Direct spectroscopic detection and EPR investigation of a ground state triplet phenyl oxenium ion

Article abstract of DOI:10.1021/jacs.5b06302

Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR³), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S₁ over 800 fs are followed by bond heterolysis in ～1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ～50 ps followed by intersystem crossing in ～300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR³ experiments, permitting a more clear assignment and identifying the triplet state as the π,π? triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ～4 K and observing a triplet species by cryogenic EPR spectroscopy.

Full text of DOI:10.1021/jacs.5b06302

Page 1 of 8
Journal of the American Chemical Society
Direct Spectroscopic Detection and EPR Investigation of a
Ground State Triplet Phenyl Oxenium Ion
1
2
3
4
MingꢀDe Li, † Toshia R. Albright,‡ Patrick J. Hanway,‡ Mingyue Liu,† Xin Lan,† Songbo Li,† Julie
5
6
7
Peterson,‡ Arthur H. Winter*,‡ and David Lee Phillips*†
8
9
†Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., P. R. China
‡Department of Chemistry, Iowa State University, 2101d Hach Hall, Ames, Iowa 50011, United States
10
11
12
13
14
15
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
KEYWORDS triplet, oxenium ions, time-resolved spectroscopy, matrix EPR spectroscopy
Supporting Information Placeholder
ABSTRACT: Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the
reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have
allowed these shortꢀlived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors,
but all of the studies to date have focused on aryloxenium ions having closedꢀshell singlet ground state configurations. To study
alternative spin configurations, we synthesized a photoprecursor to the mꢀdimethylamino phenyloxenium ion, which is predicted by
both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of
femtosecond and nanosecond transient absorption spectroscopy, nanosecond timeꢀresolved Resonance Raman spectroscopy
(nsꢀTR3), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the
complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion.
Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S1 over 800 fs
are followed by bond heterolysis in ~1 ps, generating a hot closedꢀshell singlet oxenium ion that undergoes vibrational cooling in
~50 ps followed by intersystem crossing in ~300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid
trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom
abstractions on the microsecond time domain to ultimately yield the reduced mꢀdimethylaminophenol as the only detectable stable
photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to
related known species. The triplet oxenium ion was also detected in the nsꢀTR3 experiments, permitting a more clear assignment
and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by
photolysis of the photoprecursor in an ethanol glass at ~4 K and observing a triplet species by cryogenic EPR spectroscopy.
poly(phenyl)ether (PPE), an industrial thermoplastic.12ꢀ14 In
INTRODUCTION
Oxenium ions are reactive intermediates of formula RꢀO+.
addition, the enzymatic oxidative mechanisms of phenols to
quinones are proposed to involve the intermediacy of
These species are the isoelectronic oxygen analog of the
discrete oxenium ions,15 Despite their chemical importance,
more familiar nitrene class of intermediates, but bear a
formal positive charge on a hypovalent oxygen. Like
mechanistic studies of oxenium ions are limited.
nitrenes, oxenium ions can be stabilized by transition
metals,1 acting as novel ligands, but in their free form they
are shortꢀlived in solution, as might be expected of a species
forced to suffer from such an inharmonious electronic
arrangement. Of more synthetic relevance, these transient
species have often been proposed as intermediates in a
number of synthetically useful oxidation reactions of
phenols,2ꢀ6 including the oxidative Hosomi−Sakurai
Previous experimental studies of aryloxenium ions have
been performed by either photochemical or thermal
generation methods. In the 1970s and early 1980s,
Abramovitch and Okamoto used thermolytic methods to
generate aryloxenium ions. Studies of their stable end
products permitted indirect characterizations of their
reactivity.13,14 Later, Novak and coworkers investigated the
reactivities of aryloxenium ions using both thermal and
photochemical generation methods.16 Only recently have
aryloxenium ions been directly detected by laser flash
photolysis in solution. In 2007, Novak, Platz, and coworkers
reported the first direct detection of an aryloxenium ion in
reaction7,8
electrochemical oxidations of phenols and phenolates,10,11
and plethora of other phenolic oxidations and
and
Wagner−Meerwin
transposition,9
a
tautomerization reactions. They are also suggested to be key
intermediates in the industrial production of materials like
ACS Paragon Plus Environment

Journal of the American Chemical Society
solution (a biphenylyl oxenium ion) using laser flash
Page 2 of 8
1
2
photolysis, providing definitive evidence that these reactive
ions are in fact discrete intermediates in solution. 17
3
4
5
6
7
8
9
Like nitrenes, oxenium ions can adopt different electronic
configurations. The electronic state energies of phenyl
oxenium ions and some simple substituted derivatives have
previously been computed using the CASPT2//CASSCF
computational method.18ꢀ20 These computational studies
suggested that oxenium ions undergo large changes in the
electronic state orderings by changing the substituent
attached to the formally positive oxygen. For instance, the
simplest oxenium ion, OH+, has degenerate frontier orbitals
and a triplet ground state with a gap of 54 kcal/mol to the
lowest energy singlet state.21 Substituting the hydrogen with
a phenyl ring leads to the lowestꢀenergy state being a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Overview of this work and prior work.
All of the prior studies that directly detected aryloxenium
ions studied oxenium ions that had closedꢀshell singlet
ground states. We were interested in detecting a triplet
ground state oxenium ion in order to understand the
spinꢀselective reactivity of oxenium ions and to be able to
compare the lifetimes, properties, and reaction types of
triplet oxenium ions to their closedꢀshell singlet counterparts.
closedꢀshell singlet ground state by
~ 20 kcal/mol.
Computations suggest most simple aryloxenium ions are
closedꢀshell singlet ground state species,19 although
heteroaryl oxenium ions may adopt alternative electronic
20
states. The singlet ground state of the phenyl oxenium ion
is supported by both photoelectron spectroscopy22 and the
results from these highꢀlevel computational methods.
