Temperature dependence of interactions between stable piperidine-1-yloxyl derivatives and a semicrystalline ionic liquid

Article abstract of DOI:10.1002/cphc.200900977

The stable 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives with hydrogen-bond-forming (-OH, -OSO₃H), anionic (-OSO₃ bearing K⁺ or [K(18-crown-6)]⁺ as counter ion), or cationic (-N⁺(CH₃)₃ bearing I^-, BF^-₄, PF^-₆ or N^- (SO₂CF₃)₂ as counter ion) substituents are investigated in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide over a wide temperature range. The temperature dependence of the viscosity of the ionic liquid is well described by the Vogel-Fulcher-Tammann equation. Interestingly, the temperature dependence of the rotational correlation time of the spin probes substituted with either a hydrogen-bond-forming group or an ionic substituent can be described using the Stokes-Einstein equation. In contrast, the temperature dependence of the rotational correlation time of the spin probe without an additional substituent at the 4-position to the nitroxyl group does not follow this trend. The activation energy for the mobility of the unsubstituted spin probe, determined from an Arrhenius plot of the spin-probe mobility in the ionic liquid above the melting temperature, is comparable with the activation energy for the viscous flow of the ionic liquid, but is higher for spin probes bearing an additional substituent at the 4-position. Quantum chemical calculations of the spin probes using the 6-31G+d method give information about the rotational volume of the spin probes and the spin density at the nitrogen atom of the radical structure as a function of the substituent at the spin probes in the presence and absence of a counter ion. The results of these calculations help in understanding the effect of the additional substituent on the experimentally determined isotropic hyperfine coupling constant.

Full text of DOI:10.1002/cphc.200900977

DOI: 10.1002/cphc.200900977
Temperature Dependence of Interactions Between Stable
Piperidine-1-yloxyl Derivatives and a Semicrystalline Ionic
Liquid**
Veronika Strehmel,*[a] Hans Rexhausen,[a] Peter Strauch,[b] and Bernd Strehmel[c]
The stable 2,2,6,6-tetramethylpiperidine-1-yloxyl and its deriva-
tives with hydrogen-bond-forming (-OH, -OSO3H), anionic
trend. The activation energy for the mobility of the unsubsti-
tuted spin probe, determined from an Arrhenius plot of the
spin-probe mobility in the ionic liquid above the melting tem-
perature, is comparable with the activation energy for the vis-
cous flow of the ionic liquid, but is higher for spin probes bear-
ing an additional substituent at the 4-position. Quantum
chemical calculations of the spin probes using the 6-31G+d
method give information about the rotational volume of the
spin probes and the spin density at the nitrogen atom of the
radical structure as a function of the substituent at the spin
probes in the presence and absence of a counter ion. The re-
sults of these calculations help in understanding the effect of
the additional substituent on the experimentally determined
isotropic hyperfine coupling constant.
ꢀ
(-OSO3 bearing K+ or [K(18-crown-6)]+ as counter ion), or cat-
ꢀ
ionic (-N+(CH3)3 bearing Iꢀ, BF4ꢀ, PF6 or Nꢀ(SO2CF3)2 as coun-
ter ion) substituents are investigated in 1-butyl-3-methylimid-
azolium bis(trifluoromethylsulfonyl)imide over a wide tempera-
ture range. The temperature dependence of the viscosity of
the ionic liquid is well described by the Vogel–Fulcher–Tam-
mann equation. Interestingly, the temperature dependence of
the rotational correlation time of the spin probes substituted
with either a hydrogen-bond-forming group or an ionic sub-
stituent can be described using the Stokes–Einstein equation.
In contrast, the temperature dependence of the rotational cor-
relation time of the spin probe without an additional substitu-
ent at the 4-position to the nitroxyl group does not follow this
1. Introduction
Ionic liquids have gained importance in various fields of appli-
cation, such as batteries, fuel cells, solar cells, stationary phases
in gas chromatography, reaction media for electrodeposition
of metals, and solvents for chemical reactions in organic and
polymer chemistry.[1–6,7a–n] Examples of chemical reactions are
polymerizations occurring via a radical mechanism.[7] Free radi-
cal polymerization and even controlled radical polymerizations,
such as atom-transfer radical polymerization, reversible addi-
tion–fragmentation chain-transfer polymerization, and nitro-
xide-mediated polymerization, have been investigated in ionic
liquids.[7a–n] The control mechanism is realized in nitroxide-
mediated polymerization by using, for example, 2,2,6,6-tetra-
methylpiperidine-1-yloxyl derivatives.[7o–s]
between the solute and the individual ions of the ionic liquid
are dominating factors of influence on the mobility of a solute
in an ionic liquid. 1-Butyl-3-methylimidazolium bis(trifluorome-
thylsulfonyl)imide (IL; Scheme 1a) has received increased at-
tention among the ionic liquids because of its low melting
point, low viscosity, and high conductivity.[11] Therefore, this
ionic liquid may become important for applications in electro-
chemistry and additionally as a solvent in chemical reactions.
Furthermore, stable radicals comprise models for reactive
radical species formed during chemical reactions. These stable
radicals have also been used as spin probes to discover the vis-
cosity- and polarity-related parameters of ionic liquids that in-
The mobility of the radicals in the ionic liquids is important
for the molecular weight of the polymers obtained. The broad
variability of the chemical structure of ionic liquids composed
of various cations and anions results in a broad variability of
their properties. This allows tailoring of an ionic liquid to fulfill
desired special applications. The use of ionic liquids as solvents
for chemical reactions requires ionic liquids with a broad liquid
range and a moderate viscosity to guarantee optimal reactant
mixing and diffusion of both reactants and reaction products.
The properties of ionic liquids, such as viscosity and self-dif-
fusion of the individual ions of the ionic liquid, depend strong-
ly on temperature.[8,9] Furthermore, the mobility of solutes in
an ionic liquid is temperature dependent as well.[8b,10] The
structure and properties of the ionic liquid and the interactions
[a] Dr. V. Strehmel, Dr. H. Rexhausen
Institute of Chemistry, Applied Polymer Chemistry
University of Potsdam
Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm (Germany)
Fax: (+49)3319775036
E-mail: vstrehme@uni-potsdam.de
[b] Prof. P. Strauch
Institute of Chemistry, Inorganic Chemistry
University of Potsdam
Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm (Germany)
[c] Dr. B. Strehmel
Research and Development
Kodak Graphic Communications GmbH
An der Bahn 80, 37520 Osterode (Germany)
[**] Semicrystalline ionic liquid: 1-butyl-3-methylimidazolium bis(trifluorome-
thylsulfonyl)imide.
