˚ . The structure is bound
together by hydrogen bonds, which are either strong water–water
hydrogen bonds (2.70 A˚ ) or weak water–nitrate oxygen hydrogen
bonds (2.90–2.95 A˚ ). Figs. 1 and 2 show the reported crystal struc-
Fig. 2. Hydrogen bond scheme in Th(NO3)4·5H2O.
ture and hydrogen bond scheme in Th(NO ) ·5H O respectively.
Reproduced from Ref. [22] with the IUCr’s permission; http://journal.iucr.org/.
3
4
2

B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
407
Site1
Site2
Site1
Site2
1
00 K
A)
130 K
(
(
A)
Site3
Site3
YZ
Z X
Y
XZ
0
Y
X
Y Z
+1 MI
ZX
Y
XZ
Y
X
-
1
-1
0
+1 MI
100 K
(
B)
130 K
(
B)
X
Y
Z
Z
X
Y
M= -1
M = +1
3
300
3350
3400
3450
3500
3550
H (GAUSS)
3
350
3375
3400
3
475
3500
Fig. 3. EPR spectra of NO2 in gamma irradiated Th(NO3)4·5H2O at 100 K recorded
4
using receiver gain RG = 4 × 10 , modulation amplitude MA = 1G, microwave power
3300
3350
3400
3450
3500
3550
setting 6.3 mW (A) second derivative spectrum, (B) first derivative spectrum; black
colour represents experimental spectrum and red colour represents simulated
spectrum (the simulated spectrum was constructed by considering three sites of
NO2 using EPR parameters; Site1, gx = 2.0042, gy = 1.9911, gz = 2.0020, Ax = 54.20 G,
Ay = 48.50 G and Az = 65.25 G, Site2, gx = 2.0042, gy = 1.9911, gz = 2.0020, Ax = 54.20 G,
Ay = 48.50 G and Az = 67.85 G, Site3, gx = 2.0045, gy = 1.9911, gz = 2.0015, Ax = 54.20 G,
Ay = 49.05 G and Az = 72.45 G). The weight factors used for simulations are 0.5:1.0 for
rotating and rigid NO2 respectively. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of the article.)
H (GAUSS)
Fig. 4. EPR spectra of NO2 in gamma irradiated Th(NO3)4·5H2O recorded at 130 K
4
using RG = 2 × 10 , MA = 1 G, microwave power setting 6.3 mW (A) second derivative
spectrum, (B) first derivative spectrum; black colour represents experimental spec-
trum and red colour represents simulated spectrum (the simulated spectrum was
constructed by considering three in equivalent NO2 using EPR parameters at 130 K:
Site1, gx = 2.0030, gy = 1.9911, gz = 2.0020, Ax = 56.35 G, Ay = 49.45 G and Az = 62.00 G,
Site2, gx = 2.0030, gy = 1.9911, gz = 2.0020, Ax = 56.35 G, Ay = 49.45 G and Az = 66.70 G,
Site3, gx = 2.0030, gy = 1.9911, gz = 2.0012, Ax = 56.35 G, Ay = 49.45 G and Az = 71.30 G).
The weight factors used for simulations are 0.5:1.0 for rotating and rigid NO2 respec-
tively. The inset shows the enlarged portion of MI = −1 and MI = +1 transitions of
NO2 of Site1. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of the article.)
therefore, be attributed to any one of the oxygen based centers, viz.,
O , O , O3 and O2+. These radical ions can be distinguished on
−
−
−
2
the basis of the trend in their g values [25–27]. None of these rad-
icals is known to give isotropic signal. The derivative line shape of
the signal at g = 2.0092 in the present case suggests that it is the
perpendicular component. The other components are expected to
be much weak in intensity (parallel component, in case of axial
Figs. 3(B) and 4(B) show the EPR spectra of NO2 in gamma
irradiated Th(NO ) ·5H O recorded in first derivative presentation
3
4
2
−
symmetry). The O radical ion formed due to irradiation of various
at 100 K and 130 K respectively. The EPR bands corresponding to
MI = +1 and MI = −1 transitions show unresolved hyperfine struc-
ture in the first derivative presentation of the spectra. However,
the structure gets resolved better in the second derivative presen-
tation (Figs. 3(A) and 4(A)). It is seen that the Z component of the
MI = −1 transition, which is a relatively distinct signal free from
other overlapping transitions (X component of MI = −1 transition),
showed a triplet in the second derivative presentation indicating
the presence of NO2 molecules at least in three magnetically non-
equivalent sites having different geometry, i.e., bond lengths and
bond angles. This would result in NO2 molecules having different
hyperfine coupling constants (A values) and g factors. The geometry
of NO2 is depicted in Fig. 5.
