Laser-flash photolysis of naphthyl diselenides; Reactivities of naphthylseleno radicals

Article abstract of DOI:10.1002/(SICI)1097-4601(1998)30:3<193::AID-KIN4>3.0.CO;2-N

Transient absorption spectra of 1-naphthylseleno (1-NaphSe^·), and 2-naphthylseleno (2-NaphSe^·) radicals, which are generated by laser-flash photolysis of the corresponding diselenides, were observed. The reactions of 1-NaphSe^·, and 2-NaphSe^· with 2-methyl-1,3-butadiene and α-methylstyrene were investigated by following the decay rates of these seleno radicals. By both steady-state and laser-flash photolysis, it is proved that these seleno radicals add to alkenes in a reversible manner. The reaction rate constants for such reversible addition reactions were determined by conducting the reaction in the presence of O₂, which traps selectively the carbon-centered radicals formed by the addition reaction of the seleno radicals to the alkenes. The reactivity of 2-NaphSe^· is higher than that of 1-NaphSe^·, both of which are less reactive than PhSe^·. These reactivities were interpreted with the properties of SOMO calculated by MO method.

Full text of DOI:10.1002/(SICI)1097-4601(1998)30:3<193::AID-KIN4>3.0.CO;2-N

Laser-Flash Photolysis
of Naphthyl Diselenides;
Reactivities of
Naphthylseleno Radicals
MAKSUDUL M. ALAM,1 OSAMU ITO,1 YOSHINORI KOGA,2 AKIHIKO OUCHI2
1Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai, 980-77, Japan
2National Institute of Materials and Chemical Research, AIST, MITI, Tsukuba, Ibaraki 305, Japan
Received 18 December 1996; accepted 21 April 1997
ABSTRACT: Transient absorption spectra of 1-naphthylseleno (1-NaphSeи), and 2-naphthyl-
seleno (2-NaphSeи) radicals, which are generated by laser-flash photolysis of the correspond-
ing diselenides, were observed. The reactions of 1-NaphSeи, and 2-NaphSeи with 2-methyl-
1,3-butadiene and ␣-methylstyrene were investigated by following the decay rates of these
seleno radicals. By both steady-state and laser-flash photolysis, it is proved that these seleno
radicals add to alkenes in a reversible manner. The reaction rate constants for such reversible
addition reactions were determined by conducting the reaction in the presence of O2, which
traps selectively the carbon-centered radicals formed by the addition reaction of the seleno
radicals to the alkenes. The reactivity of 2-NaphSeи is higher than that of 1-NaphSeи, both of
which are less reactive than PhSeи. These reactivities were interpreted with the properties of
SOMO calculated by MO method. ᭧ 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 193–200, 1998.
INTRODUCTION
tolysis studies have been reported on the selenium-
centered radicals [13–15]. One of the important prop-
erties of the selenium-centered radicals is their reac-
tivities to alkenes; reversible addition process has been
reported by observing cis-trans isomerization of stil-
bene [16,17]. Therefore, it is interesting to observe
such reversibility in direct kinetic study such as laser-
flash photolysis method. Comparison of the reactivi-
ties of the selenium-centered radicals with correspond-
ing sulfur analogues is also important for the better
understanding of the selenium-centered radicals. For
the comparison of different radicals, it is essential to
determine the absolute rate constants of the radicals in
a common reaction. Such rate constants of thio radi-
cals in the addition reaction to alkenes, in which the
addition reaction proceeded in a reversible fashion,
have been directly obtained by the application of flash
photolysis [18–20].
Organic seleno radicals have been recognized as im-
portant intermediates in the photochemistry of organic
selenium-compounds [1,2]. In general, flash-photoly-
sis studies are useful to obtain direct information about
the properties of the free radicals [3–6]. Among the
chalcogen-atom centered radicals, the organic thio
radicals have been studied extensively by the flash
(laser)-photolysis [7–12]. However, in spite of the im-
portance of the organic selenium-centered radicals in
the various fields of chemistry, only a few flash pho-
Correspondence to: O. Ito
Contract grant Sponsor: Grant-in-Aid on Priority-Area-Re-
search on “Photoreaction Dynamics” from the Ministry of Educa-
tion, Science, Sports and Culture
Contract grant number: (No. 08218207)
᭧ 1998 John Wiley & Sons, Inc
CCC 0538-8066/98/030193-08
In this study, we report the formation of 1-

194
ALAM ET AL.
naphthylseleno radical (1-NaphSeи), 2-naphthylse-
leno radical (2-NaphSeи) by a nano-second laser pho-
tolysis of the corresponding diselenides. By following
the decay behavior of these seleno radicals, we deter-
mined the absolute rate constants of these radicals in
the addition reaction to alkenes. We also performed
some MO calculations to confirm the electronic prop-
erties of these free radicals.