2182
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 2182 – 2190

Interactions Between Piperidine-1-yloxyl Derivatives and Ionic Liquid
0.5 wt% as determined by thermogravimetric analysis. The
broad liquid range of this ionic liquid covers the temperature
region from 272 to 570 K. Furthermore, IL exhibits a semicrys-
talline morphology (Figure 1). The heating scan in the differen-
Figure 1. DSC curve of IL obtained at a heating rate of 5 Kminꢀ1 (Tg: 186 K;
Trecryst: 232 K; Tm: 272 K).
Scheme 1. a) 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
(IL). b) Substituted piperidine-1-yloxyl derivatives used for investigation of
IL.
tial scanning calorimetry (DSC) curve shows glass transition (Tg)
at 186 K, recrystallization (Trecryst) at 232 K, and melting (Tm) of
the crystal structures formed at 272 K. No crystallization occurs
during cooling, neither with a cooling rate of 5 Kminꢀ1 nor of
1 Kminꢀ1. This metastable system can be supercooled from a
melt to form a glassy state. Perturbation of the supercooled
liquid state by heating results in transformation of the super-
cooled liquid state into a more stable state bearing a lower
free energy by recrystallization from the supercooled liquid
state, thus resulting in crystal structures. The temperature
regime strongly influences crystal formation.[14] This is the main
difference from the glass-forming 1-butyl-3-methylimidazolium
tetrafluoroborate, which was discussed in a previous paper.[8b]
Although semicrystallinity is mainly discussed in the field of
polymers, this effect may also play an increasing role in the
field of ionic liquids.[15] The semicrystallinity makes it interest-
ing to explore the spin-probe mobility in different phases/tem-
peratures of this ionic liquid.
dicate interactions between the spin probes and the ionic liq-
uids on a molecular level. Various spin probes of different
structure have been investigated in ionic liquids.[12] Recently,
2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives with various
substituents at the 4-position with respect to the nitroxyl
group were successfully used to investigate imidazolium tetra-
fluoroborates and imidazolium hexafluorophosphates.[8b,12f,g,j–l]
Temperature-dependent measurements on the glass-forming
1-butyl-3-methylimidazolium tetrafluoroborate show that both
the additional substituent at the spin probe and the tempera-
ture used for the experiments are important factors of influ-
ence on the spin-probe rotation. This is expressed by the aver-
age rotational correlation time (trot) and the hyperfine coupling
constant related to nitrogen [Aiso(14N)].[8b,13]
Herein, we discuss the temperature-dependent mobility of
2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives with distinct
substituents at the 4-position in IL, which is an example of a
semicrystalline ionic liquid. The structural variation at the 4-po-
sition of the nitroxyl radical covers hydrogen, hydroxy, sulfuric
acid, cationic, and anionic substituents to provide additional
information about the interactions between such stable radi-
cals and the ionic liquid. In addition, the formation of hydro-
gen-bonding or ionic interactions between the spin probe and
the ionic liquid include additional interactions. This can be ex-
plored in these experiments. Moreover, quantum chemical cal-
culations provide information about the rotational volume of
the spin probes and the spin density distribution.
Furthermore, the viscosity of the ionic liquid strongly de-
pends on the temperature used for the measurement. The
Vogel–Fulcher–Tammann equation [Eq. (1)]:
694:8
T ꢀ 167
ð1Þ
lnðhÞ ¼ ꢀ1:608 þ
where h is in mPas, well describes this behavior over a temper-
ature (T) range between 278 and 373 K (Figure 2). The parame-
ters of Equation (1) obtained in the fit depicted in Figure 2
result in R2 =0.9989.
2.2. Selection of Spin Probes
2. Results and Discussion
Piperidine-1-yloxyl derivatives bearing different polar substitu-
ents at the 4-position with respect to the nitroxyl radical struc-
ture were selected for mobility investigations (Scheme 1b).
The interactions of the substituted piperidine-1-yloxyl deriva-
tives (2–4) with the ionic liquid are discussed and compared
with those of the unsubstituted piperidine-1-yloxyl (1). ESR
spectra exhibit the influence of the ionic liquid on the mobility
2.1. Thermal Properties of the Ionic Liquid
The liquid range of ionic liquids is very important for their ap-
plication as solvents in chemical reactions.[7e] The liquid state
of the ionic liquid IL (Scheme 1a) is limited by its melting
point and the temperature, where mass loss is lower than
ChemPhysChem 2010, 11, 2182 – 2190
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
2183

V. Strehmel et al.
dered rotation of this spin probe. The line broadening is great-
er in the case of 3a substituted by a sulfuric acid group (Fig-
ure 3c), which comprises a relatively strong acid. Therefore, ad-
ditional hydrogen bonding is possible with the anion of the
ionic liquid. Additional ionic interactions exist between the sul-
fate group and the imidazolium ion. These two kinds of inter-
actions may cause the stronger line broadening of 3a com-
pared to 2. The anionic spin probes 3b and 3c show a stron-
ger line broadening than 3a because they possess an ionic
substituent building up additional ionic interactions with the
imidazolium ion (Figure 3d). The anionic spin probes 3b and
3c differ in the size of the counter ion, which is either potassi-
um or potassium complexed with 18-crown-6, respectively.
Furthermore, complexation of the potassium ion by crown
ether results in an improved solubility of 3c in the ionic liquid
compared to 3b.[12l] This gives a huge benefit in using such
compounds in a wide variety of applications where dissolution
in the target matrix may cause a problem.
Figure 2. Vogel–Fulcher–Tammann plot of the temperature dependence of
the viscosity (h) of IL using Equation (1).
of the piperidine-1-yloxyl derivatives. A significant line broad-
ening begins upon dissolving the piperidine-1-yloxyl deriva-
tives in ionic liquids compared to organic solvents, for exam-
ple, dimethyl sulfoxide (DMSO).[8b,12j–l] Figure 3 depicts the ESR
spectra of 1, 2, 3a, 3c, and 4d. The structural differences of
the spin probes result in differences in the interactions be-
tween the individual spin probes and the ionic liquids. The ESR
spectrum of 1 displays the interactions between the radical
structure and the ionic liquid (Figure 3a). The ESR spectra of all
the other spin probes show a line broadening relative to the
spectrum of 1. This line broadening is attributed to the addi-
tional interactions with the substituent at the 4-position of the
spin probe and the ionic liquid. A slight line broadening occurs
in the case of 2 substituted by a hydroxyl group (Figure 3b).
