A study of the tautomers of N-salicylidene-p-X-aniline compounds in methanol

Article abstract of DOI:10.1039/b101041g

We have studied the enol-imine→keto-amine tautomeric equilibrium of N-salicylidene-p-X-aniline compounds with X = Me, OMe, NMe₂ as electron donor substituents and X = COMe, CN and NO₂ as electron acceptor substituents. The equilibrium constants (K_tau^o) and standard thermodynamic properties ΔG_tau^o, ΔH_tau^o and ΔS_tau^o were measured and calculated in methanol solution at various temperatures, by means of excitation fluorescence spectroscopy. We have analyzed the p-phenylaniline substitution effect on K_tau^o and the thermodynamic properties through the Hammett parameters σ. We have performed molecular orbital calculations at the semiempirical AM1 and ab initio HF/3-21G levels to interpret the experimental results, explicitly including the solute-solvent interaction through the formation of an intermolecular hydrogen bond between the salicylidene and methanol molecules. These computational results show a good correlation with the experimental values. An interpretation of the experimental values of TΔS^o, based on changes in the molecular structure produced in the enol-imine→keto-amine tautomeric reaction, is proposed.

Full text of DOI:10.1039/b101041g

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A study of the tautomers of N-salicylidene-p-X-aniline compounds
in methanol
Victor Vargas and Luis Amigo
2
Laboratory of Luminescence and Molecular Structure, Department of Chemistry,
Faculty of Sciences, University of Chile, Casilla 653, Santiago, Chile. E-mail: victor@uchile.cl
Received (in Cambridge, UK) 30th January 2001, Accepted 11th April 2001
First published as an Advance Article on the web 31st May 2001
We have studied the enol-imine→keto-amine tautomeric equilibrium of N-salicylidene-p-X-aniline compounds with
X = Me, OMe, NMe2 as electron donor substituents and X = COMe, CN and NO2 as electron acceptor substituents.
The equilibrium constants (KtЊau) and standard thermodynamic properties ∆GtЊau, ∆HtЊau and ∆StЊau were measured and
calculated in methanol solution at various temperatures, by means of excitation fluorescence spectroscopy. We have
analyzed the p-phenylaniline substitution effect on KtЊau and the thermodynamic properties through the Hammett
parameters σ. We have performed molecular orbital calculations at the semiempirical AM1 and ab initio HF/3-21G
levels to interpret the experimental results, explicitly including the solute–solvent interaction through the formation
of an intermolecular hydrogen bond between the salicylidene and methanol molecules. These computational results
show a good correlation with the experimental values. An interpretation of the experimental values of T∆SЊ, based
on changes in the molecular structure produced in the enol-imine→keto-amine tautomeric reaction, is proposed.
is to analyze, from experimental and theoretical points of
view, the effect of para substitution in the phenylaniline ring of
N-salicylidene-p-X-anilines on Ktau constants and the thermo-
dynamic properties.
Introduction
Intramolecular proton transfer reactions in the ground or
excited electronic states have been the subject of several experi-
mental and theoretical studies.1–6 Schiff bases of N-salicylidene-
In order to obtain greater insight into the tautomerisation
p-X-aniline are model systems presenting intramolecular
process, we have performed semiempirical AM1 and ab initio
proton transfer between the O and N atoms, leading to an
molecular orbital calculations for the proton transfer reaction.
enol-imine keto-amine equilibrium, as shown in Fig. 1.
The specific interaction with the solvent is modeled by means of
an intermolecular hydrogen bond, for the enol–methanol and
keto–methanol systems. Based on these results, we propose an
explanation for the experimental values of T∆SЊ, observed in
This tautomerism, that induces interesting photo- and thermo-
chromism properties,7 depends on the electronic structure,
temperature and the polar nature of the solvent.
