Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes

Article abstract of DOI:10.1139/v03-052

A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF ₃, -CN (meta and para only), -CH₃, -OCH₃, and -NO₂) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for parasubstituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.

Full text of DOI:10.1139/v03-052

575
Subs titue nt e ffe cts in oxime radical cations . 1.
Photos e ns itize d re actions of ace tophe none
oxime s
H.J . Pe te r de Lijs e r, J a s on S. Kim , Suza nne M. Mc Grorty, a nd Erin M. Ulloa
Abstract: A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF3, -CN (meta and para only), -CH3, -OCH3,
and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state
photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calcula-
tions were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was
analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP),
the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experi-
ments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion,
which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence
for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The mea-
sured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil.
This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric,
polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for para-
substituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates.
The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates
supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured
substituent effects are transition state effects rather than ground state effects. At this point all of the available data
suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of
the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime
radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction
with little selectivity as far as substituent effects are concerned.
Key words: oxime, radical cation, iminoxyl radical, electron transfe5r8, 5substituent effect.
Résumé : On a synthétisé une grande variété d’oximes d’acétophénones substituées en ortho-, méta- et para- (-H, -F,
Cl, -CF3, -CN (uniquement en méta- et para-), -CH3, -OCH3 et -NO2) qu’on a étudiées par des expériences de photo-
lyse éclair au laser (PEL) et de photolyse à l’état stationnaire dans l’acétonitrile, en utilisant le chloranile comme pho-
tosensibilisateur. De plus, on a effectué des calculs semiempiriques (AM1) sur les espèces neutres, les cations radicaux
et les radicaux iminoxyles correspondants. On a analysé les données en fonction des potentiels électrochimiques maxi-
maux des oximes, les vitesses de désactivation du chloranile triplet (PEL), des potentiels d’ionisation calculés et des
conversions mesurées des oximes dans les expériences de photolyse à l’état stationnaire. La photolyse des oximes en
présence de chloranile conduit à la formation de l’anion radical du chloranile qui réagit rapidement avec le cation radi-
cal de l’oxime pour conduire à la formation du radical de la semiquinone et à un radical iminoxyle. On a obtenu des
indications relatives à la formation de l’anion radical du chloranile et du radical de la semiquinone à partir des études
de PEL. Les vitesses mesurées de désactivation obtenues à partir des études de PEL correspondent aux vitesses de
transfert d’électron des oximes vers le chloranile triplet. On a pu établir une corrélation entre ces données et diverses
constantes de radicaux et de substituants polaires. Les études de Hammett suggèrent que les effets stériques, polaires et
radicalaires sont importants pour les oximes des acétophénones substituées en ortho-, que les effets polaires sont im-
portants pour les oximes des acétophénones substituées en para- et que la stabilisation radicalaire est plus importante
que les effets polaires pour les substrats substitués en méta-. Les potentiels d’oxydation calculés des oximes présentent
une excellente corrélation avec les vitesses de désactivation mesurées ce qui supporte une voie de transfert électro-
nique. Sur la base des densités de charge calculées, on en conclut que les effets de substituants mesurés sont des effets
liés aux états de transition plutôt que des effets liés aux états fondamentaux. À ce point, toutes les données disponibles
Received 16 December 2002. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 21 May 2003.
This paper is dedicated to Professor Donald R. Arnold for his contributions to chemistry.
H.J.P. de Lijser,1 J.S. Kim, S.M. McGrorty, and E.M. Ulloa. Department of Chemistry and Biochemistry, California State
University Fullerton, Fullerton, CA 92834, U.S.A.
1Corresponding author (e-mail: pdelijser@fullerton.edu).
Can. J. Chem. 81: 575–585 (2003)
doi: 10.1139/V03-052
© 2003 NRC Canada

576
Can. J. Chem. Vol. 81, 2003
suggèrent que la conversion des oximes est contrôlée par deux réactions énergétiquement opposées, soit l’oxydation de
l’oxime neutre qui est favorable aux oximes portant des substituants électrodonneurs ou la déprotonation du cation ra-
dical de l’oxime qui est favorable aux oximes portant des substituants électroattracteurs. Le résultat global est une
réaction qui, du point de vue des effets de substituants, ne présente pas beaucoup de sélectivité.
Mots clés : oxime, cation radical, radical iminoxyle, transfert d’électron, effet de substituant.
[Traduit par la Rédaction] de Lijser et al.
Scheme 1.
Introduc tion
Oximes and oxime ethers have found use as protection
groups for carbonyl compounds (1). The regeneration of
carbonyl compounds form oximes can be achieved using a
variety of methods, including oxidative, reductive, and pho-
tochemical techniques (2, 3). Of specific interest to us are
the photochemical techniques (3) as well as other studies fo-
cusing on the photochemical behavior of these compounds
(4). Haley and Yates (3) studied the photohydrolysis of ox-
imes and concluded that the reactive excited state was the
lowest singlet state since triplet sensitization did not result in
the formation of any products, with the exception of syn–
anti isomerization. The latter observation is consistent with
Padwa and Albert’s (4j) work on the triplet-sensitized photo-
lysis of oxime ethers. It is interesting to note, however, that
the measured quantum yields in the aqueous photohydrolysis
reactions were typically low and certain substituted sub-
strates did not react at all. We recently reported on our stud-
ies involving the photosensitized regeneration of carbonyl
compounds from oximes (5). Photolysis of oximes in the
presence of the triplet sensitizer chloranil resulted in forma-
tion of the corresponding carbonyl compounds in reasonable
to good yields. The data is most consistent with an electron
transfer mechanism and we have proposed that the oxime
radical cation reacts rapidly with the sensitizer (chloranil)
radical anion to form the iminoxyl radical and the hydro-
chloranil (semiquinone) radical (Scheme 1).