RESULTS AND DISCUSSION
Transient Absorption Spectra
The steady-state absorption spectra of (NaphSe)2 and
(PhSe)2 are shown in Figure 1. The absorption peaks
around 300 nm of (NaphSe)2 are ascribed to the naph-
thalene moiety. The absorption tails extending from
about 350 to 400 nm are attributed to Se9Se moiety.
For phenyl diselenide (PhSe)2, the absorption bands at
240 nm is due to the phenyl moiety and broad band
from 300 to 400 nm to Se9Se moiety. Thus, laser
photolysis was performed with 355 nm light, which
predominantly excites Se9Se moiety.
EXPERIMENTAL
Materials
The transient absorption spectra observed by the
laser photolysis of (1-NaphSe)2 with 355 nm light are
shown in Figure 2, in which absorption peaks at 680
and 420 nm appear. The shape of the time-resolved
spectra in the region of 500–750 nm measured by
MCPD detector is similar to that measured by PM de-
tector. In the wavelength region longer than 800 nm,
no appreciable transient absorption was observed. The
main absorption peaks at 420 nm and 680 nm are sim-
ilar to those of 1-naphthylthio and 1-naphthyloxy rad-
icals [25,26]. These absorption bands were observed
even in the presence of O2, suggesting that
Naphthyl diselenides (NaphSe)2 were prepared from
commercially available bromonaphthalenes following
the procedure reported for the preparation of phenyl
diselenide [21]. They were used after purification by
recrystallization from benzene. Commercially avail-
able phenyl diselenide was used as received. 2-
Methyl-1,3-butadiene (isoprene) and ␣-methylstyrene
were used after distillation. Solvents were of spectro-
scopic grade. The O2-concentration in the solution was
calculated by the Henry law using the reported coef-
ficients [22].
и
1-NaphSe is not reactive to O2. This observation is
similar to that of PhSeи [13,27]. The low reactivity to
O2 was also reported for the aryloxy and arylthio rad-
icals [15,16]. Thus, the absorption bands in Figure 2
Apparatus
Steady-light photolysis were performed in THF solu-
tions of 0.02 mM diselenides with Xe9Hg lamp
(150 W) by irradiating the light longer than 270 nm at
23ЊC; the UV absorption changes were followed by a
UV/Vis spectrometer.
и
are attributed to 1-NaphSe (eq. (1)).
h␯(355 nm)
J NaphSeи
1/2 NaphSe—SeNaph I
(1)
kb
Laser-flash photolysis apparatus was a standard de-
sign with Nd:YAG laser of 6 ns duration. Diselenides
of 0.2–2 mM were photolyzed with THG light
(355 nm). Since the absorbance Ͻ1 at 355 nm of
1 mM of diselenides, the transient species may be
formed homogeneously in 10 cm rectangular optical
cell. The monitoring light, which was perpendicular to
the laser light, was selected with a monochromator.
The transient spectra were measured with a multi-
channel photodiode array (MCPD) with an image in-
tensifier [23]. The time profiles were followed by a
photomultiplier (PM) system. All the measurements
were performed at 23ЊC.
Naph; 1-Naph and 2-Naph
MO Calculation
The MO calculations were performed with PM3
method using MOPAC Version 6 presented by Japan
Program Exchange Association [24].
Figure 1 Absorption spectra of diselenides (0.02 mM) in
THF in 10 mm of optical cell length.

NAPHTHYL DISELENIDES
195
Figure 2 Transient absorption spectra obtained after laser
photolysis (355 nm) of 1 mM of (1-NaphSe)2 in deaerated
THF (MCPD-detector): Insert: transient spectra measured by
PM-detector and decay profiles at 420 and 680 nm in Ar-
bubbled solution.
