excited-state hypersurface,2
0-26
the mechanism of this pho-
that such an assignment to the type of hydrogen-donating
group (O-H vs C-H), solely on the basis of a kinetic solvent
effect, may not be compulsory. In particular, we observed
sizable solvent effects also in hydrogen abstractions from
C-H and even Sn-H bonds, which calls for a generalization.
Accurate measurements of rate constants for hydrogen
abstraction, as required for the investigation of solvent
effects, depend critically on the direct spectroscopic and time-
resolved monitoring of the abstracting species. In our
experimental studies (cf. Supporting Information), we em-
ployed two types of reactive species, which fulfill the
prerequiste of direct monitoring and which additionally
exhibit a clear-cut radical-like reactivity and high hydrogen
abstracting propensity (Figure 1). The first species, the
toreaction, which involves a transition state for hydrogen
atom abstraction, has been established in detail through
21-24
19,21,24,26-31
comprehensive computational
and experimental
investigations. In particular, DBO is known to undergo pure
hydrogen abstraction reactions without notable complications
due to competitive charge-transfer. The latter mechanism
is commonly encountered for alternative excited-state radical
models, e.g., n,π* triplet-excited benzophenone.
addition, hydrogen abstractions by singlet-excited DBO can
be monitored with very high precision by time-resolved
fluorescence. The choice of the reactive states (electrophilic
for cumyloxyl vs nucleophilic for DBO) as well as the
broad range of hydrogen donors (electrophilic C-H in
chloroform and O-H in alcohols vs nucleophilic C-H in
alcohols, ethers, and alkanes, or Sn-H in tin hydrides) were
3
1
1
4
20,31,32
In
3
0
1
6,19-21,24,29
meant to allow further generalizations.
In analogy to previous studies,
3
,29
the rate constants for
hydrogen abstraction (k ) were taken as the scavenging rate
H
constants for cumyloxyl radicals (Table 1, Figure 2) and as
Table 1. Scavenging Rate Constants for Cumyloxyl Radicals
7
-1 -1 a
kH/(10 M
s )
scavenger
phenol
tri-n-butyltin hydride
n-hexane
benzene
acetonitrile
0.58
40
90 [72]b
37
8.9 [7.5]
28
26
b
1
1
,3-cyclohexadiene
,4-cyclohexadiene
6.5
5.4
1.4
0.62
0.72
5.3
5.9
0.73
0.28
0.47
Figure 1. Conceptual relationship between reactive species and
spectroscopic probes with high hydrogen abstraction propensity:
oxygen centered (left), nitrogen centered (right), ground-state
radicals (top), and n,π*-excited states (bottom).
6.7
1.6
1.0
hexamethylbenzene
c
d
diphenylmethanol
tetrahydrofuran
0.83
a
0% estimated error. b Value in cyclohexane. c Data measured for
1
d
•
tert-butoxyl radicals. Measured in n-hexane/benzene (2:1).
cumyloxyl radical (CumO ), shows, as first observed by
Lusztyk and co-workers,15 a characteristic transient absorp-
tion in the visible range, which can be conveniently generated
by 308 nm laser-flash photolysis of dicumylperoxide. The
second species studied is the n,π* singlet-excited state of
the fluorescence quenching rate constants for singlet-excited
DBO (Table 2, Figure 3). The data were determined in the
gas phase, n-hexane, benzene, and acetonitrile. Selected
measurements in the more viscous cyclohexane (values in
square brackets) showed that viscosity effects on the rate
2,3-diazabicyclo[2.2.2]oct-2-ene (DBO). Although the hy-
drogen abstractions of singlet-excited DBO (as well as
16-21
singlet-excited ketones) proceed with low quantum yields,
owing to the intervention of a conical intersection on the
(21) Nau, W. M.; Greiner, G.; Rau, H.; Olivucci, M.; Robb, M. A. Ber.
Bunsenges. Phys. Chem. 1998, 102, 486-492.
(
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, 3665-3668.
14) Previous measurements were also done by indirect monitoring or
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2002, 1, 537-546.
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253, 92-96.
•
by following absorption changes of (persistent) DPPH radicals. The latter
are only reactive with the most potent hydrogen donors and react on a
comparatively slow timescale (minutes). Our efforts with DPPH radicals
led to poorly reproducible results, presumably due to interfering complica-
tions such as hydrogen back transfer or recombination of product radicals
with DPPH .
•
•
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