Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations

Article abstract of DOI:10.1021/ja029623y

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)₃³⁺ (bpy = 2,2′-bipyridin

Full text of DOI:10.1021/ja029623y

Published on Web 04/01/2003
Mechanisms of Electron-Transfer Oxidation of NADH
Analogues and Chemiluminescence. Detection of the Keto
and Enol Radical Cations
Shunichi Fukuzumi,* Osamu Inada, and Tomoyoshi Suenobu
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation (JST),
Suita, Osaka 565-0871, Japan
Received December 7, 2002; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been
successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from
3
+
3+
BNAH to Fe(bpy)
BNAH and the dideuterated compound (BNAH-4,4′-d
form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects
3
(bpy ) 2,2′-bipyridine) and Ru(bpy)
3
. The ESR spectra of the radical cations of
2
) indicate that the observed radical cation is the keto
•
+
of the keto form of BNAH were determined from the kinetic analysis of the electron-transfer reactions. In
the case of electron transfer from BNAH to Ru(bpy)
observed in the second electron-transfer step from BNA , produced by the deprotonation of the keto form
of BNAH , to Ru(bpy)
3+
2+*
3
, the chemiluminescence due to Ru(bpy)
3
was
•
•
+
3+
2+*
3
. The observation of chemiluminescence due to Ru(bpy)
3
provides compelling
evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer
reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to
2
+*
3+
Ru(bpy)
3
has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)
3
. This is ascribed to
•
+
the facile C-C bond cleavage in 4-t-BuBNAH . In the laser flash photolysis of a deaerated MeCN solution
of BNAH and CHBr
of the keto form of BNAH , and the enol form was tautomerized to the keto form. The rate of intramolecular
proton transfer in the enol form to produce the keto form of BNAH was determined from the decay of the
absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic
isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce
the enol form.
•
+
3
, the transient absorption spectrum of the enol form of BNAH was detected instead
•+
•
+
+
Introduction
NADH and NAD via the sequential transfer of electron, proton,
and electron. Gebicki et al. have reported that certain classes
of radical cations can undergo spontaneous tautomerization and
that such radical cation tautomerization may also occur for
The redox chemistry of 1,4-dihydronicotinamide derivatives
has attracted considerable interest in terms of the key role of
dihydronicotinamide nucleotide coenzymes (NADH) in a num-
1,2
ber of biological redox processes including respiratory chain.
(
5) (a) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Org. Chem. 1984, 49, 3571.
b) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305 and references therein.
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722. (b) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am. Chem.
(
NADH acts as the source of two electrons and a proton, which
3
is equivalent to a hydride ion. In many cases, the two-electron
4
oxidation of NADH and analogues is started by an electron
Soc. 2001, 122, 4286. (c) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka,
T.; Itoh, S.; Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 1998, 120, 8060. (d)
Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J. Am. Chem. Soc. 1989, 111,
transfer to a substrate, when the radical cations of NADH and
analogues should be formed.4
-9
NADH radical cations are
1
497. (e) Liu, Y. C.; Li, B.; Guo, Q. X. Tetrahedron 1995, 51, 9671. (f)
important reactive intermediates in the redox conversion between
Ohno, A.; Ishikawa, Y.; Yamazaki, N.; Okamura, M.; Kawai, Y. J. Am.
Chem. Soc. 1998, 120, 1186.
(
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(
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(
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(
b) Fukuzumi, S. In AdVances in Electron-Transfer Chemistry; Mariano,
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4808
9
J. AM. CHEM. SOC. 2003, 125, 4808-4816
10.1021/ja029623y CCC: $25.00 © 2003 American Chemical Society

Electron-Transfer Oxidation of NADH Analogues
A R T I C L E S
Scheme 1
and the kinetic isotope effect. The thermal electron-transfer
mechanism of NADH as well as the chemiluminescence
mechanism in the electron-transfer reactions with Ru(bpy)33
have been clarified by detailed kinetic analysis. We have also
succeeded in detecting the ESR spectra of the keto form of
radical cations of NADH analogues. Thus, the present study
provides comprehensive electron-transfer mechanisms of NADH
analogues.
+
radical cations generated from NADH model compounds
(
Scheme 1).1
0,11
Experimental Section
The transient absorption spectrum of the radical cation of
Material. Preparation of 1-benzyl-1,4-dihydronicotinamide (BNAH)
NADH in an aqueous solution has been detected using bi-
photonic one-electron oxidation of NADH upon laser excitation,
5b
was described previously. The dideuterated compound, 1-benzyl-1,4-
2
dihydro [4,4′- H
2
] nicotinamide (BNAH-4,4′-d
2
), was prepared from
12
exhibiting an absorption maximum at 540 nm. Similar transient
absorption bands were reported for NADH model compounds
upon laser photolysis of an NADH model compound with an
electron acceptor.13 We have previously reported the formation
of radical cations of NADH model compounds in the one-
electron oxidation of NADH model compounds with one-
electron oxidants and the deprotonation rates of the radical
19
monodeuterated compound (BNAH-4-d
1
)
by three cycles of oxidation
with p-chloranil in dimethylformamide and reduction with dithionite
in deuterium oxide.20 The 4-tert-butylated and 4-ethylated BNAH (4-
t-BuBNAH and 4-EtBNAH) were prepared by the Grignard reaction
+
-
21,22
with BNA Cl and purified by recrystallization from ethanol.
Tris(2,2′-bipyridyl)ruthenium(III) hexafluorophosphate [Ru(bpy) (PF
6 3
and tris(2,2′-bipyridyl)iron(III) hexafluorophosphate [Fe(bpy) (PF )
3
6 3
) ]
]
3
23
were prepared according to the literature. Tris(2,2′-bipyridyl)ruthe-
nium(II) chloride hexahydrate [Ru(bpy)
5
b,14
cations.