An important issue that has not yet been addressed is
whether the formation of the iminoxyl radicals from oximes
via an electron transfer pathway is a two-step process or a
concerted proton-coupled electron transfer (PCET) process
(6). Both oxime radical cations and iminoxyl radicals are
important intermediates in chemical and biochemical pro-
cesses (7), however, despite the large amount of data avail-
able on the structure of these species (7, 8), little is known
about their reactivity. A complete understanding of the
mechanistic aspects of these reactions and the intermediates
involved is therefore desirable.
Scheme 2b, more detailed studies are necessary to support
this hypothesis.
Structure–reactivity studies have been of great value for
the exploration of mechanistic problems and the involvement
of reactive intermediates (10). A number of linear free en-
ergy relationships (LFERs) for dealing with different inter-
mediates (cations, anions, radicals) in both ground state and
excited state reactions have been developed over the past
decades (11). It is well established that when studying aro-
matic substrates, only the meta- and para-substituted com-
pounds are considered. In general, steric effects and
hydrogen bonding prohibit the true electronic nature of
ortho-substituent effects and, therefore, are most often not
considered when discussing substituent effects. A variety of
specific scales for dealing with ortho-substituents have been
developed (12). For example, Trible and Traynham (12c) re-
ported a method for measuring electronic effects of ortho-
substituents by means of NMR in DMSO solution using
ortho-substituted phenols. This scale is known as the σ−o or
∆δo scale. A value for o-NO2 was not available as it did
show significant hydrogen bonding even in DMSO solution,
however, a value of 1.20 was reported earlier by Dietrich et
al. (13). Separation of steric and electronic effects has been
an active field of study, initiated by Taft (14) who proposed
Es and Es° values. Charton (15) indicated that the Taft Es
values are a linear function of the van der Waals radii and
independent of electronic effects, whereas the Es° values are
a function of electronic effects only without any significant
contribution of steric effects. A new steric parameter (Ese)
which was corrected for electronic effects, was introduced
by Unger and Hansch (16).
Although reasonable correlations are obtained for most
ionic reactions by using the traditional polar substituent con-
stants such as σm, σp (referred to here as σpol ), and σ+, those
involving radicals are often much poorer. To deal more ef-
fectively with this problem, a variety of new scales that spe-
cifically deal with free radicals (e.g., σα · (17a–17d), σrad
(17e, 17f), σJ· (17g–17j), and σJJ· (17l)) were developed.
Typically, dual parameter correlations are done for free radi-
cal reactions, which then reveal the relative importance of
the substituent effect on the radical species (17).
The electronic structure of iminoxyl radicals has been
proposed as that shown in Scheme 2a (8c, 8f).
According to this description, which is based on ESR
measurements, no spin density is available on carbon. Sev-
eral other mechanistic interpretations involving iminoxyl
radicals have suggested the possibility of a resonance struc-
ture as shown in Scheme 2b (9a, 9c).
Although the formation of carbonyl compounds from
oxime radical cations (Scheme 3) can be most easily ex-
plained on the basis of the resonance structure shown in
The use of LFER studies in radical ion reactions is much
more limited and no specific scales have been developed.
© 2003 NRC Canada

de Lijser et al.
577
Scheme 2.
chloranil radical anion shows a sharp absorption around
445 nm, whereas the semiquinone radical is observed at
433 nm. These spectra are consistent with those reported in
the literature (21). Attempts to observe the oxime radical
cation directly via photoionization (308 nm) were unsuc-
cessful and a direct kinetic analysis (observing the rate of
disappearance of the oxime radical cation) was therefore not
possible.
Photolysis of an argon-purged solution containing aceto-
phenone oxime and chloranil resulted in the formation of the
chloranil triplet, which was rapidly quenched by aceto-
phenone oxime (Fig. 1). At approximately 400 ns after the
pulse, two new species were formed. We assigned these two
signals to the chloranil radical anion (445 nm) and the semi-
quinone radical (433 nm). At longer times, the strong signal
of the semiquinone radical was obvious, however, the shoul-
der at 445 nm indicated the presence of the chloranil radical
anion. Similar spectra were obtained for a variety of other
oximes, suggesting that the mode of action for the initial
steps is always the same. On the basis of these results, we
concluded that the initial steps involved in the chloranil-
sensitized photolysis of oximes are electron transfer from
the oxime to triplet chloranil to form a triplet radical ion
pair, followed by deprotonation to form the iminoxyl radical
and the semiquinone radical. Because of the presence of the
chloranil radical anion in these spectra, we believe that nei-
ther hydrogen atom transfer nor proton-coupled electron
transfer are important pathways in these reactions.
Scheme 3.
For reactions involving a radical cation species, the observed
rates are most often correlated with the Brown–Okamoto σ+
parameter (18), owing to the fact that stabilization of the
positive charge is typically more important then that of the
radical. The effect depends on the type of reaction studied,
and both small and large effects have been reported (19).
For example, Nave and Trahanovsky (19a) found a ρ-value
of –2.0 for the one-electron oxidative cleavage of 2-aryl-1-
phenylethanols using cerium(IV), whereas Fox and Chen
(19e) found a relatively small reaction constant (ρ+ = –0.56)
for the TiO2-photocatalyzed oxidative cleavage of olefins.
Clearly, the use of a structure–reactivity study is expected to
aid in the development of an understanding of complicated
mechanistic problems.
