On the constituents of aqueous polyselenide electrolytes: A combined theoretical and Raman spectroscopic study

Article abstract of DOI:10.1021/ja9836186

The speciation of aqueous polyselenide solutions at high pH values has been investigated by Raman and UV/vis absorption spectroscopy. Complementary, ab initio molecular orbital calculations at the HF/6-31+G* level of theory have been carried out on the vibrational frequencies of polyselenide dianions Se_x^2- (x = 2-4), polyselenide radical anions Se_x^- (x = 2, 3), and protonated polyselenides HSe_x^- (x = 1-3) in order to substantiate the assignment of the experimental Raman spectra. Although the optical spectra suggest the successive formation of Se₂^2-, Se₃^2-, and Se₄^2- with progressive oxidation of the solutions, only two characteristic Raman bands are observed at 269 and 324 cm^-1. A sequence of overtones of the 269 cm^-1 band is observed after excitation with blue light. Likewise strong overtones of the 324 cm^-1 band appear after excitation with green light. The 269 cm^-1 feature is assigned to Se₄^2- while the 324 cm^-1 band is attributed to the radical anion Se₂^-. The occurrence of polyselenide radicals in aqueous solution is without precedent, and the formation of Se^2- is ascribed to a photolytic process in the electrolyte. The only protonated species observed in this study is HSe^-, which is characterized by a Raman band at 2303 cm^-1.

Full text of DOI:10.1021/ja9836186

J. Am. Chem. Soc. 1999, 121, 4461-4467
4461
On the Constituents of Aqueous Polyselenide Electrolytes: A
Combined Theoretical and Raman Spectroscopic Study
A. Goldbach,† J. Johnson, D. Meisel,‡ L. A. Curtiss, and Marie-Louise Saboungi*
Contribution from the Argonne National Laboratory, Argonne, Illinois 60439
ReceiVed October 14, 1998
Abstract: The speciation of aqueous polyselenide solutions at high pH values has been investigated by Raman
and UV/vis absorption spectroscopy. Complementary, ab initio molecular orbital calculations at the HF/6-
31+G* level of theory have been carried out on the vibrational frequencies of polyselenide dianions Sex2- (x
) 2-4), polyselenide radical anions Sex- (x ) 2, 3), and protonated polyselenides HSex- (x ) 1-3) in order
to substantiate the assignment of the experimental Raman spectra. Although the optical spectra suggest the
2-
successive formation of Se22-, Se32-, and Se4 with progressive oxidation of the solutions, only two
characteristic Raman bands are observed at 269 and 324 cm-1. A sequence of overtones of the 269 cm-1 band
is observed after excitation with blue light. Likewise strong overtones of the 324 cm-1 band appear after
excitation with green light. The 269 cm-1 feature is assigned to Se4 while the 324 cm-1 band is attributed
2-
to the radical anion Se2-. The occurrence of polyselenide radicals in aqueous solution is without precedent,
-
and the formation of Se2 is ascribed to a photolytic process in the electrolyte. The only protonated species
observed in this study is HSe-, which is characterized by a Raman band at 2303 cm-1
.
2-
Se32- + Se2- / 2Se2
(1)
(2)
Introduction
Aqueous polyselenide solutions are frequently used as
electrolytes in high-efficiency photoelectrochemical cells with
photoanodes made of cadmium chalcogenides or gallium
arsenide.1-3 Despite the technical interest, these polyselenide
electrolytes are relatively poorly characterized. Fairly compli-
cated equilibria4-7 have been reported between various proto-
nated and deprotonated polyselenides, but so far, the actual
composition of the solutions is still open to question. Lyons
and Young4 carried out the most comprehensive investigation,
2Se42- + Se2- a 3Se3
2-
These values are in fair agreement with those by Lyons and
Young,4 pK23 ) -0.3 and pK34 ) -3.5. A recent electrospray
mass spectroscopy (ESMS) study by Raymond et al.7 (i)
indicated the presence of Se5 and protonated polyselenides
that have not been taken into consideration in earlier studies
2-
2-
and (ii) found Se4 to be predominant at high pH regardless
2-
of the nature of the cation in the solution. In stark contrast to
the earlier spectrophotometric studies,4,6 Raymond et al. con-
clude that both equilibrium constants K23 and K34 are <1 for
pH >9.7 A recent 77Se NMR study on Sex2- in various solvents
by Cusick and Dance8 identified oxidized Se species with x )
but no distinction between protonated and deprotonated Sex
was made. They reported a mean oxidation state of the Se-
saturated solution equivalent to that of Se3.82- in good agreement
with earlier work,5 but their 77Se NMR experiments required
highly concentrated solutions and yielded only one broad signal
for electrolytes in different oxidation states while their ESR
results did not reveal any paramagnetic species.4 However, the
optical absorption curves and the results of potentiometric
titrations were analyzed independently assuming the formation
of four single species Sex2- with x ) 1-4 characterized by
formal redox potentials.4 Licht and Forouzan6 performed a
subsequent spectrophotometric study taking into account the
equilibrium between HSe- and Se2- and determined the
equilibrium constants, pK23 ) -0.7 and pK34 ) -4, for:
2-
3-6, but no evidence was found for the existence of Se2 in
water.