Figure 4 Transient absorption spectra obtained after laser
photolysis (355 nm) of 1 mM (PhSe)2 in deaerated THF
(MCPD-detector): Insert: Decay profiles at 490 nm in Ar-
and O2-saturated solution.
ical [11]. Thus, the absorption bands in Figure 4 are
attributed to PhSeи (eq. (2)).
For (2-NaphSe)2, the main absorption peaks appeared
at 490 and 680 nm (Fig. 3). At the shorter region than
420 nm, other transient absorption band seems to be
hidden. The spectral feature is similar to those of
2-naphthylthio and 2-naphthyloxy radicals [25,26].
Thus, these absorption bands are attributed to
2-NaphSeи (eq. (1)).
With the delay time, the absorption band at
490 nm decays without shifting the absorption max-
ima. With decay of the absorption at 440 nm, on the
other hand, the absorption maximum tends to shift to
the shorter wavelength. Since this shift was observed
only when the concentration of (PhSe)2 was higher
than 1 mM, this may be related to the radical adducts
The transient absorption spectra observed by the
laser photolysis of (PhSe)2 with 355 nm-light show
peaks at 440 and 490 nm (Fig. 4). The decay profiles
и
such as PhSe9Se (Ph)SePh (eq. (2)) as suggested by
Burkey and Griller for PhSи [28].
и
suggests that PhSe is low reactive to O2, which is in
The decay time-profiles of PhSeи
in THF are
accord with the observed low reactivity to O2 for
aryloxy radicals, arylthio radicals, and arylseleno rad-
shown (Fig. 5) with changing the concentration of
(PhSe)2, with which the initial absorption intensities
Figure 3 Transient absorption spectra obtained after laser
photolysis (355 nm) of 1 mM (2-NaphSe)2 in deaerated THF
(MCPD-detector): Insert: Transient spectra measured by
PM-detector.
Figure 5 Concentration dependence of decay profiles of
PhSe at 485 nm in THF: (a) 2; (b) 1; (c) 0.5; and (d)
0.2 mM. Insert: (A) First-order plots and (B) second-order
plots.
●

196
ALAM ET AL.
Table I Rate Constants (kb) for Recombination of
Seleno Radicals (RSeи)
increase. In the inserted first-order plots (Fig. 5), each
decay curve can not be fitted with a single exponential,
which indicates that after the initial reaction of
␩
2kb/⑀ (␭max),
PhSeи
with (PhSe)2, the equilibrium may be estab-
Ϫ
1
RSeи
Solvent
cP
cm s (nm)
lished. By the plots of the slopes obtained from the
initial part of the first-order plots with the concentra-
tion of (PhSe)2, the second-order rate constant was
1-NaphSeи
1-NaphSeи
2-NaphSeи
2-NaphSeи
PhSeи
THF
CH
THF
CH
THF
CH
0.55
0.98
0.55
0.98
0.55
0.98
1.3 ϫ 106 (410)
5.5 ϫ 105 (420)
6.7 ϫ 106 (480)
4.1 ϫ 106 (480)
8.2 ϫ 106 (480)
6.4 ϫ 106 (480)
1
1
evaluated to be ka ϭ 1.6 ϫ 108 MϪ sϪ . In second-
order plots, good linear relations were obtained with
the same slopes even the concentration of (PhSe)2 was
varied. This suggests that the self-recombination of
PhSeи becomes the rate determining step, since the
reverse reaction of the radical adduct is very fast. This
implies that it is difficult to observe the radical adduct
in the considerable high concentration.
PhSeи
CH; cyclohexane. kD ϭ 1.2 ϫ 1010 MϪ sϪ in THF and kD
ϭ
1
1
6.8 ϫ 109 MϪ sϪ1 in CH. On assuming kD ϭ kb, ⑀ can be evaluated.
From the ⑀ values, initial concentration of the seleno radical by a
laser exposure for 1 mM of diselenide with 10 mJ laser pulse is
evaluated to be ca. 0.01 mM.
1
h␯(355 nm)
ϩ[PhSe9SePh], ka
J PhSeи I
1/2 PhSe9SePh I
J
a
kb
Ϫ[PhSe9SePh], kϪ
tained. Thus, it was difficult to evaluate the ⑀ values
of the seleno radicals. It is to be suggested that
the kb values are close to the diffusion-controlled
limits (kD), which can be calculated to be
PhSe9 и
SePh
&
(2)
SePh
1
1
1.2 ϫ 1010 MϪ sϪ in THF and to be 6.8 ϫ
1
1
109 MϪ sϪ in cyclohexane by Debye’s equation
(kD ϭ 8 RT/3000␩ where ␩ is solvent viscosity) [29].