In previous studies carried out on 2-hydroxy-1-naphth-
the enol-imine→keto-amine tautomeric reaction.
aldehyde Schiff bases, the tautomeric equilibrium constants,
KtЊau, were determined by NMR.8,9 This technique can be used
Experimental and computational methods
with precision only for similar relative concentrations of the
different species in solution.
Compounds 1–7 were synthesized by a condensation procedure,
stirring equimolar quantities of salicylaldehyde and the aniline
in methanol solution. Double crystallizations were carried out in
methanol at low temperature followed by vacuum sublimation.
The compounds under study were characterized by IR and
1H-NMR spectroscopy. Solutes and solvents were purchased
from Aldrich. Methanol for the emission study was Fischer
HPLC grade, with a low fluorescence background.
This paper presents a study of the enol-imine→keto-amine
tautomeric reaction of compounds 1–7 in methanol solution
(see Fig. 1). The equilibrium tautomeric constants (KtЊau), and
the thermodynamic standard properties, free energy (∆GtЊau),
enthalpy (∆HtЊau) and entropy (∆StЊau), are determined by means
of excitation fluorescence spectroscopy.10 The aim of this paper
The absorption spectra were obtained in a Perkin-Elmer
Lambda 11 UV-VIS spectrophotometer in a 10 mm quartz cell.
Excitation fluorescence spectra, corrected by Rhodamine G,
were obtained in the ISS-PC Photon counting spectrofluoro-
meter, coupled to a thermoregulated methanol bath. The total
intensity is detected across a 550 nm cut-on filter and the
temperature was measured in the cell sample compartment by
using an iron-constantan thermocouple.
All the geometric structures for enol- and keto–methanol
complexes were optimized by means of the semiempirical
molecular orbital AM1 method11 using the WinMopac
package.12 The enthalpy for the tautomeric reaction (∆Htau) is
directly calculated as the difference of the heat of formation of
both tautomers. Ab initio calculations were performed with
Gaussian 98 software,13 at the restricted Hartree–Fock level,
using the standard 3-21G basis set and fully optimizing the
structure of the solute–methanol complex. The frequency
option was used for thermodynamic calculations. These
Fig. 1 The N-salicylidene-p-X-aniline series studied in this paper.
1124
J. Chem. Soc., Perkin Trans. 2, 2001, 1124–1129
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101041g

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Fig. 2 A. Absorption spectra of compounds 5 (X = Me) and 7 (X =
NMe2), in cyclohexane (——) and methanol (- -) solvent. B. Excitation
fluorescence spectra of compound 5 (X = Me) and 7 (X = NMe2) in
cyclohexane (——) and methanol (- -) solvent.
calculations were further improved by extending the basis set to
6-311G* and performing calculations at that level but keeping
the optimized 3-21G structures (HF/ 6-311G*// 3-21G).
Fig. 3 A. Excitation fluorescence spectra of compound 5 in methanol
solution as a function of temperature. B. Logarithmic relationship
between Iketo/Ienol and the reciprocal of the absolute temperature for
compound 5 in methanol solution. Iketo and Ienol were measured from
Fig. 3-A spectra at 450 and 345 nm, respectively.
Results and discussion
a. Spectroscopic results
These spectra show decreasing intensity with increasing tem-
perature. This change is more evident in the UV band (330 nm).
The other compounds under study present a similar spectral
intensity pattern for the UV and VIS bands at different
temperatures.
The enol-imine→keto-amine reaction is characterized by
the equilibrium tautomeric constants (KtЊau) at 298 K. In our
experimental scheme, KtЊau is calculated from the excitation
spectra intensity by eqn. (1), and the free standard energy
(∆GtЊau) is calculated by using eqn. (2).