Photosensitized reactions of ortho-, meta-, and para-
substituted acetophenone oximes
The results of the photosensitized reactions of the ortho-,
meta-, and para-substituted acetophenone oximes are listed
in Table 1. Accurate peak potentials (Ep) could not be ob-
tained for the o-CF3, o-Cl, and o-CH3 derivatives. Reason-
able numbers (based on electronic properties) were obtained
for o-F, o-OCH3, and o-NO2 acetophenone oximes. A possi-
ble reason for the large positive numbers for the o-CF3, o-Cl,
and o-CH3 derivatives is a steric effect. These three groups
are unable to orient themselves in such a manner that the
oxime moiety can become coplanar with the aromatic ring
upon ionization. This was confirmed by semiempirical
(AM1) calculations (22). The calculated dihedral angles for
these three radical ions are 33.7° (CF3), 28.9° (Cl), and
33.5° (CH3). This is significantly larger than acetophenone
oxime (7.9°) or the o-F derivative (17.3°). The exception is
o-NO2 acetophenone oxime for which the calculated dihed-
ral angle is 70.3°. Further analysis of the structure revealed
that the NO2 group is coplanar with the aromatic ring and
lined up such that one of the oxygens of the nitro group is
directed towards the carbon of the oxime moiety (distance is
2.5 Å) providing some stabilization. This particular orienta-
tion results in the presence of large charge and spin densities
on the oxime carbon.
Here, we report on a detailed Hammett study involving a
series of ortho-, meta-, and para-substituted acetophenone
oximes to examine the effects of substituents on the reactiv-
ity of the oxime radical cations and iminoxyl radicals. To
obtain as much detailed information as possible, we have in-
cluded all but one (4-tert-butyl) of the groups necessary for
a complete assessment of the reactivity for both radical and
ionic reactions, as suggested by Exner (20) and Dust and Ar-
nold (17a), respectively.
Re s ults a nd dis c us s ion
Laser flash photolysis studies of oximes
Formation of iminoxyl radicals from oximes via photosen-
sitized reactions can potentially result from three pathways:
(i) electron transfer followed by proton transfer; (ii) proton-
coupled electron transfer; or (iii) hydrogen atom transfer. If
the intermediate radical cation – radical anion pair has a
long enough lifetime, it should be possible to distinguish
among pathways (i) and (ii) or (iii) using spectroscopic
methods. A nanosecond laser flash photolysis experiment
would most likely not be able to differentiate between path-
ways (ii) and (iii). The intermediates of interest, triplet
chloranil, chloranil radical anion, and semiquinone radical,
were each generated separately and their spectra were re-
corded. The chloranil radical anion was obtained by photo-
lysis of a chloranil–biphenyl mixture (in acetonitrile) and the
semiquinone radical was obtained from photolysis of a
chloranil–toluene mixture. The spectrum of triplet chloranil
shows a maximum of 508 nm and a shoulder at 480 nm. The
The calculated ∆GET values (using the Weller equation
(23)) for several of the substituted oximes are positive (en-
dothermic) suggesting that electron transfer is not favorable.
However, the results from nanosecond laser flash photolysis
experiments do not agree with that conclusion. Quenching of
the excited triplet state of chloranil (monitored at 510 nm) is
fast (4 × 107 – 1 × 1010 M–1 s–1) for all ortho-substituted
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
Fig. 1. Time-resolved (ns) spectra resulting from the quenching
of triplet chloranil by acetophenone oxime in argon-purged
acetonitrile solution showing the spectral bands of triplet
chloranil (λmax = 508 nm), chloranil radical anion (shoulder at
λmax = 445 nm), and the semiquinone radical (λmax = 433 nm).
Delay times are 380, 390, 400, 425, 450, 475, and 500 ns (in-
creasing curves).
react slower than oximes with electron-donating groups such
as -OCH3. To explore the behavior of these substituted ox-
imes further, the energy required for ionization of the neutral
oxime (∆∆Hf) was calculated for each oxime. The results are
listed in Table 1 and a plot of the calculated ionization po-
tentials against the measured kq values (Fig. 3) reveals that
there is an excellent correlation (r2 = 0.85). It must be noted
that some of the measured rates are close to the diffusion-
controlled limit, however, there is no sign of these rates lev-
eling off. One could argue that the value of p-OCH3 is prob-
ably the closest to that limit, however, when that point is
omitted the slope of the line does not change significantly,
nor does the r2 value change much. This correlation supports
the idea that the diffusion-controlled limit is not reached and
we are confident that the measured rate constants reflect the
influence of the substituent on the initial electron transfer
process. Comparison of the ortho- and para-values of ∆∆Hf
(Table 1) is also consistent with a steric effect. The ∆∆Hf
values for the ortho-substituted oximes are consistently
higher than those of the para-substituted oximes (with the
exception of the nitro-substituted oximes).
Analysis of the data for the meta-substituted oximes in
Table 1 indicates that no correlation exists between the mea-
sured peak potentials and the quenching rates (kq). The exact
reason for this is unclear and at this point we will not dis-
cuss these observations. There is an excellent correlation
between the measured kq values and the calculated ∆∆Hf
values. The initial (electron transfer) reaction is fast for all
the meta-substituted oximes and the differences are clearly
due to electronic effects. An energy transfer pathway is ruled
out as no syn–anti isomerization was observed for any of
these oximes. Interesting electronic effects were observed
for the m-OCH3 derivative. Analysis of the spin and charge
distributions from the AM1 calculations reveals that there is
an unusually large (positive) spin density and negative
charge on the carbon of the oxime moiety. In addition, the
dihedral angle (–33.2°) is larger than for all the other sub-
stituents, which suggests that the large peak potential (mea-
sured) is possibly due to the fact that coplanarity cannot be
achieved (as was observed for certain ortho-substrates). The
calculated C=N bond length is much shorter than in any of
the other oximes, the N—O bond length is much longer, and
the O—H bond length is shorter than in the other oxime rad-
ical cations. Also, the calculated charge density on the hy-
droxyl proton is smaller than usual and is only slightly larger
than that of the hydroxyl proton in the neutral oxime. These
results suggest that oxidation does not occur on the oxime
moiety but rather somewhere else in the molecule. Several
other conformers were examined as well, all of which dis-
played the same characteristics, suggesting that electronic
effects rather than conformational effects cause this phenom-
enon.