However, definitive assignment of radical cations
2+
3
3
] and bromoform (CHBr )
of NADH model compounds has yet to be made. In addition,
there has been no report on the ESR spectra of radical cations
of NADH analogues containing a nicotinamide moiety, although
ESR spectra of radical cations of dihydroacridine derivatives
which do not have a nicotinamide moiety have been fully
were obtained from Aldrich. Acetonitrile (MeCN) and propionitrile
24
(
EtCN) as solvents were purified and dried by the standard procedure.
Stopped-Flow Measurements. The transient absorption spectra in
-4
the oxidation of BNAH (BNAH-4,4′-d
2
) (1.0 × 10 M) or 4-t-
-
4
3+ -4
BuBNAH (1.0 × 10 M) with Fe(bpy)3 (1.5 × 10 M) were
measured using a UNISOKU RSP-601 stopped-flow rapid scan
spectrophotometer equipped with the MOS-type high sensitive photo-
diode array under deaerated conditions. The gate time for the measure-
ment was set at 2 ms. Typically, deaerated MeCN solutions were
transferred to the spectrophotometric cell by means of a glass syringe
which had earlier been purged with a stream of argon. The transient
15
characterized. The ESR detection of radical cations of NADH
analogues would provide definitive information on the structure
of the radical cation as to whether it is the keto form or the
enol form.
On the other hand, NADH is known to produce chemi-
3+
luminescence when reacted with Ru(bpy)3 (bpy ) 2,2′-
absorption spectra in the oxidation of BNAH (BNAH-4,4′-d
2
) (6.7 ×
16
bipyridine). Such chemiluminescence has been utilized for the
determination of other analytes which consume or produce these
coenzymes.17 However, the mechanism of chemiluminescence
has yet to be clarified.
-5
3+
-5
1
0
M) with Ru(bpy)
3
(7.5 × 10 M) were measured in the same
way.
ESR Measurements. The ESR measurements were performed on
a JEOL JES-FA100 ESR spectrometer. Deaerated MeCN solutions of
-
3
3+
-2
We report herein the detection of both the keto and the enol
forms of radical cations of NADH analogues in photoinduced
BNAH (BNAH-4,4′-d
2
) (8.3 × 10 M) and Fe(bpy)
3
(1.0 × 10
M) under an atmospheric pressure of argon were mixed in a flat ESR
cell by using a JEOL ES-EMCNT1 rapid mixing flow apparatus. The
18
electron-transfer reactions using the laser flash technique. The
dynamics of keto-enol tautomerization has been successfully
monitored to determine the intramolecular proton-transfer rate
3
-1
flow rate was 1.9 cm s , and the mixing time before reaching the
ESR cell is about several hundred milliseconds. The decay of the ESR
signal was measured at a fixed magnetic field, when the flow was
stopped. The ESR spectra were recorded under nonsaturating microwave
power conditions. The magnitude of modulation was chosen to optimize
the resolution and the signal-to-noise (S/N) ratio of the observed spectra.
The g value and hyperfine splitting constants (hfc) were calibrated by
using an Mn2 marker.
(
10) (a) Gebicki, J.; Bally, T. Acc. Chem. Res. 1997, 30, 477. (b) Gebicki, J.
Pure Appl. Chem. 1995, 67, 55. (c) Gebicki, J.; Marcinek, A.; Adamus, J.;
Paneth, P.; Rogowski, J. J. Am. Chem. Soc. 1996, 118, 691.
(
11) (a) Marcineck, A.; Adamus, J.; Huben, K.; Gebicki, J.; Bartczak, T. J.;
Bednarek, P.; Bally, T. J. Am. Chem. Soc. 2000, 122, 437. (b) Marcinek,
A.; Rogowski, J.; Adamus, J.; Gebicki, J.; Benarek, P.; Bally, T. J. Phys.
Chem. A 2000, 104, 718.
+
Chemiluminescence. The steady-state chemiluminescence spectrum
was measured using a SHIMADZU spectrofluorophotometer (RF-5000).
(
12) (a) Czochralska, B.; Lindqvist, L. Chem. Phys. Lett. 1983, 101, 297. (b)
Lindqvist, L.; Czochralska, B.; Grigorov, I. Chem. Phys. Lett. 1985, 119,
4
94.
3+
-2
A deaerated MeCN solution of Ru(bpy)
3
(1.0 × 10 M) was injected
(
13) Anne, A.; Hapiot, P.; Moiroux, J.; Neta, P.; Sav e´ ant, J.-M. J. Am. Chem.
Soc. 1992, 114, 4694.
-
2
into a deaerated solution of BNAH (5.0 × 10 M) using a Harvard
(
14) (a) Fukuzumi, S.; Kondo, Y.; Tanaka, T. Chem. Lett. 1982, 1591. (b)
Fukuzumi, S.; Kondo, Y.; Tanaka, T. J. Chem. Soc., Perkin Trans. 2 1984,
Apparatus UL STD 3101-1 syringe pump with the constant flow rate
-1
of 9 µL min . Decay rates of chemiluminescence were measured by
6
73.
-
4
(
(
(
15) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J. Am. Chem.
mixing deaerated solutions of BNAH (BNAH-4,4′-d
2
) (5.0 × 10 M)
Soc. 1993, 115, 8960.
3+
-3
and deaerated solutions of Ru(bpy)
3
(1.0 × 10 M) using a
16) (a) Downey, T. M.; Nieman, T. A. Anal. Chem. 1992, 64, 261. (b) Uchikura,
K.; Kirisawa, M. Chromatography 1992, 13, 257.
17) (a) Martin, A. F.; Nieman, T. A. Anal. Chim. Acta 1993, 281, 475. (b)
Yokoyama, K.; Sasaki, K.; Ikebukuro, K.; Takeuchi, I.; Karube, Y.; Tokitsu,
Y.; Masuda, Y. Talanta 1994, 41, 1035. (c) Jameison, F.; Sanchez, R. I.;
Dong, L.; Leland, J. K.; Yost, D.; Martin, M. T. Anal. Chem. 1996, 68,
(19) (a) Anderson, A. G., Jr.; Berkelhammer, G. J. Am. Chem. Soc. 1958, 80,
992. (b) Mauzerall, D.; Westheimer, F. H. J. Am. Chem. Soc. 1955, 77,
2261.
