Activation volumes for: Cis -to- trans isomerisation reactions of azophenols: A clear mechanistic indicator?

Article abstract of DOI:10.1039/c7cp07349f

The thermal cis-to-trans isomerisation reaction of a series of hydroxy-substituted azo derivatives was studied kinetico-mechanistically as a function of temperature and pressure in order to investigate the possible role of the solvent in controlling the isomerisation mechanism, viz. inversion versus rotation. The variation of the observed first order rate constants for kinetic runs carried out at different temperatures and pressures was used to determine the thermal activation parameters ΔH^? and ΔS^?, and the pressure activation parameter ΔV^?. In addition, some experiments with deuterated species or solvents were also performed. The reported results could be interpreted as indicative of a changeover from an inversion mechanism for non-polar solvents to a rotational mechanism for polar solvents, capable of hydrogen bonding, for some of the systems studied. However, the operation of a rotational mechanism in all studied cases can account more consistently for the data observed.

Full text of DOI:10.1039/c7cp07349f

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Physical Chemistry Chemical Physics
Activation volumes for cisꢀtoꢀtrans isomerisation reactions of
azophenols. A clear mechanistic indicator?
Jaume GarciaꢀAmorós,a Grzegorz Stopa,b Grazyna Stochel,b Rudi van Eldik,b,c* Manuel
Martínezd* and Dolores Velascoa*
a
Grup de Materials Orgànics, Institut de Nanociència i Nanotecnologia (IN2UB),
Departament de Química Inorgànica i Orgànica (Secció de Química Orgànica), Universitat
de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain
b Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland
c
Department of Chemistry and Pharmacy, University of Erlangen-Nuremberg, Egerlandstr.
1, 91058 Erlangen, Germany
d
Departament de Química Inorgànica i Orgànica (Secció de Química Inorgànica), Facultat
de Química, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain
Jaume GarciaꢀAmorós: jgarciaamoros@ub.edu, ORCID 0000ꢀ0002ꢀ3554ꢀ7300
Grzegorz Stopa: stopa@chemia.uj.edu.pl, ORCID 0000ꢀ0003ꢀ0091ꢀ7786
Grazyna Stochel: stochel@chemia.uj.edu.pl, ORCID 0000ꢀ0002ꢀ9502ꢀ6371
Rudi van Eldik: rudi.vaneldik@fau.de, ORCID 0000ꢀ0003ꢀ4271ꢀ0118
Manuel Martínez: manel.martinez@qi.ub.es, ORCID 0000ꢀ0002ꢀ6289ꢀ4586
Dolores Velasco: dvelasco@ub.edu, ORCID 0000ꢀ0002ꢀ9357ꢀ5860
Abstract: The thermal cisꢀtoꢀtrans isomerisation reaction of a series of hydroxyꢀsubstituted
azo derivatives were studied kineticoꢀmechanistically as a function of temperature and
pressure in order to investigate the possible role of the solvent in controlling the isomerisation
mechanism, viz. inversion versus rotation. The variation of the observed first order rate
constants for kinetic runs carried out at different temperatures and pressures were used to
determine the thermal activation parameters ∆H≠ and ∆S≠, and the pressure activation
parameter ꢀV≠. In addition, some experiments with deuterated species or solvents were also
performed. The reported results could be interpreted as indicative of a changeover from an
inversion mechanism for nonꢀpolar solvents to a rotation mechanism for polar solvents,
capable of hydrogen bonding, for some of the systems studied. However, the operation of a
rotation mechanism in all studied cases can account more consistently for the data observed.
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Introduction
Controlling the properties of materials by means of suitable external stimuli, e.g. heat,
electric and magnetic fields, solvent gradients, and the absence or presence of ligands, is
receiving a great deal of attention since it gives rise to novel materials with unique fascinating
abilities and properties. Amongst all possible inputs, light, a clean, cheap and ubiquitous
energy source, is one of the most used since it can be applied remotely to determined areas of
the specimen in a quick and wireless fashion. On this basis, the design and study of lightꢀ
sensitive organic molecules that could be suitable for such task is essential.
