10458
J. Phys. Chem. A 2010, 114, 10458–10466
Side Reactions of Nitroxide-Mediated Polymerization: N-O versus O-C Cleavage of
Alkoxyamines
Jennifer L. Hodgson,† Luke B. Roskop,‡ Mark S. Gordon,‡ Ching Yeh Lin,† and
Michelle L. Coote*,†
Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, Research
School of Chemistry, Australian National UniVersity, Canberra, ACT 0200, Australia, and Department of
Chemistry, Iowa State UniVersity, Ames, Iowa 50011
ReceiVed: July 12, 2010; ReVised Manuscript ReceiVed: August 18, 2010
Free energies for the homolysis of the NO-C and N-OC bonds were compared for a large number of
alkoxyamines at 298 and 393 K, both in the gas phase and in toluene solution. On this basis, the scope of the
N-OC homolysis side reaction in nitroxide-mediated polymerization was determined. It was found that the
free energies of NO-C and N-OC homolysis are not correlated, with NO-C homolysis being more dependent
upon the properties of the alkyl fragment and N-OC homolysis being more dependent upon the structure of
the aminyl fragment. Acyclic alkoxyamines and those bearing the indoline functionality have lower free
energies of N-OC homolysis than other cyclic alkoxyamines, with the five-membered pyrrolidine and
isoindoline derivatives showing lower free energies than the six-membered piperidine derivatives. For most
nitroxides, N-OC homolysis is normally favored above NO-C homolysis only when a heteroatom that is R
to the NOC carbon center stabilizes the NO-C bond and/or the released alkyl radical is not sufficiently
stabilized. As part of this work, accurate methods for the calculation of free energies for the homolysis of
alkoxyamines were determined. Accurate thermodynamic parameters to within 4.5 kJ mol-1 of experimental
values were found using an ONIOM approximation to G3(MP2)-RAD combined with PCM solvation energies
at the B3-LYP/6-31G(d) level.
1. Introduction
The dissociation of an alkoxyamine into a nitroxide radical
and an alkyl radical (Scheme 1) as well as the reverse, a
combination reaction of these two species, is central to many
industrially relevant processes. These types of reactions are
important when nitroxides are utilized as radical traps, as control
agents in radical polymerization, and as stabilizers of polymer
coatings. The control reaction of nitroxide-mediated polymerization (NMP) involves the reversible trapping of a growing
polymer chain by a nitroxide to form a dormant alkoxyamine,
thereby preventing uncontrolled chain growth. A similar trapping
reaction also makes up part of the Denisov cycle, which involves
the catalytic protection of polymer coatings against photooxidative damage by hindered amine light stabilizers (HALS)
(Scheme 2).1,2
When alkoxyamines are used in NMP and also when they
are formed from hindered amine light stabilizers as part of the
Denisov cycle, competition exists in the homolysis of alkoxyamines
between the desired dissociation at the NO-C bond and the
dissociation at the N-OC bond. N-OC homolysis occurs as
an unwanted side reaction, leading to the formation of aminyl
radicals and highly reactive and possibly polymer-damaging free
alkoxyl radicals. An understanding of the effects of the chemical
structure of the nitroxide and the propagating radical on the
competition between these two processes would therefore be
extremely useful. In particular, it would assist in the design and
selection of nitroxides that were more resistant to the unwanted
N-OC homolysis process.
* Corresponding author. E-mail: mcoote@rsc.anu.edu.au.
†
Australian National University.
‡
Iowa State University.
SCHEME 1: Reversible Homolysis of Alkoxyamine at
the R′R′′NO-R Bond, Forming the Control Reaction for
Nitroxide-Mediated Polymerization
SCHEME 2: Simplified Mechanism of the Denisov
Cycle, Showing Protection Using a Piperidine HALS
(where X is typically H or an alkyl chain)
Previously, this competition has been studied by Siri et al.3
and Gigmes et al.4 for several piperidine- and indoline-based
alkoxyamines using density functional theory (DFT) as well as
experimental methods. The main finding for the piperidine-ring
systems was that NO-C homolysis has lower bond dissociation
energies (BDEs) than N-OC except in the case of a vinyl
acetate monomer-substituted alkoxyamine, which had an NO-C
bond stabilized by an anomeric interaction. For the studied
indoline alkoxyamines, it was concluded that N-OC homolysis
was preferred except for alkoxyamines substituted with an
C(CH3)2CO2CH2CH3 alkyl fragment. However, it would be
useful to extend this study to include a larger range of alkyl
fragments as well as acyclic and other common cyclic nitroxide
species.
10.1021/jp1064165 2010 American Chemical Society
Published on Web 09/02/2010
Side Reactions of Nitroxide-Mediated Polymerization
SCHEME 3: Nitroxide and Alkyl Radical Species in This
Study Formed by the NO-C Bond Homolysis of
Alkoxyamines
Additionally, in these previous studies,3,4 it was found that
the DFT and semiempirical methods used showed large variations from each other and from the experimental BDEs, although
the DFT methods were able to model the NO-C versus N-OC
competition successfully. Elsewhere, we have shown that a
variety of DFT methods failed to model even the qualitative
trends in NO-C BDEs of a series of alkoxyamines.5 Although
it is expected that the DFT methods used in these previous
studies may provide a reasonable measure of the difference in
BDEs for NO-C and N-OC bond homolysis, it is likely that
higher-level calculations will be required for the quantitative
measurement of these BDEs and an understanding of structurereactivity trends within a series.
