Selective induced polarization through electron transfer in acetone and pyrazole ester derivatives via C-H...O=C interaction

Article abstract of DOI:10.1039/c4nj00679h

A set of organic compounds (pyrazole ester derivatives, viz. 5-[3-(substituted)-propoxy]-3-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid methyl ester and 5-[2-(substituted)-ethoxy]-3-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid methyl ester) was synthesized and their affinity and stability towards the acetone molecule were tested by NMR. Further, the host-guest complex formed has been studied by cyclic voltammetric titration. Interestingly, the acetone molecule selectively bound to the methyl group of the pyrazole moiety and stabilized the system. These findings are also supported by quantum chemical calculations using the DFT/B-LYP/TZVPP method. Apart from hydrogen bonding, an important role in stabilizing the complex is also played by the C-H...π interaction governed by dispersion energy. The study proves that a methyl-substituted pyrazol ester can act as a receptor for acetone.

Full text of DOI:10.1039/c4nj00679h

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Selective induced polarization through electron
transfer in acetone and pyrazole ester derivatives
Cite this: New J. Chem., 2014,
38, 4885
Q
via C–Hꢀ ꢀ ꢀO C interaction†
Ashish Kumar Tewari,*a Priyanka Srivastava,a Ved P. Singh,a Praveen Singh,a
Ranjeet Kumar,a Ranjana S. Khanna,a Pankaj Srivastava,a
Ramachandran Gnanasekaranb and Pavel Hobzab
A set of organic compounds (pyrazole ester derivatives, viz. 5-[3-(substituted)-propoxy]-3-methyl-1-
phenyl-1H-pyrazole-4-carboxylic acid methyl ester and 5-[2-(substituted)-ethoxy]-3-methyl-1-phenyl-
1H-pyrazole-4-carboxylic acid methyl ester) was synthesized and their affinity and stability towards the
acetone molecule were tested by NMR. Further, the host–guest complex formed has been studied by
cyclic voltammetric titration. Interestingly, the acetone molecule selectively bound to the methyl group
of the pyrazole moiety and stabilized the system. These findings are also supported by quantum
chemical calculations using the DFT/B-LYP/TZVPP method. Apart from hydrogen bonding, an important
role in stabilizing the complex is also played by the C–Hꢀ ꢀ ꢀp interaction governed by dispersion energy.
The study proves that a methyl-substituted pyrazol ester can act as a receptor for acetone.
Received (in Victoria, Australia)
28th April 2014,
Accepted 29th July 2014
DOI: 10.1039/c4nj00679h
www.rsc.org/njc
three complexes formed between ethylcalix[4]resorcinarene
(CECRA) and acetone molecules suggests a recognition mecha-
1. Introduction
The question of Jeffrey and Saenger of whether C–Hꢀ ꢀ ꢀOQC nism in which an acetone molecule is captured as a guest in the
interaction can be referred to as hydrogen bond interaction cavity of a CECRA host molecule through C–Hꢀ ꢀ ꢀp interactions
even though there is every reason to suspect that the carbon between the methyl groups of acetone and the phenyl rings of
atom is not electronegative and may even carry a net positive CECRA.6 In the case of C–Hꢀ ꢀ ꢀO hydrogen bonds, the strength
charge was denied by Pauling and accepted by Pimentel and of the bond depends on the nature of the other groups
McClellan.1 A refinement of the latter definition led to quanti- attached; even weakly polar C–H groups of methyl7 may give
fication by Steiner and Saenger, who in 1993 considered a a carbonyl function forming a weak C–Hꢀ ꢀ ꢀO hydrogen bond. In
hydrogen bond as ‘any cohesive interaction X–Hꢀ ꢀ ꢀY, where H addition, C5–H in the 4,6-dimethyl pyrimidine-2-one derivative
carries a positive and Y a negative (partial or full) charge and is a rich donor, whereas O, N, and p are acceptors in this model
the charge on X is more negative than on H’.2,3 Recently, a more system.8 The crystal structure of various organic compounds
general definition has been given by Arunan et al.,4 namely ‘The usually shows C–Hꢀ ꢀ ꢀO weak interactions.9–14 These bonds also
hydrogen bond is an attractive interaction between a hydrogen play an important role in native and misfolded proteins.15
atom from a molecule or a molecular fragment X–H in which Recently, 1-(2-naphthyl)-3-methyl-5-pyrazolone coupled with
X is more electronegative than H, and an atom or a group of capillary electrophoresis with diode array detection for the
atoms in the same or a different molecule, in which there is determination of carbohydrates such as mannose, galacturonic
evidence of bond formation’.