oxide matrices exhibit typically axial symmetry of g tensor, that
−
is, gx = gy = g⊥ > gz = g|| ∼ 2.00 (O on MgO: g|| = 2.0013, g⊥ = 2.042;
−
ZnO: g|| = 2.0023 and g⊥ = 2.0210) whereas in the case of O radical
2
ion, the reported g values have either axial or nearly axial sym-
metry, i.e., gx ∼ gy ∼ 2.00, gz > 2.00. The difference in gx and gy is
−
−
very small (O ion on SnO : gz = 2.025, gx = 2.009 gy = 2.004; O
2
2
2
−
NaO :gz = 2.175, gx = 2.00, gy = 2.00). In the case of O3 radical ion in
2
nitrates, the g values are axially symmetric (Sr(NO3)2, g|| = 2.0033,
g⊥ = 2.0135; Ba(NO ) g|| = 2.0042, g⊥ = 2.0138). The reported g
3
2,
values for O in O +BF /O AsF
+
−
+
−
are g = 2.00, g = 1.98 and
2
2
4
2
6
1
2
g3 = 1.75 (shows very large anisotropy). Based on these reported
literature we conclude that the observed signal at g = 2.0092 is due
−
to O . The presence of these two paramagnetic species (NO2 and
The EPR parameters for the NO2 molecule have been precisely
determined from the calculated spectra, obtained with BRUKER
WINEPR SimFonia program based on perturbation theory [28]. The
2
−
O2 ) was further confirmed from microwave power dependence
and temperature dependence of the EPR spectra.

4
08
B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
200 K
Fig. 5. Geometry of NO2 molecule.
spectral simulation of the temperature dependent EPR spectra was
also carried out using the same program. The theoretical EPR signals
were calculated using the spin Hamiltonian
3300
3350
3400
3450
3500
3550
H (GAUSS)
H = ˇgHS + SAI
Fig. 6. EPR spectra of NO2 in gamma irradiated Th(NO3)4·5H2O at 200 K recorded
4
using RG = 2 × 10 , MA = 1G, microwave power setting 6.3 mW in first deriva-
where ˇ is the Bohr magneton, g is the spectroscopic splitting fac-
tor, H is the applied magnetic field, S is the total spin of the electron
tive presentation; black colour represents experimental spectrum and red colour
represents simulated spectrum. The simulated spectrum was constructed by consid-
ering NO2 having axial symmetry g|| = 1.9917, g⊥ = 2.0025, A|| = 50.95 G, A⊥ = 59.55 G;
line widths ꢂ|| = 3.0 G and ꢂ⊥ = 5.50 G for rotating NO2; orthorhombic symme-
try gx = 2.0042, gy = 1.9917, gz = 2.0015, Ax = 54.20 G, Ay = 50.95 G and Az = 67.85 G;
ꢂx = 2.5 G, ꢂy = 3.0 G, ꢂz = 2.5 G for rigid NO2 and g⊥ = 2.0092 and ꢂ⊥ = 3.0 G for O2
radical. The weight factors used for simulations are 0.5:1.0:0.25 for rotating NO2,
(
S = 1/2), A is the hyperfine tensor and I is the nuclear spin. Here the
value of the electronic spin used is S = 1/2 and the nuclear spin is
that of nitrogen which is I = 1. In polycrystalline samples, EPR sig-
nals have been simulated by generating 9000 random orientations
of the magnetic field and by summing the corresponding absorption
signals. The final signal was obtained by performing a convolu-
tion (Gaussian or Lorentzian line shape) of each transition line,
adding all contributions and calculating the first derivative/second
derivative of the signal; the line width of each component has been
optimized in order to obtain the best fit in accordance with the
observed experimental values.
−
−
rigid NO2 and O2 ion respectively.
to the concentrations or number of spins of NO2 in those sites.
The ratio of the weight factors for NO2 in Site1 and Site2 remained
nearly constant (0.5:1) in the entire temperature range. The com-
parison between the experimental spectra and calculated spectra
are shown in Figs. 3, 4 and 6.