acetophenone oximes (Table 1). A typical decay trace and
quenching plot are shown in Fig. 2. The discrepancy be-
tween the ∆GET and quenching rates (kq) can (in part) be
clarified by the results of the product studies. Previously, it
was suggested that the photochemistry (photohydrolysis) of
oximes occurs via their singlet states as triplet sensitization
did not result in the formation of products (3). Results ob-
tained by us2 and others (3, 4j), suggest that triplet sensitiza-
tion (energy transfer) results in syn–anti isomerization in
oximes, whereas electron transfer results in the formation of
carbonyl compounds and other products3 (5). Similar to our
previous studies on the photosensitized electron transfer re-
actions of oximes, the major product formed in these reac-
tions is the corresponding carbonyl compound (5). However,
it is important to note that photolysis of the o-NO2, o-CH3,
and o-Cl oximes also resulted in large amounts of syn–anti
isomerization, suggesting that both electron transfer and en-
ergy transfer pathways are important in these reactions. The
fact that two of these oximes had peak potentials more posi-
tive than 2.5 V and all three oxime radical cations had large
dihedral angles is consistent with the idea that in order for
electron transfer to take place favorably, the substituent must
be able to become coplanar with the aromatic ring upon ion-
ization.
The data obtained for the para-substituted oximes clearly
shows that the major trends are different from those of the
ortho- and meta-substituted oximes and actually reflect the
true (polar) nature of the substituents much better. This is
not unexpected; many radical ion reactions correlate well
Analysis of the laser flash photolysis (LFP) data reveals
that it is consistent with substituent effects as predicted on
the basis of electronic properties, i.e., oximes with electron-
withdrawing groups such as -NO2 and -CF3 are expected to
2 S.E. Ng’wono, B.V. Marquez, A. Park, and H.J.P de Lijser. Unpublished results.
3 H.J.P. de Lijser, J.S. Kim, S.M. McGrorty, and E.M. Ulloa. More detailed studies on the product formation under these conditions are un-
derway and will be published elsewhere. Manuscript in preparation.
© 2003 NRC Canada

de Lijser et al.
Table 1. Reactivity data for ortho-substituted acetophenone oximes.
579
∆GET (kcal mol–1)
kqb (M–1 s–1)
Conversion (%)c
∆∆Hf (kcal mol–1)d
a
Substituent
Ep (V)
H
CF3
1.65
>2.50
2.20
2.05
>2.50
2.06
1.93
2.26
2.26
1.95
>2.50
1.95
1.50
1.52
2.19
1.27
1.99
2.16
2.03
2.15
2.19
–11.5
>8.0
+1.2
–2.3
>8.0
–2.1
–5.1
+3.7
+2.5
–4.6
>8.0
–4.6
–15.0
–14.5
+0.9
–20.3
–3.7
+0.2
–2.8
0.0
9.5 × 109
4.7 × 107
1.4 × 109
2.5 × 109
4.5 × 109
6.4 × 109
9.4 × 109
1.1 × 1010
5.6 × 109
8.5 × 109
6.5 × 109
1.1 × 1010
1.1 × 1010
1.2 × 1010
1.3 × 1010
1.8 × 1010
6.7 × 107
6.5 × 108
3.2 × 109
1.5 × 109
3.5 × 109
18
43
14
20
48
21
28
80
19
23
54
18
14
92
22
55
33
20
17
56
26
193.04
204.63
201.81
203.54
198.57
196.73
194.35
194.92
198.04
194.01
195.10
191.60
189.25
185.92
188.73
183.47
206.45
206.04
208.19
200.43
200.41
o
m
p
o
m
p
o
m
p
o
m
p
o
m
p
o
m
p
Cl
F
CH3
OCH3
NO2
CN
m
p
+0.9
aPeak potential measured by cyclic voltammetry.
bRate of triplet chloranil quenching.
cConversion determined by calibrated GC-FID after 20 min of photolysis except for ortho-substituted oximes which were subjected
to 2 h photolysis.
dCalculated using semiempirical (AM1) methods; the number is the difference between the calculated heats of formation of the
radical cation and the neutral species.
Fig. 2. Decay of the chloranil triplet signal (at 510 nm) in the
presence of acetophenone oxime (1.6 × 10–4 M). (Inset) Determi-
nation of the rate constant for quenching triplet chloranil by
acetophenone oxime.