In the present work, we use high-level ab initio molecular
orbital calculations to examine the effects of chemical structure
on the NO-C and N-OC bond dissociation energies for a broad
range of alkoxyamines, with a view to predicting when the
N-OC breaking side reaction is likely to be important in NMP
and other applications. The various alkoxyamines in this study
are illustrated by the set of nitroxide and alkyl radicals formed
by the NO-C bond homolysis of alkoxyamines shown in
Scheme 3. They include five- and six-membered cyclic alkoxyamines as well as acyclic species, with the alkyl fragments
designed to provide models of common polymeric chains. As
part of this work, we also conduct an assessment study with
the aim of identifying and benchmarking reliable theoretical
procedures for studying this process.
2. Theoretical Procedures
Standard ab initio molecular orbital6 and density functional7
calculations in this work were carried out using Gaussian 03,8
MOLPRO 2000.6,9 and GAMESS.10 Calculations on radicals
were performed with an unrestricted wave function except in
cases designated with an R prefix where a restricted open-shell
wave function was used.
To find a suitable procedure with which to model the
homolysis of alkoxyamines, the performance of a variety of
levels of theory was assessed for a set of reactions for which
experimental kinetic and thermodynamic parameters have been
determined. Accurate methods for the calculation of geometries,
gas-phase single-point energies, and free energies of solvation
were determined, and the calculated equilibrium constant of the
reaction was compared with experimental results. Geometries
were optimized using various methods and basis sets, and the
results were benchmarked against the high-level QCISD/6-
J. Phys. Chem. A, Vol. 114, No. 38, 2010 10459
31G(d) method. To assess the effect of the level of theory used
for geometry optimization on the resulting trapping enthalpies,
single-point energy calculations were then carried out on all
the geometries at a consistent level of theory, the QCISD/631G(d) level.
Gas-phase enthalpies for the trapping of several different
small carbon-, oxygen-, and nitrogen-centered radical species
were calculated using very high level composite methods from
the W1,11 CBS,12 G4,13 and G314,15 families and compared with
experimental enthalpies in order to determine an accurate
method for the calculation of gas-phase single-point energies.
Enthalpies for the trapping of several different alkyl radical
species by the nitroxide 2,2,6,6,-tetramethylpiperidin-1-yloxyl
(TEMPO), using species optimized at the B3-LYP/6-31G(d)
level, were calculated over a wide variety of levels of theory to
determine suitable lower-cost methods for larger systems. The
methods assessed include a variety of DFT methods, RMP2,
and several ONIOM-type16 methods. In ONIOM, a chemical
reaction is divided into a core section that includes the reaction
center and principal substituents and an outer section, which is
the rest of the chemical system. The core system is calculated
at a high level of theory and also at a lower level of theory, but
the full chemical system is calculated only at the lower level.
We have previously shown that this method is suitable for
reactions involving nitroxides.5,17-19 Three lower-cost methods
were tested for the calculation of the full system in the ONIOM
calculations (RMP2/6-311+G(3df,2p), BMK/6-311+G(3df,2p),
and B3-LYP/6-311+G(3df,2p)), and G3(MP2)-RAD was the
high level of theory used to study the core section.
The free energy of solvation, ∆Gsolv, quantifies the free-energy
difference between a gas-phase system and the same system in
a solvent medium. Several procedures for the evaluation of the
free energy of solvation were examined. These included
polarized continuum models PCM20 and CPCM,21 as implemented in the Gaussian 03 software package.8 These were
employed at both the HF/6-31G(d) and B3-LYP/6-31G(d) levels
of theory using the recommended radii for these calculations
(i.e., the united atom topological model applied to radii
optimized for the HF/6-31G(d) and PBE0/6-31G(d) levels of
theory, respectively).22 For these calculations, the molecular
structures of all species were reoptimized in the presence of
the solvent field. In addition, free energies of solvation were
also computed using the COSMO-RS23 model, a method that
describes the interactions in a fluid as local interactions of
molecular surfaces. COSMO-RS free energies of solvation were
calculated on gas-phase geometries via the ADF package,24 using
the Becke-Perdew (BP86) functional and triple-ζ-quality basis
sets augmented by one set of polarization functions (TZP).
Default values were used for the parameters describing the
atomic cavity radii, the radius of the probing sphere, and the
cavity construction.
In addition to studying the thermodynamics of the homolysis
reactions, we also calculated the full reaction pathway for a
model system to examine whether kinetics should follow
thermodynamics. For this purpose, multiconfigurational selfconsistent field (MCSCF) wave functions were constructed and
single-point energies along the path were calculated at the
MRMP2/6-31G(d)//FORS-MCSCF(2,2)/6-31G(d) level for both
NO-C and N-OC homolysis of the 1-isopropyl-2,2,6,6-tetramethyl-piperidine (TEMPOCH(CH3)2) species. This level of
theory is a multireference second-order perturbative treatment
of orbitals initially optimized using a multiconfigurational selfconsistent wave function of the fully optimized reaction space
type. The T1 diagnostic values25 for the various species along
10460
Hodgson et al.
J. Phys. Chem. A, Vol. 114, No. 38, 2010
this path were also determined. So as to facilitate the calculation
of thermal and entropic corrections, frequencies along these
reaction paths were calculated at the FORS-MCSCF(2,2)/631G(d) level; because these were nonstationary points, the
projected frequencies were used.