Thus,
we
synthesized
the
mꢀdimethylamino
phenylhydroxylamine (mꢀDMAP) salt 1, which we
anticipated could generate a ground state triplet aryloxenium
ion 32 upon photolysis. The different electronic
configurations possible for the aryloxenium ion are depicted
in Figure 2.
Following Novak and Platz’s detection of a substituted
biphenylyl oxenium ion derivative, we used selected
protonated hydroxylamine salts as novel photoprecursors to
generate both the parent phenyloxenium ion and the
biphenylyl oxenium ion as
a
product from the
photoheterolysis of the aryl hydroxylamine tetrafluoroborate
salt. 23 In addition to a concomitant homolytic process, these
hydroxylamine salts also undergo heterolytic scission of the
OꢀN bond to lead to the generation of the singlet
aryloxenium ion and neutral ammonia.23,24 These results
allowed comparisons to the neutral photoprecursor studied
by Novak, Platz and coworkers,25 highlighting the unique
differences in the photochemistry/photophysics of the
precursors.26 To summarize the prior results, the
aryloxenium ions studied thus far have had closedꢀshell
singlet ground states and lifetimes of a few nanoseconds in
solution, typically reacting with nucleophiles (e.g. ammonia,
chloride, water) at the ortho and para positions to generate
ringꢀsubstituted phenols as the ultimate stable photoproducts.
See Figure 1. To date, essentially nothing is known about the
reactivity of triplet aryloxenium ions.
Figure 2. Possible schematic electronic configurations
considered for the mꢀdimethylamino phenyloxenium ion 2.
Computations suggest the ground configuration is the π,π*
triplet configuration and the lowest energy singlet state is the
closedꢀshell configuration.
Computational studies have suggested that simple ring
substituents could lead to significant changes in the
Here, we report a combined femtosecond transient
absorbance (fsꢀTA), nanosecond transient absorbance
(nsꢀTA), and nanosecond timeꢀresolved Resonance Raman
(nsꢀTR3) spectroscopic investigation of the photophysics and
photochemistry of mꢀDMAP 1 that allowed us to directly
observe the formation of the ground state triplet
phenyloxenium ion after ultraviolet photoꢀexcitation in a
MeCN:H2O solution. We also studied the triplet ion by
cryogenic matrix photolysis of the photoprecursor in an
EtOH glass and observed a triplet species in the electron
paramagnetic resonance (EPR) spectrum at 4 K, providing
strong evidence for this ion having a triplet ground state. To
our knowledge this is the first direct observation and
reactivity study of a ground state triplet oxenium ion in
solution using LFP, and the first EPR detection of a triplet
aryloxenium ion.
19
singletꢀtriplet energy gap (ΔEST). A recent computational
study on the effect of meta substitution on the ΔEST of
phenyloxenium ions found that substituting the meta
positions of the phenyloxenium ions with pi donors (e.g.
NH2) stabilizes an mꢀxylyleneꢀlike π,π* triplet state in
preference to the singlet state.18 For the mꢀamino
phenyloxenium ion, the ground state was computed to be the
triplet state by DFT computations, but was computed to have
essentially degenerate singlet and triplet energies at the
CASPT2/pVTZ level of theory. Consequently, the ground
state of the mꢀamino phenyloxenium ion cannot be predicted
with certainty. Although not explicitly considered in that
study, our thought was that the mꢀdimethylamino
phenyloxenium, bearing a stronger meta pi donor than an
amino substituent, might have a more definitively triplet
ground state.
2
ACS Paragon Plus Environment

Page 3 of 8
Journal of the American Chemical Society
Experimental and Computational Methods
The mꢀdimethylamino phenylhydroxylamine
The laser beams were lightly focused and lined up so that
they merged together onto a flowing argon purged sample
solution. pulse delay generator was utilized to
1
2
3
4
5
6
7
8
1
A
photoprecursor was synthesized, purified, and characterized
via standard methods (see Supporting Information for
synthetic procedures and spectra). Spectroscopic grade
acetonitrile (MeCN) and deionized water were used to
prepare the sample solutions for use in the timeꢀresolved
spectroscopy experiments. In all of the timeꢀresolved
experiments, the sample solutions were purged with argon
for 30 min. During the experiments, the sample solutions
were also purged with argon.
electronically set the time delay between the pump and probe
laser pulses. The Raman scattered signal was collected using
a backscattering geometry and observed by a liquid
nitrogenꢀcooled chargeꢀcoupled device (CCD) detector. The
nsꢀTR3 spectra shown here were found from subtraction of
an appropriately scaled probeꢀbeforeꢀpump spectrum from
the correlated pumpꢀprobe resonance Raman spectrum to
mostly get rid of nonꢀtransient bands. The Raman bands of
MeCN were used to calibrate the Raman shifts with an
estimated uncertainty of 5 cmꢀ1.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A.
Femtosecond Transient Absorption (fs-TA)
Experiments. The fsꢀTA experiments were performed by
employing an experimental setup and methods detailed
previously.27 Only a brief description is provided here.
D. Product and Matrix isolation EPR Studies.
Photolysis studies were performed as follows: addition of 10
mg of mꢀdimethylaminophenylhydroxylamine hydrochloride
1 and 5ꢀ10 mg of sodium acetate trihydrate (internal standard)
were added to 0.8 mL of deuterium oxide and placed in a
FsꢀTA measurements were accomplished using
a
femtosecond regenerative amplified Ti:sapphire laser system
in which the amplifier was seeded with the 120 fs laser
pulses from an oscillator laser system. The laser probe pulse
was produced by utilizing ~5% of the amplified 800 nm laser
pulses to generate a whiteꢀlight continuum (340ꢀ700 nm) in a
CaF2 crystal and then this probe beam was split into two
parts before traversing the sample. One probe laser beam
goes through the sample while the other probe laser beam
goes to the reference spectrometer in order to monitor the
fluctuations in the probe beam intensity. For the experiments
discussed in this work, a 40 ml solution was flowed through
a 2 mm pathꢀlength cuvette to avoid problems with
interference from photolysis of secondary photoproducts.