The best fit of the calculated EPR spectra for the three sites were
obtained by assuming that the molecule is nearly rigid at about
1
00 K (resulting in orthorhombic g and A). The EPR parameters used
3.2. Temperature dependence of the EPR spectra of NO2
for the calculated spectrum are listed in Table 1 along with those
reported in literature. The present g values are very close to those
Fig. 7 shows the EPR spectra of NO molecule obtained in gamma
2
reported for NO molecule trapped in irradiated inorganic nitrates.
irradiated Th(NO ) ·5H O at various temperatures in the range
2
3
4
2
However, the A values are slightly different from the reported val-
ues indicating that the NO2 geometry (the bond angle of O–N–O)
might be modified by the matrix. In case of NO2 the unpaired elec-
tron occupying a 4a1 molecular orbital constructed from nitrogen
100–300 K. The line shape (g and A tensor) of the EPR spectra
was strongly temperature dependent in the range 100–190 K. The
changes in the spectra were reversible upon varying the temper-
atures with regard to line shape and intensity. The spectra due
to NO2 at all the sites at 100 K showed characteristics of a nearly
rigid molecule. The rigidity of NO2 molecule observed at 100 K in
the present investigations is in contrast to that reported for NO2
adsorbed on different surfaces, where NO2 is in rigid form at very
low temperatures (4.2 K). The rigidity of NO2 at the higher temper-
ature of 100 K observed in the present studies is possibly due to the
presence of hydrogen bonds. On increasing the temperature from
100 K to 130 K, the EPR bands corresponding to MI = ± 1 transitions
showed well resolved hyperfine structure which could easily be
deconvoluted to that due to the three sites by recording the second
derivative spectra. On further heating the sample in the temper-
ature range 190–300 K, the EPR line shape remained unchanged.
Fig. 8 shows the plots of experimental line position vs. tempera-
2
s and 2pz, and oxygen 2pz atomic orbitals having the form
ꢀ
(4a ) = a ꢀN(2s) + azꢀN(2p ) + b ꢀ (2p )
1
S
z
1
O
z
2
2
2
where a , a and b represents the unpaired electron density in the
s
z
1
nitrogen 2s, 2pz and oxygen 2pz orbitals. The contribution of the
nitrogen 2s orbital to the 4a1 molecular orbital can be evaluated
from the isotropic part of nitrogen hyperfine splitting (Aiso) by the
following expression
ꢀ
ꢁ
8
3
ꢁ
2
2
Aiso = g ␤
|ꢀN(2S)(O)| .as
N
N
On the other hand, the anisotropic part in the contribution nitro-
gen 2pz orbital to the 4a1 molecular orbital can be calculated from
the dipolar interaction. From the estimated spin densities, aN(2s)
and aN(2pz), the ONO angle can be calculated assuming the equa-
ture for Site1 and Site . The experimental resonance positions for
2
Site could not be followed because of their weak intensity and are
3
−
1
2
−1/2
where k2 = a (2p )/a (2s). The calculated
2
N
2
N
tion 2 cos (k + 2)
not shown in the figure. It was seen that, with increase in tempera-
z
value of ONO angle of the NO2 molecule in the gas phase was
reported to be 135. The bond angle is matrix dependent and with
variation in hyperfine coupling constant the angle changes [29,34].
Similarly EPR spectra were calculated at different temperatures
ture, the X and Z components of the EPR spectrum due to Site shift
1
gradually towards their common center for all three MI transitions
and they collapse into one broad band at about 170 K. Above 100 K,
the line positions corresponding to the Y component also shift from
the original position towards the central part; however the degree
of shift is much less than that of the X and Z components. This
indicates that the rate of rotational motion about the Y axis (O–O)
direction is much greater than that about X and Z axes in this tem-
(
100 K, 130 K and 200 K). In the simulation, the individual spec-
tra were given different weightages so as to get the best fit with
the experimental spectra. The weight factors are related to the sig-
nal intensities of NO2 at different sites which are in turn related

B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
409
Table 1
EPR parameter (g values and 14N hyperfine coupling constants in G) of NO2 in various matrices.
Matrix
gx
gy
gz
giso
Ax
Ay
Az
Aiso
Ref.