Having identified accurate methods on the basis of the
assessment study, these were then used to calculate the free
energies of NO-C and N-OC bond homolysis for a large
number of alkoxyamines, both in the gas phase and in toluene,
at room temperature and at the experimentally relevant temperature of 393.15 K (120 °C). The gas-phase free energies were
calculated using the standard textbook formulas for the statistical
thermodynamics of an ideal gas under the harmonic oscillator
rigid rotor approximation.26,27 These calculations were carried
out using our in-house program TChem.28 Frequency calculations carried out at the B3-LYP/6-31G(d) level were scaled via
the appropriate factors.29 The solution-phase free energies were
calculated as the sum of the corresponding gas-phase free
energy, the calculated free energy of solvation, and a correction
term RT ln(RT/P), where R is the ideal gas constant, T is the
reaction temperature, and P is the standard pressure of 1 atm
(101.325 kPa). The correction term takes account of the fact
that the solvation energy is computed for the passage from 1
atm (g) to 1 mol L-1(soln).30
To facilitate direct comparisons between experimental and
calculated solution-phase free energies of homolysis, experimental equilibrium constants K ) kc/kd were obtained by
combining independently measured values of the rate coefficients for the forward, kc, and reverse, kd, trapping reactions.
The experimental free energy was then found from the equation
K ) (c°)∆n exp(-∆G/RT), where T is the reaction temperature
in Kelvin, R is the ideal gas constant (8.3143 J mol-1K-1), c°
is the standard unit of concentration (1 mol L-1 for the solutionphase data), and ∆n is the change in the number of moles on
reaction (1 for homolysis).
3. Results and Discussion
3.1. Geometry Optimization. To establish whether differences in the procedure used to optimize molecular geometries
will have any significant effect on the calculated energetics of
nitroxide reactions, single-point energies in the gas phase were
calculated at a consistent level of theory (QCISD/6-31G(d)) for
the optimized geometries of various methoxyamines and dissociated products. These energies, with B3-LYP/6-31G(d) frequencies, were used to calculate O-C bond homolysis reaction
enthalpies, shown in Table 1. Optimizations were performed
using a range of levels of theory from the computationally lower
cost Hartree-Fock and density functional theory (DFT) methods
to more demanding MP2 and QCISD calculations, with both
small and larger basis sets. The results were benchmarked
against the QCISD/6-31G(d) method.
In line with previous studies showing that low-cost methods
perform very well in geometry optimizations of nitroxides,31
all of the lower-cost methods yielded reasonably accurate
geometries for the calculation of alkoxyamine dissociation
reactions when compared with the benchmark level of theory,
though there are some minor differences among the methods
used. DFT method B3-LYP/6-31G(d) appears to offer the best
compromise between accuracy and computational cost. The
enthalpies of homolysis, calculated from geometries optimized
at the B3-LYP/6-31G(d) level, show a difference of less than
0.3 kJ mol-1 from the corresponding benchmark values. Thus,
B3-LYP/6-31G(d) was adopted for the determination of the
geometries of molecules in this work.
TABLE 1: Effect of the Level of Theory Used for Geometry
Optimization in the Gas-Phase Enthalpy of Homolysis
(at 0 K) of Alkoxyaminesa
a
In all cases, homolysis enthalpies were calculated using QCISD/
6-31G(d) single-point energies, performed on species optimized
using various wave functions and basis sets.
3.2. Gas-Phase Single-Point Energy. To identify an appropriate method for the calculations of gas-phase single-point
energies, it would be advantageous to assess various homolysis
reactions over a wide variety of levels of theory and compare
the results with experimental data. However, at the highest levels
of theory, only the calculation of very small systems is possible
and corresponding gas-phase experimental data are unavailable
for prototypical alkoxyamine homolysis reactions. There are,
however, limited experimental data for the gas-phase trapping
of some similar systems in the form of experimental heats of
formation for the individual reactants and products that make
up homolysis reactions forming oxygen- and nitrogen-centered
radicals.32 These data points allowed the gas-phase homolysis
enthalpies of theses reactions to be determined at various high
levels of theory and compared directly with experimental results.
Calculated and experimental enthalpies of the reactions of small
molecular species are shown in Table 2. Calculation methods
include very high level method W1U11 as well as high-level
composite methods such as CBS-QB312 and methods from the
G3 family.14,15 Enthalpies calculated using the recently reported
G413 method were also determined and are included in Table
2. Studies related to this work have shown that this method
does not offer significant improvement over the various G3
methods for radical addition and abstraction reactions.33
Reactions examined in Table 2 include the dissociation of
small alkoxyamine H2NOCH3 into H2NO · and · CH3 as well as
homolysis reactions producing similar small oxygen- and
nitrogen-centered radicals. Enthalpies, calculated using the
highest theoretical method assessed (W1U), show a mean
absolute deviation (MAD) from an experimental value of 3.6
kJ mol-1. The respective MAD for the G3(MP2)-RAD values
is only marginally larger, at 5.6 kJ mol-1, despite its considerably lower computational cost. Moreover, for the homolysis
reaction forming a methyl radical and oxygen-centered radical
H3CO · (which is most relevant to the present work), the two
G3 methods show the smallest deviations from the experimental
values. The G3(MP2)-RAD method therefore provides a suitable
benchmark level of theory for alkoxyamine homolysis.
Side Reactions of Nitroxide-Mediated Polymerization
J. Phys. Chem. A, Vol. 114, No. 38, 2010 10461
TABLE 2: Calculateda and Experimentalb Enthalpies (298 K) for Gas-Phase Radical-Trapping Reactions
enthalpy of trapping (kJ mol-1)
reaction
CBS-QB3
G3(MP2)-RAD
G3X
G4
W1U
exp.