This flowing sample was excited by a 267 nm pump laser
beam. An absorbance of 1 at 267 nm was used for the
sample solutions for the fsꢀTA experiments in order to
maintain the same number of photons being absorbed for the
same irradiating conditions for the samples.
1
quartz NMR tube. An initial H NMR was taken with a 90°
angle and a relaxation delay of 60 seconds. The solution
was then degassed for 30 minutes (under argon) and
photolyzed for 1 h in a Rayonet photoreactor fitted with 254
nm bulbs. At different time intervals of photolysis, NMR
spectra were taken with the previous parameters. At 44%
conversion of starting material, 89% mass balance of
3ꢀdimethylaminophenol was observed. No other products
were seen by 1H NMR (see Supporting Information for
spectra changes upon photolysis) and the remaining mass
balance could be accounted for by formation of an insoluble
colored tar.
Matrix EPR studies were performed using an Xꢀband EPR
spectrometer with a temperature control unit capable of
achieving near liquid helium temperatures (~4 K) and
equipped with
a UVꢀemitting mercury light source
channeled into the cavity using a fiber optic cable. More
detailed information on the EPR settings and EPR simulation
parameters can be found in the Supporting Information.
E. Computational Studies. We computed the
singletꢀtriplet gap (ꢀEST) of the mꢀdimethylamino
phenyloxenium ion 2 using density functional theory
(B3LYP/6ꢀ31G(d,p)). The ꢀEST at the DFT level of theory
was computed to be +12.5 kcal/mol in favor of the triplet
state. Because we found a RHFꢀUHF instability in the
singlet state by DFT, for added confidence an
MRMP2(8,8)/6ꢀ31G(d)//MCSCF(8,8)/6ꢀ31G(d) calculation
was also performed using a complete pi active space. At this
level of theory, the ꢀEST is computed to be +9.3 kcal/mol in
favor of the triplet state and the lowest energy singlet state is
found to be the closed shell configuration. Thus, the
computational studies make a clear prediction that the triplet
state is the lowest energy electronic configuration for 2, with
the triplet state being a π,π* configuration. This π,π* triplet
configuration can be arrived at conceptually by starting with
the closedꢀshell singlet state and transferring an electron
from the nitrogen lone pair to the empty π* orbital associated
with oxenium ion center. See Figure 2. The TDꢀDFT
methodology was performed to predict the UVꢀvis
absorption spectra of the candidate transient species
generated from the photolysis of the photoprecursor
B. Nanosecond Transient Absorption (ns-TA)
Experiment. The nsꢀTA experiments were performed using
a commercial laser flash photolysis apparatus. The fourth
harmonic output of an Nd:YAG laser supplied the 266 nm
laser pump pulse. The probe light came from a 450 W Xenon
lamp. The 266 nm pump laser beam photoꢀexcited the
sample and at a right angle the probe light from the Xenon
lamp traversed the sample held in a 1 cm flowing quartz cell.
The transmitted probe light was then detected by a single
detector (for kinetic analysis) or by an array detector (for
spectral analysis). The changes in the light transmission
signals were normally converted into changes of optical
density (ꢁOD) and these signals were then analyzed by a
monochromator equipped with a photomultiplier to detect
the light. Unless specified otherwise, the nsꢀTA experiments
were performed in argon purged solutions and the sample
solutions were made up to have an absorbance of 1 at 266
nm.
C. Nanosecond Time-Resolved Resonance Raman
(ns-TR3) Experiments. The nsꢀTR3 experiments were
performed employing an experimental setup and methods
detailed previously and only a brief account is provided here.
28
The fourth harmonic of a Nd:YAG nanosecond pulsed
laser supplied the 266 nm pump wavelength and the 355 nm
probe wavelength came from the third harmonic of a second
Nd:YAG laser. The pump pulse photoꢀexcited the sample to
start the photochemical processes and the probe pulse
monitored the sample and the intermediate species formed.
(
TDꢀB3LYP/6ꢀ311G(2d,p)) 29
.
GaussSum software was
utilized to simulate the UV−vis spectra.30 To predict the TR3
spectra, secondꢀorder MøllerꢀPlesset perturbation theory
(MP2) with a 6ꢀ311G(d,p) basis set was employed to
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 8
Figure 3. Shown are the fsꢀTA spectra of species produced in a
optimize the structures and predict the Raman spectra of key
intermediates. A Lorentzian function with a 15 cmꢀ1
bandwidth for the vibrational frequencies and a frequency
scaling factor of 0.974 was used in the comparison of the
calculated results with the experimental spectra.31 No
imaginary frequency modes were observed at the stationary
states of the optimized structures. All of the calculations
were done using the Gaussian 03/09 program32 except for the
MRMP2//MCSCF computations, which were computed with
the GAMESS software. 33
1
2
3
4
5
6
7
8
MeCN:H2O 1:1 solution acquired after 266 nm irradiation of the
precursor mꢀDMAP (Top left) LFP from 0.96 ps to 1.6 ps; (Top
right) from 1.6 ps to 12 ps; (Bottom left) from 19 ps to 2990 ps;
Bottom right shows the kinetics of the characteristic fsꢀTA
absorption bands observed at 357 nm, 484 nm for the fsꢀTA
spectra observed after 266 nm photoexcitation of mꢀDMAP 1 in
a MeCN:H2O 1:1 solution. See text for more details.
Subsequently, the two bands decay over ~550 ps to generate
a new band at ~485 nm. This conversion is attributed to
9
1
intersystem crossing of the singlet oxenium ion 2 to the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
RESULTS AND DISCUSSION
triplet phenyloxenium ion 2. Finally, this band converts to a
new band over ~ 1 µs to a band at 494 nm, attributed to the
mꢀdimethylamino phenol radical cation (see Figure 7), which
slowly decays into a final product at 357 nm. An isosbestic
point at 394 nm indicates a clean conversion between the
latter two species.