The development of organic synthesis has rendered over the years a wide range of organic
photochromes.1 In this context, the easy and totally reversible interꢀconversion between the
two isomers of azobenzene of different stability, viz. trans and cis, in combination with the
possibility of the cis isomer to revert back to the initial state thermally in the dark,2 render
azobenzenes and their substituted derivatives to be the photochromes of common choice for
switching purposes. Indeed, the integration of azo dyes into different host materials has
already allowed to access a diversity of applications, covering both materials science and the
biomedical field.3−24
Since the first observation of the azobenzene isomerisation by Hartley in 1937, the
mechanism of the thermal cisꢀtoꢀtrans isomerisation of azobenzenes and, more specifically
azophenols, has attracted much attention, not without some controversy. Actually, the kinetics
and mechanism of the thermal back reaction of hydroxyꢀsubstituted azo dyes has so far been
the subject of many theoretical and experimental investigations. As found for conventional
azobenzenes, two different possible mechanisms have been proposed for the process.25−29 The
first one involves the rotation around the N−N bond, while the second one implies inversion,
inꢀplane lateral shift, via a linear transitionꢀstate. For phenolic azo chromophores, the rate of
their thermal back reaction has been found to dramatically depend on the number and position
of hydroxyl substituents within the covalent backbone of the azo platform, and surprisingly,
also on the solvent and general environment in which the azo photochrome is embedded.30−32
2

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In this respect, we have been involved for some time in the effect that the environment has
on the thermal cisꢀtoꢀtrans isomerisation reaction of an important number of specially and
carefully designed azo derivatives. In particular, activation volumes, derived from kinetic
measurements at different hydrostatic pressures, have been used as an efficient and useful tool
for the mechanistic assignment of the processes.3,23,24,30,31,33 In this paper we report new
insight obtained for the description of some isomerisation processes of azophenols using
kineticoꢀmechanistic studies. The involvement of the solvent in these processes has been
studied in a comprehensive manner using a range of solvents going from nonꢀpolar aprotic
(toluene) to polar aprotic (acetonitrile) and to polar protic (ethanol or methanol). The results
obtained by means of high pressure kinetics, have allowed us to gain further insight into the
isomerisation mechanism by which the reactions occur, as well as the polar or hydrogenꢀ
bonded nature of the interactions taking place in the transition state. Thus, investigation of the
structural and environmental factors governing the thermal cisꢀtoꢀtrans isomerisation kinetics
of this particular type of azo chromophores is essential to guide the future design of
photoactive analogues with the purpose of creating new lightꢀtriggered materials with the
desired abilities and functionalities.
Results and Discussion
The structures of the azo derivatives analysed in this study are summarized in Scheme 1. In
the first instance, two alkoxyꢀsubstituted azo derivatives (1 and 2) have been studied for the
sake of comparison for systems where hydrogen bonding is less likely. Then, two distinct
types of azophenolic dyes were selected for the kineticoꢀmechanistic studies. Azophenol 3
and its pushꢀpull paraꢀsubstituted cyanoꢀ and nitroꢀsubstituted counterparts, 4 and 5, as well
as the corresponding orthoꢀsubstituted analogues, 6 and 7, were chosen.
All the paraꢀalkoxyꢀ and paraꢀhydroxyꢀsubstituted azo derivatives used in this study
exhibit the typical spectral profile of azobenzenes, i.e. a strong band with a maximum
between 350 and 380 nm, assigned to the symmetryꢀallowed πꢀπ* transition of the trans
isomer, and a weaker broad band peaking at ca. 450 nm, associated with a symmetryꢀ
forbidden nꢀπ* transition (Figure 1a). In the figure it is also clear that the orthoꢀsubstituted
3

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azo compounds 6 and 7 show an extra band due to intramolecular hydrogen bonding
established between the hydroxyl substituent and one of the azo nitrogens.30
The fraction of the cis forms of the azo derivatives shown in Scheme 1 was increased via
photoꢀirradiation at λIrrad (see Experimental Section). Starting from that point, the kinetics of
their thermal back cisꢀtoꢀtrans isomerisation (Figure 1b) was studied in toluene, acetonitrile
and ethanol (or methanol) as solvents. Whereas for toluene no polarity or hydrogen bonding
interaction with the solvent is feasible, the stabilization of a polar transition state is possible
for acetonitrile. Furthermore, for the two protic alcohol solvents, the presence of hydroxyl
groups opens up the possibility of hydrogen bonding with the azo chromophore.