The absorption spectra of compounds 1–7 in solution, in the
300 to 500 nm region, are characterized by a broad and intense
band centered at 340 nm. A spectral shift to 380 nm is in agree-
ment with an increasing electron donor strength of the sub-
stituents.14 In Fig. 2-A we compare the absorption spectra of
compounds 5 (X = Me) and 7 (X = NMe2) in methanol and
cyclohexane solutions. In methanolic solution, we observe a
new weak absorption band in the 450 to 500 nm region, which is
absent in cyclohexane solution. The same band was observed
by Dudek and Dudek15 in N-salicylidene-o-toluidine, when
they added small amounts of propionic acid to 3-methyl-
pentane solution. They assign this absorption band to a
cis-keto-aniline tautomer.
The corrected fluorescence excitation spectra for the same
substituted compounds in cyclohexane and methanol solutions
are presented in Fig. 2-B. With only slight differences, they
recover the absorption spectra presented in Fig. 2-A, i.e.,
in cyclohexane solutions, we observe only one band with a max-
imum located within the 340–380 nm region, which can be
associated with the enol species. In methanol we observe the
enol band plus a low absorption one, in the 450–500 nm region,
which is associated with the keto form.
Iketo
KtЊau =
(1)
(2)
Ienol
∆GtЊau = ϪRT ln KtЊau
The standard enthalpy (∆HtЊau) and standard entropy (∆StЊau),
for the tautomeric reaction are obtained from the linear
regression, eqn. (3).
Iketo
ln ͩ ͪ
Ienol
Ϫ∆HtЊau 1 ∆StЊau
=
ϩ
(3)
R
T
R
Due to the very high sensitivity of the fluorescence technique,
we have used excitation fluorescence spectroscopy to measure
with better precision the intensity of the maximum UV and VIS
bands. For the low intensity band we have obtained a ratio
signal/noise of about 100. Furthermore, since at low optical
densities (OD <0.1), the fluorescence intensity is proportional
to the sample concentration, we have assumed that for the
excitation fluorescence in methanol solution, the maximum
intensities of the UV (Ienol) and VIS (Iketo) bands are pro-
portional to the concentration of the enol and keto species,
respectively.
Fig. 3-B shows the plot of ln( Iketo/Ienol) against the reciprocal
of the absolute temperature. For all the compounds in this
study, a good linear regression is obtained, which allows the
calculation of ∆HtЊau and ∆StЊau directly as the slope and the
intercept of eqn. (3), respectively.
In Table 1 we display the KtЊau, ∆GtЊau, ∆HtЊau and ϪT∆SЊtau
values obtained with that procedure. We note that these data do
not present a large dependence on the variation of the acceptor
or donor capacity of the substituents in the phenylaniline
group. They do present, however, a clear trend: KtЊau decreases
and ∆GtЊau, ∆HtЊau increase when the para substituent in the
aniline ring goes from the dimethylamine donor group to
the nitro acceptor group.
Fig. 3-A presents the excitation fluorescence spectra at dif-
ferent temperatures of compound 5 in methanol solutions.
J. Chem. Soc., Perkin Trans. 2, 2001, 1124–1129
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Table 1 Experimental equilibrium tautomeric constants, and standard
thermodynamic properties (kJ molϪ1) of N-salicylidene-p-X-aniline in
methanol solution
a
X
σp
10Ϫ2 KtЊau
∆GtЊau
∆HtЊau
ϪT∆StЊau
-NMe2
-OMe
-Me
-H
-COMe
-CN
Ϫ0.63 6.48 1.22 6.78 0.46 3.80 0.21 2.95 0.25
Ϫ0.28 4.86 0.98 7.49 0.50 5.01 0.17 2.48 0.25
Ϫ0.14 7.23 1.53 6.49 0.52 4.55 0.31 1.94 0.15
Ϫ0.00 5.73 1.31 7.08 0.56 5.25 0.30 1.88 0.21
0.47 3.00 0.82 8.68 0.67 6.25 0.38 2.43 0.25
0.70 2.04 0.61 9.64 0.73 7.10 0.57 2.53 0.27
0.81 2.19 0.62 9.46 0.70 6.37 0.44 3.08 0.41
-NO2
a From ref. 17.