Here we use Raman spectroscopy for the structural identifica-
tion of Sex2- in aqueous solution. Lyons and Young dismissed
this optical technique in their investigation on account of the
intense color of concentrated polyselenide solutions.4 However,
the well-separated optical transitions of Sex2- with x ) 2-4 in
the visible region4,6 provide in principle a very selective
detection method which can be exploited by adjusting the
excitation wavelength to produce resonance. To consolidate the
assignment of the experimental Raman spectra, we have
performed ab initio molecular orbital calculations on the
structure and vibrational spectra of polyselenide species in
various states of oxidation and protonation.
* Corresponding author. E-mail: mls@anl.gov.
† Present address: Institute of Physical Chemistry, University of Marburg,
Marburg, Germany.
‡ Present address: Department of Chemistry, Notre Dame University,
Indiana.
(1) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am.
Chem. Soc. 1977, 99, 2839.
(2) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J. Am.
Chem. Soc. 1977, 99, 2848.
(3) Licht, S. Sol. Energy Mater. Sol. Cells 1995, 38, 305.
(4) Lyons, L. E.; Young, T. L. Austr. J. Chem. 1986, 39, 511.
(5) Zaitseva, I. G.; Greiver, T. N. J. Appl. Chem. USSR 1967, 40, 1923.
(6) Licht, S.; Forouzan, F. J. Electrochem. Soc. 1995, 142, 1546.
(7) Raymond, C. C.; Dick, D. L.; Dorhout, P. K. Inorg. Chem. 1997,
36, 2678.
Experimental Section
Preparation of Polyselenides. We used aqueous solutions of H2Se
at high pH and added calculated amounts of H2O2 to generate
polyselenide solutions of varying mean oxidation state. All solutions
were prepared in a glovebox under nitrogen atmosphere. A typical
(8) Cusick, J.; Dance, I. Polyhedron 1991, 10, 2629.
10.1021/ja9836186 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/27/1999

4462 J. Am. Chem. Soc., Vol. 121, No. 18, 1999
Goldbach et al.
Table 1. Composition of the Polyselenide Solutions Used in the
Present Studya
mean Se
0.08 M
0.10 M
2 M
oxidation equivalent H2Se soln, H2O2 soln, KOH soln, H2O,
level
polyselenide
Se2-
mL
mL
mL
mL
-2
-1
-2/3
-1/2
12.5
25
37.5
50
0
10
20
30
25
25
25
25
62.5
40
17.5
0
2-
Se2
Se3
2-
2-
Se4
a The solutions were designed to give optimum concentrations of
the respective polyselenide.
preparation is described below. Stock solutions of 2 M KOH and 0.1
M H2O2 were deoxygenated by bubbling Ar gas through them for 15-
20 min. Saturated H2Se solutions were prepared by bubbling the gas
into deoxygenated water in a septum sealed flask. At 28 °C and under
atmospheric H2Se pressure the concentration of this solution is 8.4 ×
10-2 M.9 Then, appropriate amounts of these solutions (Table 1) were
combined to obtain 0.01 M Sex2- with nominal mean selenium oxidation
states of -2, -1, -0.67, and -0.5 (i.e. x ) 1, 2, 3, 4), respectively, in
0.5 M OH- solution (pH 13.7).
Figure 1. HF/6-31+G* optimized geometries of Se3-, Se32-, and Se42-
The bond lengths of Se2- and Se22- are 2.260 and 2.415 Å, respectively.
.
For the spectroscopy measurements the polyselenide solutions were
filled under N2 atmosphere in sealed quartz cuvettes. Prior to the Raman
measurement an absorption spectrum of each solution was recorded
with a Varian Cary 5 spectrophotometer. The setup for the Raman
measurement consisted of a triple Jobin Yvon T64000 spectrometer
equipped with an optical multichannel analyzer.10 Multiple lines in the
visible range of the emission spectrum of an argon and a krypton ion
laser were used for excitation: 457.1, 476.2, 488.0, 514.5, 530.9, 568.2,
647.1, and 676.4 nm. The resolution of the instrument was set to 3
cm-1. Due to the strong coloration of the solutions, the penetration
depth of the excitation light was very small and the Raman spectra
had to be recorded in 45° backscattering geometry.
Computational Methods. Ab initio molecular orbital calculations
were performed using the Gaussian 94 computer program.11 The
Hartree-Fock (HF) method with the 6-31+G* basis set12 was used in
the determination of the structures and harmonic vibrational frequencies
of all clusters. The frequencies presented here are scaled by the factor
0.893.13
-
-
Figure 2. HF/6-31+G* optimized geometries of HSe2 and HSe3
.
The bond lengths of HSe- is 1.475 Å.
Theoretical Frequencies
2-
Optimized geometries of polyselenides Sex- (x ) 2,3), Sex
experimental values: 329 and 328.5 cm-1 for Se2 implanted
in zeolite Y10,17 and KI,15 respectively.
-
-
(x ) 2-4), and HSex (x ) 2, 3) are shown in Figures 1 and
2 while their relative stabilities and vibrational frequencies are
The triselenide radical anion Se3- has been studied in a couple
of earlier computational investigations.14,18 In agreement with
grouped in Tables 2 and 3. We are aware of only one earlier
-
2
Heinemann et al.14 we find a C2V geometry and a B1 ground
theoretical investigation dealing with the radical anion Se2
.