While the alkylated azo dye 1 exhibits a slow thermal back isomerisation in all types of
organic solvents (k = 4.5×10–6 s–1 in toluene and ethanol at 298 K), the corresponding free
azophenol 3 presents a much faster and solvent dependent back isomerisation process (k = 4.9
and 6.0×10–4 s–1 in ethanol and toluene, respectively). The same behaviour is observed for the
2 and 4 pair (see Table 1). When comparing the reactions for these same compounds in
acetonitrile (nonꢀprotic polar solvent), an identical kind of acceleration is observed.34−36 Thus,
two possible scenarios can be considered; either the operating mechanism is rotation for all
systems in all the solvents used (with stabilization of chargeꢀtransfer in the transition state
depending on the nature of the dye), or a switch from inversion to rotation mechanism occurs
for polar solvents capable of stabilizing charge transfer situations (Scheme 2). Furthermore,
the possible role of the −OH group in 3 and 4 establishing intermolecular interactions, either
with the cage of the surrounding solvent molecules or with neighbouring azophenolic units,
cannot be ruled out, but is not evident from the kinetic data collected in Table 1.
Table 2 collects the pressure activation parameters ∆V≠ determined for the systems studied
and Figure 2 represents some typical pressure dependent kinetic data for compound 5. The
value of ∆V≠ has been used in this work as a reliable indicator for the type of thermal
isomerisation mechanism operating for these molecules. In fact, this particular activation
parameter has been widely employed to evaluate the degree of association and electrostriction
in a great number of reactions involving substitution, redox, and different isomerization
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processes.37−40 Its advantage versus activation entropy relies in the fact that the value of ∆V≠
derives from the slope of a plot within the range of the data used, instead of an intercept
outside the experimental temperature range, having thus a smaller mathematical error. The
data in Table 2 clearly show that the values of ∆V≠ are practically zero for all the azo
derivatives when the reactions are carried out in toluene, indicating the nonꢀinvolvement of
the solvent in the transition state via which the reaction occurs. In addition, the values
obtained for the alkoxyꢀsubstituted compounds 1 and 2 in polar and protic solvents, zero in
both cases, definitely indicate that for these two derivatives an inversion mechanism is
operative, with no formation of polar transition states during the process.
Interestingly, as seen in Table 2, for azophenols 3 to 5 the values of ∆V≠ depend
dramatically on the solvent used. Clearly, either a polar transition state, whose stabilization
depends on the polarity of the medium, or a changeover from an inversion (in the nonꢀpolar
medium, toluene) to a rotation mechanism (in the polar media, ethanol or acetonitrile) is
occurring. In this respect, the fact that for the alkoxy derivatives 1 and 2 the mechanism is
definitely established as inversion, could suggest the same behaviour for compounds 3, 4 and
5 in nonꢀpolar solvents. However, from the data shown in Table 2, a rotational mechanism
could be claimed to be preferred in polar media.37−39 For example in the case of acetonitrile, a
polar nonꢀprotic solvent, the values determined for the activation volumes are negative in all
cases, in good agreement with the electrostriction suffered by the solvent molecules for a
chargeꢀseparated transition state. Furthermore, for compounds 4 and 5, both with a strong
pushꢀpull electronic configuration, the activation volumes are even more negative than for the
parent derivative 3, in line with the higher polarizing capability of the pushꢀpull species, and
thus with the expected higher polar character of the transition state.
Given the fact that the ∆V≠/∆S≠ correlation trend has been frequently associated with the
formation of outerꢀsphere hydrogen bonded aggregates involved in the transition state,41−43
systems 1ꢀ5 were also studied as a function of temperature to determine the thermal activation
parameters associated with the process (Table 3). The enthalpy and entropy of activation for
the thermal back isomerisation of alkoxyꢀsubstituted azo derivatives 1 and 2 are ca. 95 kJ
mol−1 and −30 J K−1 mol−1, respectively, suggesting that the process is enthalpy controlled. In
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the case of the phenolic analogues 3ꢀ5, the values are around 30 kJ mol−1 and −150 J K−1
mol−1 for the same parameters. Indeed, these values suggest a less enthalpy controlled, but
strongly entropy controlled transition state for the thermal isomerisation reaction, which can
be easily associated with an effective solvation process by the polar solvent molecules, in
agreement with a chargeꢀseparated nature of the transition state.