acid, glucuronic acid, rhamnose, glucose, galactose, xylose,
Molecular recognition processes can be said to hinge on arabinose and fucose has been developed.16
isotropic interactions, the most important among which is the
In the present study, we synthesised the derivatives of
hydrogen bond.5 An X-ray single crystal structure analysis of 3-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid methyl ester
and explored its role as a host for the acetone guest molecule
through a weak intermolecular C–HꢀꢀꢀO hydrogen bond with
a Department of Chemistry, Faculty of Science, Banaras Hindu University,
the support of NMR, cyclic voltammetric (CV) and theoretical
studies. The varying chemical shift in the 1H NMR titration of
experimental compounds in CDCl3 solvent with varying concentra-
Varanasi-221 005, India. E-mail: tashish2002@yahoo.com
b Institute of Organic Chemistry and Biochemistry,
Academy of Sciences of the Czech Republic, 16610 Prague, Czech Republic
tions of acetone indicated the formation of host–guest complexa-
tion. In CH2Cl2 solution, the complex formation between guest and
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj00679h
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Table 1 1H NMR titration data of compounds with acetone
Compound no.
Z
n
0.2 ml acetone
2.45
0.4 ml acetone
2.59
0.6 ml acetone
2.78
0.8 ml acetone
2.90
1.0 ml acetone
Dd
1
3
2.93
2.96
0.48
2
3
3
2
2.46
2.42
2.67
2.91
2.86
2.95
2.93
2.95
0.50
0.53
2.95
4
5
2
2
2.43
2.47
2.77
2.66
2.89
2.86
2.93
2.91
2.94
2.94
0.51
0.47
host molecules indicated a one-electron transfer mechanism in CV
studies. The best way to understand this complex formation
theoretically is by means of analysing the interaction energy, its
correlation with the bond length and frequency shift with respect to
the localised C–HꢀꢀꢀO hydrogen bonding. In a usual hydrogen-
bond linear complex like X–HꢀꢀꢀY, the formation of the hydrogen
bond makes the covalent X–H bond weaker with a concomitant
decrease of the X–H stretch frequency (red shift).17 Our analysis
thus focuses on the study of the interaction energy, frequency
shift and C–HꢀꢀꢀO hydrogen-bond distances in complexes of
compounds 1–5 (Table 1) placed in the acetone environment.
The aim of the present work is to study the selectivity and stability
of the acetone guest molecule upon complex formation by under-
standing the role of the C–HꢀꢀꢀO hydrogen bond along with other
effects such as p-stacking and steric hindrance, etc. using experi-
mental (NMR) and theoretical results.
Scheme 1 Synthesis of ethylene and propylene linked pyridine and
phthalimide substituted pyrazole ester derivatives (1–5).
ester;19 the subsequent treatment of this halide with a corre-
sponding heterocyclic compound in the presence of a weak base,
potassium carbonate, in dimethyl formamide, resulted in the
formation of desired products. The compounds were characterised
by spectroscopic techniques (Scheme 1).