447.8
443.0
438.8
441.4
447.9
452.7 ( 1.3
H2NCH3 f H2N + CH3
357.0
346.8
348.5
350.1
354.8
358.6 ( 2.1
CH3OH f H3CO · + · H
437.9
435.9
430.3
431.5
438.4
436.0 ( 3.8
H3COCH3 f H3CO · + · CH3
356.3
347.5
348.5
347.1
351.5
348.1 ( 4.2
H2NOCH3 f H2NO · + · CH3
240.1
237.4
237.0
235.3
237.6
4.1
5.6
7.5
6.3
3.6
·
·
NH3 f H2N + H
·
-1 c
MAD (kJ mol )
·
a
0
b
Calculated at various levels of theory using literature-recommended geometries. Taken from ref 32. Where multiple values are provided in
this reference, the recommended values are quoted. c Mean absolute deviation from experimental values.
Figure 1. Mean and maximum absolute deviations of various lowcost methods from G3(MP2)-RAD benchmark values for the gas-phase
enthalpies of NO-C homolysis (at 0 K) of various TEMPO-alkyl
alkoxyamines. Single-point calculations were made with 6-311+G(3df,2p)
basis sets for species optimized at the B3-LYP/6-31G(d) level.
Although G3(MP2)-RAD is a relatively computationally
efficient high-level composite method, this type of calculation
is currently limited to systems of less than around 17 to 18 nonhydrogen atoms using our computational resources. Therefore,
suitable lower-cost methods are required for reactions involving
larger polymeric and nitroxide radicals. Recently, we have
examined the use of various low-cost DFT, MP2, and ONIOM
procedures in the calculation of a broad range of radical
reactions, including a small test set of alkoxyamine BDEs, with
a view to finding a suitable method to use for larger systems
for which G3(MP2)-RAD calculations are impractical.5 We have
now updated the alkoxyamine test set to include calculations
using some more recently developed DFT methods (M05-2X,
M06-2X, M08-HX, and M08-SO), where a number of these
were recently shown to yield improved results for these and/or
related reactions.34 A complete set of the results is shown in
Figure 1; the corresponding BDEs at the various levels of theory
are provided in Table S2 of the Supporting Information.
In Figure 1, it is seen that, in general, the DFT treatments
fail to reproduce the G3(MP2)-RAD benchmark enthalpies,
having instead large and unpredictable deviations with MADs
of over 25 kJ mol-1 and in some cases over 80 kJ mol-1.
Consistent with the recent work of Truhlar et al.,34 more success
is seen for the newly developed M05-2X, M06-2X, M08-HX,
and M08-SO series of DFT methods, with average errors still
in the range of 7-15 kJ mol-1. Of these, the M06-2X method
showed the best performance for this test set, possibly because
these particular reactions were part of the test set used to
parametrize this functional. However, even for this functional
the maximum error is still 17.8 kJ mol-1. The RMP2 method
shows better performance than most of the DFT methods, but
the enthalpy values still differ from the benchmark values by
an average of 8 kJ mol-1 and have a maximum deviation of
9.5 kJ mol-1 It is therefore clear that the DFT and RMP2
methods assessed are not suitable as accurate, low-cost methods
for reactions in this study.
It has previously been shown that an ONIOM method35 can
been used to approximate the G3(MP2)-RAD energies of radical
reactions at a much lower computational cost.36 Deviations in
homolysis enthalpies of alkoxyamines calculated using ONIOM
methods from G3(MP2)-RAD values are shown in Figure 1. In
this approach, the full reaction system was calculated using a
method such as B3-LYP, BMK, or RMP2 with the 6-311+
G(3df,2p) basis set whereas the core system was calculated at
G3(MP2)-RAD. The core system was set up to include
substituents up to the β position in the nitroxide so that in the
core the nitroxide was represented by the small dimethyl
nitroxide species, (CH3)2NO · . In all reactions studied, the use
of an ONIOM correction leads to greatly improved results. Thus,
for example, at the B3-LYP/6-311+G(3df,2p) level of theory
the MAD value decreases from 67.0 to 12.4 kJ mol-1 when the
ONIOM correction is applied; for BMK, the MAD value
improves from 25.8 to 6.3 kJ mol-1, although the maximum
deviation still exceeds 10 kJ mol-1. Not surprisingly, given its
performance above, the best results are obtained when the
remote substituent effects are measured using RMP2/6311+G(3df,2p). In this case, the MAD, relative to full G3(MP2)RAD calculations, is only 4.7 kJ mol-1 and the maximum
deviation is only 5.6 kJ mol-1. Therefore, when the computational cost of G3(MP2)-RAD becomes excessive, it is expected
that an ONIOM approximation, in which the full nitroxide
system is represented by dimethyl nitroxide, will still give a
reasonable approximation to the full G3(MP2)-RAD calculated
energy.
3.3. Free Energy of Solvation. To assess the level of theory
used to calculate free energies of solvation for the species in
this work, calculated total free energies of reaction for
alkoxyamine homolysis reactions were compared with experimental values found from measured rate constants.37-40 Gasphase energies were calculated at the benchmark G3(MP2)RAD//B3-LYP/6-31G(d) level and added to free energies of
solvation calculated using different levels of theory, basis sets,
and solvation models. Because the accuracy of the gas-phase
calculation has been separately validated, it was expected that
any additional differences would be caused by differences in
the free energies of solvation. Three methods of calculating
solvation effects were assessed: polarized continuum models
PCM18 and CPCM21 and COSMO-RS.23 Because the experimental measurements were performed at 393 K in tert-butyl
benzene (tBB), the ab initio reaction free energies were also
calculated at 393 K in the closely related solvent toluene.
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Hodgson et al.