Heinemann et al.14 calculated the structure of the X 2Πg ground
state at the MRCI and the CCSD(T) level of approximation and
reported bond distances of 2.306 (MRCI) and 2.290 Å (CCSD-
(T)), slightly larger than our value of 2.260 Å which is in good
agreement with the experimental bond distances deduced from
photoluminescence spectra of Se2- implanted into KI (r ) 2.25
Å)15 and sodalite (r ) 2.26 Å).16 The scaled HF/6-31+G*
vibrational frequency of 326 cm-1 compares favorably with
state with a bond distance of 2.256 Å, slightly shorter than the
CCSD(T) value of 2.284 Å14 and a bond angle of 112.4° close
to their value of 113.5°. Basch performed complete active space
multi-configuration self-consistent field (CAS-MCSCF) ab initio
calculations18 which yielded two low-lying states of C2V
2
geometry with the B2 state (r ) 2.35 Å, R ) 115.8°) favored
2
by 16.6 kcal/mol over the B1 state (r ) 2.39 Å, R ) 84.1°).
At the HF/6-31G* level, the 2B2 state (r ) 2.333 Å, R ) 83.6°)
2
lies 12.4 kcal/mol above the B1 state. No experimental data
(9) Fogg, P. G. T.; Young, C. L., Eds. IUPAC Solubility Data Series,
Vol. 32, Hydrogen Sulfide, Deuterium Sulfide, and Hydrogen Selenide;
Pergamon Press: Oxford, 1988.
(10) Goldbach, A.; Iton, L.; Grimsditch, M.; Saboungi, M.-L. J. Am.
Chem. Soc. 1996, 118, 2004.
(11) Frisch, M. J. et al. GAUSSIAN 94; Gaussian Inc.: Pittsburgh, PA,
1995.
(12) Binning, R. C., Jr.; Curtiss, L. A. J. Comput. Chem. 1990, 11, 1206.
(13) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley,
J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J.
Quantum Chem. Symp. 1981, 15, 269.
are available on the geometry of Se3-, but a Raman band has
-
2-
been reported at 405 cm-1 for Se3 generated from Se6 in
acetone solution.19 This frequency is notably larger than the
highest value of the three scaled HF/6-31+G* vibrational modes
of the B1 state (107, 235, and 293 cm-1) and different from
any of the B2 state vibrational modes at 152, 267, and 471
2
2
cm-1
.
(17) Goldbach, A.; Grimsditch, M.; Iton, L.; Saboungi, M.-L. J. Phys.
Chem. B 1997, 101, 330.
(18) Basch, H. Chem. Phys. Lett. 1989, 157, 129.
(19) Teller, R. G.; Krause, L. J.; Haushalter, R. C. Inorg. Chem. 1983,
22, 1809.
(14) Heinemann, C.; Koch, W.; Lindner, G.-G.; Reinen, D.; Widmark,
P.-O. Phys. ReV. A 1996, 54, 1979.
(15) Rolfe, J. J. Chem. Phys. 1968, 49, 4193.
(16) G.-G. Lindner, Ph.D. Thesis, Marburg, 1993.

Aqueous Polyselenide Electrolytes
J. Am. Chem. Soc., Vol. 121, No. 18, 1999 4463
Table 2. Calculated HF/6-31+G(d) Frequencies for Sex- and Sex2-
assess the influence of the aqueous environment, the complex
of the diselenide dianion with one water molecule was studied
and found to have a C2V structure with the two hydrogen atoms
interacting with the two selenium atoms at a Se-H distance of
3.34 Å. Changes in the Se-Se bond length and the stretching
frequencies were negligible.
We have here considered a C2V and two ring geometries for
the triselenide dianion Se32-. Both ring variants (D3h and one
with a double bond) are approximately 60 kcal/mol higher in
energy than the open C2V structure and can be thus dismissed.
The calculated bond distance for Se32-(C2V) is 2.369 Å, which
is in good agreement with the experimental values for the bulk
triselenides K2Se3 (2.382 Å), Rb2Se3 (2.383 Å), and Cs2Se3
(2.358 Å).22 However, the calculated angle of Se32-(C2V) of
110.9° is considerably less compressed than in the condensed
phases: K2Se3 (102.5°), Rb2Se3 (103.1°), and Cs2Se3 (103.6°).22
All three computed vibrational modes (scaled: 92 cm-1, 247
cm-1, 271 cm-1) of the C2V geometry are Raman active with
the two high-frequency vibrations forming a characteristic
double band of equal intensity.