2. The preparation of compounds
Pyrazole ester derivatives, viz. 5-[3-(substituted)-propoxy]-3-methyl-
1-phenyl-1H-pyrazole-4-carboxylic acid methyl ester (1 & 2)
[Table 1], 5-[2-(substituted)-ethoxy]-3-methyl-1-phenyl-1H-pyrazole-
3. Computational methods
4-carboxylic acid methyl ester (3, 4 & 5) [Table 1], have been All the systems presented in Table 1 were optimised in vacuum
synthesised first by synthesising dithioacetals of 3-methyl-1- (see the ESI†) and in implicit acetone solvent (e = 20.7) using
phenyl-pyrazole-5-one18 by its treatment with carbon disulphide the DFT-D method (the B-LYP functional with the TZVPP basis
in the presence of a strong base followed by its treatment with an set20 and Grimme’s dispersion (D3) correction21 included). This
appropriate alkyl halide. Dithioacetals were then hydrolysed in the method is known to describe different types of noncovalent
presence of a strong base in a corresponding alcohol. Pyrazole interactions reasonably well.22
esters thus formed were treated with 1,n-dibromo alkanes (n = 2, 3)
To understand the nature of the acetone solvent interaction,
in excess to synthesise the polymethylene halide of pyrazole an acetone molecule was explicitly connected to the hydrogen
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The experiments were carried out in chloroform as solvent.
Although the interaction of chloroform with acetone is well
known,25 in the present studies the interaction of the 3-methyl
substituent with acetone is observed to be more favorable,
because the oxygen of the carbonyl group is not available to
interact with chloroform (calculated interaction energy between
acetone and chloroform is ꢁ0.11 kcal molꢁ1). The dielectric
constant of acetone (20.7) is more compared to chloroform
(4.8), which favours the interaction of acetone with the systems
under study. The other reason is also due to the presence of the
ester group at the carbon atom adjacent to that of the methyl
group in the pyrazole-ring system. The presence of the ester
group at position 4 of the pyrazole-ring system makes it easier
for the 3-methyl group to undergo no bond resonance and thus
to interact with acetone (Scheme 2).
Fig. 1 The chemical structure of the compounds with an acetone mole-
cule used for the theoretical study.
NMR studies26–28 are powerful enough to understand such
type of weak interactions. 1H NMR titration experiments of
compounds 1–5 (Table 1) with acetone yielded a dramatic change
in the chemical shift of 3-methyl protons in all the compounds.
During the NMR titration experiments of compound 1, when
acetone was added, the chemical shift of the 3-methyl proton was
altered while the rest of the peaks remained unaffected (Table 1),
indicating the intermolecular interaction of 3-methyl proton and
acetone. With increasing concentration of acetone, a change in
the chemical shift of the 3-methyl proton of the pyrazole nucleus
was observed without any significant change in the chemical shift
of any other proton. This change in the chemical shift of the
3-methyl proton in compound 1 was observed to be 0.48 ppm,
which is significant in the identification of any kind of weak
interactions between the molecules. Titrations were carried out
by increasing the amount of acetone in the solution of compound
1 in CDCl3. Upon addition of 0.2 ml of acetone, there was no
change in the chemical shift of the respective proton, whereas
upon further addition of 0.2 ml of acetone (up to a total acetone
volume of 0.4 ml), the change in the chemical shift of the
3-methyl proton was observed to be 0.14 ppm; addition of
another 0.2 ml of acetone (a total acetone volume of 0.6 ml) led
to a change in the chemical shift of 0.33 ppm as compared to the
chemical shift of the 3-methyl proton of compound 1, and this
increase in the chemical shift change was further continued with
the addition of another 0.2 ml of acetone (a total acetone volume
of 0.8 ml), where it was 0.45 ppm. The total addition of 1 ml of
acetone in the titration mixture increased the chemical shift of
the 3-methyl proton of compound 1 by 0.48 ppm. Further
addition of acetone to the titration mixture did not cause any
significant change in the chemical shift of the respective protons.
In order to understand the strength of such interactions, these
experiments were carried out on some other similar molecules
and yielded parallel results. The 1H NMR titration of compound 2
with acetone also showed a reasonable amount of change in the
chemical shift for the 3-methyl proton of the pyrazole nucleus.