Additional hydrogen bonding between the hydroxyl group
and the anion of the ionic liquid may cause the slightly hin-
The ionic spin probes 3b and 3c as well as 4a–d display the
strongest line broadening in the ESR spectra (see Figure 3d
and e for examples), which shows the strong interaction with
the surrounding IL. The line broadening in the ESR spectra of
3b, 3c, and 4a–d is attributed to additional interactions be-
tween the ionic substituent at the spin probe and either the
cation or anion of the ionic liquid. The anionic spin probes 3b
and 3c bear the potassium ion as counter ion, which is com-
plexed with crown ether in the case of 3c. The spin probes 3b
and 3c exhibit different molar volumes as long as one consid-
ers them as ion pairs that should interact differently with the
surrounding IL. An increase in the molar volume should result
in a broadening of the ESR lines. In the case of ion exchange
with the surrounding IL, one obtains spin probes with a coun-
ter ion originating from the IL. Under such circumstances the
ESR spectra must be similar for 3b and 3c. From this finding
one can conclude that counter-ion exchange occurs with the
imidazolium ion of the ionic liquid. Otherwise, one would
expect broader lines in the case of 3c in comparison with 3b
attributable to the larger volume of the potassium crown ether
complex relative to potassium. The cationic spin probes 4a–d
differ in their anion structure. The size of the anions increases
in the order: Iꢀ <BF4ꢀ <PF6ꢀ <NTf2ꢀ. This may cause differen-
ces in the anion exchange with the bis(trifluoromethylsulfony-
ꢀ
l)imide (NTf2 ) anion of the ionic liquid.
2.3. Molecular Considerations of the Spin Probes
The spin probes selected differ regarding their size and func-
tionality, in particular considering the substitution at the 4-po-
sition of the nitroxyl radical. Density functional theory (DFT)
calculations provide a deeper insight into the molecular struc-
ture depending on the substitution pattern and counter ion as
well as electron distribution at the radical nitrogen atom. The
latter responsibly tunes the hyperfine coupling constant relat-
ed to nitrogen, AN. The higher the electron density at the radi-
cal nitrogen, the larger the hyperfine coupling constant.[13]
Table 1 summarizes the results obtained based on the
6-31G+d method. Compared to the results obtained in a pre-
Figure 3. ESR spectra of the spin probes a) 1, b) 2, c) 3a, d) 3c, and e) 4d in
the ionic liquid at 283 K.
2184
www.chemphyschem.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 2182 – 2190

Interactions Between Piperidine-1-yloxyl Derivatives and Ionic Liquid
fonyl group interacts with the counter ion. Data obtained for
3c and 3-BMIm are more or less comparable with 1 that has
only hydrogen atoms at the 4-position of the nitroxyl radical. A
similar trend was also observed for the cationic probes 4. The
fact that spin and charge density change upon switching the
nature of the counter ion must also influence the dynamics of
the spin probe if the NO group interacts with the surrounding
ionic liquid. Furthermore, the mentioned ion exchange can
also interfere with the behavior of the spin probe because an-
other rotational volume has to be considered. Thus, the
probes 3-BMIm and 4d exhibit ion pairs as a result of an ex-
changed cation and anion by the surrounding matrix, respec-
tively. This must be experimentally observable in the viscosity-
dependent experiments providing information about the rota-
tional volume as long as the probe exhibits Stokes–Einstein be-
havior. Because 3b and 3c have different rotational volumes,
there should be a difference in the rotational volume observed
in the experiment as long as both spin probes exist as ion
pairs. Ion exchange with the ions of the surrounding matrix
changes the situation.
Table 1. Summary of calculation results obtained by applying density
functional theory using the 6-31G+d method.
calc
Spin Probe[a]
Vrot [ꢁ3][b]
1[c]
q [N][d]
AN [G][e]
1
2
208
285
329
325
359
627
513
326
482
325
0.4592
0.4597
0.4832
0.4557
0.4460
0.4651
0.4681
0.4695
0.4457
0.4452
0.2830
0.2275
0.1849
0.1773
0.1823
0.2962
0.1856
0.1986
0.2059
0.2699
13.76
13.57
13.80
13.52
13.58
13.70
13.65
13.29
13.40
13.66
3-without
3a
3b
3c
3-BMIm
4-without
4b
4d
[a] Structure of the spin probe according to Scheme 1. [b] This was con-
verted from the molar volume obtained in the calculation. [c] Spin densi-
ty at the NO group. [d] Partial charge of the nitrogen at the NO group.
[e] Fermi Hyperfine coupling constant.
vious study,[8b] we chose a less expensive calculation method
because some of the spin probes are too large for stronger
basis sets. Thus, the results obtained allow a better comparison
of the entire data set but keeping in mind that more accuracy
is not possible for the larger probes; that is, 3c and any anion-
ic probe 3 bearing as counter ion the cation of the ionic liquid,
which is 1-butyl-3-methylimidazolium (BMIm). Therefore, the
latter is called 3-BMIm. Such a probe molecule can be formed
in an ion-exchange process by dissolution of the ionic spin
probes 3b or 3c in the ionic liquid. Ion exchange can also
occur with the anions of the ionic liquid. Spin probe 4d com-
prises the anion of the ionic liquid.
2.4. Dynamic Behavior of the Spin Probes
The rotational correlation time trot is a quantitative measure for
the spin-probe mobility embedded in the ionic liquid. This is
calculated from the average rate constant for rotational diffu-
sion (RBAR) using Equation (2):
trot ¼ 10ꢀRBAR
ð2Þ
The results obtained for the charged spin probes with no
counter ion (3 and 4) show the same trend as previously pub-
lished with a larger basis set.[8b] That is, the charge of the sub-
stituent in the 4-position influences the spin density at the ni-
trogen atom, which finally has an impact on the charge on this
atom, and the Fermi hyperfine splitting constant. Thus, a sub-
stituent with a negative charge results in a higher spin density
on the nitrogen atom, while a substituent with positive charge
directs the spin density in the opposite direction; that is, the
nitrogen exhibits less spin density compared to 1 that has only
hydrogen at the 4-position. The fact that substitution in the
4-position to the nitroxyl radical changes both the spin density
and the charge of the nitrogen of the NO group can be dis-
cussed as a certain result of hyperconjugation, because there
are four s bonds between the radical nitrogen and the
charged substituent. Under these circumstances, incorporation
of different counter ions must also have an impact on the spin
density of the NO group because the counter ion compensates
a part of the charge of the substituent in the 4-position. There-
fore, less charge is available for interaction between the sub-
stituent at the 4-position and the NO group.