Anions
relative
energya
ωcalc
raman
activity
IR
activity
(cm-1
)
-
Se2
365
120
263
328
171
300
530
273
103
276
303
50
s
s
vs
vs
s
Se3- (2B1)
0.0
w
vs
w
vw
vw
vs
Se3- (2B2)
12.4
s
vs
m
m
s
2-
Se2
Se32- (C2V)
ia
ia
w
m
w
m
w
s
m
m
m
ia
s
s
Se42- (C2)
0.0
1.3
m
m
w
s
93
115
277
293
312
32
s
m
ia
ia
m
m
ia
ia
Se42- (trans)
67
Two HF/6-31+G* structures of Se42- are separated by about
1 kcal/mol, a C2 conformer being energetically slightly more
favorable than the C2h trans conformer. The cis structure is more
than 12 kcal/mol higher in energy than the C2 conformer and it
is only a saddle point on the potential energy surface as indicated
by an imaginary frequency. Most notably for the C2 structure
the terminal bonds are slightly longer (rt ) 2.352 Å) than the
center distance (rc ) 2.340 Å). The bond angle R of this
conformer comes to 109.7° and the dihedral angle γ amounts
to 98.4°. These geometrical parameters compare reasonably with
a Se42-(C2) molecule identified in the ethylenediamine (en)
stabilized barium salt [Ba-4en][Se4]: rt ) 2.329 Å, rc ) 2.342
Å, R ) 112.1°, and γ ) 98.4°.23 Experimental vibrational
spectra of this compound have not been measured, but the
theoretical calculations indicate three fairly intense Raman bands
between 246 and 278 cm-1 (Table 2). A crystal structure
106
299
303
305
55i
ia
ia
Se42- (cis)
12.6
51
134
246
304
305
a Relative energy for different species with more than one structure
reported. In kcal/mol.
Table 3. Calculated HF/6-31+G(d) Frequencies for HSex- Anions
rel
ωcalc
raman
activity
IR
activity
energya
(cm-1
)
HSe-
HSe2
2474
293
768
2514
85
-
s
w
m
s
2-
containing the trans-Se4 molecule has been synthesized
w
vs
w
w
s
yielding24 rt ) 2.312 Å, rc ) 2.397 Å, and R ) 103.4° in good
agreement with the calculated values for the Se42-(C2h) iso-
mer: rt ) 2.350 Å, rc ) 2.382 Å, and R ) 102.3°. While
experimental data on the vibrational spectra are not available,
our calculations indicate a single high-frequency Raman band
around 267 cm-1 for this highly symmetric structure.
HSe3- (trans)
3.0
0.0
m
w
m
w
m
vs
w
m
m
m
m
s
93
292
300
767
2528
103
222
291
300
785
2544
s
m
vs
w
w
s
m
w
vs
HSe3- (gauche)
No structural information is available on protonated polyse-
lenides HSex-, x >1, but our calculations indicate that the
bonding of a hydrogen atom to a terminal Se atom is hardly
affected by the length of the chain. The H-Se bond length is
virtually constant for all structures considered (Figure 2), and
accordingly the frequencies for the stretching mode vary by less
than 3% from 2209 cm-1 for HSe- to 2265 cm-1 for the gauche
a Relative energy for different species with more than one structure
reported. In kcal/mol.
-
form of HSe3 and are smaller than the stretching modes of
H2Se in the gas phase: ν1 ) 2345 cm-1 and ν3 ) 2358 cm-1 25
.
We are not aware of any theoretical studies of polyselenide
dianions, but experimental structural parameters are available
for various bulk compounds featuring polyselenide chains Sex2-
in the range 2 e x e 6. Our calculated Se22- bond distance of
2.415 Å agrees well with the values reported for Na2Se2 (2.43
Å)20 and K2Se2 (2.40 Å)20 while the calculated frequency of
244 cm-1 is slightly smaller than the measured value of 253
The stretching mode should be dominant in the Raman spectrum
of any HSex- species (Table 3). A second characteristic Raman
active mode of the higher polyselenides HSex-, x > 1, should
appear around 700 cm-1 (Table 3). The calculated vibrational
-
spectra of the two HSe3 geometries considered here are
virtually identical, the gauche form being more stable than the
trans isomer by approximately 3 kcal/mol.
cm-1 for the bulk K2Se2 but in remarkable agreement with
20
Se2 confined in the channels of the zeolite cancrinite.21 To
2-
(22) Bo¨ttcher, P. Z. Anorg. Allg. Chem. 1980, 461, 13.
(23) Ko¨nig, T.; Eisenmann, B.; Scha¨fer, H. Z. Naturforsch. 1982, 37b,
1245.
(24) Brese, N. E.; Randall, C. R.; Ibers, J. A. Inorg. Chem. 1988, 27,
940.
(20) Bo¨ttcher, P.; Getzschmann, J.; Keller, R. Z. Anorg. Allg. Chem. 1993,
619, 476.
(21) Lindner, G.-G.; Hoffman, K.; Witke, K.; Reinen, D.; Heinemann,
C.; Koch, W. J. Solid State Chem. 1996, 126, 50.
(25) Palik, E. D. J. Mol. Spectrosc. 1959, 3, 259.

4464 J. Am. Chem. Soc., Vol. 121, No. 18, 1999
Goldbach et al.
Figure 3. UV/vis spectra of the polyselenide solutions with the degree
of oxidation increasing from left to right. The spectra are dominated
by the 430 nm band of Se22- (left), the 330, 380, and 530 nm transitions
of Se33- (middle), and the two bands of Se42- at 380 and 470 nm (right),
respectively.
Figure 4. Raman spectrum of a polyselenide solution with mean Se
oxidation state -0.67, i.e., Se32- is predominant. At an excitation energy
of 530 nm three overtones of the 324 cm-1 band are observed at 645,
965, and 1287 cm-1
.