NaNO2 (77 K)
NaNO2 (293 K)
AgNO3 (77 K)
KNO3 (77 K)
Sr(NO3)2 (304 K)
Pb(NO3)2 (77 K)
Ice (77 K)
CH3NO2 (77 K)
N2O4 (77 K)
Neon (4 K)
NO2 (gas) (293 K)
Vycor (4.8 K)
Zeolite (4.8 K)
Silica gel (77 K)
Porous clay
Cs2Pu(NO3)6 (300 K)
Th(NO3)4·5H2O (Site1) (100 K)
Th(NO3)4·5H2O (Site2) (100 K)
Th(NO3)4·5H2O (Site3) (100 K)
2.0057
2.0059
2.0090
2.0055
2.0054
2.004
2.0066
2.0047
2.0054
2.0062
2.0062
2.0051
2.0051
2.004
1.9910
2.0052
1.9978
1.9932
1.9976
1.9950
1.9920
1.9915
1.9913
1.9920
1.9910
1.9913
1.9913
1.9907
1.9921
2.0015
2.0055
2.0039
1.9996
1.9976
1.9950
2.0022
2.0011
2.0015
2.0030
2.0020
2.0017
2.0017
2.004
1.9994
2.0055
2.0036
1.9994
2.0002
1.998
2.0003
1.9991
1.9994
2.0004
1.9997
1.9994
1.9994
1.9996
1.9999
2.0016
1.9991
1.9991
1.9990
49.3
36.4
51.1
48.8
49.0
50.0
50.6
47.4
50.3
51.8
45.6
50.1
50.1
59.2
52.5
46.5
33.0
48.3
50.4
56.0
57.0
49.9
44.4
48.1
45.6
45.0
45.8
45.8
48.9
49.0
67.8
50.0
69.8
63.0
56
57.0
70.3
65.1
67.2
61.8
66.1
65.5
65.5
59.2
64.9
54.5
39.8
56.4
54.1
53.7
54.7
56.9
52.3
55.2
53.1
52.2
53.8
53.8
55.8
55.5
58.6
55.98
56.85
58.57
[9]
[10]
[11]
[8]
[13]
[12]
[29]
[30]
[31]
[32]
[33]
[17]
[17]
[34]
[35]
2.0055
2.0020
[2]
a
2.0042
2.0042
2.0045
1.9911
1.9911
1.9911
2.0020
2.0020
2.0015
54.20
54.20
54.20
48.50
48.50
49.05
65.25
67.85
72.45
a
a
a
Polycrystalline sample, present study.
perature range. The Y component gradually becomes more diffused,
when the temperature is increased above 230 K. This is reflected in
the increased rotational motion about the X and Z axes and also
perature range of observation (Z component of the Site is shown in
2
Fig. 8) indicating that Site2 remains rigid in this range. Fig. 6 shows
the experimental spectrum recorded at 200 K along with the sim-
ulated spectrum generated by considering the contributions from
two signals, one having axial symmetry of g and A tensors (due to
increased line width with temperature. However, for Site , the EPR
2
spectrum almost remained unchanged throughout the entire tem-
Fig. 7. Temperature dependence of the experimental spectra of NO2 in gamma irradiated Th(NO3)4·5H2O in the temperature range between 100 and 300 K. The intensities
given are blown to the full scale to follow precise resonance position.

4
10
B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
The EPR of NO2 adsorbed on surfaces was the subject of exten-
2
2
2
2
1
1
1
1
1
60
40
20
00
80
60
40
20
00
sive investigation in the recent past with special emphasis given to
understanding the molecular motion of NO2 in these porous mate-
rials. A study on NO2 adsorbed in synthetic zeolites was reported
by Pietrzak and Wood [36]. They observed temperature dependent
EPR spectra, which were explained in terms of hindered rotation of
the molecule. However, they reported that there was no preferred
axis of the rotational freedom for NO2 adsorbed in synthetic zeo-
lites. It is interesting that similar studies on silica–gel system [31]
show a distinct contrast in that on the silica gel surface, NO prefers
2
to rotate about the O–O bond direction. In this case, the hydrogen
bonds formed between the oxygen of the NO2 molecule and OH
groups on the silica gel surface play an important role. Shiotani and
Site
Site
Z
X
M= -1
Y
X Z M= 0
Y
8
0
Freed [17] investigated the EPR spectra of NO adsorbed on crushed
2
Vycor glass and on a zeolite surface and they analyzed the data in
terms of the molecular dynamics of NO2 using a theory developed
for slow motional EPR spectra. The theory was successfully applied
for understanding the temperature dependent EPR line shapes of
nitroxides and the VO2+ ion in isotropic and anisotropic liquids [17
and references therein]. It was shown by them that below 77 K,
the rotational motion of the NO2 is axially symmetric about the Y
axis (parallel to the O–O inter-nuclear axis). In the present work,
we have shown that NO at Site exhibited temperature dependent
-
80 -70 -60 -50 -40 -30 -20 -10
0
10 20
H-H (GAUSS)
0
Fig. 8. Plot of experimental resonance position vs. temperature of MI = 0 and MI = −1
transitions of NO2. H0 stands for a magnetic field strength of 3425 G. A microwave
frequency of 9.60 GHz was used. MI = +1 transition is not shown in figure. The Z
component of Site2 is independent of temperature indicating that Site2 is rigid.