In Table 1 we also display the standard entropy values. Note
that the enol→keto tautomeric reaction has an important
entropy contribution at 298 K (20–40% of the free standard
energy). This contribution is higher in more polar substituted
compounds (NMe2 and NO2) than in less polar compounds.
These entropy values will be discussed later along with the
results of the computational calculations.
b. Hammett analysis
In order to characterize the substituent effect on KtЊau and
thermodynamic properties, we used the Hammett analysis,
where σp, the Hammett parameter, is related to KtЊau through
eqn. (4).16
log KtЊau = ρσp ϩ log KtHau
(4)
Fig.
4
A. Linear correlation of log Ktau against σp Hammett
In this equation, KtHau is the equilibrium tautomeric
constant for N-salicylidene-p-X-aniline, and ρ, the reaction
constant, is the slope appearing in the log KtЊau vs. σp plot.
parameters. B. Linear correlation of ∆GЊtau against σp Hammett
parameters. C. Linear correlation of ∆HtЊau against σp Hammett
parameters.
17
Fig. 4-A shows a good correlation of log KtЊau with σp
(r = 0.91). The ρ and log KtHau constants are Ϫ0.38 0.08 and
Ϫ1.34 0.25, respectively; the latter value is in good agreement
with that for log KtЊau for the unsubstituted system determined
from Table 1.
The reaction constants ρ have been interpreted as a measure
of the sensitivity of the equilibrium to the electronic substituent
effect, which is, by definition, 1.00 for the dissociation of
benzoic acid in water at 25 ЊC.16 If ρ > 0, the electron acceptor
substituent favours the right hand of the equilibrium equation.
For the salicylidene compounds ρ < 0, thereby indicating that
an electron acceptor favours the left side of the equilibrium,
i.e., the enol species. An electron donating substituent favours
the right hand side of the equilibrium reaction, i.e., the
keto species. On the other hand, small values of ρ can be inter-
preted as a low sensitivity of the tautomeric equilibrium to
the substituent effects, probably due to lower through-bond
interactions with the OH group.
Fig. 4 also includes the correlation of Hammett parameters
with the standard free energy (Fig. 4-B) and enthalpy
(Fig. 4-C), respectively. We have found the values 2.20 0.47,
7.65 0.23 for the slope and intercept in ∆GtЊau, and 2.03 0.30,
5.22 0.15 in ∆HЊtau, respectively. These relationships found
for ∆GtЊau and ∆HtЊau with the σp parameters suggest that the
entropic changes in the tautomeric reaction present a linear
dependence with the Hammett parameter of the substituent,
(ϪT∆StЊau = 0.2σp ϩ 2.4). Due to the small value of the slope
(close to zero) the experimental entropic changes in the
salicylidene derivatives could not be explained solely on the
basis of the electronic nature of the substituent (see Table 1).
Fig.
5
Intermolecular hydrogen bonding of N-salicylidene-p-X-
anilines with a methanol molecule, for enol and keto tautomers.
calculations we have assumed that the specific solute–solvent
interaction can be described by means of an intermolecular
hydrogen bond between the solute and a methanol molecule
(solute–methanol hydrogen bonded complex). Fig. 5 shows the
interaction between the solute molecule, in both tautomeric
forms with methanol. Fig. 6 displays the optimized geometric
structures of N-salicylidene-p-X-aniline–methanol hydrogen
bonded complex for the enol and keto configurations, deter-
mined from the AM1 method and ab initio with HF/3-21G
method.
The formation energy of each hydrogen bonded solute–
solvent complex, for the enol–methanol (∆Eehb) and keto–
methanol (∆Ekhb) tautomeric species, is given by eqns. (5) and (6),
respectively.