(20) Caughey, W.; Schellenberg, K. A. J. Org. Chem. 1966, 31, 1978.
(21) Anne, A. Heterocycles 1992, 34, 2331.
(22) Takada, N.; Itoh, S.; Fukuzumi, S. Chem. Lett. 1996, 1103.
(23) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(24) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals;
Butterworth-Heinemann: Oxford, 1988.
1
298. (d) Gerardi, R. D.; Barnett, N. W.; Lewis, S. W. Anal. Chim. Acta
999, 378, 1. (e) Lee, W.-Y.; Nieman, T. A. Anal. Chem. 1995, 67, 1789.
1
(
18) A part of the present results has appeared as a preliminary communication,
see: Fukuzumi, S.; Inada, O.; Suenobu, T. J. Am. Chem. Soc. 2002, 124,
1
4538.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003 4809

A R T I C L E S
Fukuzumi et al.
UNISOKU RSP-601 stopped-flow rapid scan spectrophotometer equipped
with the MOS-type high sensitive photodiode array and with the light
source switched off under deaerated conditions. The gate time for the
measurement was set at 2 ms. Typically, deaerated MeCN solutions
were transferred to the spectrophotometric cell by means of a glass
syringe which had earlier been purged with a stream of argon.
Laser Flash Photolysis Measurements. The measurements of
transient absorption spectra in the photooxidation of BNAH, BNAH-
4
,4′-d
2 3
, 4-EtBNAH, and 4-t-BuBNAH with CHBr were performed
according to the following procedures. The degassed EtCN solution
-
4
containing BNAH (BNAH-4,4′-d
×
2
) (4.0 × 10 M), 4-EtBNAH (4.0
-
4
-4
10 M), or 4-t-BuBNAH (4.0 × 10 M) and CHBr
3
(0.25 M) was
excited by a Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm) at λ
355 nm with a power of 20 mJ per pulse. The transient absorption
)
spectra (Figures 8a, S5, and S6) for the photoinduced electron-transfer
reactions were measured by using a continuous Xe-flash lamp (EPS,
XN-20MS, 12 mJ/pulse) as a probe light and a photodiode array S3904-
1
024F with gated image intensifier C2925-01 (Hamamatsu Photonics)
Figure 1. Differential spectral change in thermal electron transfer from
-
5
3+
-5
as a detector. Time courses of the transient absorption spectra were
measured by using a continuous Xe-lamp (150 W) and an InGaAs-
PIN photodiode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a photomultiplier
tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032,
BNAH (6.7 × 10 M) to Ru(bpy)3 (7.5 × 10 M) in deaerated MeCN
at 298 K; time interval: 20 ms.
V)28 and Ru(bpy)33+ (Ered vs SCE ) 1.24 V). The differential
absorption spectra were recorded by subtracting the final
absorption spectrum from the observed spectra during the
0
29
3
00 MHz). The transient spectra were recorded using fresh solutions
in each laser excitation. The measurements of transient absorption
spectra in the photooxidation of BNAH and 4-t-BuBNAH with
Ru(bpy)
3+
electron-transfer oxidation of BNAH by Ru(bpy)3 as shown
in Figure 1 (see Supporting Information S1 for the original
spectra). The new absorption band at 380 nm appears upon
2+
3
were performed according to the following procedures. The
-
3
degassed MeCN solution containing BNAH (3.0 × 10 M) or 4-t-
3
+
mixing MeCN solutions of BNAH and Ru(bpy)3 and disap-
pears accompanied by the appearance of the absorption band
-
2
2+
-4
BuBNAH (1.0 × 10 M) and Ru(bpy)
3
(1.0 × 10 M) was excited
by a Panther OPO pumped by Nd:YAG laser (Continuum, SLII-10,
2
+
at 450 nm due to Ru(bpy)3 . The absorption band at 380 nm
4
-6 ns fwhm) at λ ) 450 nm with the power of 10 mJ per pulse. The
•
+
photoinduced electron-transfer reactions were monitored by using a
continuous Xe-lamp (150 W) and an InGaAs-PIN photodiode (Hama-
matsu 2949) as a probe light and a detector, respectively. The output
from the photodiodes and a photomultiplier tube was recorded with a
digitizing oscilloscope (Tektronix, TDS3032, 300 MHz).
can be assigned to BNAH . The same absorption band due to
•
+
BNAH was observed in the electron-transfer oxidation of
3+ 18
•+
BNAH by Fe(bpy)3
. The decay rate of BNAH obeys first-
order kinetics (Figure 2a), coinciding with the rate of formation
2+
of Fe(bpy)3 (Figure 2b). When BNAH is replaced by BNAH-
Theoretical Calculation. Density-functional theory (DFT) calcula-
tions were performed on a COMPAQ DS20E computer. Geometry
optimizations were carried out using the B3LYP functional and 6-31G*
4
,4′-d2, the kinetic isotope effects were observed for both the
•
+
2+
decay rate of BNAH and the rate of formation of Fe(bpy)3
as shown in Table 1. This indicates that the decay of BNAH
occurs via deprotonation to produce BNA , which is rapidly
oxidized by Fe(bpy)3 (Scheme 2).
•+
2
5,26
basis set
with the unrestricted Hartree-Fock (UHF) formalism as
•
2
7
implemented in the Gaussian 98 program.