Fig. 3. Correlation between the measured quenching rates (LFP)
of triplet chloranil and the calculated (AM1) ionization potentials
(∆∆Hf) of the acetophenone oximes.
tive peak potentials are obtained relative to the unsubstituted
compound) and electron-withdrawing substituents deactivate
the molecule; the overall range (∆∆GET) spans more than
21 kcal mol–1. On the basis of the measured peak potentials,
calculated ∆GET values, the measured kq values, and the
with the σ+ parameter (18), suggesting a strong polar effect
in these reactions. A reasonable correlation (r2 = 0.84) is ob-
served between the measured Ep and kq values. Electron-
donating substituents activate the molecule (i.e., less posi-
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580
Can. J. Chem. Vol. 81, 2003
calculated ∆∆Hf values, we conclude that quenching of trip-
let chloranil proceeds predominantly via an electron transfer
process to form the oxime radical cation. However, as indi-
cated above, a competitive energy transfer pathway for the
o-NO2, o-CH3, and o-Cl derivatives cannot be ruled out.
small, which is not unusual for radical ion reactions, and
supports the formation of a positively charged species in the
transition state. Poor correlations are observed when using
the radical substituent constants. Clearly, radical stabilizing
effects are not of significant importance for these deriva-
tives. Dual parameter correlations do not significantly im-
prove these results, however, there is some variation in the
ρrad/ρpol ratios. Using σ+ and σrad gives ρrad/ρpol = 1.0,
whereas using σ+ and σα gives ρrad&/ρpol = 3.7.
When analyzing kinetic data of this kind it is common
practice to combine the results of the meta- and para-sub-
stituents. When doing so for the data presented here (Ta-
ble 4), the results emphasize the observed trends for the
individual series; however, overall the correlations are worse
than for the individual series. This is most likely due to the
fact that different stabilizing effects are important for each
series of compounds. Poor correlations are observed when
using kq and the radical substituent constants and good cor-
relations when using polar substituent constants or a dual
parameter correlation. However, as stated above, a much
better insight into these reactions is obtained when using in-
dividual series rather than a mixed set.
It has been suggested that substituent effects are often
ground state effects rather than an effect on the transition
state of the reaction under investigation, especially when
considering phenols and similar species (24). To see if this is
also true for these oximes, we have calculated the spin and
charge distributions in the ground state (neutral) and in the
radical cation species. The calculations show that both sets
of compounds are affected by the substituents. In general,
the radical ion species is more susceptible to these effects,
however, large effects are also seen in the calculated charge
densities for the neutral ground state species with -NO2 and
-OCH3 substituents. The calculated charge densities on the
carbon of the oxime moiety of the radical ion seem to accu-
rately reflect the substituent effects. For the ortho-substi-
tuted substrates, a plot of the charge density against the σo−
values gives an excellent correlation (r2 = 0.90), whereas a
plot of charge density of the neutral compound against the
same σ−o values gives a poor correlation (Fig. 4a). The slopes
of the two lines (0.13 vs. 0.01) are significantly different and
we conclude that for the ortho-substrates ground state effects
are less important than the effect of the substituent in the
transition state of the electron transfer reaction. The calcula-
tions for the meta-substituted oximes show the familiar pat-
tern of larger (more positive) charge densities on the carbon
of the oxime moiety when electron-withdrawing groups are
present, as can be seen from the correlation (slope = 0.08)
between these calculated charge densities and the polar σ
values (σpol ), which is acceptable when the m-OCH3 point is
omitted (Fig. 4b). A better correlation is observed when us-
ing the ground state charge densities of the same carbon and
plotting them against σpol (r2 = 0.91), however, the slope
(0.008) is smaller by a factor of 10. This clearly demon-
strates that polar effects are more important for the radical
ions of the meta-substituted oximes but not as much for the
neutral ground state molecules. A reasonable correlation
(r2 = 0.80) also exists between the calculated spin densities
at the carbon of the oxime moiety and the σrad constants (ig-
noring the m-OCH3 point). The slope in this case (–0.84) is
much larger than that observed for the charge density,
At this point we believe that the measured kq values most
accurately represent the substituent effect on the initial elec-
tron transfer process. As such, we have used this particular
e
set of data to determine the influence of steric (Es ), polar
(σ−o, σmb, σpol ), and radical (σα , σrad, σJJ·) effects on this ini-
tial process. Since no ortho-substituent constants are avail-
able for these scales, we have opted to use the data available
for the para-substituents. The substituent constants used in
this study are listed in Table 2.
Correlation of the quenching rates with the σ−o substituent
constants for ortho-substituents reveals a reasonable correla-
tion (ρ = –1.76, r2 = 0.81; Table 3) and suggests that there is
a significant polar electronic contribution from the ortho-
substituents. Correlation of the data with the steric parameter
Ese shows that steric effects are important (ρ = +1.00, r2 =
0.81), however, a dual parameter fit using both Ese and σo−
does not improve the correlation. To determine if there was a
radical stabilization effect in these reactions, the data was
correlated with both σ−o and the (para) radical substituent co-
efficients σrad, σJJ·, and σα ·, however, the correlations were
significantly worse when including σrad or σJJ· and only a
small improvement was observed when using σ−o and σα (ρ·/
ρo = 3.3, r2 = 0.81) suggesting that radical effects are poten-
tially important for ortho-substituents. Care has to be taken
to interpret this data, as no ortho-substituent constants are
available. The fact that better correlations are observed when
using the σα · constants rather than the σrad or σJJ· constants
is most likely due to the fact that the σα · constants are de-
rived from benzylic substrates and take into account the
deviation of the half-empty orbital from the plane of the aro-
matic ring and the resulting decreasing spin delocalization.
If this is correct, it would suggest that a resonance structure
such as shown in Scheme 2b has a significant contribution.