The latter is determined from the spectra by using the method
of Budil et al.,[16] and shows a different pattern of linewidth de-
pending on both spin-probe structure and mobility in the
ionic liquid. The higher the mobility of the nitroxyl radical in
the matrix, the smaller the linewidth of the ESR lines becomes.
Table 2 summarizes the rotational correlation times for the
spin probes for measurements at room temperature. The
lowest value and therefore highest mobility exists in the case
Table 2. Summary of data describing the dynamics of different spin
probes in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluorome-
thylsulfonyl)imide. These are the average rotational correlation time trot
,
the activation energy Ea, the rotational volume Vrot, and the ratio between
experimental determined rotational volume and calculated volume ob-
calc
tained from density functional theory calculations Vrot/Vrot
.
Vrot
[b]
Spin Probe
trot[a] [ns]
Ea [kJmolꢀ1
]
Vrot [ꢁ3]
calc
V
rot
1
2
0.9
2.0
5.9
6.7
6.5
8.8
7.8
7.8
7.7
14
36
16
25
26
24
26
22
19
No Stokes-Einstein behavior
75
0.26
0.45
0.43
0.24
–
3a
3b
3c
4a
4b
4c
4d
145
156
153
147
128
177
131
Our calculations even prove that this assumption holds for
all charged ions bearing either a positively or negatively
charged group at the 4-position. Consideration of the data ob-
0.39
–
0.27
ꢀ
tained for the radicals exhibiting an -OSO3 group show a de-
[a] Taken at room temperature. [b] Calculated from the slope of Equa-
tion (3).
crease of the spin density of the nitrogen at the NO group if a
counter ion is introduced. Thus, most of the charge of the sul-
ChemPhysChem 2010, 11, 2182 – 2190
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
2185

V. Strehmel et al.
of 1 bearing no substituent in the 4-position. Thus, this probe
interacts with the ionic liquid only via the radical structure (NO
group). Additional interactions via an additional substituent
placed at the 4-position are missing in the case of 1. All other
spin probes exhibit higher trot values and therefore lower mo-
bility in IL compared to 1. This is caused by interactions be-
tween the additional substituent at the 4-position to the ni-
troxyl radical with the ionic liquid. Thus, this substituent signifi-
cantly interferes with the rotation of the spin probe. Stronger
interactions are observed if the substituent bears an ionic
moiety (3 and 4), while incorporation of a hydroxyl group (2)
only slightly decreases the spin-probe mobility. The spin probe
3a contains with the sulfuric acid group a strong acidic sub-
stituent, which causes an average rotational correlation time
between the trot values of the neutral probe 2 and the anionic
probes 3b and 3c.
Furthermore, the form of the ESR spectra and the linewidth
of the three lines strongly depend on temperature. The spin-
probe mobility is almost frozen at low temperatures (Fig-
ures 4a,b and Figures 5a,b) showing nearly no changes of
Figure 5. ESR spectra of the spin probe 4d in the ionic liquid at a) 170,
b) 260, c) 280, and d) 380 K.
Typical Arrhenius diagrams for the trot values calculated from
the shape of the ESR spectra are shown in Figure 6a and b for
the spin probes 3b and 4d, respectively. These probes deviate
from the expected linear behavior at specific temperatures.
Above the melting point, the curves exhibit the expected
linear behavior in the Arrhenius diagram for both spin probes.
On the other hand, nearly no mobility changes are observed
when the sample temperature passes the glass transition tem-
Figure 4. ESR spectra of the spin probe 3c in the ionic liquid at a) 170,
b) 260, c) 280, and d) 380 K.
spectral shape, and therefore trot is large exhibiting nearly no
changes at the glass transition of the semicrystalline ionic
liquid. Surprisingly, the spin-probe mobility strongly changes
above the melting point of the ionic liquid upon heating at
280 K (Figure 4c and Figure 5c). A further increase of tempera-
ture results in an additional increase of the spin-probe mobili-
ty, which results in smaller lines of the ESR spectra (Figure 4d
and Figure 5d).
Figure 6. Arrhenius plots of the rotational correlation time trot obtained at
different temperatures for a) probe 3b and b) probe 4d in the ionic liquid.
2186
www.chemphyschem.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 2182 – 2190

Interactions Between Piperidine-1-yloxyl Derivatives and Ionic Liquid
perature (Tg) of the ionic liquid. Because there are only slight
increases of the rotational mobility above Tg in the case of the
spin probe 3b, the probe can only monitor a small decrease of
trot, which possibly ranges within the experimental fluctuation
of our experimental setup. Comparable results were recently
discussed for the spin-probe mobility in the glass-forming
ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate.[8b]
After recrystallization is finished mobility starts in the case of
the spin probe 3b, although the mobility of 4d stays frozen
up to the melting point. However, the straight line in the Ar-
rhenius plot describing the mobility of 3b in the melt is not
valid for the mobility of this spin probe in the temperature
region between recrystallization and melting of the crystal
structures formed. Furthermore, these plots show that the mo-
bility of the cationic spin probe 4d, which undergoes strong
ionic interactions with the anion of the ionic liquid, is more
hindered in the ionic liquid in comparison with the anionic
spin probe 3b, which interacts with the cation of the ionic
liquid.
cate similar surroundings of these spin probes, although the
higher trot value of 4a may be a hint for an incomplete anion
exchange of the Iꢀ by the bis(trifluoromethylsulfonyl)imide
under the experimental conditions chosen.
The anionic spin probes 3b and 3c exhibit similar activation
energies. This is again a hint for a similar behavior of both
probes in the ionic liquid. Because these probes exhibit a dif-
ferent rotational volume, the fact that both of them exhibit
similar dynamics even supports the hypothesis of ion ex-
change with the surrounding ionic liquid. Similar activation en-
ergies are also obtained for the cationic spin probes 4a and
4b. The spin probe 4c shows a slightly lower activation
energy than 4a and 4b. Furthermore, the lowest activation
energy in the row of the cationic spin probes was obtained for
4d, which has the same counter ion as the ionic liquid. Com-
parison of the activation energy determined by the mobility of
the spin probes (Table 2) and the value for the activation
energy of the viscous flow (Eh) published in the literature
(14 kJmolꢀ1) shows that only spin probe 1 exhibits a compara-
ble agreement.[17] The activation energy for the spin-probe mo-
bility is higher in all examples bearing either a hydrogen-bond-
forming substituent or an ionic substituent at the 4-position,
which indicates that additional interactions exist requiring ad-
ditional activation energy. Thus, the activation energy mea-
sured by the spin probes is a sum of the activation energy re-
lated to the viscous flow and an additional intrinsic activation
energy that does not depend on viscosity.