3
+
Judging from the highly negative oxidation potential of BNA•
Results and Discussion
0
(
Eox vs SCE ) -1.08 V), which is equivalent to the reduction
Thermal Electron-Transfer Oxidation of BNAH. Thermal
electron-transfer oxidation of an NADH analogue (BNAH) was
examined using Fe(bpy)33 and Ru(bpy)3 as one-electron
+ 5b
•
potential of BNA , the electron transfer from BNA to
3+
0
28
Fe(bpy)3 (Ered vs SCE ) 1.06 V) is highly exergonic, and
+
3+
thus it is expected to be diffusion-limited. In such a case, the
oxidants in MeCN. The initial electron transfer from BNAH to
2+
rate-limiting step for formation of Fe(bpy)3
is the de-
Fe(bpy)33 and Ru(bpy)3 is too rapid to be monitored using
a stopped-flow technique. This is consistent with the largely
negative free energy change of electron transfer from BNAH
+
3+
•+
protonation from BNAH as observed experimentally. The
Eyring plots of the deprotonation rate constant (k) of BNAH
•+
(
Figure 3) afforded the activation parameters listed in Table 2.
0
5b
3+
0
(
Eox vs SCE ) 0.57 V) to Fe(bpy)3 (Ered vs SCE ) 1.06
When BNAH is replaced by 4-t-BuBNAH where the hydro-
gen at the C(4) position is substituted by a tert-butyl group, no
transient absorption spectrum due to 4-t-BuBNAH• has been
detected (see Supporting Information S2). We have previously
reported that cleavage of the C(9)-C bond of a radical cation
of an NADH analogue, 9-tert-butyl-10-methyl-9,10-dihydro-
acridine [AcrH(t-Bu)], occurs selectively rather than the cleavage
of the C(9)-H bond in the electron-transfer oxidation of AcrH(t-
(
(
(
25) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.;
+
Parr, R. G. Phys. ReV. B 1988, 37, 785.
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785.
4
810 J. AM. CHEM. SOC. VOL. 125, NO. 16, 2003
9

Electron-Transfer Oxidation of NADH Analogues
A R T I C L E S
Scheme 2
Table 2. Activation Parameters of Deprotonation and
•
+
•+
Tautomerization of BNAH and BNAH-4,4′-d2
∆
Hqa
kcal mol-1)
∆Sqa
∆Hqb
(kcal mol-1)
∆Sqb
(cal K-1 mol-1
)
(
(cal K-1 mol-1
)
BNAH•+
BNAH-4,4′-d2
3.1
4.1
42
39
1.9
2.8
29
25
•
+
a
Determined by an Eyring plot of deprotonation rates.18 Determined
b
by an Eyring plot of tautomerization rates.
Figure 2. (a) Time course of absorbance change at 380 nm for the reaction
-
4
3+
-4
of BNAH (1.0 × 10 M) with Fe(bpy)3 (1.5 × 10 M) in deaerated
MeCN at 298 K. Inset: First-order plot based on the absorption change at
3
80 nm. (b) Time course of absorbance change at 520 nm for the reaction
-
4
3+
-4
of BNAH (1.0 × 10 M) with Fe(bpy)3 (1.5 × 10 M) in deaerated
MeCN at 298 K. Inset: First-order plot based on the absorption change at
5
20 nm.
•
+
Table 1. Rate Constants (k) of Deprotonation of BNAH and
•+
Figure 3. Eyring plots of the deprotonation rate constant (k) of BNAH
•
+
•+
BNAH-4,4′-d2 in the Reaction of BNAH or BNAH-4,4′-d2 (1.0 ×
•+
(O) and BNAH-4,4′-d2 (b) in deaerated MeCN.
-
4
3+
-4
10
M) with Fe(bpy)3 (1.5 × 10 M) in Deaerated MeCN
1
-
, s-1
Scheme 3
T, K
k
H
, s
k
D
H D
k /k
2
2
2
2
98
78
58
43
24
15
8.5
6.0
18
9.6
5.1
3.1
1.3
1.5
1.7
1.9
Bu) by Fe3 complexes.15 One-electron oxidation of 4-t-
+
BuBNAH is also known to result in the selective C(4)-C bond
•
+
•
+
cleavage of 4-t-BuBNAH to give t-Bu and BNA (Scheme
3
6
c
), both of which have no absorption bands in the visible
region. Such an ultrafast carbon-carbon bond cleavage is also
known for benzpinacol radical cations.30 In such a case, two-
electron oxidation of 4-t-BuBNAH occurs with 2 equiv of
•
+
3+
0
C-C bond cleavage in 4-t-BuBNAH , to Fe(bpy)3 (Ered vs
2
8
SCE ) 1.06 V) is as rapid as the initial electron transfer from
0
22
3+
4
-t-BuBNAH (Eox vs SCE ) 0.71 V) to Fe(bpy)3 . The two-
3+
•
Fe(bpy)3 (Scheme 3). The second electron transfer from t-Bu
electron oxidation mechanism of 4-t-BuBNAH, including the
trapping of the t-Bu radical with oxygen, has previously been
0
31
(
Eox vs SCE ) 0.09 V), which is generated via the facile
•
2
2
(
30) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120,
542.
31) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc. 1988,
10, 132.
reported.
6
•+
ESR Detection of the Keto Form of BNAH . The relatively
long lifetime of the radical cation of BNAH in Figure 1 enabled
(
1
J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003 4811

A R T I C L E S
Fukuzumi et al.
•
+
Figure 5. Decay of the ESR signal intensity of BNAH at g ) 2.0195
-
3
3+
observed in the reaction of BNAH (8.3 × 10 M) with Fe(bpy)3 (1.0 ×
-
2
10
M) in deaerated MeCN at 233 K. Inset: First-order plot.
G) are assigned to the two protons at the C(4) position, because
the hfc value is significantly larger than the maximum hfc value
2
(
23.04 G) of proton bonding to the sp carbon (C(4) in enol
34
form). Such large values at the C4 position are consistent with
those of NADH expected by photo-CIDNP experiments, which
35
•
+
confirm that BNAH detected by ESR is the keto form.