2
1
rotational motion along Y axis that is O–O bond direction, Site1)
and the second one having orthorhombic g and A tensors (due to
rigidity of NO2 molecule, Site ).
EPR spectra, which can be explained by the restricted rotation of
the NO2 molecule, where the preferred axis of rotation is the O–O
bond direction. On the other hand, NO2 at Site2 was found to be
rigid. From the reported crystal structure, it is seen that thorium is
coordinated to three oxygen atoms of three water molecules and
eight oxygen atoms from four bidentate nitrate groups. The remain-
ing water molecules, which are not bound directly to thorium
(two non-coordinated), are involved in the stacking of thorium
nitrate groups with hydrogen bonding. From the hydrogen bond
scheme shown in the reported crystal structure, there are two types
of hydrogen bonds (i) two “strong” water-water hydrogen bonds
O(1)–H(1)· · ·O(3) and O(2)–H(2)· · ·O(3) with bond lengths 2.698 A
2
The temperature dependence of the EPR spectra of NO2 in the
range 100–300 K was studied by keeping the instrumental param-
eters (such as receiver gain, modulation amplitude and microwave
power) identical. An important observation was that the EPR sig-
nal intensity of NO2 determined by double integration decreased
considerably in the range 140–300 K which was found to be much
greater than that expected from Curie–Weiss law. On the other
hand, the decrease in intensity followed Curie–Weiss law in the
temperature range 100–140 K. The decrease in intensity was not
due to thermal destruction of NO2 as evident from the reversible
nature of the signal intensity variations (Fig. 9). The changes in the
spectra were completely reversible upon varying temperature, i.e.,
the line shape and the line intensities were totally reproducible on
cooling. A similar behavior was also observed for NO2 adsorbed on
˚
and 2.697 A respectively and (ii) the three weak water–nitrate
oxygen hydrogen bonds O(2)–H(3)· · ·O(22), O(3)–H(4)· · ·O(23) and
O(3)–H(5)· · ·O(13) with bond lengths 2.953, 2.901 and 2.946 A
respectively. The different strengths of hydrogen bonding in the
crystal structure of Th(NO ) ·5H O is in accordance with infrared
˚
˚
3
4
2
silica gel and zeolites [17]. The room temperature spectrum of NO2
becomes a triplet with equal intensities indicating that at room
temperature, rotation around the X and Z directions becomes sig-
nificant. In addition, the perpendicular component of O2 radical
observations by Cho and Wadsworth [37], where a broad band at
−1
ca 3100 cm was attributed to strongly hydrogen bonded water,
whereas weakly hydrogen bonded water was evidenced by a band
−
−1
at 3440 cm
.
became prominent at 300 K.
It may be mentioned that in thorium nitrate pentahydrate, NO3
acts as bidentate ligand with one free oxygen (not bound to tho-
rium) which is involved in hydrogen bonding. The mobility of NO2
at Site is suppressed completely over the entire temperature range
2
of observation if the NO2 is bound to thorium through two oxygen
atoms (NO2 is formed by cleavage of one of the oxygens of NO3
2
1
1
00
50
00
which are hydrogen bonded to H O, i.e., NO2 is formed by break-
2
ing N· · ·O(23), N· · ·O(13) or N· · ·O(23)). Another possibility could
be that one oxygen atom is bound to thorium on one side and a
hydrogen bonded oxygen on the other side (NO is formed by cleav-
2
age of one of the coordinated oxygen atoms, i.e., NO2 is formed
by breaking N· · ·O(21), N· · ·O(22); N· · ·O(11), N· · ·O(12), N· · ·O(22),
N· · ·O(21)). Both types of NO did not show any temperature depen-
2
dent changes in EPR spectrum and it remained rigid throughout the
5
0
temperature range of observation. On the other hand, if NO at Site1
2
is formed from NO3 trapped at interstitial sites (non coordinated
NO3 group) or NO3 adsorbed on the surface of Th(NO ) ·5H O,
3
4
2
0
it will be weakly bound in the lattice and will be amenable to
molecular dynamics and exhibit increased rotational mobility with
increase in temperature. These NO2 showed strong temperature
dependent changes in the EPR spectra in the range 100–200 K, as
it was more flexible towards motion (restricted rotation around
80
100 120 140 160 180 200 220 240 260
TEMPERATURE (K)
Fig. 9. Plot of intensity × temperature (a.u.) vs. temperature (K).