Experimental Results
Figure 3 shows optoelectronic spectra of typical polyselenide
solutions used in this investigation. From left to right the mean
oxidation state of the solutions increases. At first as expected,
the UV/vis spectrum is dominated by the 430 nm band of the
diselenide dianion Se22- (left).4,6 With increasing oxidation level
2-
the three bands attributed to Se3 become prominent at 330,
380, and 530 nm (middle).4,6 Finally, the highest oxidation level
bands at 380 and 470 nm which have been previously assigned
to Se4 (right)4,6 prevail. Thus, by scanning the Raman
2-
excitation energy through the visible range of the spectrum, we
can resonantly excite the triselenide dianion with green light
(530 nm band) and the tetraselenide dianion with blue light (470
nm band). Unfortunately, we have not been able to excite the
diselenide dianion (430 nm band) in the same way in the UV
range near 400 nm since the laser emission lines at 407 and
413 nm of the Ar ion laser could not be sufficiently stabilized.
Figure 5. Raman spectrum of a polyselenide solution with mean Se
oxidation state -0.5, i.e., Se42- is predominant. At an excitation energy
of 476 nm two broad overtones of the 269 cm-1 band are observed at
Raman measurements on species in solution face the par-
ticular problem of the overriding light scattering by the solvent.
Due to the hindered motion of molecules in the liquid, the
translational and rotational degrees of freedom of gas-phase
molecules become restricted translations and librational modes,
respectively, which contribute to the Raman spectra. In liquid
water very intense Raman bands related to the restricted
translations are centered in the vicinity of 65 and 160 cm-1
and the librational modes cause very broad Raman scattering
538 and 810 cm-1
.
overtones of the 324 cm-1 band. The markedly different
broadening of the overtones is especially noteworthy. Since
natural selenium consists of six isotopes with the fraction of
the most abundant 80Se amounting to only 0.50, the energy
spread of vibrational levels due to isotopic substitution of the
polyselenides increases rapidly with the vibrational excitation
giving rise to the observed broadening of the overtones. In
addition, this energy spread increases with the number of
selenium atoms in the polyselenide species. The broader
overtones of the 269 cm-1 band compared to the 324 cm-1 band
indicate a larger number of selenium atoms in the responsible
polyselenide species: the 269 cm-1 band originates from a
species that includes at least three Se atoms.
between 300 and 900 cm-1 26
. The intrinsic modes of the water
molecules give rise to broad bands around 1600 cm-1 (bending
vibrations) and between 3000 and 3800 cm-1 (stretching
vibrations).26 In practice, this means that Raman bands of the
solute unless very intense are completely hidden below 200
cm-1 and in the regions of the intrinsic water Raman bands.
Figures 4 and 5 show Raman spectra of polyselenide solutions
excited at 530.9 and 476.2 nm, respectively; both spectra include
bands at 269, 324, 645, 965, and 1287 cm-1 which are not due
to water scattering. In Figure 5, the strong fluorescence above
800 cm-1 is probably related to the formation of colloidal
selenium at the highest levels of oxidation; the two additional
relatively broad bands at 538 and 810 cm-1 are not due to water
scattering and can be readily attributed to overtones of the 269
cm-1 band. Likewise, the 645, 965, and 1287 cm-1 bands are
Another remarkable feature in Figure 4 is the well-defined
band at 2303 cm-1 close to the gas-phase stretching frequencies
of H2Se (ν1 ) 2345 cm-1, ν3 ) 2358 cm-1 25
) and the calculated
-
H-Se stretching modes of HSex species (Table 3). For
clarification, we measured Raman spectra of H2Se in aqueous
solution and in a highly basic solution (Figure 6). While the
aqueous solution becomes acidic upon dissolution of H2Se (pKa1
) 3.8),27-29 even at pH 14 HSe- still exists (pKa2 ) 15.0).28,29
(26) Carey, D. M.; Korenowski, G. M. J. Chem. Phys. 1998, 108, 2669.
(27) Hagisawa, H. Abstr. Rikwagaku-Kenkyu-Ih 1941, 20, 30.

Aqueous Polyselenide Electrolytes
J. Am. Chem. Soc., Vol. 121, No. 18, 1999 4465
Figure 6. Raman spectra of aqueous solutions of H2Se (pH 4) and
HSe- (pH 12.3). While one broad stretching mode of H2Se is found at
2330 cm-1, the corresponding vibrational mode of HSe- is slightly
red-shifted to 2303 cm-1
.
Figure 8. Raman spectra of polyselenide solutions with varying mean
oxidation state at excitation energies 476, 530, and 647 nm. The bottom
traces result from H2Se solutions with no H2O2 added, i.e., Se oxidation
state -2. The remaining traces from bottom to top correspond to
solutions with a mean Se oxidation state of -1, -0.67, and -0.5,
respectively.
Below 500 nm, the intensity of this band increases at a faster
pace and the first overtone becomes visible at 538 cm-1 with a
maximum in intensity at 457.1 nm. Even though the excitation
band for the resonant amplification of the 269 cm-1 band is
not fully determined, it is clear that the corresponding polyse-
lenide is resonantly excited in the blue region of the spectrum.
In contrast, the 324 cm-1 band and its first overtone at 645
cm-1 becomes weakly perceptible only at 568 nm, reaches a
maximum intensity around 530 nm, and then fades when the
excitation energy is shifted to the blue. Therefore, the second
polyselenide has an absorption band in the green region of the
spectrum with a maximum around 530 nm.
