ESR study on gold atoms produced in γ -irradiated MTHF and 10 M NaOH aqueous solutions containing Au + ions at 77 K

Article abstract of DOI:10.1016/S0009-2614(00)00770-3

MTHF and 10 M NaOH aqueous solutions containing Au⁺ ions were γ-irradiated at 77 K. ESR measurements at 73 K revealed the production and trapping of Au⁰ atoms in the solid solutions. This is the first ESR report of Au⁰ ato

Full text of DOI:10.1016/S0009-2614(00)00770-3

18 August 2000
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Chemical Physics Letters 326 2000 299–303
www.elsevier.nlrlocatercplett
ESR study on gold atoms produced in g-irradiated MTHF and 10
M NaOH aqueous solutions containing Auq ions at 77 K
Hirotomo Hase a,), Yoko Miyatake b, Yoshiki Miyamoto a
Research Reactor Institute, Kyoto UniÕersity, Kumatori-cho, Sennan-gun, Osaka, 590-0494, Japan
a
b
Department of Mechanical Engineering, Faculty of Engineering Science, Osaka, UniÕersity, Toyonaka 1-3, Osaka 560-8531, Japan
Received 19 April 2000; in final form 5 June 2000
Abstract
MTHF and 10 M NaOH aqueous solutions containing Auq ions were g-irradiated at 77 K. ESR measurements at 73 K
revealed the production and trapping of Au0 atoms in the solid solutions. This is the first ESR report of Au0 atoms which are
produced radiation–chemically by reducing Auq ions. The gJ and hyperfine splitting values of Au0 atoms were determined.
q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
In fact, a variety of experimental studies have
0
Ž
.
been carried out for silver Ag atoms, which are
produced radiation–chemically by reducing Agq ions
in aqueous and organic solid solutions at low tem-
0
Ž
.
The 1B metal atoms M are produced by radia-
tion–chemical conversion from Mq ions in solid
solutions by the following reactions:
w
x
peratures, by means of optical absorption 1,2 , ESR
Sqqey
1
w
x
w
x
S
Ž .
3–5 and fluorescence lifetime measurements 6,7 .
eyqMq M0
2
Ž .
These studies elucidated the structure of trapping
sites of Ag0 atoms and the formation mechanism of
complexes between Agq ions and ligand molecules
in the solid solutions.
where S and ey stand for solvent molecules and
mobile electrons, respectively, the latter being gener-
ated by ionization of solvent molecules. The metal
atoms thus produced are not free atoms, but are
perturbed in some degree by their environments. The
nature of the atoms which are produced by this
method and trapped in solid solutions is also ex-
pected to be different from that in rare-gas matrixes.
ESR studies were previously reported for gold
0
Ž
.
Au atoms trapped in several polar and non-polar
w x
w x
solids 8 and solid rare-gas matrixes 9 by simulta-
neous condensation at 77 K of gold and solvent
vapor. It has long been considered that spectroscopic
studies of Au0 atoms produced in a variety of aque-
ous and non-aqueous solid solutions would give
further information on the nature in common and
peculiar to 1B metal atoms as for the structure of
)
Corresponding author. Fax: q81 724 51 2472; e-mail:
hase@HL.rri.kyoto-u.ac.jp
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
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PII: S0009-2614 00 00770-3