Table 2 summarizes the parameters obtained from the Arrhe-
nius plot above the melting point. The calculated activation
energy Ea only slightly differs for the charged spin probes 3b,
3c, and 4. Ea has the lowest value for 1, which only interacts
via the radical structure (NO) with the ionic liquid. Surprisingly,
the highest activation energy was obtained in the case of 2
that has the capability to form additional hydrogen bonds
with the ionic liquid. In contrast, the activation energy is signif-
icantly smaller for 3a compared to 2, although additional hy-
drogen bonding to the anion of the ionic liquid is possible in
the case of 3a as long as this probe does not dissociate into
the corresponding ions. Nevertheless, 3a possesses a higher
acidity than 2, and perhaps it must be considered as a probe
molecule that dissociates into its respective ions in the ionic
liquid. Therefore, 3a can be discussed as a spin probe exhibit-
ing partially the property of an anion. The partial ionic proper-
ty of 3a becomes clearer by comparing the rotational correla-
tion time of 3a with those of 3b and 3c (Table 2). The trot
value of 3a is closer to those of 3b and 3c, which indicates
additional interactions between the anionic substituent of the
spin probe and the cation of the ionic liquid. Thus, a structure
based on ion exchange between the probes 3a–c and the
ionic liquid favors the description of our dynamics observed in
the ESR experiments; this is 3-BMIm. Furthermore, the anionic
spin probes 3b and 3c show similar trot values. This is a further
hint that cation exchange occurs between the counter ions of
the spin probes with the imidazolium ion of the ionic liquid. If
no counter-ion exchange occurs, differences should be found
in the trot values of 3b and 3c because these cations differ in
their size, which should result in differences in the rotational
mobility as well. However, this is not the case.
2.6. Stokes–Einstein Behavior
Interestingly, the temperature dependence of trot shows
Stokes–Einstein behavior [Eq. (3)] for the probes 2–4:
Vm ꢁ p h
ð3Þ
t ¼
ꢁ
T
kB
We observed in a previous study that Stokes–Einstein behavior
did not occur in 1-butyl-3-methylimidazolium tetrafluorobora-
te.[8b] Microviscosity theory described the data obtained in the
imidazolium tetrafluoroborate.
The spin probes 2–4 can additionally interact with the ionic
liquid via the substituent placed at the 4-position. In these ex-
amples, trot is proportional to the macroscopic viscosity of the
IL discussed in this work. The slope results in the rotational
volume of the probe. Each probe possesses a different molar
volume if one considers these probes as pairs with the counter
ion (Table 1). However, no differences in the slope of Equa-
tion (3) were observed for the probes 3b and 3c (Figure 7).
Both spin probes possess the largest differences regarding
their rotational volume if one includes the counter ion. The
fact that both probes exhibit a similar slope indicates that the
rotational volume must be the same for both probes, which
again supports the idea of ion exchange between the spin
probe and the corresponding ions of the surrounding ionic
liquid. In the case of no ion exchange between the spin probe
and the ionic liquid, one would expect similar data by dividing
the rotational volume of the probe by the value calculated
The highest trot value is obtained for 4a in the ionic liquid,
which is also slightly higher than the trot value of the spin
probes 4b–d. These trot values are similar, thus indicating the
similarity of the anion exchange of the spin probes 4b and 4c
with the bis(trifluoromethylsulfonyl)imide anion of the ionic
liquid. The spin probe 4d possesses the same anion as the
ionic liquid. Similarities in the trot values between 4b–d indi-
ChemPhysChem 2010, 11, 2182 – 2190
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
2187

V. Strehmel et al.
sion behavior according to the Spernol–Gierer–Wirtz model,[8b]
the diffusion of the dissolved spin-probe molecule switches to
Stokes–Einstein diffusion just by changing to the bis(trifluoro-
methylsulfonyl)imide anion.
2.7. Probing of Micropolarity
The hyperfine coupling constant is a measure for the micropo-
larity of a surrounding matrix.[13] Because the radical electron is
localized at either the oxygen or nitrogen atom, our experi-
ment can see only the nitrogen, and therefore the interaction
between the radical electron and this atom. The larger the hy-
perfine coupling constant related to nitrogen AN, the larger the
electron density of the unpaired electron at the nitrogen
atom.[13]
The isotropic hyperfine coupling constants [Aiso(14N)] ob-
tained for the spin probes are summarized in Table 3. Data
show similar Aiso(14N) values for the neutral probes 1 and 2 as
Figure 7. Stokes–Einstein plot [Eq. (3)] of rotational correlation time (trot
)
&
*
versus macroscopic viscosity (h) for the spin probes 3b ( ) and 3c ( ) in IL.
from the DFT calculations (Vrot/Vrotcalc, Table 1). However, we ob-
served the opposite behavior (compare last column of Table 2),
which means that this assumption cannot hold. All charged
probes 3a–c and 4a–d exhibit a similar rotational volume of
about 148 ꢁ3 on average. This data set is only describable if a
probe structure with similar rotational volume responsibly de-
scribes the rotational dynamics, which is only the case for
probes with no counter ion (3-without and 4-without). An ion
pair based on an ion exchange, which can be either 3-BMIm
or 4d, must result in a different slope of Equation (3) and
therefore in a distinct rotational volume. No counter ion
means for our understanding that, within the experiment, only
a structure appears with a volume that is comparable to those
of 3-without and 4-without. It is also clear to us that to each
ion belongs a counter ion. There would be several interpreta-
tions possible to explain this phenomenon, which we would
not like to continue based on our available data.
Table 3. Summary of hyperfine coupling constants Aiso(14N) obtained
from the ESR spectra of the spin probes 1–4 dissolved in 1-butyl-3-meth-
ylimidazolium bis(trifluoromethylsulfonyl)imide at room temperature.
Spin Probe
Aiso(14N) [G]
Spin Probe
Aiso(14N) [G]
Spin Probe
Aiso(14N) [G]
1
2
16.11
3a
16.11
4a
16.01
3b
16.14
4b
3c
16.11
4c
4d
15.87
15.84
15.84
15.84
well as for the probes with a sulfate group in the 4-position no
matter whether a smaller (3b) or larger cation (3c) functions
as counter ion. These results even support our findings dis-
closed in the viscosity-dependent measurements that a cation
of almost similar size must be available in the case of 3a–c.