∆Eehb = Eehb Ϫ (Ee ϩ Emet
)
(5)
(6)
c. Molecular orbital calculations
Formation of the hydrogen bonded complex. To interpret the
experimental results, we have used the semiempirical AM1
method and the ab initio Hartree–Fock method to estimate the
thermodynamic standard functions ∆HЊ, ∆GЊ and ∆SЊ. In both
∆Ekhb = Ekhb Ϫ (Ek ϩ Emet
)
Eehb and Ekhb are the total energies of the fully optimized
hydrogen-bonded complexes of the enol and keto forms,
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Table 3 AM1 (first row) and 6-311G*//3-21G (second row) standard
thermodynamic properties calculations, variation of the phenylaniline
dihedral angle, ∆φЊ, and absolute dipole moment changes, ∆µ, for
the tautomeric reaction of the N-salicylidene-p-X-aniline–methanol
hydrogen bond complexes
Table 2 AM1 Heat of formation and 6-311G*//3–21G MO energy
calculations for enol (e) and keto (k) N-salicylidene-p-X-aniline and
N-salicyl-p-X-aniline–methanol hydrogen bond complexes (hb)
AM1/kJ molϪ1
6-311G*//3-21Ga/au
a
∆GtЊau
∆HtЊau
T∆StЊau
∆φЊ(φketo
)
∆µ/debyeb
Ee
121.75
Ϫ116.98
Ϫ129.66
Ϫ12.63
Ϫ627.880314
Ϫ742.887668
Ϫ742.902068
Ϫ0.0144
Ee ϩ Emet
Eehb
∆Eehb = Eehb Ϫ (Ee ϩ Emet
-NMe2
-OMe
-Me
2.702
5.916
3.414
7.739
1.452
7.142
1.464
7.167
1.460
8.217
3.243
11.180
3.807
12.247
3.653
5.535
4.485
7.037
2.686
6.452
2.686
6.661
2.569
5.828
4.201
10.263
4.778
11.255
0.825 Ϫ7.4 (26.5)
Ϫ0.525 Ϫ4.0 (22.6)
1.148 Ϫ3.1 (28.8)
2.637
2.332
2.250
1.792
1.954
1.389
1.618
1.018
1.222
1.170
1.393
1.028
)
37.81 kJ molϪ1
Ϫ627.869401
Ϫ742.876755
Ϫ742.899528
Ϫ0.0228
Ek
Ek ϩ Emet
Ekhb
140.37
Ϫ98.32
Ϫ126.98
Ϫ28.66
Ϫ0.605
0.792
Ϫ1.045
0.794
Ϫ0.805
0.680
Ϫ0.996
0.961
2.0 (19.5)
4.1 (21.7)
9.4 (18.6)
5.4 (21.5)
10.3 (19.0)
12.8 (17.5)
18.7 (11.7)
12.7 (13.6)
19.1 (11.8)
∆Ekhb = Ekhb Ϫ (Ek ϩ Emet
)
-H
Ϫ59.79 kJ molϪ1
-COMe
-CN
a The ab initio calculation including the ZPE thermal corrections.
Ϫ2.247
1.108
-NO2
18.8 (12.8) Ϫ1.279
24.3 (9.3) Ϫ0.835
Ϫ2.554
a HF/3-21G calculations. b HF/6-311G*//3-21G calculations.
it is not possible to observe the visible band at 450 nm associ-
ated with the keto species, which is probably due to the fact that
in a non-polar solvent the energy between both species is about
20 kJ molϪ1 or more. The equilibrium should then be shifted to
the enol form and the absorption spectra should present only
the one band associated with this species. On the other hand,
in a protic solvent, the energy difference is reduced to 4 kJ
molϪ1, and a small fraction of the keto species is present in the
solution, and therefore a weak absorption band in the visible
region appears.
Thermodynamics calculations. For each compound under
study, we have calculated the total energy for the optimized
hydrogen bonded complex of the enol and keto species. In the
evaluation of the thermodynamics properties HЊ(hb), GЊ(hb)
and SЊ(hb), we have used the option “thermo”21 and “fre-
quency”19 in both AM1 and ab initio, calculations, respectively.