Deuterium substitution at these positions permits an experi-
mental verification of the assignment of the observed radical
species, because a single deuteron gives a triplet (instead of
doublet) hyperfine pattern and the deuteron splitting should
decrease by the magnetogyric ratio of proton to deuterium
34
(
0.153). In fact, deuterium substitution of two hydrogen atoms
•+
at the C(4) of BNAH results in drastic changes in the splitting
pattern from the spectrum in Figure 4a to that in Figure 4c,
where BNAH is substituted by BNAH-4,4′-d2. The computer
simulation spectrum using the same hfc values except for the
deuterium (I ) 1) at the C(4) position, which are reduced by a
factor of 0.153, agrees well with the observed ESR spectrum
•
+
of BNAH-4,4′-d2 (Figure 4d). Such an agreement confirms
the hfc assignment in Figure 4. When 4-ethyl substituted BNAH
•
+
Figure 4. (a) ESR spectrum of the keto form of BNAH generated by
(
4-EtBNAH) was used in place of BNAH, the ESR spectrum
-
3
3+
-2
oxidation of BNAH (8.3 × 10 M) with Fe(bpy)3 (1.0 × 10 M) in
•
+
of 4-EtBNAH was observed at g ) 2.0029 in the electron-
transfer oxidation of 4-EtBNAH with Fe(bpy)3 in deaerated
deaerated MeCN at 233 K and (b) the computer simulation spectrum with
•
+
3+
the hfc values. (c) ESR spectrum of the keto form of BNAH-4,4′-d2
-
3
3+
generated by oxidation of BNAH-4,4′-d2 (8.3 × 10 M) with Fe(bpy)3
MeCN (Figure 4e). The hfc values are assigned as shown in
-
2
(
1.0 × 10 M) in deaerated MeCN at 233 K and (d) the computer
3
3
Figure 4f. A large hfc value (50.4 G) assigned to a proton at
simulation spectrum with the hfc values. (e) ESR spectrum of the keto form
•+
-3
•+
of 4-EtBNAH generated by oxidation of 4-EtBNAH (8.3 × 10 M) with
the C(4) position further confirms that 4-EtBNAH exists as
3
+
-2
Fe(bpy)3 (1.0 × 10 M) in deaerated MeCN at 233 K and (f) the
the keto form rather than the enol form. In the case of 4-t-
computer simulation spectrum with the hfc values.
•+
BuBNAH, no ESR spectrum of 4-t-BuBNAH was detected
•+ 22
due to the facile C-C bond cleavage in 4-t-BuBNAH .
•
+
us to detect the ESR spectrum for the first time in the electron-
transfer oxidation of BNAH with Fe(bpy)33+ in deaerated MeCN
by applying a rapid-mixing ESR technique as shown in Figure
a.
The g-value is 2.0031, which is slightly larger than the free
To confirm that the keto form of BNAH observed in the
ESR spectrum in Figure 4 is the same as that observed in the
(
32) (a) Gauld, J. W.; Eriksson, L. A.; Radom, L. J. Phys. Chem. A 1997, 101,
18
4
1
352. (b) Lassmann, G.; Eriksson, L. A.; Lendzian, F.; Lubitz, W. J. Phys.
Chem. A 2000, 104, 9144.
(
33) The assignment of the hfc values is made by comparison with the hfc values
predicted by the DFT calculation (B3LYP functional and 6-31G* basis
set). The calculated hfc values are obtained as 1.3 (H2), 57.0 (H4), 9.7
spin value, indicating the contribution of spin-orbit coupling
due to electron spin at the nitrogen and oxygen atoms. The
hyperfine coupling constants (hfc) are determined by comparison
of the observed spectrum with the computer simulated spectrum
as shown in Figure 4b. By comparing the hfc values with the
•
+
(
H5), 1.9 (H6), 12.8 (CH
2
Ph), and 8.9 G (N1) for BNAH , and 1.1 (H2),
52.5 (H4), 9.0 (H5), 2.0 (H6), 11.0 (CH Ph), and 8.8 G (N1) for
-EtBNAH , which are used for assignment of the observed hfc values.
2
•
+
4
(
34) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance:
Elementary Theory and Practical Applications; John Wiley & Sons: New
York, 1994.
3
2
spin densities obtained by the DFT calculation, we assigned
(35) Hore, P. J.; Volbeda, A.; Dijkstra, K.; Kaptein, R. J. Am. Chem. Soc. 1982,
104, 6262.
the hfc values as shown in Figure 4b.33 Large hfc values (53.0
4812 J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003

Electron-Transfer Oxidation of NADH Analogues
A R T I C L E S
Scheme 4
transient absorption spectrum in Figure 1, the decay dynamics
of the ESR signal was examined. The decay rate obeyed first-
order kinetics as shown in Figure 5, and the first-order decay
-1
rate constant (k) at 233 K was determined as 3.4 s . This value
agrees within experimental error with the extrapolated value
from the Eyring plot in Figure 3 for the deprotonation rate
•
+
-1
constant of BNAH (kH ) 4.0 s ) determined from the decay
dynamics of the transient absorption band at λmax ) 380 nm.
Such an agreement confirms that the transient species detected
by ESR in Figure 4 is the same as that detected by UV-vis in
Figure 1 and that both species are definitely assigned to the
•
+ 36
keto form of BNAH . However, the λmax value of the keto
•+
form of BNAH is quite different from the reported λmax value
•
+
of NADH (540 nm) observed in biphotonic one-electron
12
oxidation of NADH upon laser excitation.