J. Phys. Chem. A, Vol. 114, No. 38, 2010
TABLE 3: Effect of the Level of Theory Used for Free Energies of Solvation on the Free Energy (kJ mol-1) of the Homolysis
at the NO-C Bond (at 298 K) of Alkoxyaminesa
solvation model
PCMb
CPCMb
COSMO-RSc
alkoxyamine
HF/6-31G(d)
B3-LYP/6-31G(d)
HF/6-31G(d)
B3-LYP/6-31G(d)
BP86/TZP
exp.d
1k
2k
2l
2m
2n
2p
4l
4m
MADe
88.2
98.0
87.0
73.8
119.0
81.5
78.9
72.5
4.1
87.4
97.0
85.9
72.6
118.0
80.3
77.9
71.3
4.5
87.4
97.2
85.8
72.7
118.5
80.6
77.9
71.6
4.4
86.2
95.9
84.6
71.3
117.2
79.2
76.6
69.9
5.0
88.9
98.0
89.6
80.4
118.1
79.5
88.0
75.8
4.1
90.6
101.6
87.9
66.3
123.8
78.5
89.0
72.0
0
a
Incorporating G3(MP2)-RAD//B3-LYP/6-31G(d) gas-phase energies. b Calculated using the united atom topological model with the
recommended optimization of radii (i.e., UAHF and UAKS keywords for HF/6-31G(d) and B3-LYP/6-31G(d), respectively).22 c Geometry not
reoptimized in solvent. d Calculated from experimental rate constants for homolysis and combination using K ) kc/kd and K ) exp(-∆G/RT).
e
Mean absolute deviation from experimental values.
TABLE 4: Comparison of Calculated and Literature Gas-Phase (298 K) Bond Dissociation Energies (BDEs) (kJ mol-1)
PM3a
alkoxyamine
2a
2k
2l
2m
2o
2p
2q
5a
5l
5p
O-C break
B3P86/6-311++G(d,p)//B3-LYP/6-31G(d)
N-O break
diff.
O-C break
N-O break
176a
119a
108a
87a
121a
189a
diff.
13
174a
66
180a
59
179a
173b
118b
102b,d
176a
141b
137b
126b,d
G3(MP2)-RAD//B3-LYP/6-31G(d)
O-C break
N-O break
205
162
169
161
182
169
235
210c
195
216
231
238
238
223
242
228
237
192c
214
226
diff.
26
76
69
62
60
59
2
-18
19
10
161
130
105
118
86
142
107
112
130
-54
-18
25
117
107
31
-35
Absolute Values
MADe
max. deviationf
56
93
118
131
64
130
61
114
65
100
7
14
(0)
(0)
(0)
(0)
(0)
(0)
Relative valuesg
MADe
max. deviationf
36
54
14
33
51
59
20
53
15
36
8
17
(0)
(0)
(0)
(0)
(0)
(0)
-3
-32
19
24
a
Reference 3. b Reference 4. c Calculated for model system 5a. d Calculated for a C(CH3)2CO2CH2CH3 alkyl fragment. e Mean absolute
deviation from G3(MP2)-RAD values. f Maximum deviation from G3(MP2)-RAD values. g Relative values calculated using the 2l system as the
reference.
The calculated solution-phase free energies for homolysis
reactions of a range of alkoxyamines are shown along with the
available experimental data37-40 in Table 3. As Table 3 shows,
there is little variation in the reaction free energies when
different methods of calculating the solvation energy are used.
The MADs of the calculated free energies from the experimental
values vary between 4.1 and 5.0 kJ mol-1. The level of theory,
the basis set, and the solvation model have very little effect on
the value of the free energy of solvation. As such, the method
chosen for the calculation of free energies of solvation in
nitroxide trapping reactions appears to be fairly arbitrary for
these reactions. The PCM solvation model at the B3-LYP/631G(d) level was chosen for further calculations in this study
because it gives good solvation energies and has likewise
performed well in our studies of the redox potentials of nitroxide
radicals18,19 while remaining computationally efficient.
3.4. Relative Errors in NO-C and N-OC BDEs. In the
assessment study described above, it was found that in order to
reproduce experimental free energies for O-C bond homolysis
in alkoxyamines high-level composite methods are required.
Previous investigations of the competition between NO-C and
N-OC in akoxyamines have been performed using lower-cost
semiempirical and B3P86 methods on the basis that these lower
cost methods may be able to describe the qualitative trends in
the data.3 It is therefore of interest to see how well these perform
against our G3(MP2)-RAD values (Table 4).
From Table 4, it is clear that, as expected, neither of these
low-level methods can reproduce the absolute values of the
NO-C and N-OC BDEs with MADs in excess of 50 kJ mol-1
and maximum absolute deviations in excess of 100 kJ mol-1.
If we next examine the relative values of the BDEs within either
the NO-C or N-OC series (taking 2l as our reference in each
case), then we note that these errors, while smaller, are still
unacceptably large, having MADs ranging from 14 to 36 kJ
mol-1 and maximum deviations ranging from 33 to 54 kJ mol-1.
Because an error of just 6 kJ mol-1 alters the equilibrium
constant by approximately 1 order of magnitude at room
temperature, neither of these low levels of theory is suitable
for studying substituent effects in these reactions.
Finally, if we instead examine the difference in the NO-C
and N-OC BDEs for the same alkoxyamines, then the semiempirical PM3 method still has unacceptably large errors. (For the
absolute values, the MAD is 64 kJ mol-1 and the maximum
deviation is 130 kJ mol-1.) However, the B3P86 calculations
do benefit from some systematic error cancellation and perform
reasonably well, with an MAD (for the absolute values) of just
Side Reactions of Nitroxide-Mediated Polymerization
Figure 2. Enthalpy surface (gas phase, 298.15 K) for the homolysis
of 1-isopropoxy-2,2,6,6-tetramethyl-piperidine calculated at the MRMP2/
6-31G(d)//FORS-MCSCF(2,2)/6-31G(d) level.