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300
H. Hase et al.rChemical Physics Letters 326 2000 299–303
trapping sites and interaction with their environments
such as ligand molecules and Auq atoms in solid
solutions, if they could be successfully produced by
the radiation–chemical method at low temperatures.
However, such comparative studies have been unsuc-
cessful because of the instability and the insolubility
of Auq monovalent compounds in many aqueous
and non-aqueous solvents.
In this study, we used two sets of solute–solvent
Ž
.
combination: AuClP C2 H5 in 2-methyltetrahydro-
3
Ž
.
furan MTHF and AuCN in 10 M NaOH and have
successfully produced Au0 atoms radiation–chem-
ically in these solid solutions at 77 K. ESR character-
istics of Au0 atoms in the solid solutions are ob-
tained and are compared with those of Au0 atoms
which were produced by the condensation method
Fig. 1. ESR spectra of Au0 atoms produced radiation–chemically
Ž .
Ž
.
.
in MTHF A and 10 M NaOH aqueous B solutions containing
Ž
AuClP C2 H5 and AuCN, respectively. The solute concentration
3
w x
was 2=10y2 M. The g-irradiation was carried out at 77 K and
the dose was typically 60 kGy. ESR measurements were done at
73 K.
8 . This pioneer study will initiate a variety of
experimental and theoretical studies of Au0 atoms in
the solid solutions hereafter.
Ž
.
Ž
.
mT are 165.2, 433.2 for MTHF and 157.9, 406.8
2. Experimental
for 10 M NaOH aqueous solutions. The values of the
magnetic field, mutual spacing of these lines are
similar to those of the corresponding two lines out of
four lines due to Au0 atoms which are produced by
the condensation method in various matrixes 8,9 .
Thus we conclude that Auq ions are reduced by
g-radiolysis of the solid solutions to be converted to
Au0 atoms and trapped in the matrixes at 77 K. The
observed lines at H1 and H2 are attributed to the
ESR transitions Fs1, ms1 Fs2, ms2 and
Fs2, msy2 Fs2, msy1 of Au atoms,
respectively, where m is the magnetic quantum num-
ber of the total moment of Au0 atoms FsJqI in
which J and I are, respectively, the electronic and
nuclear angular momenta. The alternative interpreta-
tion of the spectra is possible, if the hyperfine split-
Ž
.
AuClP C2 H5 of the purity claimed to be more
3
than 97% and the reagent grade AuCN and NaOH
were used. Reagent grade MTHF was distilled frac-
tionally and then dried over Na and K alloy. The
w
x
Ž
.
concentration of both AuClP C2 H5 in MTHF and
3
AuCN in 10 M NaOH was 2=10y2 M. The solu-
tions were first bubbled by N2 gas for a few minutes
and transferred into ESR tubes and then cooled to 77
K by immersing them into liquid nitrogen.. The
g-irradiation of the frozen samples was carried out at
the dose rate of 30 kGy hy1 at 77 K. The total dose
was typically 60 kGy for each sample. X-band ESR
measurements were carried out at 73 K by bubbling
helium gas into liquid nitrogen in an ESR Dewar to
avoid bubbling from liquid nitrogen.
Ž
.
0
Ž
.
w
x
ting is larger than the spectrometer frequency 8,9 :
Ž
The line at HsH1 is the ‘NMR’ transition Fs1,
.
msy1 Fs2, msy2 and the line at HsH2
3. Results and discussion
Ž
is the ESR transition Fs2, msy2 Fs2, m
. w
x
sy1 8,9 . However, we ascribe the two lines
observed at H1 and H2 to the ESR transitions, since
the calculated hyperfine splitting values are always
The ESR spectra of the irradiated MTHF and 10
M NaOH aqueous solutions are shown in Fig. 1 A
Ž .
Ž .
Ž
.
and B , respectively. In each spectrum, two separate
lower than the working frequency see Table 1 . The
ESR lines at H1 and H2 are accompanied by satellite
lines which are more clearly resolved in MTHF than
Ž
Ž
.
Ž
.
lines were observed at low- H1 and high- H2 mag-
netic fields. The values of H1, H2 in the units of
.

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H. Hase et al.rChemical Physics Letters 326 2000 299–303
301
Table 1
Comparison of the values of a and gJ of Au0 atoms which are radiation–chemically produced in this study
Ž .
Ž .
a
with those for the
Ž
.
Ž
condensation method reported by Zhitnikov et al. b . The set of transitions Fs1, ms1 Fs2, ms2 and Fs2, msy2 Fs2,
.
msy1 was used for the calculation of a and gJ values by using the Breit–Rabi equation in this study.
Ž .
d a rafree
Matrix
a
gJ
Ž
.
Ž .
%
MHz
a
MTHF
5025.3"6.5
5144.7"4.0
5909.0"5.7
6105.6"3.2
4146.7"13.1
6107.1"1.0
y17.7
y15.8
y3.24
y0.02
y32.1
2.107"0.001
2.257"0.002
2.00422"0.00060
2.00306"0.00015
2.07150"0.00100
2.00412"0.00012
a
10 M-NaOH
b
C11H24
b
H2O
b
C2 H5OH
b
Free
a
This work.
w x
Ref. 8 .
b
Ž .
Ž .
10 M NaOH aqueous solution. It is plausible that the
satellites are due to Au0 atoms trapped in an envi-
ronment different from that of a majority of Au0
atoms. However, this should be further clarified.
It seems that the line observed at Hs264 mT for
MTHF solution includes the third line due to the
values of ni and Hi in Eq. 3 and Eq. 4 . The least
squares fitting calculation was used and the values of
a and gJ thus obtained are listed in Table 1 together
w x
with the values of Zhitnikov et al. 8 The relative
Ž .
shifts of the values of a, i.e. d a rafree where
Ž .
d a sayafree are also listed in Table 1.
transition Fs1, ms0 Fs2, ms1 of Au0
atoms, although its linewidth is more broadened than
expected. The corresponding line was not distin-
guished for 10 M NaOH aqueous solution according
to the large signal due to radicals of the solution. The
It is seen from Table 1 that the values of a and
Ž
.
Ž
gJ derived from the transitions Fs1, ms1
F
.
Ž
s2, ms2 and Fs2, msy2 Fs2, ms
.
y1 in this study are rather closer to those reported
for C2 H5OH than other matrixes. The large negative
shifts of a were observed for MTHF and 10 M
NaOH aqueous solutions in this study. The present
results indicate that the electronic delocalization onto
ligand molecules amounts to ca 16% for both solid
solutions and that the Van der Waals type interaction
between Au0 atoms and ligand molecules is domi-
nant.
Ž
fourth line due to the transition Fs1, msy1
.
Fs2, ms0 is expected to appear at around 340
mT, but the signal due to radicals of the solutions is
too intense for the Au0 line in this magnetic field
region to be distinguished.
w
x
From the Breit–Rabi relation 10 , the following
equations are derived for the ESR transitions at
HsH1 and H2:
As shown in Table 1, the values of gJ for MTHF
and 10 M NaOH aqueous solutions are much larger
than that of the free Au0 atoms, that is, the sign of
the deviation from the gJ value of the free atom,
D gJ , is positive. The same is true for Au0 atoms
condensed in C2 H5OH matrix in which D gJ of
1
gI bH1
2
1
1
n1 sa
n2 sa
1qx qx2
q
y
1qx
1yx
q
q
,
2 Ž
.
2 Ž
.
.
1
1
1
2
½
½
5
h
3
Ž .
1
2
w x
silver atoms is negative on the contrary 8 . When
gI bH2
1
1
2 Ž
1yx qx2
,
2
the wavefunction of a Au0 atom in its S1r2 state is
.
2 Ž
2
2
5
h
admixed into the p orbitals of the ligand molecules,
a strong spin–orbit interaction is caused so that a
shift of gJ occurs. In this case D gJ becomes nega-
4
Ž .
g yg bH
Ž
.
J
I
i
where xi s
for is1, 2 and a is the
w
x
tive 11 . Now it should be kept in mind that the
ha
hyperfine splitting value. The values of a and gJ are
values of a and gJ for Au0 atoms in this study as
w x
numerically obtained by substituting experimental
well as those reported 8,9 were calculated from the