Chemiluminescence in Electron Transfer from BNAH to
3+
3+
Ru(bpy)3 . When Fe(bpy)3 is replaced by a stronger oxidant,
Ru(bpy)33 , virtually the same electron-transfer mechanism in
Scheme 2 is applied to the electron transfer from BNAH to
Ru(bpy)33 as shown in Scheme 4. In this case, however, the
+
+
2+
luminescence of Ru(bpy)3 (λmax ) 610 nm) is observed during
3+
the electron transfer from BNAH to Ru(bpy)3 as shown in
Figure 6a. The chemiluminescence spectrum in Figure 6a agrees
with the emission spectrum of Ru(bpy)32 (* denotes the
excited state) (Figure 6b). Figure 7a shows the dynamics of the
chemiluminescence observed in the electron transfer from
Figure 6. (a) Chemiluminescence spectrum observed during mixing BNAH
+*
-2
3+
-2
(
5.0 × 10 M) with Ru(bpy)3 (1.0 × 10 M) in deaerated MeCN at
2+*
298 K and (b) emission spectrum of Ru(bpy)3
nm.
with excitation at 452
3+
BNAH to Ru(bpy)3 . As in the case of electron transfer from
chemiluminescence of Ru(bpy)32 is observed until all BNAH
+*
3+
BNAH to Fe(bpy)3 , the initial electron transfer from BNAH
to Ru(bpy)33+ is too rapid to be monitored using a stopped-
molecules are consumed (see TOC).
When BNAH is replaced by BNAH-4,4′-d2, the kinetic
flow technique because of the largely negative free energy
0
isotope effects (k /k ) 1.4) are observed for both the decay
change of electron transfer from BNAH (Eox vs SCE ) 0.57
H
D
V) to Ru(bpy)3 (Ered0 vs SCE ) 1.24 V). The chemi-
5
b
3+
29
rate of the luminescence intensity and the rate of formation of
2
+
2
+*
H D
Ru(bpy)3 as shown in Figure 7b. The k /k value at 298 K
luminescence of Ru(bpy)3
is observed upon mixing MeCN
solutions of BNAH and Ru(bpy)33 , and the decay of the
+
agrees well with the value observed for the deprotonation of
•
+
BNAH (Table 1).
luminescence intensity at 610 nm obeys first-order kinetics (see
Supporting Information S3), which coincides with the formation
According to Scheme 4, the rate of deprotonation of the keto
2+
•+
rate of Ru(bpy)3 monitored at 450 nm (Figure 7a). When
BNAH is mixed with Ru(bpy)33 in an aqueous solution, BNAH
is solubilized into an aqueous solution slowly as the oxidation
of BNAH by Ru(bpy)33 proceeds. In such a case, the persistent
form of BNAH is the same as the formation rate of
+
2+
•
0
Ru(bpy)3 . The electron transfer from BNA (Eox vs SCE )
5
b
3+
0
29
-1.08 V) to Ru(bpy)3 (Ered vs SCE ) 1.24 V) is highly
+
0
exergonic (∆Get ) -2.32 eV) when the driving force of elec-
tron transfer is larger than the excitation energy of Ru(bpy)3
2+
(
36) The concentration of BNAH•+ detected by the ESR spectrum in Figure 4a
is much smaller than the concentration detected by the transient absorption
spectrum because of the short lifetime of BNAH•+ (0.24 s) and the longer
mixing time required for the ESR flow measurements (about several
hundreds of milliseconds) than the mixing time for the transient absorption
measurements (about several tens of milliseconds).
•
(
2.12 eV). In such a case, electron transfer from BNA to
3+
2+*
Ru(bpy)3 to produce Ru(bpy)3
is still exergonic (-0.20
3+
eV). The initial electron transfer from BNAH to Ru(bpy)3 is
not exergonic enough to produce Ru(bpy)3 . Thus, the
2
+*
J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003 4813

A R T I C L E S
Fukuzumi et al.
for the inVerted region has, however, been scarce.43 The
chemiluminescence observed in electron transfer from BNAH
3
+
to Ru(bpy)3 provides compelling evidence of the occurrence
of the Marcus inverted region even for an intermolecular
electron-transfer reaction.
When BNAH is replaced by 4-t-BuBNAH, no chemilumi-
2
+*
nescence due to Ru(bpy)3
has been observed in the electron
3+
transfer from 4-t-BuBNAH to Ru(bpy)3 . This is again ascribed
to the facile C-C bond cleavage in 4-t-BuBNAH (Scheme
). Electron transfer from t-Bu (Eox vs SCE ) 0.09 V) to
Ru(bpy)3 (Ered vs SCE ) 1.24 V) is not exergonic enough
•+
•
0
31
3
3
+
0
29
2
+*
to produce Ru(bpy)3
.
Formation of Radical Cations of NADH Analogues via
Photoinduced Electron Transfer. The singlet excited state of
1
*
BNAH ( BNAH ) can act as an extremely strong electron donor
judging from the highly negative one-electron oxidation potential
0
*
5b
1
*
(
Eox vs SCE ) -2.60 V). An electron transfer from BNAH
to CHBr3 (Ered vs SCE ) -1.55 V) is highly exergonic (∆Get0
-1.05 eV). Thus, laser excitation of an EtCN solution of
BNAH and bromoform (CHBr3) results in photoinduced electron
0
44
)
1
*
•+
transfer from BNAH to CHBr3 to produce BNAH (eq 1).
The transient absorption spectra in the visible region are
observed by the laser flash photolysis of a deaerated EtCN
-
4
solution of BNAH (4.0 × 10 M) and CHBr3 (0.25 M) with
3
55 nm laser light as shown in Figure 8a. The new absorption
appears at λmax ) 460 nm upon the laser excitation, and the
decay of this absorption band is accompanied by the appearance
of a new absorption band at 400 nm with isosbestic points. The
rate of disappearance of the absorption band at 460 nm obeys
first-order kinetics (see Supporting Information S4), coinciding
with the rate of appearance of the absorption band at 400 nm
(Figure 8b). Virtually the same transient absorption spectra were
observed in the photoinduced electron transfer from 4-EtBNAH
to CHBr3 (see Supporting Information S5).
Figure 7. (a) Time courses of emission intensity change at 610 nm and
-
4
absorption change at 450 nm for the reaction of BNAH (5.0 × 10 M)
3
+
-3
with Ru(bpy)3 (1.0 × 10 M) in deaerated MeCN at 298 K and (b) time
courses of emission intensity change at 610 nm and absorption change at
-
4
3+
4
50 nm for the reaction of BNAH-4,4′-d2 (5.0 × 10 M) with Ru(bpy)3
-3
(
1.0 × 10 M) in deaerated MeCN at 298 K.