7 kJ mol-1, though the maximum deviation is still a little high
at 14 kJ mol-1. Moreover, in this case the relative values (i.e.,
trends) in these values actually have slightly larger levels of
error. Although the success of B3P86 in reproducing the NO-C
versus N-OC BDE differences is promising and may be
improved through the use of more sophisticated DFT methods,
for the present work we have retained the use of high-level
methods so that both the absolute and relative values of the
BDEs that we report are reliable.
3.5. Homolysis Reaction Pathways. Reaction pathways
involving the homolysis of a closed-shell species into two radical
species usually involve at least two significant configurations
(i.e., the bonding and antibonding configurations) and are thus
likely to be imperfectly described by a single reference wave
function. To calculate the complete reaction pathway accurately,
a multireference treatment is therefore usually required. In this
case, the minimum description requires an active space including
the N-OC (or NO-C) σ bonding and antibonding orbitals along
with two active electrons. The multiconfigurational selfconsistent field (MCSCF) wave functions were constructed and
used to calculate the energies along the path of the homolysis
reaction of the alkoxyamine 1-isopropoxy-2,2,6,6-tetramethylpiperidine (TEMPO-CH(CH3)2) into both TEMPO and an
isopropyl radical species •CH(CH3)2 and into a 2,2,6,6-tetramethyl-piperidinyl radical and an isopropoxy radical species
•
OCH(CH3)2.
Figure 2 shows the gas-phase reaction enthalpy curve (298.15
K) of the homolysis of TEMPO-CH(CH3)2 at both the O-C
and N-O bonds. The active space used for both reactions
includes two active electrons and two active orbitals (2, 2)
consisting of the σ bonding and σ* antibonding orbitals of the
forming bond. To account for dynamic correlation effects,
single-point energies along the path were calculated at the
MRMP2/6-31G(d)//FORS-MCSCF(2,2)/6-31G(d) level. The
inclusion of lone pairs on the nitrogen and oxygen species into
the active space was shown to be unnecessary by a calculation
of the O-C breaking homolysis reaction pathway at the
MRMP2/6-31G(d)//FORS-MCSCF(8,5)/6-31G(d) level. The
MAD of the (2, 2) active space from the (8, 5) active space
was only 2.9 kJ mol-1 (Figure S1 in the Supporting Information). The corresponding free energies of homolysis are shown
in Figure 3. The extrapolation of these curves to infinity can be
compared with the free energies calculated using G3(MP2)RAD. In the case of the O-C homolysis, the deviations are
relatively small (4 kJ mol-1); for N-O homolysis, the deviations
are slightly larger (10 kJ mol-1), indicating that a larger basis
J. Phys. Chem. A, Vol. 114, No. 38, 2010 10463
Figure 3. Free-energy surface (298.15 K, gas phase) for the homolysis
of 1-isopropoxy-2,2,6,6-tetramethyl-piperidine calculated at the MRMP2/
6-31G(d)//FORS-MCSCF(2,2)/6-31G(d) level. The corresponding
G3(MP2)-RAD free energies of homolysis are shown on the right y
axis for purposes of comparison.
set would be necessary for a chemically accurate treatment of
the reaction pathway. Nonetheless, the agreement is close
enough to support the use of these curves in a qualitative analysis
of the dissociation.
The reaction surface of the O-C and N-O homolysis
reactions shown in Figure 2 is a smooth curve, as expected,
with no enthalpic barrier. Small free-energy barriers exist
because of the effects of entropy, occurring at separations of
around 2.6 and 3.0 Å for the O-C and N-O bond-breaking
reactions, respectively (Figure 3). However, in general it appears
that these effects are minor, at least at synthetically relevant
reaction temperatures, and it is the thermochemistry of the
process that ultimately determines whether N-OC or NO-C
homolysis occurs.
Although the kinetic parameters of the reaction require a
multireference treatment, as shown above, single-reference
methods such as G3(MP2)-RAD are suitable for calculations
of the thermodynamic parameters of these types of trapping
reactions. This is because energies of only the reactant and
product species are required. The T1 diagnostic value25 for the
TEMPO-CH(CH3)2 alkoxyamine is 0.0051, confirming that
ground-state alkoxyamine does not require a multireference
treatment.
3.6. Structure-Reactivity Trends in Homolysis. Table 5
shows the free energies of homolysis of various alkoxyamines
at the NO-C or N-OC bond at 298.15 and 393.15 K with
calculations performed both in the gas phase and in toluene.
For the TEMPO-based alkoxyamines, we have plotted this
difference as a function of the alkyl fragment for each of the
sets of reaction conditions (Figure 4). From this Figure (and
also the broader data set in Table 5), it is clear that both
temperature and the presence of solvent have very little effect
on the relative preference for NO-C versus N-OC homolysis.
For the TEMPO series (Figure 4), we see that the NO-C
bond is usually most likely to break, and the alternative
N-OC dissociation is normally energetically disfavored by
more than 50 kJ mol-1. Not surprisingly, the inclusion of
substituents (such as phenyl, carbonyl, and cyano) that
stabilize the alkyl radical product of NO-C homolysis
strongly enhances this preference. At the other extreme, in
cases where a heteroatom is present in the position R to the
(N-O-C) carbon center, N-OC homolysis becomes the
preferred pathway. As discussed in earlier studies of NO-C
homolysis, this is because one of the lone pairs on the nitroxyl
oxygen lies antiperiplanar to the carbon-heteroatom bond,
10464
Hodgson et al.