B. Rajeswari et al. / Spectrochimica Acta Part A 79 (2011) 405–411
411
O–O bond direction) and it exhibited mobility. It may be noted
that EPR investigations on thorium oxalate containing NO3 as an
impurity (incorporated during sample preparation by oxalate pre-
cipitation from nitric acid) gave NO2 on irradiation and exhibited
similar rotational mobility as observed in the present case [38].
[4] V. Natarajan, T.K. Seshagiri, M.D. Sastry, Radiat. Phys. Chem. 38 (1991)
365.
−
[
5] T.K. Seshagiri, V. Natarajan, M.D. Sastry, Nucl. Tracks Radiat. Meas. 21 (1993)
71.
[6] M.K. Bhide, R.M. Kadam, V. Natarajan, Y. Babu, M.D. Sastry, Chem. Phys. Lett.
32 (2000) 98.
2
3
[
[
7] M.V. Krishanamurthy, Radiochim. Acta 22 (1975) 177.
8] H. Zeldes, Paramagn. Resonan. Proc. Intern. Conf. Ist., vol. 2, 1962, p. 264.
4
. Conclusion
[9] H. Zeldes, R. Livingston, J. Chem. Phys. 35 (1961) 563.
10] J. Tateno, K. Gesi, J. Chem. Phys. 40 (1964) 1317.
11] W.C. Mosley, W.G. Moulton, J. Chem. Phys. 43 (1965) 1207.
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[
[
[
EPR studies on gamma irradiated Th(NO ) ·5H O gave evidence
3
4
2
for the stabilization of NO2 at three different sites with slightly
different spin Hamiltonian parameters. NO2 at one of these sites
exhibited molecular dynamics transforming from a slow motion
region to fast motion region with increase in temperature thereby
leading to a change in EPR spectrum from orthorhombic to axial.
This indicated preferred rotation of NO2 molecule about the O–O
bond direction. The NO2 molecule at the second site was found to
be rigid throughout the entire temperature range. The mobility of
NO2 at Site2 is suppressed if it is bound to thorium either through
two oxygen atoms or by one oxygen atom bound to thorium on one
side and a hydrogen bonded oxygen on the other side. On the other
hand, if NO at Site is formed from NO trapped at interstitial sites
(
1995) 673.
[
15] M. Nagata, H. Yahiro, M. Shiotani, M. Lindgren, A. Lund, Chem. Phys. Lett. 256
(1996) 27.
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[
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Berlin Heidelberg, New York, 1977.
[
[
[
[
[
2
1
3
(
non coordinated NO group) or NO adsorbed on the surface, it will
3 3
be weakly bound in the lattice and will be amenable to molecular
dynamics resulting in the observed change in the EPR spectra from
orthorhombic to axial.
[
27] M.D. Sastry, Chapter X, in: N.M. Gupta, V.B. Kartha, R.A. Rajadhyaksha (Eds.),
Spectroscopic Methods in Heterogeneous Catalysis, Tata McGraw Hill Publish-
ing Company Ltd., New Delhi, 1991, and references therein.
Acknowledgements
[
28] R.T. Weber, WINEPR SimFonia Manual, Ver. 1.2, Bruker Instruments, Inc., Bil-
lerica, MA, 1995, and references therein.
Authors wish to thank Dr. V.K. Manchanda, Head, Radiochem-
istry Division, BARC for his keen interest and encouragement during
the course of the present study. Thanks are also due to Dr. A.K.
Tyagi, Chemistry Division, BARC for providing the XRD data of the
samples.
[29] P.W. Atkins, N. Keen, M.C.R. Symons, J. Chem. Soc. (1962) 2873.
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[
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