Correlation of the log of the relative rate (log (kX/kH) for
the meta-substituted oximes with a variety of substituent
constants reveals that both polar and radical stabilizing ef-
fects are important, however, the magnitude of the radical
stabilization constant is much larger (Table 3). These analy-
ses indicate that both polar subsituent constants (σpol and
σmb) give similar results, whereas the results from the radi-
cal substituent constants are significantly different. The best
results are obtained when using Creary’s radical (σrad) sub-
stituent constants. A dual parameter correlation using σpol
and σrad gives a value of 4.2 for the ratio ρrad/ρpol . Similarly,
a value of 6.2 is obtained when using σmb and σrad. All of
these data clearly indicate that radical effects are important
in stabilizing the transition states of the electron transfer re-
actions of meta-substituted acetophenone oximes. This is
consistent with the observation by Jiang and Ji (17l) who
noted a meta-(radical) substituent effect in the thermal cyclo-
addition reactions of α,β,β-trifluorostyrenes, which was inde-
pendent of a polar effect.
Correlation of the quenching rates (kq) for the para-substi-
tuted oximes with the familiar polar and radical substituent
constants shows that good results are obtained when using
polar substituent constants (Table 3). The slope is typically
© 2003 NRC Canada

de Lijser et al.
581
Table 2. Substituent constants used for Hammett studies of ortho-, meta-, and para-substituted acetophenone oximes.
c
d
e
σ−oa
Eseb
o
σ rad
m
σpol
m
σJJ·e
σmb
m
σα · f
o
p
p
m
p
p
m
p
H
0
0
0
0
0
0
0
0
0
0
0
0
CH3
OCH3
CF3
NO2
Cl
–0.13
–0.37
0.81
1.20g
0.50
1.18
0.29
–1.24
–0.55
–2.4
–2.52
–0.97
–0.51
–0.46
0.03
–0.02
–0.07
–0.11
–0.04
–0.12
–0.05
0.11
0.24
0.08
0.57
0.12
0.46
–0.069
0.115
0.43
0.71
0.373
0.56
–0.17
–0.268
0.54
0.778
0.227
0.66
0
0.1
0.15
0.23
–0.01
0.36
0.22
0.42
–0.2
–0.11
0.39
0.69
0.12
0.89
0.23
–0.29
–0.77
0.49
0.86
0.11
–0.001
0.001
0.034
0.001
–0.07
0.001
–0.05
0.11
0.03
–0.014
–0.001
–0.039
–0.018
0.017
0.043
–0.011
CN
F
0.86
–0.24
–0.08
0.337
0.062
–0.02
aM.T. Tribble and J.G. Traynham. J. Am. Chem. Soc. 91, 379 (1969).
bS.H. Unger and C. Hansch. Prog. Phys. Org. Chem. 12, 91 (1976).
cX. Creary. J. Org. Chem. 45, 280 (1980); X. Creary, M. Mehrsheikh-Mohammadi, and S. McDonald. J. Org. Chem. 52, 3254 (1987).
dD.H. McDaniel and H.C. Brown. J. Org. Chem. 23, 420 (1958).
eX.-K. Jiang and G.-Z. Ji. J. Org. Chem. 57, 6051 (1992).
fJ.M. Dust and D.R. Arnold. J. Am. Chem. Soc. 105, 1221 (1983); D.D.M. Wayner and D.R. Arnold. Can. J. Chem. 62, 1167 (1984); D.D.M. Wayner
and D.R. Arnold. Can. J. Chem. 63, 2378 (1985); A.M. de P. Nicholas and D.R. Arnold. Can. J. Chem. 64, 270 (1986).
gM.W. Dietrich, J.S. Nash, and R.E. Keller. Anal. Chem. 38, 1479 (1966).
Table 3. Hammett data for ortho-, meta-, and para-substituted
acetophenone oximes.
as for the radical ions (r2 = 0.70 and 0.79, respectively),
however, the slopes are significantly different (Fig. 4c).
Good correlations are also found when using the σ+ con-
stants. These results suggest that both ground state effects
and stabilization of the transition state of the electron trans-
fer pathways are important. Similar to the results for the
ortho- and meta-substituted oximes, the ground state effects
are less important than the transition state effects. This
observation is consistent with O—H bond dissociation en-
ergy (BDE) data for a series of substituted benzaldehyde
oximes as calculated by Bordwell and Ji (9a). The BDE val-
ues show little variation with different substituents (e.g., p-
OCH3 (87.8 kcal mol–1), p-CN (87.8 kcal mol–1, p-CF3
(87.9 kcal mol–1)), which would be expected for compounds
for which ground state effects are not important. Although it
is not obvious from any of these correlations, the p-OCH3
substituted oxime shows a somewhat abnormal behavior,
similar to that observed for the m-OCH3 derivative. The
C=N bond length is significantly shorter than for most other
para-substituted oxime radical cations and the calculated
N—O bond length is much longer. These results are not as
drastic as those observed for the meta-derivative, however, it
seems to suggest that oxidation elsewhere in the molecule is
competitive with oxidation of the oxime moiety. These re-
sults are supported by a plot of the log of the relative charge
or spin densities against the different substituent constants.
In all cases, the point for the p-OCH3 derivative substan-
tially deviates from the best-fit line.
In contrast to these observations are the results from the
calculations on the iminoxyl radicals. The calculated spin
densities on the carbon of the oxime moiety are all quite
similar and do not reflect any significant substituent effects.
A large amount of (negative) spin density is present on the
carbon and large amounts of positive spin density are pres-
ent on both the nitrogen and the oxygen. Similar results
were obtained for the meta- and para-substituted iminoxyl
radicals. The fact that all of the calculated parameters for the
m-OCH3 derivative are similar to those of the other meta-
substituted oximes, confirms that the unusual behavior noted
before is due to electronic effects in the radical cation. De-
protonation of the meta-methoxyacetophenone oxime radical
Parameter
σo−
Ese
ortho
meta
para
–1.76 (0.81)
+1.00 (0.81)
σpol
–1.63 (0.79)
–1.15 (0.82)
–0.73 (0.88)
–0.49 (0.83)
σmb
σ+
σJJ·
–0.54 (0.91)
–0.36 (0.04)
+1.08 (0.02)
+8.46 (0.79)
+20.0 (0.59)
σ rad
–0.62 (0.20)
+0.20 (0.00)
σα ·
Table 4. Hammett data for meta- and para-substituted
acetophenone oximes (combined).