J. Phys. Chem. A, Vol. 114, No. 38, 2010
TABLE 5: Calculated Gas- and Solution-Phase Free Energies of the Bond Homolysis of NO-C and N-OC Bonds (kJ mol-1)a
298 K
gas phase
393 K
solution phase (toluene)
gas phase
solution phase (toluene)
alkoxyamine
O-C break
N-O break
O-C break
N-O break
O-C break
N-O break
O-C break
N-O break
1a
1f
1g
1kb
1lb
1ob
1pb
1qb
1r
1sb
1t
2a
2b
2c
2d
2e
2f
2g
2h
2i
2jb
2kb
2lb
2mb
2nb
2ob
2pb
2qb
2r
2sb
2tb
3a
3f
3g
3lb
3ob
3pb
3qb
3r
3sb
3tb
4a
4f
4g
4lb
4mb
4ob
4pb
4qb
4rb
4sb
4tb
5ab
5lc
5pc
144.5
162.2
183.4
99.5
90.1
107.1
90.0
160.0
145.5
103.5
115.9
151.8
155.3
158.6
146.1
132.3
168.5
190.1
119.7
115.3
93.1
108.7
105.0
92.7
129.1
118.3
99.7
169.3
156.1
115.0
130.7
140.8
165.5
183.6
96.8
110.6
92.2
158.6
156.3
108.8
120.4
142.4
166.7
183.8
100.2
92.9
110.8
92.8
161.6
146.8
108.3
121.3
156.6
132.4
142.1
152.5
144.2
146.0
157.3
148.7
155.1
140.3
152.3
146.9
153.6
146.3
169.7
167.8
172.9
161.6
143.1
160.3
162.6
183.3
180.2
162.3
176.7
173.8
154.9
177.1
176.4
160.2
171.8
167.4
175.3
171.2
157.8
156.3
155.1
168.7
171.9
155.9
164.2
166.5
172.2
164.1
158.2
156.4
154.2
171.4
157.6
171.4
155.7
166.4
167.9
171.0
164.3
131.3
152.3
153.7
155.3
166.2
190.4
97.8
84.4
103.3
84.9
155.5
148.1
101.4
112.7
162.3
159.3
158.9
147.5
132.9
172.9
197.6
126.7
115.7
91.6
107.0
98.8
86.0
129.2
113.5
93.6
164.3
158.4
112.2
126.6
150.2
168.2
189.1
87.2
102.8
83.8
151.4
155.6
103.1
116.2
152.3
170.0
189.7
91.0
85.1
103.8
86.4
155.4
144.2
103.4
117.0
139.6
112.6
118.6
158.4
148.1
150.5
155.5
145.6
153.9
137.5
150.1
148.8
153.0
147.2
174.9
169.4
173.6
161.8
142.7
164.2
167.2
185.7
181.2
162.2
174.5
169.7
150.2
175.4
173.9
156.0
168.7
168.6
173.6
170.9
160.6
157.5
156.7
160.1
165.3
148.2
157.9
163.8
166.6
162.5
161.4
157.8
155.8
162.9
150.4
165.3
149.7
160.8
162.8
165.7
162.3
112.1
132.4
130.7
127.1
143.2
164.4
81.7
69.3
86.5
68.3
139.5
124.6
82.9
95.1
134.7
135.4
139.3
124.8
111.8
149.1
171.0
102.3
95.3
72.0
91.5
84.7
70.9
110.7
98.2
77.9
148.5
135.7
95.1
111.0
124.3
146.4
164.2
76.8
90.8
70.2
137.7
136.2
89.1
100.9
125.6
147.2
164.5
80.6
71.1
91.1
70.6
141.0
126.8
88.4
101.4
164.7
119.4
127.5
132.3
123.5
126.0
136.7
127.2
133.3
118.3
131.4
125.5
131.9
125.0
150.3
146.9
153.7
141.1
123.0
139.9
143.0
164.2
160.0
141.3
157.1
153.3
139.8
156.7
155.7
138.7
151.1
147.0
154.8
151.5
140.1
137.2
136.4
149.7
152.6
135.3
144.6
147.5
153.1
145.8
140.2
136.9
135.5
152.7
143.5
152.1
134.8
147.1
148.9
151.6
145.5
131.3
138.8
138.3
146.1
154.4
178.9
87.4
71.1
90.2
71.8
142.4
134.9
88.3
99.8
153.4
148.1
146.5
134.1
121.0
160.9
186.1
117.0
103.4
79.0
97.0
86.0
72.6
118.0
100.7
80.3
150.8
145.6
99.7
114.6
141.6
156.2
177.1
73.8
89.7
69.8
137.2
142.5
90.2
104.3
143.5
157.7
177.7
77.9
71.3
90.7
72.0
141.7
130.4
90.3
104.6
155.5
106.2
112.2
146.0
137.2
138.1
142.2
132.0
140.0
123.4
136.7
135.3
138.8
133.2
163.2
157.5
161.9
149.5
131.2
153.7
155.3
174.0
169.3
149.7
162.1
157.1
143.8
162.5
160.8
142.2
155.3
156.0
160.5
158.2
150.1
147.7
145.0
147.8
152.7
134.6
144.7
151.7
153.9
150.8
150.8
147.9
144.3
151.0
144.5
152.8
135.8
148.2
150.2
153.1
150.2
119.3
125.6
122.5
a
G3(MP2)-RAD//B3-LYP/6-31G(d) values except where noted with the free energy of solvation calculated using PCM at the B3-LYP/
6-31G(d) level using the united atom topological model with the recommended optimization of radii (UAKS keyword in Gaussian). b iONIOM
approximation of G3(MP2)-RAD//B3-LYP/6-31G(d) values including (CH3)2NO-alkyl in the core system, with the full system calculated at the
RMP2/6-311+G(3df,2p) level. c ONIOM approximation of G3(MP2)-RAD//B3-LYP/6-31G(d) values including (Me)2NOR in the core system,
1-alkyl-2,2-dimethylindoline as the middle system calculated at the RMP2/6-311+G(3df,2p) level, and the full system calculated at the RMP2/
6-311+G(2d) level.