Figure 7. Raman spectra of a polyselenide solution with mean Se
oxidation state -0.67 at various excitation energies. The order of the
excitation energies from the bottom (676 nm) to the top trace (457
nm) is indicated. The 269 and 324 cm-1 bands are resonantly enhanced
in the blue and green regions of the spectrum, respectively.
Results shown in Figure 3 provide additional support to the
2-
earlier spectrophotometric studies suggesting that Se22-, Se3
,
According to Figure 6, H2Se in aqueous solution is characterized
by a Raman band at 2330 cm-1 whereas the HSe- band shifts
to 2303 cm-1 in perfect agreement with the Raman band
displayed in Figure 4.
The bands at 269 and 324 cm-1 originate from two different
polyselenide species, which can be resonantly excited in
different regions of the visible range of the spectrum. To outline
the excitation profiles of both species, we repeated the measure-
ments at all excitation energies provided by the argon and
krypton ion lasers. The corresponding low-frequency-range
spectra are assembled in Figure 7. The band around 269 cm-1
is evident at all excitation energies and its intensity increases
with the frequency of the excitation radiation above 500 nm.
and Se42- are successively generated as majority polyselenides
in solution by the gradual additions of the oxidizing agent.
Figure 8 shows Raman spectra of these polyselenide solutions
at increasing oxidation levels recorded at 647.1, 530.9, and 476.2
nm, respectively. Unfortunately, even in the absence of H2O2,
our selenide solutions have already a weak yellow color
indicating the formation of traces of polyselenides generated
by residual oxygen in the starting solution. Despite this, no
polyselenide Raman band is observed in the starting solution
at 647.1 nm excitation (Figure 8), but the 324 cm-1 band
is already noticeable at 530.9 and 476.2 nm. Upon addition
of a small amount of H2O2, a shoulder emerges at 269 cm-1
at 647.1 nm excitation and the 324 cm-1 band gains intensity
at 530.9 and 476.2 nm. Further addition of H2O2 leads to
(28) Wood, R. H. J. Am. Chem. Soc. 1958, 80, 1559.
(29) Levy, D. E.; Myers, R. J. J. Phys. Chem. 1990, 94, 7842.

4466 J. Am. Chem. Soc., Vol. 121, No. 18, 1999
Goldbach et al.
continuous enhancement of both bands except that the 324 cm-1
band is noticeably absent from the spectra excited with red light.
The absence of this band at 647.1 nm excitation even at the
highest oxidation levels implies a very low concentration of
the responsible polyselenide relative to that responsible for the
269 cm-1 band. On the other hand, the simultaneous growth of
both bands upon progressive oxidation suggests an interactive
link between the two species.
of HSe- was deduced from titration curves, while the more
recent studies identified it by its UV spectrum. The 2303 cm-1
Raman band (Figure 7) is the first structural manifestation of
HSe- in aqueous solution. Among the polyselenides only the
dissociation constants of H2Se2 are known:32 Ka1 ) 2 × 10-2
mol L-1 and Ka2 ) 5 × 10-10 mol L-1. The corresponding
dissociation constants of H2Se3 and larger hydrogenated poly-
selenides are expected to be in the same range or even larger.
It is reasonable to assume then that only fully ionized polyse-
lenides will exist at pH 13.7. Notwithstanding, Raymond et al.7
Discussion
-
observed small amounts of HSe3 by mass spectroscopy in
The assignment of the two overtone sequences starting with
the 269 and the 324 cm-1 bands is straightforward from the
evidence obtained here. The band at 269 cm-1 could not be
aqueous solutions, particularly in the presence of NH4+: this
is not in contradiction with the above assumption since the
ammonium ions neutralize some of the base and lead to the
formation of a buffered solution at pH 10. At such low pH
values, the polyselenide equilibria may still sustain noticeable
concentrations of the hydrogenated forms. However, since we
observe only a single Raman band in the vicinity of 2303 cm-1
that originates from HSe- it should be emphasized that the
hydrogenated polyselenides do not play a significant role in the
electrolytes investigated here.
2-
-
attributed to either Se2 or HSe2 because of the overtone
broadening mentioned above. The triselenide Se32- is an unlikely
2-
candidate since the only probable Se3 geometry C2V is
expected to give rise to two Raman bands of fairly equal
intensities near 250 and 270 cm-1 (Table 2). In addition, one
expects the maximum of the excitation profile of Se32- around
530 nm following from the reported absorption spectra4,6 while
the 269 cm-1 band is resonantly enhanced in the blue region of
the spectrum (Figure 7). Similarly to Se32- both conformers of
2-
It is unexpected that Se4 is the only polyselenide dianion
clearly identified from our Raman results since according to
the absorption spectra (Figure 3) Se22- and Se32- should be the
predominant polyselenides.4,6 However, Raymond et al.7 indicate
a predominance of Se42- for pH >9 particularly in the presence
of K+ cations. In fact, the mass spectrum obtained for K2Se4
dissolved in 0.1 M KOH shows the tetraselenide as the only
major component in this electrolyte, whereas the dissolution of
-
HSe3 are expected to exhibit two Raman bands between 260
and 270 cm-1 (Table 3) which is not the case here. The
occurrence of high concentrations of protonated polyselenides
is rather unlikely considering the pKa values of H2Se and
2-
H2Se2 (vide infra). These considerations lead to Se4 as the
only deprotonated polyselenide species under consideration. The
C2 conformer is expected to have two or three Raman active
modes between 250 and 280 cm-1 (Table 2). On the other hand,
the computed trans isomer has only one Raman band at 267
cm-1, which is in excellent agreement with the 269 cm-1
band: this assignment is supported by the excitation profile
mapped out in Figure 5, which indicates a nonlinear enhance-
ment in the blue region of the spectrum. The absorption band
of the Se42- dianion centered around 470 nm is well established
from earlier spectrophotometric studies.4,6
2-
Na2Se4 in 0.1 M NaOH results in significant amounts of Se3
and Se52- besides the predominant Se42-.7 Thus the preeminence
of the latter dianion in polyselenide electrolytes containing KOH
is not without precedent. It should be noted that the Raman
spectra do not completely rule out the occurrence of the di-
and triselenide dianions in these solutions. The calculated
vibrational spectra indicate that most of the Raman bands of
the polyselenides considered fall into a narrow range between
240 and 270 cm-1. Some of these polyselenides may have
overlapping vibrational bands and be difficult to distinguish.