For the enol-imine→keto-amine tautomeric reaction, the
standard properties (∆PtЊau(hb)), were evaluated through ∆PtЊau-
(hb) = PЊ(khb) Ϫ PЊ(ehb), where PЊ represents any thermo-
dynamic property, HЊ, GЊ, SЊ.
In Table 3, we present the calculated ∆GtЊau(hb), ∆HtЊau(hb)
and T∆StЊau(hb) values for the tautomeric reaction at 298 K.
We observe that ∆GtЊau(hb) and ∆HtЊau(hb) values, calculated by
means of the 6-311G*//3-21G basis (second row), are system-
atically larger than the AM1 ones (first row). The greatest
difference between them was found in the nitro-substituted
compound.
In order to compare the calculated thermochemical proper-
ties with the experimental ones, we show in Figs. 4-B and 4-C
the calculated and experimental values against the Hammett
parameter. In Fig. 4-B we show the 6-311G*//3-21G ∆GtЊau(hb)
values. They are quite close to the experimental data with a
small negative deviation of 0.13 kJ molϪ1 (on average). On the
other hand, the AM1 results present a larger negative deviation
of Ϫ5.4 kJ molϪ1 (on average) with respect to the experi-
mental data, which is due to the positive value of the calculated
entropic contribution (see Table 3).
The AM1 and ab initio ∆HtЊau(hb) values are shown in the
Fig. 4-C. It can be seen that the AM1 calculations display a
negative deviation (1.9 kJ molϪ1 on average) and the ab initio
values predict a positive deviation (2.3 kJ molϪ1, on average),
with respect to the experimental values. Furthermore, we note
that the ab initio data follow the same trend with a good linear
correlation (r = 0.94). In contrast to this, the AM1 results do
not exhibit an important effect of the substituent, leading to a
poor linear correlation (r = 0.25).
Fig. 6 Optimized geometrical structures obtained from AM1 and
ab initio calculations.
respectively. Ee and Ek are the total energies of the optimized
geometry of the non-bonded enol and keto forms and Emet is
the energy of the optimized geometry of a methanol molecule.
To compare the AM1 and ab initio results, we have added, in
ab initio calculations, the thermal ZPE correction enthalpy,18,19
at 298.15 K to the total energy. This is a standard procedure,
and requires calculation of the force field. With the knowledge
of the force field and the moments of inertia even the cal-
culation of the partition function and the thermodynamic data
at room temperature is straightforward.20 The standard enthalpy
of the enol→keto tautomeric reaction is given by ∆HtЊau =
Ek Ϫ Ee for the non-bonded fragment and ∆HtЊau(hb) = Ekhb
Eehb for the H-bonded complex.
=
In Table 2 we display the AM1 and 6-311G*//3-21G
results of N-salicylidene-p-X-aniline and the N-salicylidene-p-
X-aniline–methanol hydrogen-bonded complexes. We note that
the formation energy of the keto–methanol complex (Ekhb
)
is higher than the formation energy of the enol–methanol
complex (Eehb). This higher stabilization energy of the keto
complex reduces the ∆HtЊau value, from 18.6 to 2.7 kJ molϪ1 in
AM1, and from 28.7 to 6.7 kJ molϪ1 in the ab initio calculation.
The energy values are in agreement with the absorption and
fluorescence excitation spectra shown in Fig. 2. In cyclohexane
J. Chem. Soc., Perkin Trans. 2, 2001, 1124–1129
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Fig. 8 Plot of the calculated AM1 and 6-311G*//3-21G ∆φ; and
∆µ values against σp Hammett parameters.