allowing hyperconjugation to occur between the lone pair
and the σ* antibonding orbital of the bond.3,17,41,42 This
anomeric stabilization results in a significant lengthening of
the NOC-heteroatom bond whereas the NO-C bond is
significantly shortened and strengthened. It is expected that
when anomeric stabilization of the alkoxyamine occurs,
nitroxide-mediated polymerization will be less effective
because of the N-OC homolysis side reaction.
To examine the effect of nitroxide structure, Figure 5 shows
a plot of the free energies of NO-C and N-OC homolysis for
Side Reactions of Nitroxide-Mediated Polymerization
J. Phys. Chem. A, Vol. 114, No. 38, 2010 10465
homolysis because of the resonance stabilization of the released
aminyl radical. In absolute terms, the N-OC homolysis is still
favored only for certain R groups (e.g., R ) CH3 in the systems
examined), but in all cases, the N-OC bond dissociation energy
is greatly reduced. The only other notable trend is that the
acyclic species consistently has slightly lower relative N-OC
bond dissociation energies than the remaining cyclic compounds,
presumably because its resulting aminyl radical is more easily
pyramidalized at nitrogen.
Figure 4. Relative free energies (kJ mol-1) for N-OR versus NO-R
homolysis at 393 K in toluene as a function of R for the series of
TEMPO-based alkoxyamines. A positive free-energy difference indicates that the NO-R homolysis is favored. Data for the gas and solution
(toluene) phases are shown at 298 and 393 K.
Figure 5. Free energies (393 K in toluene) of NO-C and N-OC
bond homolysis for the test set of alkoxyamines.
Figure 6. Relative free energies (kJ mol-1) for N-OR versus NO-R
homolysis at 393 K in toluene for combinations of R ) Me,
CH(CH3)Ph, and C(CH3)2COOCH3 and nitroxide structures 1-5 in
Scheme 3. A positive free-energy difference indicates that the NO-R
homolysis is favored.
the complete data set at the experimentally relevant condition
of 393 K in toluene. From this Figure, it is clear that NO-C
and N-OC homolysis are not correlated. Whereas NO-C
homolysis can be successfully modeled in terms of the polar
and radical stabilization terms describing the individual alkyl
and nitroxide radicals,17 when this same free-energy relationship
is applied to the N-OC bond homolysis in the present work
the fit is extremely poor (R2 ) 0.05). Instead, the N-OC
homolysis appears to be much more dependent upon the stability
of the released aminyl radical. To examine this further, Figure
6 shows the effect of the nitroxide structure on the relative free
energies of homolysis of the NO-C or N-OC bond at 393.15
K in toluene for three different alkyl groups, R ) Me,
CH(CH3)Ph, and C(CH3)2COOCH3. In each case, the indoline
derivative shows the greatest relative preference for N-OC
4. Conclusions
The competition of NO-C and N-OC homolysis in various
alkoxyamines was investigated using high-level computational
methods. It was found that the free energies of NO-C and
N-OC homolysis are not correlated, with NO-C homolysis
being more dependent upon the properties of the alkyl fragment
and N-OC homolysis being more dependent upon the structure
of the aminyl fragment. For piperidine-, pyrrolidine-, and
isoindoline-based nitroxides, N-OC homolysis is favored above
NO-C homolysis only in the case where a heteroatom that is
R to the NOC carbon center stabilizes the NO-C bond, though
it may become a competitive minor reaction for primary alkyl
radicals. Acyclic and indoline-type species have lower free
energies of N-OC homolysis than other cyclic species because
of the greater stability of the aminyl radical fragment and are
less resistant to this unwanted side reaction.
As part of this work, a series of assessment studies were used
to determine appropriate levels for the calculation of alkoxyamine
homolysis reactions. It was determined that G3(MP2)-RAD//
B3-LYP/6-31G(d) is an appropriate level of theory for the
calculation of gas-phase reactions and molecular properties, with
an ONIOM approximation to G3(MP2)-RAD (in which the full
system is calculated at the RMP2/6-311+G(3df,2p) level of
theory) providing good results for large systems. For reactions
calculated in solution, the effects of the solvent medium are
well described by a PCM method at the B3-LYP/6-31G(d) level.
Using these methods, deviations between calculated values of
alkoxyamine homolysis reactions and available experimental
results are around 5 kJ mol-1. On the basis of this, chemical
accuracy is achieved.
Acknowledgment. This research was undertaken on the NCI
National Facility in Canberra, Australia, which is supported by
the Australian Commonwealth Government. We gratefully
acknowledge support (to M.L.C) from the Australian Research
Council under their Centres of Excellence program and the
award (to J. L. H.) of an Australian Postgraduate Award and
an O’Donnell Young Scientist Prize from the RACI Polymer
Division. M.S.G. and L.B.R acknowledge the support of the
U.S. Air Force Office of Scientific Research.
Supporting Information Available: B3-LYP/6-31G(d)optimized geometries in the form of Gaussian archive entries,
corresponding total energies, thermal corrections, entropies, free
energies, and data used to construct to Figure 1. This material
is available free of charge via the Internet at http://pubs.acs.org.
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