The intense scattering due to the restricted translation of the
solvent (water) complicates the detection of the solute bands
and may simply blur minute differences in band positions.
The excitation profile of the 324 cm-1 band (Figure 7)
2- 4,6
matches the lowest energy absorption band attributed to Se3
.
However, this vibrational frequency is much too high to
originate from any polyselenide dianions or protonated poly-
selenides listed in Tables 2 and 3. But it is well in the range of
frequencies observed for the Se2- radical anion in various solid-
state matrices.10,15-17,30 This is rather unexpected since the earlier
ESR studies4 did not reveal any paramagnetic species in the
polyselenide electrolytes. Nevertheless, the lowest lying elec-
2-
Furthermore, the Raman spectroscopic identification of Se4
in these electrolytes does not merely rely upon the coincidence
between the experimental and the calculated position of the
Raman band of the Se42- trans isomer. It is both the resonance
enhancement below 500 nm (Figure 7) and the large energy
spread of the Vibrational oVertones due to isotopically substi-
-
tronic transition of Se2 in solid-state matrices occurs around
tuted molecules (Figure 5) that are piVotal in our assignment
500 nm as well.15,16,30,31 The actual center of the absorption band
depends on the host matrix and is 515 nm in KI,15,31 for instance.
Therefore, the 324 cm-1 band is assigned as the sole vibrational
mode of this radical anion.
2-
of the 269 cm-1 band to Se4
.
-
The identification of the Se2 radical anion in aqueous
solution establishes a new component of polyselenide electro-
lytes. So far attempts to trace polyselenide radicals in an aqueous
environment have not been successful,4 although such radicals
are suspected to play an important role in photoelectrochemical
cells utilizing polyselenide electrolytes. The redox reactions at
the surfaces of electrodes in such cells are likely to be one-
electron processes involving intermediate paramagnetic species.
The one-electron oxidation products of selenide ions, the Se-,
Protonated species were never considered to play a major
role in basic polyselenide solutions. Recently, Levy and Myers29
examined the dissociation of H2Se in water by UV spectroscopy
and established Ka2 ) (8.8 ( 0.4) × 10-16 in agreement with
the value of 10-(15.0 ( 0.6) by Wood28 and much smaller than
the value of 10-11 reported by Hagisawa.27 Thus, in aqueous
environment, fully reduced Se will mostly exist as HSe- anion
and not as naked Se2- dianion. In the early studies, the formation
2-
its acid form HSe, and its complexed form HSe2 have been
identified by pulse radiolysis.32 Nevertheless, experiments show
that the formation of polyselenides in the electrolyte is
(30) Clark, R. J. H.; Dines, T. J.; Kurmoo, M. Inorg. Chem. 1983, 22,
2766.
(31) Murata, H.; Kishigami, T.; Kato, R. J. Phys. Soc. Jpn. 1990, 59,
506.
(32) Scho¨nesho¨fer, M.; Karmann, W.; Henglein, A. Int. J. Radiat. Phys.
Chem. 1969, 1, 407.

Aqueous Polyselenide Electrolytes
J. Am. Chem. Soc., Vol. 121, No. 18, 1999 4467
prerequisite for sustained photocurrents in the above-described
electrochemical cells.1 Therefore paramagnetic polyselenide
species may be crucial in these electrode reactions. The
(x ) 2, 3), and protonated polyselenides HSex- (x ) 1-3) have
been calculated using ab initio molecular orbital theory at the
HF/6-31+G* level of theory. The computed geometries and
vibrational spectra of Sex2- and Sex- molecules are in satisfac-
tory agreement with the available experimental data. In par-
ticular, the vibrational frequencies of the Sex2- dianions do not
-
occurrence of the Se2 in aqueous solution is not completely
-
-
unexpected since the complementary S2 and S3 radicals are
known to form in aqueous electrolytes at elevated tempera-
tures.33 Also, it should be pointed out that in nonaqueous polar
solvents such as dimethylformanide (DMF), radical species
(e.g., Se2-, Se3-) have been postulated to exist in high
concentrations.34 Because the fundamental 324 cm-1 mode is
-
-
exceed 300 cm-1 while the radical anions Se2 and Se3 have
higher characteristic modes. The computed Raman spectra of
all protonated polyselenides have an intense H-Se stretching
-
vibration around 2250 cm-1. The characteristic band of HSex
-
always accompanied by overtones, the Se2 bands are due to
molecules with x g 2 lies in the vicinity of 700 cm-1
.