In order to find evidence for intermolecular hydrogen bonding that could also be a
favourable effect for the rotational mechanism, the effect of azo chromophore concentration
on the thermal cisꢀtoꢀtrans isomerisation rate in toluene was studied for paraꢀhydroxyꢀ
azobenzene 3 (Figure 3). Whereas in highly diluted solutions (< 1 mM) the isomerisation rate
constant shows an average value of 6×10−4 s−1, this value increases considerably, reaching a
value that is 130ꢀfold higher at 2.5 mM solutions. At higher concentrations the rate constant
reaches a plateau, which possibly indicates that the isomerisation step of all the azo dye
molecules is effectively assisted by hydrogen bonding promoted by a second azophenol
molecule. Nevertheless, it is clear that the solventꢀassisted process is still much faster than
that of the selfꢀassisted pathway of the azo dye (see Table 1). In this respect, the fact that a
deuterated sample of 3 in toluene shows essentially the same kinetic behaviour at low dye
concentrations (○ in Figure 3) than the nonꢀdeuterated compound 3, clearly indicates that no
intermolecular hydrogen bonding is involved in the process, both in the absence of polar
solvents or selfꢀaggregation conditions.
Further evidence that intermolecular hydrogen bonding can effectively assist the thermal
back reaction of azophenols in nonꢀprotic solvents came from a study of the thermal back
isomerisation of cisꢀ3 in methanol and d1ꢀmethanol (Figure 4). As expected, whereas in
methanol (pKa = 15.5)44 the process is characterized by a rate constant of 8.5 s−1, the value
decreases to 4.9 s−1 when d1ꢀmethanol is used. Such an isotopic influence due to the lower
acidity of the O−D bond was also found for both 4ꢀhydroxyꢀ4’ꢀmethoxyazobenzene and 4,4’ꢀ
dihydroxyazobenzene with similar results. For the former compound the rate constant values
changed from 9.4 to 6.0 s−1 on going from methanol to d1ꢀmethanol, whereas for the latter
compound these values changed from 16.4 to 8.1 s−1.
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The values of ∆V≠ (Table 2) are a definite indication of the distinct involvement of the
solvent in the cisꢀtoꢀtrans isomerisation process. For the protic polar solvents methanol and
ethanol, the activation volumes are significantly positive, which is opposite to the
electrostriction contraction expected when polar molecules solvate the charge separated
transition state generated in a rotational isomerisation mechanism. The only way to come to
terms with this observation is to refer to the involvement of directional outerꢀsphere hydrogen
bonding interactions of the polar transition state with the protic solvent molecules within the
solvation cage. This fact has already been observed in a rather large number of cases in
coordination chemistry substitution and redox processes in polar and hydrogenꢀbonding prone
solvents, water in particular.41−43 Furthermore, for compounds 4 and 5 the effect is enhanced
due to the higher pushꢀpull character of the process. Summarizing, the experimentally
determined values of the activation volumes are more positive for the systems where a larger
hydrogenꢀbonded solvent cage might be expected. That is, azo derivatives showing a pushꢀ
pull character generate a kind of domino effect that implies an increased number of protic
polar solvent molecules in the transition state of the process.
In this context, the analysis of the activation volumes determined for the orthoꢀsubstituted
azophenols 6 and 7 is even more revealing. Table 2 clearly shows that the values of ꢀV≠ for
the back cisꢀtoꢀtrans isomerisation of these derivatives, both in protic and nonꢀprotic polar
solvents, are consistently negative in line with pure electrostriction contraction in a rotational
chargeꢀseparated mechanism. Given the fact that for these azo derivatives hydrogen bonding
can be established efficiently in an intramolecular way, as a consequence of the placement of
the hydroxyl group in the ortho position, the thermal isomerisation step occurs via a rotation
mechanism, independently of the solvent used.
Adding up, ꢀV≠ for the orthoꢀhydroxy azo compounds 6 and 7 in toluene are ca. 0 even
evolving through a rotational mechanism (as we have shown, see above), and the alkoxyꢀ
substituted compounds 1 and 2, have values of ꢀV≠ ≈ 0 in toluene, due to the fact that they
operate via an inversion mechanism. Consequently, the mechanism by which azo derivatives
thermally isomerise cannot be unambiguously determined with the sole aid of the activation
volumes and, thus, other activation parameters must be considered. In this way, from data
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reported in Table 2 and 3, it can be proposed that the isomerisation mechanism for paraꢀ
hydroxy azo derivatives 3ꢀ5 is rotation in polar solvents (ethanol and acetonitrile) and that in
nonꢀpolar solvents (toluene) it could be essentially the same, since the thermal activation
parameters reported, seem to be independent of solvent polarity and within the range for wellꢀ
established rotation mechanisms.