When BNAH is replaced by 4-t-BuBNAH where the hydro-
gen at the C(4) position is substituted by a tert-butyl group, no
observation of the strong Ru(bpy)32+* emission indicates that
•
3+
electron transfer from BNA to Ru(bpy)3 occurs to produce
Ru(bpy)32 (∆Get ) -0.20 eV) as well as Ru(bpy)32+ (∆Get
+*
0
0
(40) (a) Osuka, A.; Noya, G.; Taniguchi, S.; Okada, T.; Nishimura, Y.;
Yamazaki, I.; Mataga, N. Chem.-Eur. J. 2000, 6, 33. (b) Mataga, N.;
Chosrowjan, H.; Shibata, Y.; Yoshida, N.; Osuka, A.; Kikuzawa, T.; Okada,
T. J. Am. Chem. Soc. 2001, 123, 12422.
)
-2.32 eV). The occurrence of an electron-transfer process
to produce the excited state with a smaller driving force as
compared with a process to produce the ground state with a
much larger driving force is a clear indication of the occurrence
of a Marcus inverted region where the ET rate (ket) decreases
(
41) (a) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607. (b) Fukuzumi,
S.; Guldi, D. M. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 2, pp 270-337. (c) Imahori, H.; Guldi, D.
M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J. Am.
Chem. Soc. 2001, 123, 6617. (d) Fukuzumi, S.; Ohkubo, K.; Imahori, H.;
Shao, J.; Ou, Z.; Zheng, G.; Chen, Y.; Pandey, P. K.; Fujitsuka, M.; Ito,
O.; Kadish, K. M. J. Am. Chem. Soc. 2001, 123, 10676. (e) Fukuzumi, S.;
Yoshida, Y.; Urano, T.; Suenobu, T.; Imahori, H. J. Am. Chem. Soc. 2001,
0
as the ET driving force (-∆Get ) increases for the strongly
0
37
exergonic region (-∆Get < λ). The Marcus inVerted region
has been, after a long controversy, well-established through
123, 11331.
studies on a number of donor/acceptor systems with fixed
(
42) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000;
Vol. 8, pp 153-190. (b) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem.
Res. 2001, 34, 40. (c) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18.
(d) Verhoeven, J. W. In Electron Transfer-From Isolated Molecules to
Biomolecules; Jortner, J., Bixon, M., Eds.; John Wiley & Sons: New York,
1999; Part 1, pp 603-644.
distances.38-42
In intermolecular ET reactions, decisive evidence
(
37) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus, R.
A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (c) Marcus, R. A.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
(38) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984,
(43) There have been considerable discussions on the reasons for the non-
observance in the intermolecular electron-transfer reactions, see: (a) Weller,
A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 590. (b) Suppan, P. Top.
Curr. Chem. 1992, 163, 95. (c) Efrima, S.; Bixon, M. Chem. Phys. Lett.
1974, 25, 34. (d) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. Am. Chem.
Soc. 1984, 106, 6858. (e) Barzykin, A. V.; Frantsuzov, P. A.; Seki, K.;
Tachiya, M. AdV. Chem. Phys. 2002, 123, 511 and references therein.
(44) Bonesi, S. M.; Erra-Balsells, R. J. Chem. Soc., Perkin Trans. 2 2000, 1583.
106, 3047. (b) Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (c) Gould,
I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522. (d) McLendon, G. Acc.
Chem. Res. 1988, 21, 160. (e) Winkler, J. R.; Gray, H. B. Chem. ReV.
1
992, 92, 369. (f) McLendon, G.; Hake, R. Chem. ReV. 1992, 92, 481.
(
39) Mataga, N.; Miyasaka, H. In Electron Transfer from Isolated Molecules to
Biomolecules Part 2; Jortner, J., Bixon, M., Eds.; Wiley: New York, 1999;
p 431.
4814 J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003

Electron-Transfer Oxidation of NADH Analogues
A R T I C L E S
Scheme 5
•+
Table 3. Rate Constant (k) of Tautomerization of BNAH and
•
+
BNAH-4,4′-d2 in Deaerated EtCN and the Kinetic Deuterium
Isotope Effect (kH/kD)
T, K
10-
5
k
H
, s-1
10-5
k
D
, s-1
H D
k /k
2
2
2
2
2
98
83
68
43
28
3.2
2.7
2.2
1.6
1.2
1.7
1.2
0.9
0.5
0.3
1.9
2.2
2.4
3.2
4.1
Figure 9. Eyring plots of the keto-enol tautomerization rate constant (k)
•
+
•+
of BNAH (O) and BNAH-4,4′-d2 (b) in deaerated EtCN.
the keto form (380 nm), being closer to the reported absorption
•
+
band of NADH (λmax ) 540 nm) produced by the biphotonic
1
2,46
laser excitation of NADH in an aqueous solution.
Thus, the
Figure 8. (a) Transient absorption spectra for the photoinduced electron
•
+
1
*
-4
NADH generated in such a high energy process may also be
the enol form. However, the definitive assignment of NADH
transfer from BNAH (4.0 × 10 M) to CHBr3 (0.25 M) in deaerated
EtCN at 298 K (0.5, 3, and 10 µs after irradiation of laser pulse at λ ) 355
nm with 20 mJ/pulse). (b) Time courses of the absorption change at 400
and 460 nm.
•+
47
has yet to be made.
The conversion from the enol form to the keto form of
•+
absorption band appears in the visible region (see Supporting
BNAH is intramolecular proton transfer. In such a case, kinetic
isotope effects are expected to be observed. This is confirmed
by determining the conversion rate from the enol form to the
2
Information S6) due to the facile and ultrafast C-C bond
•
+
cleavage in 4-t-BuBNAH .