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)
302
H. Hase et al.rChemical Physics Letters 326 2000 299–303
Table 2
0
Ž
.
Ž .
Comparison of the values of the linewidths in mT of Au atoms, which are radiation–chemically produced in this study a with those for
Ž .
the condensation method reported by Zhitnikov et al. b .
Matrix
Transition
Ž
.
Ž
.
Ž
.
Ž
.
1, 1 2, 2
1, 0 2, 1
1, -1
2, 0
2, -2 2, -1
a
MTHF
4.5
2.0
4.6
2.7
7.1
–
–
3.9
2.1
4.1
–
–
2.1
1.1
2.8
6.8
9.1
3.0
1.5
7.3
a
10 M-NaOH
b
C11H24
b
H2O
b
C2 H5OH
a
This work.
w x
Ref. 8 .
b
Breit–Rabi equation that is valid for the case of
isotropic forces of interaction of the atom with the
surrounding ligand molecules. The applicability of
the equation could be tested by comparing the values
of a and gJ calculated from various pairs of lines of
a given ESR spectrum. In this context, we cannot
assess the validity in this study, because each ESR
spectrum shows only the single reliable pair of lines.
Let it suffice to say that the positive shift of D gJ
observed for MTHF and 10 M NaOH aqueous solu-
tions in this study together with the result for
C2 H5OH by Zhitnikov is now not rare and that it
may be necessary to add an anisotropic interaction
term to the Breit–Rabi equation for such cases as the
atoms trapped in some aqueous and non-aqueous,
organic solutions.
and the ions andror solvent molecules, further stud-
ies are in progress by means of optical absorption
and fluorescence spectroscopies as well as ESR mea-
surements.
4. Conclusion
The present work provides ESR characteristics of
Au0 atoms produced radiation–chemically in MTHF
and 10 M NaOH aqueous solutions at 77 K. The
hyperfine splittings showed the negative shifts, indi-
cating that the Van der Waals interaction of the
atoms with environments is dominant. The positive
value of D gJ obtained for the solutions may bring in
the necessity of adding anisotropic term to the
Breit–Rabi equation for the atoms trapped in speci-
fied matrixes. The ESR lines of Au0 atoms in the
solid solutions investigated are broadened in the
same fashion as that for ethanol, but different from
undecane and water. The nature of the inhomogene-
ity and anisotropy of the interaction of Au0 atoms
with the environment must be further investigated.
It is seen in Table 2 that the linewidth at the high
field is broader than that at the low field both for
MTHF and 10 M NaOH aqueous solutions. This is in
a similar fashion to the result for C2 H5OH, but is in
the sharp contrast to the observation in C11H24 and
w x
H2O matrixes. As inferred by Zhitnikov et al. 8 ,
the linewidth may be determined by the inhomogene-
ity and anisotropy of the interaction of Au0 atoms
with the matrix molecules. However, there is as yet
no satisfactory explanation of the discrepancies of
line broadening observed between various matrixes.
The line broadening mechanism and the cause of
the positive shift of D gJ must be elucidated here-
after for the organic matrixes of different physico-
chemical properties. In order to shed light also on the
solvation and desolvation mechanism of Auq ions
and Au0 atoms and the interaction between the atoms
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