1. Femtosecond transient absorption investigation of
m-dimethylamino phenylhydroxylamine hydrochloride 1.
Figure 3 shows the evolution of the transient absorption
of mꢀdimethylamino phenylhydroxylamine (mꢀDMAP) 1 in
an acetonitrile:water (MeCN:H2O) 1:1 mixed solution from
0.96 ps to 2990 ps. The spectra at early (Figure 1, top left),
middle (Figure 1, top right) and later delay times (Figure,
bottom left) are given separately to more easily discern the
spectral changes that take place on different time scales. First
we discuss the observations and our assignments of the
transients and then we detail how we made all of the
assignments. When using 267 nm as excitation wavelength,
the mꢀDMAP 1 can be excited to higher singlet excited states
Sn. TDꢀDFT computations indicate that the predominant
absorption of 1 at 267 nm is the S0ꢀS3 transition. See
Supporting Information. Over ~800 fs, 1 undergoes internal
conversion (IC) and evolves from Sn into the S1 excited state
(see Figure 1S and Table 1S for spectra from 450ꢀ1000 fs).
After 1 ps, the absorptions are assigned to the S1 precursor
excited state having an absorption centered at ca. 360 nm.
This band rapidly decays over ~1 ps into a new transient
having major absorptions at 485 and 627 nm, assigned to the
2.
TD-DFT calculated electronic absorption of
candidate intermediates after m-DMAP 1 photolysis
Timeꢀdependent density functional theory (TDꢀDFT)
computations have been previously demonstrated to be
useful in estimating the absorption bands for transient
species in previous work. For example, they are frequently
used for assigning absorption bands for carbenes, nitrenes
and oxenium ions.24,26,34 To help assign the transients
observed in the fsꢀTA experiments, we have performed
TDꢀB3LYP/6ꢀ311G(2d,p) calculations to estimate the UVꢀvis
absorption for some likely candidate species that could be
generated from the excited singlet state of mꢀDMAP 1.
TDꢀDFT computations were done to estimate the electronic
absorption spectra of the mꢀdimethylamino phenoxy radical,
the triplet mꢀdimethylamino phenyloxenium ion and the
singlet mꢀdimethylamino phenyloxenium ion. The results
using TDꢀB3LYP/6ꢀ311+G(2d,p) for these calculations are
shown in Figure 4. The computed absorption spectrum for
the singlet mꢀdimethylamino phenyloxenium ion gives a
single band at 418 nm in the 300 to 600 nm region and this
does not agree with the experimental spectra at early delay
time (1.6 to 12 ps) where there are one maximum transient
absorptions at about 485 nm and a tailing broad shoulder
transient absorption at 627 nm (see Figure 3). We noted that
singlet carbenes have been reported to form complexes with
solvents such as acetonitrile (and even form ylides in some
cases).35 Thus, we considered the possibility that the singlet
oxenium ion was forming a solvent complex, which might
alter its absorption profile. Indeed, by adding explicit
complexing waters (1, 3, 4 water molecules), the computed
TDꢀDFT absorption spectra of singlet oxenium ion in the
absence of water and in the presence of one water molecule
did not match with the experimental spectra at early delay
time while the simulated absorption spectra of singlet
oxenium ion water adducts in the presence of 3 and 4 water
molecules give reasonable agreement with the experimental
bands recorded at 19 ps (see Figure 4). At very earlier time,
as the slightly shift and decay of 476 nm band, the higher
wavelength 650 nm band would become relatively smaller.
TDꢀDFT calculation reveals that only water adducts of the
singlet oxenium ion rather than the gas phase isolated singlet
oxenium ion have the contribution of absorption at around
600 nm (see Figure 4). Therefore, the slightly decay of 650
nm band is likely associated by both the vibrational cooling
and the reorientation of the solvent effect of singlet oxenium
1
closedꢀshell singlet oxenium ion 2. Similarly, prior studies
of the protonated hydroxylamine photoprecursors shown in
Figure 1 underwent bond cleavage in ~2 ps.26 In the top right
of Figure 3, it can be seen that the broad bands centered at
485 nm and 627 nm slightly decayed and noticeably shifted
with time toward a shorter wavelength and a longer
wavelength respectively over ~55 ps, ultimately yielding a
sharp band at 476 nm and a broader absorption band
centered at ~650 nm. This process is attributed to vibrational
cooling as the hotꢀborn oxenium ion sheds heat to solvent, a
phenomenon that has also been seen in the prior work
following OꢀN bond cleavage.
0.008
0.006
0.004
0.002
0.008
0.006
0.004
0.002
1.6 ps
1.6 ps
12 ps
0.96 ps
350 400 450 500 550 600 650 700
Wavelength/ nm
350 400 450 500 550 600 650 700
Wavelength/ nm
1.0
357
0.012
2990 ps
357 nm
484 nm
0.8
0.6
0.4
0.2
0.0
0.010
0.008
0.006
0.004
0.002
1.0
0.8
0.6
0.4
0.2
0.0
476
19 ps
484
0
20 40 60 80 100120140
delay time/ ps
0
500 1000 1500 2000 2500 3000
delay time/ ps
350 400 450 500 550 600 650 700
Wavelength/ nm
4
ACS Paragon Plus Environment

Page 5 of 8
Journal of the American Chemical Society
mꢀdimethylamino phenyloxenium ion in the presence and
ion. Comparing to the isolated singlet oxenium ion, the
1
2
3
4
5
6
absence of explicit solvent waters (note that the simulated
spectrum goes to 300 nm to show smaller computed shoulder
band).