In Figure 6, the decay curves of 2-NaphSeи in the
presence of reactive alkenes such as 2-methyl-1,3-
butadiene (isoprene) are shown in the form of first-
For NaphSeи, the change of the transient absorption
spectra with the concentration of diselenide was not
found, because of the complex absorption bands. Al-
though the decay profiles show similar behavior to that
of PhSeи, the bulky naphthalene group may hinder the
formation of radical adducts.
и
order plot. The decay rate of 2-NaphSe was not ac-
celerated (Fig. 6(a)) compared with that in the absence
of isoprene; upward curved line in the first-order plot
indicates that the second-order process is included. In
aerated and O2-saturated solutions, on the other hand,
Linear lines were obtained for the second-order
plots for the decays of 1-NaphSeи, 2-NaphSe , and
и
PhSeи, suggesting that they predominantly recombine
into the diselenide. The slopes give the ratio of recom-
bination rate constant (kb) to molar extinction coeffi-
и
2-NaphSe showed fast decay (Fig. 6(b) and (c)). The
first-order rate constants (k1st) was separated from the
и
cient (⑀), 2kb/⑀. For 1-NaphSe , 2kb/⑀ of the 680 nm
band is about four-fold larger than that of 420 nm
band, which indicates that the ⑀ at 680 nm is 1/4 of
that at 420 nm. This is in fair agreement with the ratio
of the observed initial absorption intensities at 420 and
680 nm, which is ca. 1/3. This clearly supports that
both bands are attributed to a species, 1-NaphSeи.
The ratios of 2kb/⑀ are summarized in Table I. The
и
2kb/⑀ of 2-NaphSe in viscous cyclohexane is smaller
than that in less viscous THF, suggesting that recom-
bination reaction of the free radicals is controlled by
the diffusion rate of the two radicals. In order to
evaluate the ⑀ values, the laser-flash photolysis exper-
iments of the diselenides were performed in the pres-
ence of electron donors such as tetramethyl-p-phenyl-
enediamine (TMPD), no electron transfer reaction
from TMPD to the seleno radicals was observed. In
the presence of electron acceptor such as tetracyano-
quinodimethane (TCNQ), evidence of the electron
transfer from the seleno radical to TCNQ was not ob-
Figure 6 First-order decay plots of absorption band at
490 nm of 2-NaphSe in the presence of 50 mM of isoprene
(a) in deaerated THF, (b) in aerated solution and (c) in O2-
saturated solution. Insert: Plot of [isoprene]/k1st vs. 1/[O2].
●

NAPHTHYL DISELENIDES
197
second-order rate constant (kb) by curve-fitting
method. The k1st was proportional to both concen-
trations of isoprene and O2. Similar behavior was
observed for ␣-methylstyrene (␣-MSt)-2-NaphSeи,
isoprene-PhSeи, and ␣-MSt-PhSe . In the case of
и
1-NaphSeи, the decay rates were accelerated only
slightly on addition of these alkenes, even in O2-
saturated solution, indicating the low reactivity of
и
1-NaphSe .
This indicates that addition reaction of the seleno
radicals to the alkenes proceeds in a reversible mode
as indicated from cis-trans isomerization of stilbene
[16,17]. Here, O2 acts as a selective trapping agent for
the carbon centered radicals which are generated by
the addition reaction [30]. The selectivity of the carbon
radical trapping by O2 is quite high because O2 is re-
active only to the carbon-centered radicals [31], but
not to the seleno radicals. These processes are illus-
trated in Scheme I, in which NaphSeи adds to the ter-
minal olefinic carbon in an anti-Markovnikov manner
similar to the thio radicals [2,32].