Parameter
σpol
ρ rad
ρJJ
ρpol
ρmb
ρα ·
r2
–1.06
0.70
0.68
0.02
0.19
0.21
0.71
0.69
0.79
0.76
σmb
–0.70
σ rad
+0.29
σjj·
+0.50
σα ·
+7.37
+4.26
σpol + σjj·
σmb + σjj·
σpol + σα ·
σmb + σα ·
–1.21
–1.32
–2.49
–0.94
–1.78
–0.61 +2.98
suggesting that radical effects are more important than polar
effects for the meta-substituted acetophenone oximes. These
results are in good agreement with the Hammett correlations
discussed above. Similar results are obtained for the para-
substituted oximes. The calculated charge densities follow
the trends as expected for typical electron-donating and
electron-withdrawing groups. Correlation of the calculated
charge densities with the polar parameters σpol and σmb re-
vealed that there is a reasonable relationship between these
in the ground state (r2 = 0.92 and 0.91, respectively) as well
© 2003 NRC Canada

582
Can. J. Chem. Vol. 81, 2003
Fig. 4. Correlation between (a) the ortho-substituent constant σo−
and the calculated (AM1) charge density on the iminoxyl carbon
of the ortho-substituted acetophenone oximes; (b) the polar
(meta) substituent constant σpol and the calculated charge density
on the iminoxyl carbon of the meta-substituted acetophenone ox-
imes; (c) the polar (para) substituent constant σmb and the calcu-
lated charge density on the iminoxyl carbon of the para-
substituted acetophenone oximes.
cation leads to an iminoxyl radical with electronic properties
similar to those of other iminoxyl radicals. These results in-
dicate that: (i) substituent effects are not important for
iminoxyl radicals; and (ii) a complete electronic structure of
iminoxyl radicals must include both configurations as shown
in Scheme 2.
Steady-state photolysis of meta- and para-substituted
acetophenone oximes
Steady-state photolysis of the oximes was carried out in a
Rayonet photochemical reactor. In a typical experiment, the
oxime (0.015 M) and chloranil (0.005 M) were irradiated us-
ing black lights (λ max = 350 nm) for 20 min to keep the con-
version low. The progress of the reaction was followed by
gas chromatography with flame ionization detection (GC-
FID) and the conversion of the oximes was determined by
calibrated GC-FID. Analysis of the data in Table 1 indicates
that even at these short irradiation times significant conver-
sion occurs. No obvious relationship among conversion and
any other parameter (kq, Ep, ∆∆Hf, etc.) was found and cor-
relation of the log of the relative conversion with the usual
substituent parameters failed as well. These results can be
explained on the basis of the proposed mechanism and addi-
tional data available in the literature. Several pieces of infor-
mation are available. First, the measured quenching rates for
the reactions of the substituted oximes with triplet chloranil
are believed to represent the initial electron transfer reaction
between these two species and the variation in the measured
rate constants is due to substituent effects. As expected on
the basis of electronic properties of these substituents, the
presence of electron-donating groups increases the quench-
ing rate and the presence of electron-withdrawing groups
slows it down. Second, calculations have shown that (i) the
ionization potentials correlate well with the measured rate
constants (supporting the electron transfer pathway), (ii) the
effects are not due to ground state effects, and (iii) the sub-
stituents have no significant effect on the spin distribution of
the iminoxyl radicals. Third, Bordwell and Ji (9a) calculated
the pKa values of a variety of oximes, including several sub-
stituted benzaldehyde oximes and acetophenone oxime it-
self. For example, the calculated pKa values of the oxidized
p-CH3, p-OCH3, and p-NO2 benzaldehyde oximes are –11,
0.7, and –22, respectively. Benzaldehyde oxime radical cat-
ion and acetophenone oxime radical cation both have a pKa
value of –13. These results demonstrate that electron-donating
groups (-CH3, -OCH3) decrease the acidity, whereas electron-
withdrawing groups (-NO2) increase the acidity of the radi-
cal ions. Combining all this data and applying it to the work-
ing mechanism of the first steps of these reactions (electron
transfer followed by deprotonation of the oxime radical cat-
ion) can explain the observed conversions. Compounds with
electron-donating groups are more easily oxidized and ex-
© 2003 NRC Canada

de Lijser et al.
583
hibit fast quenching rates, however, the resulting radical ions
must overcome larger barriers for the deprotonation step.
The opposite is true for oximes with electron-withdrawing
substituents, which have slower quenching rates but will un-
dergo a faster deprotonation step because the resulting radi-
cal cations are more acidic. Clearly, this is a simplified
picture and many more factors are likely to be important in
the overall reactions of these substrates. For example, this
explanation does not yet include the follow up steps, which
are likely to be important. Oxidation of the iminoxyl radical
(Scheme 3) would result in the formation of a carbocation
species, which would be expected to show a large substi-
tuent effect. We continue to investigate the scope and limita-
tions of the photosensitized electron transfer reactions of
oximes as well as the structure and reactivity of iminoxyl
radicals.