explicitly solvated singlet oxenium ion has two new
electronic transitions whose absorption bands appear at 332
and 588 nm (see Figure 4). TDꢀDFT calculations indicate
that the electronic transition at 588 nm can be described as
charge transfer in character from water to the oxenium ion
3. ns-TR3 experiments identify the transient at 357 nm
as the π,π* triplet m-dimethylamino phenyloxenium ion
while the electronic transition at 332 nm is a π, π* transition
altered by water mixing with the HOMO (see Table 2S). It is
challenging to think of alternative structures to the singlet
oxenium ion water complexes that would follow the singlet
excited state of the photoprecursor and ultimately converts to
a transient that can be assigned to the triplet oxenium ion by
both UVꢀVis and nsꢀTR3 experiments, described later;
therefore, this transient is assigned to the singlet oxenium
ion. Additionally, because the TDꢀDFT computations of the
solvated closedꢀshell singlet oxenium ion match with the
experimental bands, we assign the electronic state as the
closedꢀshell singlet oxenium ion. This assignment is
consistent with the CASSCF calculations described earlier
that also indicate that the lowestꢀenergy singlet state is the
closedꢀshell singlet state. Based on our previous study, a
ground state singlet aryloxenium ion can be trapped by water
molecules or other nucleophiles (chloride, ammonia). In this
study, we see no trapping adducts for this oxenium ion,
which might be attributed to its short lifetime, decaying into
new absorption bands at 357 and 484 nm, which can still be
observed at 3 ns. In addition, it should be noted that allylic
strain interaction generated by the dimethylamino moiety in
the meta position may also prevent nucleophilic additions in
para and one ortho position. Although the computed
absorption spectrum for the mꢀdimethylamino phenoxy
radical and triplet mꢀdimethylamino phenyloxenium ion have
two bands at around 300 nm and 428 nm, which are in good
agreement with the experimental spectra in shape at later
delay time (Figure 3), the observation of triplet
mꢀdimethylamino phenyloxenium ion by nsꢀTR3 (described
later) indicates that this new transient at 357 and 484 nm
should be assigned to the triplet mꢀdimethylamino
phenyloxenium ion, not the mꢀdimethylamino phenoxy
7
8
9
In order to identify the transient species seen at around 3
ns, nsꢀTR3 experiments for mꢀDMAP 1 were carried out in
an aqueous acetonitrile solution. Figure 5 shows the nsꢀTR3
spectra obtained with various delay times after 266 nm
photoꢀexcitation of mꢀDMAP and using 355 nm as the probe
wavelength in a MeCN:H2O 1:1 solution. The Raman band
at 1592 cmꢀ1 gradually drops off in intensity, which is
accompanied by the growth of the Raman band at 1656 cmꢀ1.
This not only indicates that there are two transient species
that can be detected in the aqueous solution by the TR3
spectra when using 355 nm as the probe wavelength, but also
reveals that the first transient species with its main Raman
band at 1592 cmꢀ1 is the precursor of the second transient
species with a main Raman band at 1656 cmꢀ1. At 10 µs, the
1592 cmꢀ1 disappears, indicating that the spectrum at 10 µs
arises from the second species. In an attempt to isolate the
Raman spectra of the first species, the Raman spectrum
obtained at 20 ns had an appropriately scaled spectrum at 10
µs subtracted from it. Figure 6 presents the nsꢀTR3 spectrum
obtained at 20 ns with the 10 ꢁs spectrum subtracted from it
in a MeCN:H2O 1:1 solution and the calculated normal
Raman spectrum of the triplet mꢀdimethylamino
phenyloxenium ion 2 using results from MP2/6ꢀ311G(d,p)
calculations. The experimental spectrum agrees well with the
calculated normal Raman spectrum of the triplet
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
mꢀdimethylamino phenyloxenium ion 2 for its vibrational
1656
10ꢁs
1 ꢁs
1592
200ns
radical.
0.05
0.08
0.06
0.04
0.02
0.00
427
3000
2000
1000
0
100ns
20ns
0ns
6000
4000
2000
0
428
295
0.04
0.03
0.02
0.01
0.00
311
300 350 400 450 500 550 600
Wavelength/ nm
300 350 400 450 500 550 600
Wavelength/ nm
800
1000
1200
1400
1600
1800
Raman shift/ cm-1
418
0.12
0.10
0.08
0.06
0.04
0.02
0.00
8000
6000
4000
2000
0
Figure 5.Shown are nsꢀTR3 spectra obtained with various delay
times indicated next to the spectra after 266 nm photoꢀexcitation
of mꢀdimethylamino phenyloxenium and using a 355 nm as the
probe wavelength in a MeCN:H2O 1:1 solution.
350
400
450
500
550
600
650
700
0 H2O
Wavelength/ nm
1 H2O
3 H2O
4 H2O
1592
300 350 400 450 500 550 600
Wavelength/ nm
300 350 400 450 500 550 600 650 700
Wavelength/ nm
1420
*
916
1144
Figure 4. Shown are the computed electronic spectra of the
mꢀdimethylamino phenoxy radical (top left), the triplet
mꢀdimethylamino phenyloxenium ion 32 (top right) and the
1
singlet mꢀdimethylamino phenyloxenium ion 2 (bottom left)
800
1000
1200 1400
1600
1800
800
1000
1200
1400
1600
1800
Raman shift/ cm-1
Raman shift/ cm-1
from TDꢀB3LYP/6ꢀ311G(2d,p) calculations. Bottom right of the
figure shows the experimental UVꢀvis spectrum at 19 ps after
the pulse and the computed spectrum of the singlet
Figure 6. Left: comparison of the nsꢀTR3 spectrum obtained at
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 8
20 ns with an appropriately scaled 10 ꢁs spectrum subtracted
cation. The absorbance that we assign to the
1
2
3
4
5
6
7
8
from it in a MeCN:H2O 1:1 solution (red) and the calculated
normal Raman spectrum of the triplet mꢀdimethylamino
phenyloxenium ion 32 using results from MP2/6ꢀ311G(d,p)
calculations (blue) are shown. Right: comparison of the nsꢀTR3
spectrum obtained at 10 µs in a MeCN:H2O 1:1 solution (red)
and the calculated normal Raman spectrum of the
mꢀdimethylamino phenol radical cation using results from
B3LYP/6ꢀ311G(d,p) calculations (blue) are shown. Asterisks
represent solvent subtraction artifacts. See text for more details.
mꢀdimethylamino phenol radical cation at 494 nm is also
close to the known absorption of the mꢀdimethylaniline
radical cation at 470 nm. Putting together all of our
observations from the fsꢀTA, nsꢀTA, and nsꢀTR3 data, we
can map out essentially the entire arc of the photochemistry
of 1. These assignments are shown in Scheme 1.