The micropolarity obtained is generally comparable with that
of DMSO.[8b,12j–l]
Only the viscosity-dependent behavior of the spin probe 1
cannot describe the change of trot according to Equation (3).
These data rather follow the trend of Equation (4):
h
kB
ꢁ
x20
Ttrot 6pVrotf0 6pVrotf0
kB
x0Stokes
ꢁ hx
ð4Þ
¼
þ
Furthermore, the isotropic hyperfine coupling constants are
nearly independent of the temperature in the case of the spin
probes 1, 2, and 4 in the temperature region between 280 and
3808C. In contrast, the spin probe 3a substituted with the sul-
furic acid group and the anionic spin probes 3b and 3c show
a decrease in the Aiso(14N) value between 280 and 3008C, al-
though these values are nearly independent of the tempera-
ture at higher temperatures. Similar values for the hyperfine
coupling constants in the case of both the anionic spin probes
3b and 3c (Figure 8b) on the one hand and the cationic spin
probes 4a–d (Figure 8c) on the other hand support the hy-
pothesis of counter ion exchange between the counter ion of
the spin probe and the ionic liquid ion. The data obtained
clearly show that there is almost no difference in the hyperfine
splitting constants of the anionic spin probes and the cationic
spin probes, although one could expect this from the theoreti-
cal predictions (Table 1). Furthermore, the isotropic hyperfine
coupling constant for the cationic spin probes 4a–d are lower
than the Aiso(14N) values for the anionic spin probes 3b and 3c
(Table 3). This is attributed to the lower spin density at the ni-
This expression is based on the microviscosity theory by Sper-
nol, Gierer, and Wirtz, which was previously applied to the spin
probes dissolved in 1-butyl-3-methylimidazolium tetrafluoro-
borate.[8b] The results obtained show a straight line indicating
that Equation (4) is applicable to describe the viscosity de-
pendence between trot and h. The exponent x discloses the in-
fluence of the activation energy for diffusion into the free
volume (E2) on the dynamics; that is, x=1ꢀE2/Eh. We obtained
x=0.79 meaning that 79% of spin-probe molecules describe
diffusion according to the macroscopic viscous flow, while
21% of spin-probe molecules undergo diffusion by jumping
into the free volume of the matrix. Because all other probes
follow the Stokes–Einstein behavior, further discussion with 1
is not possible. This result is surprising, and comparison with
the results obtained during investigation of 1-butyl-3-methyli-
midazolium tetrafluoroborate[8b] demonstrates how the anion
of the ionic liquid strongly affects the diffusion behavior of dis-
solved probes in an ionic liquid. While the ionic liquid 1-butyl-
3-methylimidazolium tetrafluoroborate mainly causes a diffu-
2188
www.chemphyschem.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 2182 – 2190

Interactions Between Piperidine-1-yloxyl Derivatives and Ionic Liquid
ents collect more charged species in their surroundings than
the hydroxy-substituted spin probe.
The higher activation energy of the spin probes bearing an
additional substituent relative to the 2,2,6,6-tetramethylpiperi-
dine-1-yloxyl without an additional substituent indicates a
stronger hindrance in the mobility of the substituted spin
probes. Interestingly, the activation energy for the mobility of
the spin probe without additional substituent is similar to the
activation energy for the viscous flow of the ionic liquid, which
is known from the literature. From this one can conclude that
the mobility of the unsubstituted spin probe corresponds to
the viscous flow of the ionic liquid, whereas the mobility of
the charged spin probes is slower than the viscous flow. This
may be caused by the larger size of the 2,2,6,6-tetramethylpi-
peridine-1-yloxyl derivatives relative to the ions of the ionic
liquid and their stronger interactions with the individual ions
of the ionic liquid.
Experimental Section
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (IL)
was synthesized in two steps. First, 1-butyl-3-methylimidazolium
chloride was made in 80% yield from 1-methylimidazole and
1-butyl chloride under stirring at 658C. In the second step, anion
metathesis was carried out using lithium bis(trifluoromethylsulfon-
yl)imide dissolved in water. After phase separation and drying in
vacuo, the IL was obtained in 79% yield. This ionic liquid contains
300 ppm water and no halide. The spin probes 1 and 2 were pur-
chased from Aldrich and used as obtained. Synthesis of the spin
probes 3a–c and 4a–d is described elsewhere.[12j–l]
Figure 8. Hyperfine coupling constant Aiso(14N) for the spin probes a) 1 and
2, b) 3a, 3b, and 3c, and c) 4a, 4b, 4c and 4d in the ionic liquid IL ob-
tained from ESR spectra taken at different temperatures.
DSC investigation of IL using a DSC 822e apparatus from Mettler–
Toledo with heating and cooling rates of 10 Kminꢀ1 gave the glass
transition temperature (Tg) at 186 K, a recrystallization at 232 K,
and melting at 272 K. The glass transition was determined from
the point of intersection of the tangents on the DSC curve. The vis-
cosity was measured as function of temperature (Rheometer AR-
G2, TA Instruments, equipped with a Peltier plate and 40 mm cone
18 for 0.3 mL sample volume) using a shear rate of 10 sꢀ1 and a
heating rate of 4 Kminꢀ1. Furthermore, shear rate viscosity meas-
urements were carried out using shear rates between 5 and
800 sꢀ1 at 296 and 343 K. ESR spectra of the spin probes were mea-
sured in the X band with a continuous-wave spectrometer (ELEX-
SYS E500, Bruker) at 9.4 GHz. The concentration of the spin probes
was about 2ꢂ10ꢀ3 m in the ionic liquid. Average rotational correla-
tion times (trot) were calculated from the average rotational diffu-
sion rate constants, which were determined by the method of
Budil et al.[16] using the model of isotropic viscosity for the fitting
procedure.
trogen atom caused by the ammonium substituent, as was
also found by quantum chemical calculations (Table 1).
3. Conclusions
Investigation of 2,2,6,6-tetramethylpiperidine-1-yloxyl deriva-
tives dissolved in 1-butyl-3-methylimidazolium bis(trifluorome-
thylsulfonyl)imide as a function of the temperature shows dif-
ferences in the dynamic behavior depending on the substitu-
ent at the 4-position to the radical structure, which can be hy-
drogen-bond-forming substituents, ionic substituents, or just
hydrogen. From this one can conclude that the mobility of rad-
icals in an ionic liquid is strongly dependent on the tempera-
ture used for the experiments on the one hand, and on the
nature of the substituent on the other hand, and therefore on
the interactions between the spin probes and the individual
ions of the ionic liquid.