Figure 7 UV/Visible absorption spectral changes by pho-
tolysis of 0.05 mM (2-NaphSe)2 with light (Ͼ270 nm) in
THF at 23ЊC in the presence of isoprene (A) in deaerated
solution and (B) in O2-saturated solution.
ated solution, the absorption of (NaphSe)2 was only
slightly changed as shown in Figure 7(a); this corre-
sponds with the laser-flash photolysis experiments in
Figure 6(b) and (c). On the other hand, the absorption
band of (NaphSe)2 decreased rapidly in the presence
of both isoprene and O2 (Fig. 7(b)) with half-time
(t1/2) of 10 min. This phenomenon is corresponding to
the laser-flash photolysis experiment (Fig. 6). These
findings are supporting the reaction processes as
shown in Scheme I.
ϩ[CH2"CRRЈ], k1
NaphSe
NaphSe9CH29CRRЈ
9[CH2"CRRЈ], kϪ1
hv
kb
k2 ϩ[O2]
O9O
1/2 NaphSe9SeNaph
NaphSe9CH29CRRЈ
R 9CH3, RЈ 9Ph, 9CH"CH2
Scheme I
Similarly, the absorption spectral changes were ob-
served by the steady-light photolysis of (1-NaphSe)2
as shown in Figure 8. The final spectrum indicates that
As reported, cis-trans isomerization of stilbene pho-
tosensitized with (PhSe)2 strongly indicates that the
addition process is reversible [16,17]; i.e., the addition
и
reaction of PhSe to stilbene produces the carbon-
centered radical [PhSe9CHPh9CиHPh], in which
rotation around the C9C bond and successive elim-
и
ination of PhSe induces cis-trans isomerization in the
absence of O2. When O2 is present in solution, isom-
erization was inhibited [16,17], indicating that O2 traps
the carbon-centered radical as shown in Scheme I.
To confirm Scheme I, the steady-light photolysis of
(NaphSe)2 was performed. The absorption spectra
practically did not change even after the irradiation of
(NaphSe)2 for more than 25 min with Ͼ270 nm light
both in aerated and deaerated solutions. This is in ac-
cord with the laser-flash photolysis experiments,
и
showing that NaphSe , which is unreactive to O2, is
predominantly recombined back to the starting
(NaphSe)2. The small change in the spectrum by pro-
longed irradiation of more than 1 h may be due to the
photochemical reactions such as change into mono-
selenide [33,34]. In the presence of isoprene in deaer-
Figure 8 UV/Visible absorption spectral changes by pho-
tolysis of 0.05 mM (1-NaphSe)2 with light (Ͼ270 nm) in
THF at 23ЊC in the presence of isoprene (A) in deaerated
solution and (B) in O2-saturated solution.

198
ALAM ET AL.
the product also contains the naphthalene moiety. In
the presence of isoprene in deaerated solution, the ab-
sorption band of (1-NaphSe)2 was practically un-
changed during the photolysis for 90 min with
Ͼ270 nm light. In the presence of both isoprene and
O2, the absorption bands of (1-NaphSe)2 decreased in
a slower rate (t1/2 ϭ 15 min) than that of (2-NaphSe)2,
although the acceleration of the decay rate of
1-NaphSeи was not observed in the laser-flash pho-
tolysis experiments even in the presence of O2.
The steady kinetic analysis, which follows a de-
crease in diselenide or an increase in the product with
steady-light photolysis, was reported for the addition
reaction of the phenylthio radical to styrenes [35]; it
was pointed out that evaluation of the relative rate con-
stants was very difficult. Thus, only when bold as-
sumptions that irreversibility is low for conjugated
olefins were applied to the reversibility of addition
process, the relative rate constants for addition reac-
tions were evaluated [36]. In this study, on the other
hand, since we could follow the decays of NaphSeи
directly, kinetic analysis to obtain the rate constants
may be possible by the selective radical trap method
developed by us [8].
k1st ϭ k1{1 Ϫ kϪ /(kϪ ϩ k2[O2])}
1 1
ϫ [CH2 "CRRЈ] (5)
The following eq. (6) is derived from eq. (5). Thus,
the k1 value can be obtained by the extrapolation of
the plots of [CH2 "CRRЈ]/k1st against 1/[O2] as shown
in the insert of Figure 6 kϪ /k1k2 can be obtained from
1
the slope.
[CH2 "CRRЈ]/k1st ϭ 1/k1 ϩ k1/(k1k2[O2]) (6)
For isoprene and ␣-methylstyrene, the k1 values are
и
summarized in
II for 2-NaphSe
and
1-NaphSeи with PhSe . For 1-NaphSe , the depen-
Tabиle
и
dence of k1st values on [O2] is too small to be observed
by this method; thus, the k1st observed in O2-saturated
THF was put equal to k1. These k1 values are less than
about 5 ϫ 105 MϪ sϪ .