9
356
0 ꢁs
0.5 ꢁs
1 ꢁs
0.08
0.06
0.04
0.02
0.00
-0.02
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
frequency pattern. It does not agree well with alternative
candidate intermediates (12, or the radical). Therefore, the
first species detected by the nsꢀTR3 spectra in the aqueous
solution can be assigned to the triplet mꢀdimethylamino
phenyloxenium ion. Logically, it also seems reasonable to
expect the triplet state following the singlet oxenium ion, and
the computationally predicted UVꢀvis absorption spectrum
seen in the fsꢀTA studies is also in reasonable agreement
with the experimental value. The calculated structure of the
π,π* triplet mꢀdimethylamino phenyloxenium indicates a
CꢀO bond length of 1.216 Å, with the electronic density
delocalized into the benzene ring rather than in the CꢀO bond.
For an aromatic ketone (e.g such as triplet benzophenone), a
CꢀO bond length extending up to 1.32 Å has a n, π* triplet
state character and the triplet electronic density is mainly
located in the C=O group.36ꢀ42 For the second species with a
main Raman band at 1656 cmꢀ1, we tentatively assigned this
species to the mꢀdimethylamino phenyloxenium radical
cation which would be generated by the hydrogen
abstraction of the triplet oxenium ion from the surrounding
8 ꢁs
10 ꢁs
40 ꢁs
60 ꢁs
100 ꢁs
494
300 350 400 450 500 550 600 650 700
Wavelength/ nm
0.09
0.08
0.07
0.06
0.05
0.035
0.030
0.025
0.020
0.015
0.010
0
200
400
600
800
0
200
400
600
800
delay time/ ps
delay time/ ps
Figure 7. Top: nsꢀTA spectra from 0 µs to 100 µs produced in
a MeCN:H2O 1:1 solution acquired after 266 nm irradiation of
the initial precursor compound are shown. See text for more
details. Bottom: The temporal dependences of the transient
absorption intensity of initial compound in a MeCN:H2O 1:1
mixed solution at 356 (left) nm and 494 nm (right) are shown.
The solid red line indicates fittings using a biꢀexponential
function.
water molecules. Figure
6 (right) indicates that the
computationally predicted Raman spectrum of the
mꢀdimethylamino phenyloxenium radical cation is consistent
with the experimental Raman spectrum obtained at 10 µs,
although the noise in the spectrum at this time delay prevents
a clear assignment. In the nsꢀTA spectra (discussed next),
this transient absorbs at ~490 nm. Given that the
dimethylaniline radical cation absorbs at 470 nm, it seems
plausible that the mꢀhydroxy dimethylaniline radical cation
could absorb at ~490 nm, and this is the logical intermediate
following the triplet oxenium ion given that the stable end
product is the reduced mꢀdimethylaminophenol.
4. A ns-TA experimental study on the reactivity of the
triplet m-dimethylamino phenyloxenium ion.
The results from the nsꢀTR3 study on the
mꢀdimethylamino phenyloxenium indicated that the triplet
mꢀdimethylamino phenyloxenium ion is completely
consumed within about 10 µs to produce the radical cation in
an aqueous solution. The nsꢀTA experiments show that the
band at 356 nm continues to increase in intensity while the
band at 494 nm decreases in intensity (see Figure 7). There is
an isosbestic point at around 396 nm for this process
indicating a clear transformation between a precursor species
and a product species. The temporal dependences of the
transient absorption intensity of the initial species in the
MeCN:H2O 1:1 mixed solution with its bands at 356 nm and
494 nm can be fitted by a biꢀexponential function with two
time constants (τ1=1.69 µs, τ2=103 µs) (see Figure 7).
According to the nsꢀTR3 study, the 1.69 µs may be the
lifetime of the triplet mꢀdimethylamino phenyloxenium ion
and the species having a lifetime of 103 ꢁs being the radical
Scheme 1. Proposed mechanistic pathway based on
piecing together the spectroscopic data.
6. Product studies from photolysis of 1.
Additional evidence for a triplet ground state for the
mꢀdimethylamino phenyloxenium ion comes from analysis
of the photoproducts. The only detectable photoproduct of 1
in water by 1H NMR is mꢀdimethylaminophenol. This single
photoproduct observed from photolysis is in sharp contrast to
photolysis of the unsubstituted phenylhydroxylamine
hydrochloride, which gave additional nucleophile trapping
adducts (e.g. chlorophenol, hydroquinone, catechol,
protonated o/p aminophenols, etc). In general, one expects
nucleophile adducts of
a singlet phenyloxenium ion.