Fig. 7 Calculated angular and dipole contributions to T∆SЊ experi-
mental values.
means of multiple linear regression. We have obtained a =
Ϫ0.83 0.05 kJ molϪ1 debyeϪ1, b = Ϫ6.27 0.40 kJ molϪ1 radϪ1
(r = 0.99) at the AM1 level and a = Ϫ1.15 0.16 kJ molϪ1
debyeϪ1, b = Ϫ4.20 0.69 kJ molϪ1 radϪ1 (r = 0.94) at the
6-311G*//3-21G level.
Knowledge of the coefficients a and b allows us to calculate
separately the dipole and angular change contributions to the
total entropy change in the enol→keto reaction. Fig. 7 displays
both contributions to ϪT∆SЊ.
Entropy calculation model. Fig. 7 displays ϪT∆StЊau values at
298 K against σp. It is observed that the species with higher
values of σp (i.e., species 1 and 7, at both extremes of the figure)
present the higher T∆SЊ. These values are dependent only on
the absolute σp, and decrease towards a minimum for X = H. To
explain this trend, we propose a model based on the structural
changes occurring in the solute when the tautomeric reaction
takes place (vide supra).
In our N-salicylidene-p-X-aniline compounds there are two
molecular parameters that change considerably during the
Conclusions
tautomeric equilibrium. These are the angle φ (see Fig. 1) and
The high sensitivity of the fluorescence technique allows
accurate determination of the tautomeric constants for the
enol→keto reaction of the N-salicylidene-p-X-aniline com-
pounds in methanol solution.
The hydrogen bond in N-salicylidene-p-X-aniline derivatives
with a methanol solvent molecule, characterized by means of
MO calculations, has been shown to be an adequate model to
explain the absorption and the fluorescence excitation spectra
of N-salicylidene-p-X-aniline in methanol solution.
The thermodynamic standard properties of N-salicylidene-p-
X–aniline–methanol complexes, determined by means of AM1
and ab initio MO calculations are in good agreement with the
experimental values. However, the semiempirical AM1 method
results inadequately account for the effect of substituents.
When the enol→keto tautomeric reaction takes place, the
entropy changes in the molecular salicylidene species are
correctly described by means of a linear function of the ∆φ
and ∆µ molecular parameters.
→
the dipole moment |µ|. To quantify the structural molecular
changes in the intramolecular proton process, we define the
change of the dihedral angle as ∆φ = φketo Ϫ φenol and the
change of the dipole moment as ∆µ = |µketo| Ϫ |µenol|. Therefore
∆µ and ∆φ can be seen as a measure of the intramolecular
charge transfer and the molecular geometry change, respec-
tively, when the intramolecular proton transfer takes place.
Furthermore, both variables are dependent on the electronic
acceptor or donor nature of substituent in the phenylaniline
ring.
In Table 3 we present the ∆φ and ∆µ values calculated with
AM1 and 3-21G methods. Both calculations lead to similar
trends for the ∆φ and ∆µ variables. In Fig. 8 we display the
plot of ∆φ and ∆µ versus σp. These show that ∆φ increases
when the electron acceptor properties of the substituent
increase. Furthermore, ∆µ increases when the electron donor
properties increase from nitro- to dimethylamine-substituted
compounds.
In the enol-imine→keto-amine reaction in our molecular
system in methanol solution, the T∆SЊ value is a measure of
the reorganization of methanol molecules around the solute.
This reorganization in the solvent is induced by changes in the
solute molecular properties when the enol→keto reaction takes
place.
Acknowledgements
The authors acknowledge the FONDECYT Project 1970787
for financial support. We also appreciate helpful comments
from Professors Alejandro Toro-Labbé and Raúl. G. E.
Morales.
The above results suggest that the experimental T∆StЊau values
contain two contributions: one electronic, probably pro-
portional to the dipole change (∆µ), and the other a geometric
contribution, dependent on the dihedral angle change (∆φ):
Therefore the experimental values of T∆StЊau may be expressed
through eqn. (7), where a and b are coefficients obtained by
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ϪT∆S =a∆µ ϩ b∆φ
(7)
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