former pathway. The latter pathway is certainly a possibility
in the electrochemical experiments because of the timescale
in these reactions. In the photochemical experiments it
would be possible if the energetics are favorable, and on the
basis of the results obtained so far, we predict that it would
be more likely to occur in the case of oximes with electron-
withdrawing substituents as their acidity (upon one-electron
oxidation) is much larger and the deprotonation step is ex-
pected to be much faster. Future experiments will have to
clarify this issue.4
Expe rim e nta l
Materials
All chemicals other than the oximes were commercially
available. Acetonitrile (Fisher) was HPLC grade and used as
received. All oximes were prepared via standard procedures
(25) from the corresponding ketones (Aldrich or Lancaster
Synthesis, Inc.). A typical procedure consisted of combining
equal amounts (~1 g) of hydroxylamine hydrochloride and
the appropriate acetophenone with 5 mL of water and 3 mL
of 3 M NaOH. Ethanol (95%) was used to create a clear so-
lution, which was then refluxed for 60 min. The reaction so-
lution was allowed to cool, and the product was extracted
with diethyl ether. The solvent was evaporated and the crude
product collected and purified by recrystallization or column
chromatography. The purity was checked by GC-FID
Conc lus ions
This study has shown that quenching of the excited photo-
sensitizer (chloranil) by substituted acetophenone oximes is
typically fast except in those cases where there is a signifi-
cant steric effect. This steric effect is only observed with the
substituent in the ortho-position reactions and can also be
noticed from the measured peak potentials. It is believed that
bulky ortho-substituents such as -CH3, -CF3, -NO2, and -Cl
prevent the oxime moiety and aromatic ring from becoming
coplanar so that conjugation is minimized. Under these con-
ditions, energy transfer can be competitive with electron
transfer as shown by significant syn–anti isomerization. The
measured quenching rates correlate well with the calculated
(AM1) ionization potentials for the radical ions, suggesting
that the quenching represents an electron transfer pathway.
Correlation of the quenching rates with Hammett substituent
constants revealed that for the ortho-substituted oximes,
steric (Ese), polar electronic (σo−), and radical (σα ·) effects
are important. For meta-substituted oximes, both polar and
radical effects dominate the quenching rates, however, the
large magnitude of the radical reaction constants suggests
that radical effects dominate. As expected, polar effects
dominate the para-substituted oximes. It seems clear that to
predict the influence of a substituent on the photosensitized
electron transfer reactions of acetophenone oximes, the best
results will be obtained by analysis of the quenching rates
and when studying individual series of compounds. On the
basis of the correlation between calculated (AM1) charge
densities on the carbon of the oxime moiety and polar
substituent constants it is concluded that ground state effects
are relatively unimportant and the observed substituent ef-
fects represent transition state effects. Also of importance is
the observation that the quenching rates do not correlate
with the measured conversions of the oximes. This is ex-
plained in terms of two important but energetically opposing
steps: oxidation and deprotonation. Whether this process
consists of two individual steps (oxidation followed by
deprotonation) or a single proton-coupled electron transfer
step is unclear, although the LFP studies currently favor the
1
(> 98% peak area), GC–MS, H NMR, FT-IR, and melting
point determination.
Steady-state photolysis experiments and sample analysis
Appropriate amounts of the ortho-substituted aceto-
phenone oxime (0.025 M) and chloranil (0.025 M) were
weighed out and dissolved in 5 mL of acetonitrile. The stud-
ies on the meta- and para-substituted oximes utilized
0.015 M oxime and 0.005 M chloranil. The solution was
placed in a Pyrex tube and photolyzed in a Rayonet RPR-
100 photochemical reactor, equipped with 16 RPR-3500A
(black light phosphor) bulbs (λ = 350 nm) for up to 3 h. The
progress of the reactions was followed by GC-FID and the
products were identified by GC–MS. Conversion of the
starting material and product yields were determined by cali-
brated GC-FID. Sample analyses were performed on a
Hewlett-Packard 5890 series II gas chromatograph coupled
to a Hewlett-Packard 5971 series mass selective detector
(MSD) and on a Perkin-Elmer (PE) Autosystem equipped
with a flame ionization detector (FID). The HP GC–MS was
equipped with an HP-5 capillary column (30 m × 0.25 mm
i.d., film thickness 0.25 mm). The PE GC-FID was equipped
with a Chrompack CP-Sil–5-Cb capillary column (30 m ×
0.32 mm i.d, film thickness 0.25 mm).
Cyclic voltammetric measurements
Cyclic voltammetry at a sweep rate of 100 mV s–1 was
used to obtain the oxidation potential of the oximes. All
electrochemical experiments were performed using a BAS
CV-50 voltammetric analyzer and a BAS C3 electrochemis-
4 Supplementary data may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). These data
can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, U.K.; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2003 NRC Canada

584
Can. J. Chem. Vol. 81, 2003
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3
0.5–2 mJ/pulse, 4 ns pulse width) and the decay of CA at
510 nm was observed. Small amounts (10–25 uL) of the
quencher (~0.015 M oxime standard solutions in acetoni-
trile) were added to the solution after which the decay was
measured. The quenching rate was obtained from a plot of
the measured decay rates against the quencher concentration.
Detailed descriptions of the nanosecond transient absorp-
tion apparatus used for the spectral studies are given else-
where (29). Spectra were obtained by flash photolysis
(343 nm, 15 ns, 3 mJ) of argon-saturated acetonitrile solu-
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Ac knowle dge m e nts
Acknowledgement is made to the donors of the Petroleum
Research Fund, administered by the American Chemical So-
ciety, and to the Research Corporation for support of this
work. Part of this work is based upon work supported by the
National Science Foundation under Grant No. 0097795. We
are grateful to Professors J.P. Dinnocenzo (University of
Rochester) and I.R. Gould (Arizona State University) for the
use of their laser flash photolysis equipment.
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