6
ACS Paragon Plus Environment

Page 7 of 8
Journal of the American Chemical Society
Reactivity of a triplet phenyloxenium ion is less certain, but atom abstractions to yield a reduced product. One difference
1
2
3
4
5
6
7
8
by analogy with related reactive intermediates (e.g. carbenes,
nitrenes, etc), one would expect radical ion chemistry (e.g. H
atom abstraction). The parent phenyloxenium ion is a singlet
ground state ion, 23 so these nucleophile trapping adducts are
not surprising. In contrast, the reduced product formed from
photolysis of 1 might be expected from a triplet oxenium ion
as a result of sequential H atom abstraction processes
(followed by loss of a proton) rather than nuclophilic
trapping chemistry. Thermolysis studies of 1 led to insoluble
between this study and our prior studies with the protonated
hydroxylamine photoprecursors is that the photoprecursors
for making the phenyloxenium ion and biphenyloxenium ion
reported previously gave a mixture of the oxenium ion
resulting from a photoheterolysis pathway and the free
radical resulting from a photohomolysis pathway of the OꢀN
bond. In this study, we only observe the oxenium ion
resulting from the photoheterolysis pathway and do not see
transients we can attribute to the free radical. Perhaps the
lack of a concomitant homolysis pathway can be attributed
to the “meta effect”, a term coined by the late Howard
Zimmerman44 to describe the propensity of meta pi donors to
favor photoheterolysis mechanisms in preference to
photohomolysis pathways. This study also permitted the first
look at the reactivity of a triplet oxenium ion. In contrast to
the singlet state oxenium ions, which react with nucleophiles
at the o/p position on the benzene ring to generate
nucleophileꢀsubstituted phenols, the triplet state engages in
H atom abstractions to yield a reduced product. This triplet
reactivity is rather similar to triplet carbenes and triplet
nitrenium ions, which can engage in H atom abstraction
processes. The π,π* triplet state of the oxenium ion is
longerꢀlived than the closedꢀshell singlet state oxenium ions,
living for ~1.7 ꢁs in solution. By comparison, singlet
aryloxenium ions have lifetimes of a few nanoseconds in
solution. The longer lifetime of the triplet state is similar to
carbenes and nitrenes whose triplet states can be seen with
nanosecond spectroscopy, while the singlets often have
lifetimes on the order of tens of picoseconds.45 In future
work, it would be interesting to compare the reactivity and
lifetime of this π,π* triplet oxenium ion to an n,π* triplet
oxenium ion.
9
tars,
likely
from
oxidation
or
polymerization
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(mꢀdimethylaminophenol itself is an electronꢀrich species
and is unstable to oxidation).
7. Cryogenic EPR studies from photolysis of 1 in
EtOH glass indicates triplet ground state for 2. Further
evidence for the ground state of the mꢀdimethylamino
phenyloxenium being
lowꢀtemperature matrix
a
triplet state comes from
isolation experiments.
A
lowꢀtemperature EtOH glassy matrix photolysis study of 1
(at 4 K), performed in the cavity of an EPR spectrometer,
provided the spectrum shown in Figure 8. The signals in the
EPR spectrum are consistent with the formation of a triplet
diradical species as well as a monoradical impurity (perhaps
arising from a competitive homolysis pathway or from a
back electron transfer process). This EPR spectrum provides
evidence that the ground state of the oxenium ion is the
triplet state, since at 4 K essentially no thermal population of
an excited triplet state would be possible outside of having
virtually degenerate singlet and triplet energies. The value
of the D/hc parameter (0.026 cmꢀ1) seen here is very close
to that of the mꢀquinone methide D/hc parameter (0.027
43
cmꢀ1) reported by Berson and coworkers,
perhaps not
surprising given the very similar electronic structure.
ASSOCIATED CONTENT
Supporting Information. Synthetic procedures and compound
characterization data. Product studies. Absolute energies and
Cartesian coordinates. Cryogenic EPR details and simulation
parameters. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
Eꢀmail addresses: winter@iastate.edu
phillips@hku.hk
Figure 8. Xꢀband EPR spectrum of 1 irradiated in EtOH
glass at 4 K using a mercury lamp source delivered via a
ACKNOWLEDGMENTS
fiber optic cable into the cavity.
Triplet simulation
parameters: D = 280 Gauss (D/hc = 0.026 cmꢀ1), E/D =
This work was supported by grants from the Research
Grants Council of Hong Kong (HKU 7035/13P) to DLP.
Partial support from the Grants Committee Areas of
Excellence Scheme (AoE/P-03/08) and the Special
Equipment Grant (SEG HKU/07) are also gratefully
acknowledged. AHW thanks the petroleum research fund,
the NSF (1464956), the Cottrell Scholar Award from the
Research Corporation for Scientific Advancement, and the
ISU chemical instrumentation facility (CIF).
0.075 Gauss (7.0 x 10ꢀ6 cmꢀ1)
Inset: ꢀmS=2 transition.
.
CONCLUSION
In conclusion, using a combination of pulsed laser
spectroscopies we have been able to map out essentially the
complete photophysics and photochemistry pathways of 1 in
aqueous acetonitrile. This photoprecursor undergoes
a
photoheterolysis reaction to generate initially the
closedꢀshell singlet oxenium ion and then the triplet oxenium
ion after intersystem crossing, which reacts via sequential H
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 8
(27) Li, M.-D.; Ma, J.; Su, T.; Liu, M.; Yu, L.; Phillips,
REFERENCES
1
2
3
4
5
6
7
8
D. L. J. Phys. Chem. B 2012, 116, 5882.
(28) Li, M. D.; Yeung, C. S.; Guan, X.; Ma, J.; Li, W.;
Ma, C.; Phillips, D. L. Chem. Eur. J. 2011, 17, 10935.
(29)Gross, E.; Driezler, R. Phys. Rev. Lett. 1984, 52,
997.
(30) O'boyle, N. M.; Tenderholt, A. L.; Langner, K.
M. J. Comp. Chem. 2008, 29, 839.
(1) Vigalok, A.; Rybtchinski, B.; Gozin, Y.;
Koblenz, T. S.; Ben-David, Y.; Rozenberg, H.; Milstein, D.
J. Am. Chem. Soc. 2003, 125, 15692.
(2) Swenton, J. S.; Carpenter, K.; Chen, Y.; Kerns,
M. L.; Morrow, G. W. J. Org. Chem. 1993, 58, 3308.
(3) Rieker, A.; Beisswenger, R.; Regier, K.