Because the observed absorption band at 400 nm in Figure
keto form using BNAH-4,4′-d . The rate constants of intra-
•+
•+
8a is overlapped with bleaching of the absorption band due to
H
molecular proton transfer of BNAH (k ) and BNAH-4,4′-d2
BNAH, this absorption band is virtually the same as the
(k ) were determined from the decay of the absorption band at
D
•
+
absorption band at 380 nm due to the keto form of BNAH
observed in the thermal electron transfer from BNAH to
•
+
(45) The DFT calculations indicate that the enol form of BNAH is by 3.9
kcal mol- more stable than the keto form, consistent with the result in the
1
Ru(bpy)33 (Figure 1) and Fe(bpy)3
+
3+ 18
.
Thus, the absorption
10,11
gas phase.
The observed conversion from the enol to the keto form in
solution suggests that the keto form is more stabilized by solvation than
the enol form.
(46) The absorption maximum at 460 nm observed in Figure 8a is different
band observed initially at 460 nm is assigned to the enol form
•
+
of BNAH . In the highly exergonic electron transfer from
1
3
1
*
•+
from that reported previously. However, the assignment in our study is
supported by the detailed dynamics and the kinetic isotope effects and also
by the results in thermal electron-transfer reactions, monitored by the ESR
BNAH to CHBr3, the enol form of BNAH , which is higher
in energy than the keto form, may be formed initially, followed
by the conversion to the thermodynamically more stable keto
and transient absorption spectra. The difference in solvents (H
2
O and
•
+
MeCN) may cause the difference in the absorption spectra of NADH
•
+
45
•+
form of BNAH (λmax ) 400 nm) as shown in Scheme 5.
and BNAH
.
•
+
•
+
(47) We could not detect NADH in the electron-transfer oxidation of NADH
The absorption band due to the enol form of BNAH (λmax
3+
•+
by Fe(bpy)
3
in an aqueous solution because of the instability of NADH
)
460 nm) in MeCN is significantly red-shifted from that of
even by using a stopped-flow technique.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003 4815

A R T I C L E S
Fukuzumi et al.
transient absorption spectrum is observed by the laser flash
-
3
photolysis of a deaerated MeCN solution of BNAH (3.0 × 10
2
+
-4
M) and Ru(bpy)3 (1.0 × 10 M) with 450 nm laser light as
shown in Figure 10a. At 2 µs after the laser excitation, only
the absorption band at 400 nm due to the keto form is observed
+
48
together with the absorption band at 510 nm due to Ru(bpy)3 ,
produced in the electron transfer from BNAH to Ru(bpy)32+*.
2
+*
The electron transfer from BNAH to Ru(bpy)3
is slightly
0
5b
exergonic (∆Get ) -0.22 eV); therefore, the keto form of
BNAH is produced directly in contrast with the case of the
•
+
1
*
highly exergonic electron transfer from BNAH to CHBr3.
When BNAH is replaced by 4-t-BuBNAH, only the absorp-
+
tion band at 510 nm due to Ru(bpy)3 is observed as shown in
Figure 10b where no transient absorption spectrum due to 4-t-
•
+
BuBNAH is detected due to the facile C-C bond cleavage
•
+
in 4-t-BuBNAH (Scheme 2).
Summary and Conclusions
In this study, we present the first direct detection of radical
cations of NADH analogues by ESR. The ESR spectra have
clearly shown that the radical cation is the keto form. This
•
+
indicates that the keto form of BNAH is more stable than the
•+
enol form of BNAH in solution, which is opposite to the case
1
0,11
•+
in the gas phase.
The enol form of BNAH was observed
only in the case of highly exergonic electron transfer in which
•
+
direct production of the keto form of BNAH is unfavorable.
The observation of chemiluminescence when BNAH is oxidized
3
+
•
by Ru(bpy)3 indicates that BNA is formed by deprotonation
•
+
of BNAH .
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Area (Nos.
1
1228205, 13440216, and 14045249) from the Ministry of
Figure 10. (a) Transient absorption spectra in photoinduced electron
-
3
2+*
-4
Education, Culture, Sports, Science, and Technology, Japan.
transfer from BNAH (3.0 × 10 M) to Ru(bpy)3
(1.0 × 10 M) and
-4
(
b) transient absorption spectra in photoinduced electron transfer from 4-t-
Supporting Information Available: Spectral change in the
-
2
2+*
BuBNAH (1.0 × 10 M) to Ru(bpy)3
(1.0 × 10 M) in deaerated
3
+
reaction between BNAH and Ru(bpy)3 (S1), spectral change
in the reaction between 4-t-BuBNAH and Fe(bpy)33 (S2), the
first-order plots in the reaction between BNAH and Ru(bpy)33
MeCN at 298 K (2 µs after irradiation of laser pulse at λ ) 450 nm with
+
1
0 mJ/pulse).
+
460 nm or the appearance of the absorption band at 400 nm at
(
S3), the first-order plots in photoinduced electron transfer from
BNAH to CHBr3 (S4), transient absorption spectra in photo-
various temperatures. The results are summarized in Table 3.
The kinetic isotope effects (kH/kD) increase with a decrease in
the temperature. The Eyring plots of the intramolecular proton
1
*
1
*
induced electron transfer from 4-EtBNAH to CHBr3 (S5),
transient absorption spectra in photoinduced electron transfer
from 4-t-BuBNAH to CHBr3 (S6) (PDF). This material is
•
+
•+
transfer of BNAH and BNAH-4,4′-d2 are shown in Figure
. The activation parameters obtained from the Eyring plots are
1
*
9
available free of charge via the Internet at http://pubs.acs.org.
listed in Table 2.
JA029623Y
Photoinduced electron transfer from BNAH to the excited
2+
2+*
state of Ru(bpy)3 (Ru(bpy)3 ) was also examined. The
(48) Baxendale, J. H.; Fiti, M. J. Chem. Soc., Dalton Trans. 1972, 1995.
4816 J. AM. CHEM. SOC.
9
VOL. 125, NO. 16, 2003