Conclusion
The thermal cisꢀtoꢀtrans isomerisation process of a series of hydroxyl substituted azo
derivatives has been studied kineticoꢀmechanistically as a function of temperature, pressure
and dye concentration to investigate the type of isomerisation mechanism (inversion versus
rotation) taking place. Furthermore, some experiments were carried out on deuterated species
and solvents with the same purpose. Results clearly indicate that the thermal back cisꢀtoꢀtrans
isomerisation process takes place through an inversion mechanism for the nonꢀhydroxy azo
dyes, whatever solvent is used. On the contrary, for azophenols with hydroxy substitution at
the orthoꢀposition, relative to the azo function, the operational mechanism is determined to be
always rotation. Finally, for paraꢀsubstituted azophenols, the transition state is solvent
sensitive; while the isomerisation mechanism is clearly rotation in polar solvents, in nonꢀpolar
solvents the thermal and pressure activation parameters suggest that it can be essentially the
same.
Experimental Section
Kinetics. The population of cisꢀazobenzenes was generated by UV photolysis and its backꢀ
relaxation to the trans form was followed by timeꢀresolved UVꢀVis spectroscopy.
For longꢀlived cisꢀazobenzenes, the samples were irradiated with a Philips highꢀpressure
mercury lamp (total nominal power 500 W) filtered with a 0.5 M aqueous solution of
Co(NO3)2 (320 < λIrrad < 390 nm); irradiation was pursued until no further changes were
observed in the UVꢀVis spectrum of the sample (ca. 10 min). After this time the solutions
were thermostated in the dark at the desired temperature and pressure, and the thermal cisꢀtoꢀ
trans isomerisation was monitored by the change in the electronic spectrum of the sample
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(Figure 1b). Atmospheric pressure runs were monitored on a Varian Cary 50
spectrophotometer; for runs at variable pressure, a previously described pressurizing system
and pillbox cell were used which was connected to a J&M TIDAS spectrophotometer; the
systems have already been described,45 as well as the software used for the determination of
the first order rate constants involved.46,47
For shortꢀlived samples at ambient pressure, the thermal cisꢀtoꢀtrans isomerisation process
was studied by nanosecond laser flashꢀphotolysis. The cis form of the azo derivative was
generated by a Qꢀswitched NdꢀYAG laser (λIrrad = 355 nm, 5 ns pulse width, 1ꢀ10 mJ per
pulse) and the time evolution of the UVꢀVis spectrum of the sample was monitored
perpendicularly by a whiteꢀlight beam produced by a PTI 75 W Xe lamp; a postꢀsample
monochromator and a Hamamatsu R928 photomultiplier were used, and their output fed into
a digital oscilloscope through a 50 Ohm resistor. The observation wavelength was set at 370
nm in all the cases, where no isomerisation of the azo compounds took place. For runs at
elevated pressure, laser flash photolysis within the nanoꢀ to milliꢀsecond time scale was
carried out at 298 K using a LKS spectrometer (Applied Photophysics, UK) equipped with
Nd:YAG laser pump source Surelite Iꢀ10 Continuum (λIrrad = 355 nm, max 100 mJ, 5 ns pulse
width). Samples were monitored by a Xe lamp, monochromator and Hamamatsu R11568
photomultiplier. The laser line was connected to a 4ꢀwindow highꢀpressure cell in a
perpendicular arrangement using a ‘pillꢀbox’ optical cuvette and pressure gear as described
elsewhere.48 Additional spectral analyses of the solutions were made using a Perkin Elmer
Lambda 950 UVꢀvisꢀNIR spectrometer.
Unless otherwise stated, the experiments were carried out with ca. 1×10−5 M solutions of
the azoꢀdye in the corresponding solvent; no photoꢀdegradation of the compounds was
observed under the conditions of the study. For the fast reactions, observed rate constants
were derived in all cases from the corresponding absorbance versus time traces using the
standard software indicated above. For longꢀlived species, the full spectra were collected as a
function of time, and no dependence of the values of the observed rate constants on the
selected wavelengths was detected, as expected for reactions where a good retention of
isosbestic points is observed.49 All postꢀrun fittings for the determination of the thermal and
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pressure activation parameters were carried out by the standard available commercial
programs. Table S1 (ESI†) collects all the observed rate constants measured as a function of
all the variables used. In all instances, the experimental error associated to rate constants was
of less than 10 %.