Tetrahedron 1991, 47, 645.
(31)Merrick, J. P.; Moran, D.; Radom, L. The J. Phys.
Chem. A 2007, 111, 11683.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(32) Frisch, M.; et al. Gaussian, Inc., Revision A.02
Wallingford CT, 2009.
(4) Rieker, A. DECHEMA Monographien 1992,
125, 777.
(33) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.;
Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.;
Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.;
Dupuis, M.; Montgomery, J. A. J Comp. Chem. 1993, 14,
1347.
(34) Lage, M. L.; Fernández, I.; Mancheño, M. J.;
Sierra, M. A. Inorg. Chem. 2008, 47, 5253.
(35) Wang, J.; Kubicki, J.; Peng, H.; Platz, M. S. J.
Am. Chem. Soc. 2008, 130, 6604.
(36) Webb, S.; Yeh, S. W.; Philips, L. A.; Tolbert, M.;
Clark, J. J. Am. Chem. Soc. 1984, 106, 7286.
(37) Webb, S.; Philips, L. A.; Yeh, S. W.; Tolbert, L.
M.; Clark, J. J. Phys. Chem. 1986, 90, 5154.
(38) Schwartz, B. J.; Peteanu, L. A.; Harris, C. B. J.
Phys. Chem. 1992, 96, 3591.
(39) Toscano, J. P. Adv. Photochem., Vol 26 2001,
41.
(40) Du, Y.; Ma, C.; Kwok, W. M.; Xue, J.; Phillips, D.
L. J. Org. Chem. 2007, 72, 7148.
(41) Toscano, J.; Platz, M.; Moss, R.; Jones Jr, M.;
John Wiley and Sons, Inc. Hoboken, NJ: 2007, p 183.
(42) Chuang, Y. P.; Xue, J.; Du, Y.; Li, M.; An, H.-Y.;
Phillips, D. L. J. Phys. Chem. B 2009, 113, 10530.
(43) Rule, M.; Matlin, A. R.; Hilinski, E. F.;
Dougherty, D. A.; Berson, J. A. J. Am. Chem. Soc. 1979,
101, 5098.
(5) Pelter, A.; Ward, R. S. Tetrahedron 2001, 57,
273.
(6) Dimroth, K.; Umbach, W.; Thomas, H. Chem.
Ber. 1967, 100, 132.
(7) Sabot, C.; Guerard, K. C.; Canesi, S. Chem.
Commun. 2009, 2941.
(8) Sabot, C.; Commare, B.; Duceppe, M.-A.; Nahi,
S.; Guérard, K. C.; Canesi, S. Synlett 2008, 2008, 3226.
(9) Guérard, K. C.; Chapelle, C. m.; Giroux, M.-A.;
Sabot, C.; Beaulieu, M.-A.; Achache, N.; Canesi, S. Org.
Lett. 2009, 11, 4756.
(10)Williams, L. L.; Webster, R. D. J. Am. Chem. Soc.
2004, 126, 12441.
(11) Peng, H. M.; Webster, R. D. J. Org. Chem. 2008,
73, 2169.
(12) Taylor, W. I.; Battersby, A. R. Oxidative
coupling of phenols; M. Dekker New York, 1967; Vol. 1.
(13) Baesjou, P. J.; Driessen, W. L.; Challa, G.;
Reedijk, J. J. Am. Chem. Soc. 1997, 119, 12590.
(14) Gamez, P.; Gupta, S.; Reedijk, J. Comptes
Rendus Chimie 2007, 10, 295.
(15) Osborne, R. L.; Coggins, M. K.; Raner, G. M.;
Walla, M.; Dawson, J. H. Biochemistry 2009, 48, 4231.
(16) Novak, M.; Glover, S. A. J. Am. Chem. Soc.
2004, 126, 7748.
(17) Wang, Y.-T.; Jin, K. J.; Leopold, S. H.; Wang, J.;
Peng, H.-L.; Platz, M. S.; Xue, J.; Phillips, D. L.; Glover, S.
A.; Novak, M. J. Am. Chem. Soc. 2008, 130, 16021.
(18) Winter, A. H.; Falvey, D. E.; Cramer, C. J.;
Gherman, B. F. J. Am. Chem. Soc. 2007, 129, 10113.
(19) Hanway, P. J.; Winter, A. H. J. Am. Chem. Soc.
2011, 133, 5086.
(20) Hanway, P. J.; Winter, A. H. J. Phys. Chem. A
2012, 116, 9398.
(21) Katsumata, S.; Lloyd, D. Chem. Phys. Lett.
1977, 45, 519.
(22) Dewar, M. J. S.; David, D. E. J. Am. Chem. Soc.
1980, 102, 7387.
(23) Hanway, P. J.; Xue, J.; Bhattacharjee, U.; Milot,
M. J.; Ruixue, Z.; Phillips, D. L.; Winter, A. H. J. Am. Chem.
Soc. 2013, 135, 9078.
(44) Zimmerman, H. E. J. Am. Chem. Soc. 1995,
117, 8988.
(45) Reactive Intermediate Chemistry; Moss, R. A.;
Platz, M. S.; Jones, M., Eds.; Wiley, 2004.
Graphical TOC
(24) Xue, J.; Luk, H. L.; Eswaran, S.; Hadad, C. M.;
Platz, M. S. J. Phys. Chem. A 2012, 116, 5325.
(25) Wang, Y.-T.; Wang, J.; Platz, M. S.; Novak, M. J.
Am. Chem. Soc. 2007, 129, 14566.
(26) Li, M.-D.; Hanway, P. J.; Albright, T. R.; Winter,
A. H.; Phillips, D. L. J. Am. Chem. Soc. 2014, 136, 12364.
8
ACS Paragon Plus Environment