resonance enhancement and the actual concentration of the
radicals may be many orders of magnitude smaller than that of
the dianions Se42-. It is surprising that the radicals have not
been previously found by ESR spectroscopy4 which is an
extremely sensitive technique for paramagnetic species unless
The UV/vis absorption spectra of polyselenide solutions at
various mean oxidation levels are in good agreement with optical
spectra published by other groups.4,6 However, we have
observed only two Raman bands at 269 and 324 cm-1 which
can be ascribed to polyselenide species. The low-frequency
mode is assigned to Se42- on account of its resonance enhance-
ment below 500 nm (Figure 5) and the large energy spread of
the vibrational overtones which is the result of the large number
of isotopically different molecules. The high-frequency band
does not correspond to any calculated or observed experimental
vibrational mode of a polyselenide dianion but is in excellent
-
Se2 forms in situ during the Raman measurements. Because
the polyselenides Sex2- are all strongly absorbing in the visible
region of the spectrum, it is possible that the intense laser
radiation used for excitation breaks up one of these dianions to
form the radical. Since Se2- is detected even in the least oxidized
selenide solutions (Figure 8) any polyselenide dianion has to
be taken into consideration for a parent molecule:
-
agreement with the theoretical and measured values of the Se2
stretching vibration. This assignment is corroborated by the
excitation profile, which indicates an absorption band in the
green region of the visible spectrum, where the lowest lying
optical transition is known to occur. The lack of Raman
spectroscopic evidence in the present work for the previously
postulated4,6 Se22- and Se32- is consistent with the conclusions
reached from a mass spectroscopic investigation7 showing that
Se22- + hν f Se2- + e-(aq)
Se32- + hν f Se2- + Se-
(3)
(4)
(5)
-
Se42- + hν f 2Se2
2-
Se4 is the prevalent species at high pH (>9) and especially
in the presence of K+ cations, i.e., the conditions under which
the polyselenides were generated here. Also, the initially present
triselenide dianions may have been removed by a photolytically
induced decomposition leading to the formation of the para-
Se22- is the most improbable progenitor because the formation
of the radical dianion would require the release of an electron
(eq 3). Such a photoionization is highly unlikely in an aqueous
environment. In addition, the lowest energy absorption band of
Se22- is found in the violet around 430 nm.4,6 The formation of
Se2- from Se22- under 568.2 nm irradiation will hardly be very
efficient even if the onset of the 430 nm transition extends that
far into the yellow. The formation of Se2- from Se32- seems a
more plausible route (eq 4), especially when the 530 nm
absorption band attributed to this polyselenide is taken into
account. It involves the formation of the Se- radical ion which
has been previously obtained in aqueous solution by pulse
radiolysis.32 The latter is not a very stable species and dimerizes
-
magnetic Se2 anion.
The Raman spectroscopic identification of the Se2 radical
-
anion in aqueous solution is significant for the understanding
of the (electrode) processes that occur in photoelectrochemical
cells with polyselenide electrolytes. Previous attempts to locate
it in aqueous solution have been unsuccessful4 and the exclusive
observation of the 324 cm-1 band in combination with its
overtones after excitation in the (blue-)green spectral range
suggests a very low actual concentration of this radical anion.
2-
It may form either under the intense laser radiation from Se4
very fast to form Se2 .
2- 32 Likewise the Se2- radicals are prone
or from Se32-. Finally, it has been established by Raman
spectroscopy that protonated polyselenides apart from HSe- do
not play a significant role in aqueous solutions at pH 13.
to dimerization to form the omnipresent tetraselenide dianion.
As such this scenario is quite attractive since it neatly accounts
for the seemingly absent Se32-. However, at this stage the most
Furthermore, the observation of the Raman band at 2303 cm-1
,
probable precursor for Se2- appears to be the tetraselenide Se4
.
2-
even at the highest pH values in aqueous solution, lends further
support to the existence of HSe- as already suggested from the
earlier UV-spectroscopic study.29
It is the only clearly identified dianion here and its 470 nm
absorption band extends far enough into the green region of
the spectrum to support a photolytic decomposition. The
formation of Se2- can be simply achieved by symmetrical bond
cleavage of the tetraselenide with no need to invoke another
energetically less favorable species (eq 5).
Acknowledgment. This work was supported by the U.S.
Department of Energy, Division of Materials Sciences and
Division of Chemical Sciences, Office of Basic Energy Sciences,
under Contract No. W-31-109-ENG-38. A. Goldbach gratefully
acknowledges support from the Deutsche Forschungsgemein-
schaft and thanks Dr. M. Grimsditch for generous access to the
Raman spectrometer and valuable advice during the collection
of the Raman spectra.
Conclusion
The structures and vibrational frequencies of polyselenide
dianions Sex (x ) 2-4), polyselenide radical anions Sex
2-
-
(33) Giggenbach, W. F. Inorg. Chem. 1974, 13, 1724.
(34) Kanatzidis M. G.; Huang S. P. Coord. Chem. ReV. 1994, 1-2, 509.
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