The geometric structures of the spin probes were optimized with
Gaussian 03W.[18] The methods used are explained in Table 1. A fre-
quency calculation was carried out for each geometry obtained to
check that the properties calculated belonged to a minimum.
Although the spin-probe volume calculated using DFT dif-
fers for the single spin probes, the rotational volume calculat-
ed from the slope of the Stokes–Einstein plot is similar for the
charged spin probes. From this one can conclude that coun- Acknowledgements
ter-ion exchange occurs between the counter ion of the spin
probe and the individual ions of the ionic liquid. Furthermore,
the rotational volume of the charged spin probes is about
twice that of the hydroxy-substituted spin probe. From this
one can conclude that the spin probes bearing ionic substitu-
The authors gratefully acknowledge E. Gçrnitz and K. Blꢀmel
(Fraunhofer Institute for Applied Polymer Research Potsdam-
Golm) for their support during viscosity measurements and H.
Wetzel (Fraunhofer Institute for Applied Polymer Research Pots-
ChemPhysChem 2010, 11, 2182 – 2190
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
2189

V. Strehmel et al.
P. D. Price, S. G. Davis, C. Hardacre, R. G. Compton, ChemPhysChem
2005, 6, 526–533; c) D. Nama, P. G. A. Kumar, P. S. Pregosin, T. J. Geld-
bach, P. J. Dyson, Inorg. Chim. Acta 2006, 359, 1907–1911; d) M. Kanaku-
bo, K. R. Harris, N. Tsuchihashi, K. Ibuki, M. Ueno, J. Phys. Chem. B 2007,
111, 2062–2069; e) G. Annat, D. R. MacFarlane, M. Forsyth, J. Phys.
Chem. B 2007, 111, 9018–9024.
dam-Golm) for Karl–Fischer analysis of the water content in the
ionic liquid. V.S. and H.R. gratefully acknowledge the DFG for fi-
nancial support within the priority program SPP 1191.
Keywords: density functional calculations · ESR spectroscopy ·
ionic liquids · spin probes · viscosity
[10] D. Camper, C. Becker, C. Koval, R. Noble, Ind. Eng. Chem. Res. 2006, 45,
445–450.
[11] a) C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, J. F. Bren-
necke, J. Chem. Eng. Data 2004, 49, 954–964; b) H. Tokuda, K. Ishii,
M. A. B. H. Susan, S. Tsuzuki, K. Hayamizu, M. Watanabe, J. Phys. Chem. B
2006, 110, 2833–2839; c) Y. Francois, K. Zhang, A. Varenne, P. Gareil,
Anal. Chim. Acta 2006, 562, 164–170; d) H. Jin, B. O’Hare, J. Dong, S.
Arzhantsev, G. A. Baker, J. F. Wishart, A. J. Benesi, M. Maroncelli, J. Phys.
Chem. B 2008, 112, 81–92; e) D. S. Silvester, W. He, L. Aldous, C. Harda-
cre, R. G. Compton, J. Phys. Chem. C 2008, 112, 12966–12973; f) E. I.
Rogers, B. Sljukic, C. Hardacre, R. G. Compton, J. Chem. Eng. Data 2009,
54, 2049–2053; g) E. Miyako, T. Itoh, Y. Nara, T. Hirotsu, Chem. Eur. J.
2009, 15, 7520–7525; h) M. K. Mistry, S. Subianto, N. R. Choudhury, N. K.
Dutta, Langmuir 2009, 25, 9240–9251.
[12] a) M. A. M. Noel, R. D. Allendoerfer, R. A. Osteryoung, J. Phys. Chem.
1992, 96, 2391–2394; b) A. Kawai, T. Hidemori, K. Shibuya, Chem. Lett.
2004, 11, 1464–1465; c) R. G. Evans, A. J. Wain, C. Hardacre, R. G. Comp-
ton, ChemPhysChem 2005, 6, 1035–1039; d) A. Kawai, T. Hidemori, K.
Shibuya, Chem. Lett. 2005, 414, 378–383; e) A. Kawai, T. Hidemori, K.
Shibuya, Mol. Phys. 2006, 104, 1573–1579; f) V. Strehmel, A. Laschewsky,
R. Stoesser, A. Zehl, W. Herrmann, J. Phys. Org. Chem. 2006, 19, 318;
g) R. Stoesser, W. Herrmann, A. Zehl, V. Strehmel, A. Laschewsky, Chem-
PhysChem 2006, 7, 1106–1111; h) G. Grampp, D. Kattnig, B. Mladenova,
Spectrochim. Acta Part A 2006, 63, 821–825; i) R. Stoesser, W. Herrmann,
A. Zehl, A. Laschewsky, V. Strehmel, Z. Phys. Chem. 2006, 220, 1309–
1342; j) V. Strehmel, H. Rexhausen, P. Strauch, Tetrahedron Lett. 2008, 49,
586–588; k) V. Strehmel, H. Rexhausen, P. Strauch, Tetrahedron Lett.
2008, 49, 3264–3267; l) V. Strehmel, H. Rexhausen, P. Strauch, Tetrahe-
dron Lett. 2008, 49, 7143–7145.
[1] a) H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley, New York,
2005; b) W. A. Henderson, M. Herstedt, V. G. Young, S. Passerini, H. C.
De Long, P. C. Trulove, Inorg. Chem. 2006, 45, 1412–1414; c) S.-L. Chou,
J.-Z. Wang, J.-Z. Sun, D. Wexler, M. Forsyth, H.-K. Liu, D. R. MacFarlane,
S.-X. Dou, Chem. Mater. 2008, 20, 7044–7051.
[2] S. S. Sekhon, B. S. Lalia, J.-S. Park, C.-S. Kim, K. Yamada, J. Mater. Chem.
2006, 16, 2256–2265.
[3] a) Z.-S. Wang, N. Koumura, Y. Cui, M. Miyashita, S. Mori, K. Hara, Chem.
Mater. 2009, 21, 2810–2816; b) N. Cai, J. Zhang, D. Zhou, Z. Yi, J. Guo,
P. Wang, J. Phys. Chem. C 2009, 113, 4215–4221; c) A. Mishra, M. K. R.
Fischer, P. Bauerle, Angew. Chem. 2009, 121, 2510–2536; Angew. Chem.
