15640
J. Phys. Chem. 1996, 100, 15640-15643
Magnetic Field Effects on the Hydrogen Abstraction Reactions of Triplet Benzophenone
with Thiophenol in Nonviscous Homogeneous Solutions
Masanobu Wakasa and Hisaharu Hayashi*
Molecular Photochemistry Laboratory, The Institute of Physical and Chemical Research (RIKEN),
Wako, Saitama 351-01, Japan
ReceiVed: May 31, 1996; In Final Form: August 2, 1996X
The hydrogen abstraction reaction of triplet benzophenone with thiophenol at room temperature was investigated
in several nonviscous homogeneous solutions (η ) 0.29-3.33 cP) by means of a nanosecond laser flash
photolysis technique under magnetic fields of 0-10 T. The escape yield of the benzophenone ketyl radical
decreased with increasing magnetic field strength from 0 to 10 T. In 2-methyl-1-propanol (η ) 3.33 cP), the
yield at 10 T reduced to 80 ( 2% of that at 0 T. The observed magnetic field effects can be interpreted by
the ∆g mechanism when the geminate recombination process is faster than or comparable to the escape one.
Introduction
ns). We, therefore, have noticed that if an MFE is finished
within 10 ns, it is not necessary to use such confined systems
as micellar solutions, viscous solutions, and linked biradicals.
Then, we chose a reaction of benzophenone with thiophenol
and studied the MFEs in nonviscous homogeneous solution.
Since the expected radical pair has substantially a large ∆g
value11,12 and small hyperfine couplings,13-15 one can optimize
the ∆g mechanism. In this letter, we report on large MFEs
observed for the hydrogen abstraction reaction of triplet ben-
zophenone with thiophenol in several nonviscous homogeneous
solutions.
Magnetic field effects (MFEs) on the dynamic behavior of
radical pairs have been widely investigated in confined systems
such as micellar solutions, viscous solutions, and linked
biradicals.1-3 Although there have been quite many reports of
the MFEs on the dynamic behavior of radical ion pairs in
nonviscous homogeneous solutions, it has been believed to be
difficult to observe appreciable MFEs of neutral radical pairs
in nonviscous homogeneous solutions for the following rea-
sons: (1) For the appearance of appreciable MFEs, the radical
pair lifetime should be comparable to the singlet-triplet spin
evolution time of radical pairs. (2) In usual nonviscous
homogeneous solutions at room temperature, the lifetime of
neutral radical pairs (10-9-10-10 s) is much shorter than the
spin evolution time (10-8-10-9 s).1-3
To the best of our knowledge, however, there have been only
two reports on the MFEs of the dynamic behavior of neutral
radical pairs in nonviscous homogeneous solutions,4,5 although
the MFEs on the product yields were reported in the early works
of MFEs.6,7 Staerk and Razi Naqvi reported in the reaction of
triplet benzophenone with n-hexane (solvent) that a slight
decrease (-8% at 0.6 T) in the free radical yield was observed
with increasing magnetic fields (B) from 0 to 0.6 T.4 The
mechanistic interpretation of this MFE was, however, unclear
because the suggested ∆g mechanism was not substantiated in
a quantitative theoretical model calculation by Schulten and
Epstein.8 More recently, Sakaguchi and Hayashi reported more
reliable MFEs on the dynamic behavior of a neutral radical pair
in nonviscous homogeneous solutions.5 In the reaction of
triphenylphosphine, they reported that the yield of the escaped
diphenylphosphinyl radical at 1.5 T reduced to 74% of that of
at 0 T in 2-propanol. Although they concluded that the main
feature of their MFEs was ascribed to the ∆g mechanism, the
interpretation of their MFEs has still been unclear. Thus, the
study of MFEs of neutral radical pairs in nonviscous homoge-
neous solutions is now an underdeveloped research area.
Recently, we have found that the initial yields of biradicals
formed by the hydrogen abstraction reactions of benzophenone-
phenol9 or -thiophenol10 chain-linked compounds decrease with
increasing B from 0 to 1.7 T. In these reactions, the recombina-
tion rate of biradical is so fast that the generation of the MFEs
is completed within the time resolution of our apparatus (∼10
Experimental Section
Benzophenone was recrystallize from methanol. Thiophenol
was purified by vacuum distillation. Acetonitrile, methanol,
ethanol, 1-propanol, 2-propanol, 2-methyl-1-propanol, n-hexane,
and cyclohexane were used without further purification.
Nonanenitrile (CH3(CH2)7CN) was purified by vacuum distil-
lation. The concentrations of benzophenone, thiophenol in the
employed solutions were 20 × 10-3 and 120 × 10-3 mol dm-3
,
respectively.
Laser flash photolysis experiments in the absence and
presence of a magnetic field were performed on an apparatus
that was similar to that described elsewhere.16 Each nitrogen-
bubbled solution was circulated through a quartz cell (20 mL/
min). The third (355 nm) harmonic of a Quanta-Ray GCR-
100 Nd:YAG laser was used as an exciting light source. The
transient absorption was recorded by a Hewlett-Packard
HP54522A digitizing oscilloscope with a time resolution of 0.5
ns. Magnetic fields (B) up to 1.7 T were provided by a Tokin
SEE-10W electromagnet and those up to 10 T by an Oxford
37057 superconducting magnet. The lowest magnetic field
generated by a countercurrent for canceling the residual field
was less than 0.2 mT. Hereafter, the experiments under the
lowest magnetic field are denoted as those in the absence of a
magnetic field.
Results and Discussion
Laser flash photolysis was performed at room temperature
on each of the nonviscous homogeneous solution containing
benzophenone (BP) and thiophenol (PhSH). The transient
absorption bands of benzophenone ketyl (BPH•, 550 nm)17 and
X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01599-7 CCC: $12.00 © 1996 American Chemical Society