1
1
The kϪ values are also listed in Table II; the kϪ
1
1
values are calculated from the observed slope
(kϪ /k1k2) by substituting with observed k1 and re-
1
ported k2; in Table II, k2 ϭ 2 ϫ 109 MϪ sϪ was
1
1
adopted by averaging the representative reported val-
On the application of the steady-state approxima-
tion with respect to the intermediate [NaphSe9CH2
ues [31]. The kϪ values, which are in the order of
1
5 ϫ 106 Ϫ 107 MϪ sϪ , indicate that the equilibrium
of the addition is established within 0.1–0.2 ␮s.
The k1 values in Table II indicate that the addition
reactivities of the seleno radicals increase in the order;
1
1
и
9C RRЈ] in Scheme I, eq. (3) can be derived [8],
2
и
и
Ϫd[NaphSe ]/dt ϭ kb[NaphSe ]
и
и
и
1-NaphSe Ͻ 2-NaphSe Ͻ PhSe . The reactivity of
NaphSeи was found to be lower than the correspond-
ing NaphSи by one order in magnitude [26,37].
In the case of the reaction of NaphSeи with iso-
prene and ␣-methylstyrene, both the addition and H-
atom abstraction reactions would be anticipated, be-
cause both reactions produce conjugated allyl and
benzyl radicals having high resonance stability. In
order to evaluate the H-atom abstraction ability of
ϩ k1{1 Ϫ kϪ /(kϪ ϩ k2[O2])}[CH2 "CRRЈ]
1
1
ϫ [NaphSeи]
(3)
The observed actual decay curves are mixed order
of the first-order and the second-order with respect to
и
[NaphSe ] giving eq. (4),
2
и
[NaphSeи]
Ϫd[NaphSe ]/dt ϭ k2nd
NaphSeи, the decay rates of NaphSe were observed
и
и
ϩ k1st[NaphSe ] (4)
in the presence of 1,4-cyclohexadiene, which is known
as a strong hydrogen-atom donor [38]. It was observed
that the decay rates were not accelerated in the pres-
where k2nd ϭ kb, and
Table II Rate Constants for Addition Reaction of Seleno Radicals with Isoprene and ␣-Methylstyrene in THF
Isoprene ␣-Methylstyrene
k1
kϪ
1
k1
kϪ
1
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ1
RSeи
1-NaphSeи
2-NaphSeи
PhSeи
M
s
s
M
s
s
Ͻ5 ϫ 105
7.1 ϫ 106
1.8 ϫ 107
--------a
(1.0 ϫ 107)b
(4.0 ϫ 106)b
Ͻ1 ϫ 105
7.0 ϫ 105
3.0 ϫ 106
--------a
(1.6 ϫ 106)b
(1.0 ϫ 107)b
a Variation of k1st with changing [O2] was too small to observe.
b k2 ϭ 2 ϫ 109 MϪ sϪ [31].
1
1

NAPHTHYL DISELENIDES
199
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и
cay of NaphSe besides the recombination reaction
(Scheme I).
The molecular orbital (M.O.) energies and electron
и
densities of NaphSe were calculated by semiempiri-
cal restricted Hartree–Fock method using the opti-
mized geometry obtained by PM3 method [24]; the
results are shown in Figure 9. The reactivities of the
radicals can be explained from these MO parame-
ters [39]; the unpaired electron density of SOMO at
Se atom increase in the order of 1-NaphSeи Ͻ
2-NaphSeи Ͻ PhSe , which seems to be in accord
и
with the observed reactivities.
Since the S-centered radical has highly electro-
philic character, it would be expected that the seleno
radicals also have similar character in some extent. In
the addition reaction to alkenes, it has been anticipated
that the contribution of polar resonance structure in
the transition state of the reaction lowers the activation
energy [40]. The higher the electrophilicity of the se-
leno radical, the lower the SOMO energy. The order
of the SOMO energy is also in accord with the in-
creasing reactivity of the radicals obtained by the ex-
periments.
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08218207) from the Ministry of Education, Science, Sports
and Culture.
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