Int. Ed. 2009, 48, 2474–2499.
[4] a) T. Payagala, Y. Zhang, E. Wanigasekara, K. Huang, Z. S. Breitbach, P. S.
Sharma, L. M. Sidisky, D. W. Armstrong, Anal. Chem. 2009, 81, 160–173;
b) C. D. Tran, I. Mejac, J. Rebek, R. J. Hooley, Anal. Chem. 2009, 81,
1244–1254.
[5] a) F. Endres, S. Zein El Abedin, A. Y. Saad, E. M. Moustafa, N. Borissenko,
W. E. Price, G. G. Wallace, D. R. MacFarlane, P. J. Newman, A. Bund, Phys.
Chem. Chem. Phys. 2008, 10, 2189–2199; b) H. Itoh, K. Naka, Y. Chujo, J.
Am. Chem. Soc. 2004, 126, 3026–3027; c) L. Ren, L. Meng, Q. Lu, Z. Fei,
P. J. Dyson, J. Colloid Interface Sci. 2008, 323, 260–266; d) R. Bçck, Mate-
rialwiss. Werkstofftech. 2008, 39, 901–906; e) R. G. Reddy, J. Phys. Conf.
Ser. 2009, 165, 012076.
[6] a) P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Wein-
heim, 2003; b) C. E. Song, Chem. Commun. 2004, 1033–1043; c) N.
Paape, W. Wei, A. Bçsmann, C. Kolbeck, F. Maier, H.-P. Steinrꢃck, P. Was-
serscheid, P. S. Schulz, Chem. Commun. 2008, 3867–3869; d) D. Saha, A.
Saha, B. C. Ranu, Green Chem. 2009, 11, 733–737.
[13] R. Owenius, M. Engstrçm, M. Lindgren, M. Huber, J. Phys. Chem. A 2001,
105, 10967–10977.
[7] a) P. Kubisa, Prog. Polym. Sci. 2004, 29, 3–12; b) V. Strehmel, A. Laschew-
sky, H. Kraudelt, H. Wetzel, E. Gçrnitz, ACS Symp. Ser. 2005, 913, 17–36;
c) V. Strehmel, A. Laschewsky, H. Wetzel, E. Gçrnitz, Macromolecules
2006, 39, 923–930; d) V. Strehmel, A. Laschewsky, H. Wetzel, e-Polymers
2006, no. 011; e) V. Strehmel, Macromol. Symp. 2007, 254, 25–33; f) Y. S.
Vygodskii, O. A. Mel’nik, E. I. Lozinskaya, A. S. Shaplov, I. A. Malyshkina,
N. D. Gavrilova, K. A. Lyssenko, M. Y. Antipin, D. G. Golovanov, A. A. Kor-
lyukov, N. Ignat’ev, U. Welz-Biermann, Polym. Adv. Technol. 2007, 18, 50–
63; g) V. Strehmel, H. Wetzel, A. Laschewsky, E. Moldenhauer, T. Klein,
Polym. Adv. Technol. 2008, 19, 1383–1390; h) V. Strehmel, E. Reynaud, H.
Wetzel, E. Gçrnitz, A. Laschewsky, Macromol. Symp. 2009, 275–276,
242–249; i) H. Zhang, Y. Zhang, W. Liu, H. Wang, J. Appl. Polym. Sci.
2008, 110, 244–252; j) S. Perrier, T. P. Davis, A. J. Carmichael, D. M. Had-
dleton, Chem. Commun. 2002, 2226–2227; k) H. Mori, M. Yahagi, T.
Endo, Macromolecules 2009, 42, 8082–8092; l) S. Puttick, D. J. Irvine, P.
Licence, K. J. Thurecht, J. Mater. Chem. 2009, 19, 2679–2682; m) H.
Zhang, K. Hong, J. W. Mays, Polym. Bull. 2004, 52, 9–16; n) J. Ryan, F. Al-
dabbagh, P. B. Zetterlund, B. Yamada, Macromol. Rapid Commun. 2004,
25, 930–934; o) W. Huang, B. Charleux, R. Chiarelli, L. Marx, A. Rassat,
J.-P. Vairon, Macromol. Chem. Phys. 2002, 203, 1715–1723; p) M. Fischer,
B. R. Morrison, H. F. Sutoris, Y. Durant (BASF AG), DE 19727502, 1999;
q) M. Fischer, G. E. MacKee, H. F. Sutoris (BASF AG), DE 19738081, 1999;
r) M. Fischer, B. R. Morrison, H. F. Sutoris, Y. Durant (BASF AG),
WO 9900427, 1999; s) V. Sciannamea, R. Jerome, C. Detrembleur, Chem.
Rev. 2008, 108, 1104–1126.
[14] a) A. V. Blokhin, Y. U. Paulechka, A. A. Strechan, G. J. Kabo, J. Phys. Chem.
B 2008, 112, 4357–4364; b) Y. U. Paulechka, G. J. Kabo, A. V. Blokhin,
A. S. Shaplov, E. I. Lozinskaya, D. G. Golovanov, K. A. Lyssenko, A. A. Kor-
lyukov, Ya. S. Vygodskii, J. Phys. Chem. B 2009, 113, 9538–9546.
[15] G. Strobl, The Physics of Polymers, Springer, Heidelberg, 2007.
[16] D. E. Budil, S. Lee, S. Saxena, J. H. Freed, J. Magn. Reson. Ser. A 1996,
120, 155–189.
[17] S. H. Chung, R. Lopato, S. G. Greenbaum, H. Shirota, E. W. Castner, J. F.
Wishart, J. Phys. Chem. B 2007, 111, 4885–4893.
[18] Gaussian 03 (Revision D.01), M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T.
Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dan-
nenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V.
Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W.
Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussi-
an, Inc., Wallingford, CT, 2004. http://www.gaussian.com.
[8] a) G. McHale, C. Hardacre, R. Ge, N. Doy, R. W. K. Allen, J. M. MacInnes,
M. R. Bown, M. I. Newton, Anal. Chem. 2008, 80, 5806–5811; b) V. Streh-
mel, H. Rexhausen, P. Strauch, E. Gçrnitz, B. Strehmel, ChemPhysChem
2008, 9, 1294–1302.
[9] a) H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan, M. Watanabe, J.
Phys. Chem. B 2004, 108, 16593–16600; b) R. G. Evans, O. V. Klymenko,
Received: December 10, 2009
Revised: March 2, 2010
Published online on May 18, 2010
2190
www.chemphyschem.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 2182 – 2190