Acknowledgements
Financial support from the Spanish Ministerio de Economía y Competitividad (projects
CTQ2015ꢀ65707C2ꢀ1 and CTQ2015ꢀ65770ꢀP MINECO/FEDER) are acknowledged. The
authors thank Prof. Santi Nonell for his help with the flash photolysis measurements at
atmospheric pressure.
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Schemes and Figures
Scheme 1. Chemical structures of paraꢀalkoxyꢀ (1 and 2), paraꢀhydroxyꢀ (3ꢀ5) and orthoꢀ
hydroxyꢀ (6 and 7) substituted azo dyes.
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Scheme 2. Rotation and inversion transition states for the thermal back cisꢀtoꢀtrans
isomerisation occurring on azo derivatives.24
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Figure 1. a) Electronic spectra of the paraꢀ and orthoꢀsubstituted nitroꢀazophenols 5 (black)
and 7 (blue) in toluene. b) Changes in the electronic spectrum of 4 observed during the
thermal cisꢀtoꢀtrans isomerisation at 25 ºC ([4] = 2.7×10−5 M).
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Figure 2. Pressure dependence of the first order rate constants for the cisꢀtoꢀtrans
isomerisation reaction of compound 5 in the different solvents used at 298 K.
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Figure 3. Dependence of the isomerisation rate constant, k, on the azo photochrome
concentration in toluene for the paraꢀhydroxyꢀsubstituted azo dye 3 at 298 K (○ corresponds
to a deuterated sample).
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Figure 4. Transient absorption change photoꢀinduced by laser pulsed irradiation with UV
light (NdꢀYAG laser, 5 ns pulse width, 10 mJ per pulse, λIrrad = 355 nm, λObs = 370 nm) for
compound 3 in methanol (a) and d1ꢀmethanol (b).
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Tables
Table 1. Kinetic data for the thermal isomerisation process of azo dyes 1ꢀ7 in different
solvents at 298 K.
k
/s−1
Compound MePh
EtOH (MeOH)
4.0×10−6
MeCN
5.0×10−6 n.d.
1a
2b
3a
4
3.2×10−5 2.6×10−5 4.6×10−5
6.0×10−4 2.1
1.0×10−3 5.5
1.6×10−3 3.7
4.9 (8.5)
3.7×101
2.2×102 (4.2×102)
8.3×101
5
6a
1.9×101
1.9×101
3.0
4.5
1.6×101
7
n.d.: Not Determined. aData from ref. 30. bData from ref. 3.
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Table 2. Pressure activation parameters for the thermal isomerisation process of the studied
azo dyes 1ꢀ7 in different solvents.
ꢁV
≠ /cm−3 mol−1
Compound Solvent
EtOHa
MePha
EtOHb
MePhb
~0
~0
~0
~0
1
2
EtOH
MeOH
MeCN
MePh
EtOH
MeCN
MePh
EtOH
MeOH
MeCN
MePh
EtOH
MeCN
MePh
EtOH
MeCN
MePh
+3.5 ± 0.5
+3.5 ± 0.7
−2.4 ± 0.6
~0
3
+14 ± 1
−10 ± 1
~0
4
5
+13 ± 1
+9.3 ± 0.9
−13 ± 1
~0
−11 ± 1
−7.3 ± 0.6
~0
6
7
−7 ± 1
−6.0 ± 0.4
~0
aData from ref. 30. bData from ref. 3.
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Table 3. Thermal activation parameters for the thermal isomerisation process of the azo dyes
1ꢀ5 studied in different solvents. Errors rounded to whole integers.
S≠
ꢁ
H≠
ꢁ
Compound Solvent
/kJ mol−1
/J K−1 mol−1
EtOHa
95 ± 1
−33 ± 3
1
MePha
94 ± 1
−32 ± 1
EtOHb
86 ± 1
94 ± 1
94 ± 1
15 ± 1
61 ± 1
13 ± 1
10 ± 1
47 ± 3
29 ± 1
44 ± 1
−42 ± 1
2
MeCNb
MePhb
−19 ± 1
−19 ± 3
EtOHa
−186 ± 1
−103 ± 2
−171 ± 1
−200 ± 1
−145 ± 9
−117 ± 2
−151 ± 3
3
MePh
EtOH
MeCN
MePh
EtOH
MePh
4
5
aData from ref. 30. bData from ref. 3.
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Graphical Abstract
22