The gradual increase in the concentration of acetone in the
titration mixture resulted in changes in the chemical shift of
the 3-methyl proton: 0.21 ppm (0.4 ml of acetone), 0.40 ppm
(0.6 ml of acetone), 0.47 ppm (0.8 ml of acetone) and 0.50 ppm
(1 ml of acetone). No further alteration of the chemical shift was
atom of the methyl group via a weak C–Hꢀ ꢀ ꢀO hydrogen bond
(Fig. 1) and the structure of the 1 :1 complex was fully optimised
again in vacuum (see the ESI†) and implicit acetone solvent. Basis
set superposition errors (BSSEs) were corrected using counter-
poise corrections.23 All these calculations were performed using
the Turbomole Version 6.3 software package.24
4. Results and discussion
4. a. NMR titration studies
In the solid state, 5-[3-(1,3-dioxo-1,3-dihydro-isoindol-2-yl)-propoxy]-
3-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid methyl ester has
two conformations, but in solution only one conformation is
observed. This is confirmed through low-temperature NMR in
acetone, and a shift in the proton of the methyl group at position
3 is observed to be 1.6 ppm (Fig. 2).
Fig. 2 The results of the low-temperature NMR experiments in acetone
solvent.
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Scheme 2 One electron transfer reaction in cyclic voltammetric studies.
observed with another increase in the amount of acetone. Similar experiments clearly indicate the formation of hydrogen-bonded
results were observed with the NMR titration of the rest of complexes of compounds 1–5 with acetone via the hydrogen
the pyrazole-based compounds with acetone. In the case of atoms of the 3-methyl substituent.
compound 3, the addition of even 0.4 ml of acetone changed
4. b. Cyclic voltammetric study
the chemical shift of the 3-methyl proton of the pyrazole nucleus
by 0.49 ppm, and upon further addition of acetone, there was a
moderate change in the value of the chemical shift of the
respective proton. After the addition of 1 ml of acetone in the
titration mixture, the total change in the value of the chemical
shift was 0.53 ppm. In the case of compounds 4 and 5, the
changes in the value of the chemical shift with the addition of
acetone were 0.34 ppm, 0.19 ppm (0.4 ml of acetone), 0.46 ppm,
0.39 ppm (0.6 ml acetone), 0.50 ppm, 0.47 ppm (0.8 ml acetone)
and 0.51 ppm, 0.47 ppm (1 ml acetone), respectively. The titration
The electrochemical properties of compounds were investigated
by cyclic voltammetry (CV)29,30 in dichloromethane solution at a
scan rate of 100 mV sꢁ1 for understanding the intermolecular
C–Hꢀ ꢀ ꢀO hydrogen bonding between experimental molecules
and acetone. The experiments were performed in a cell having
platinum working (disk)/counter (foil) electrodes and an Ag/Ag+
reference electrode. The compounds exhibit single irreversible
reduction wave in a potential range of EPC = ꢁ1.68 to ꢁ1.75 V
(Fig. 3).
Fig. 3 Cyclic voltammograms of compounds upon titration with different concentrations of acetone indicate bonding of the acetone molecule with the
compound via a C–Hꢀ ꢀ ꢀO weak bond.
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Fig. 4 Plot of A2 vs. 1 ꢁ A2/[Comp (1–5)] for l : l complex. A = Ip/l0p.
The change in peak currents and shift in peak potentials as coefficients of the complexed and free compounds 1–5 respec-
the concentration of acetone increases can be used to evaluate tively, aE and fE are the half-wave potentials of compounds
1/2
1/2
the formation constants if the electrode reaction is reversible and 1–5 in the presence and absence of acetone.
the formation and dissociation of the complex are fast enough to
The increase in the peak currents at lower concentrations of
maintain equilibrium.31,32 The analysis of the cyclic voltammo- acetone was used to calculate Kox and the decrease in the peak
grams obtained in the presence and absence of acetone clearly currents observed at higher concentrations of acetone was used
0
show that the above two requirements are met by the acetone– to calculate Kox (eqn (1)). The shift in the peak potentials at
pyrazole ester system. The formation constants for the reduced lower concentrations of acetone was used to calculate Kred by
and oxidized pyrazole ester derivative were calculated using the employing eqn (1). Since there is no observable change in the
method reported by Osa and coworkers.31,33 The formation peak potential Ep at higher concentrations of acetone ([acetone]/
constant Kox was calculated using eqn (1) (Fig. 4) and Kred was [pyrazole ester derivative] 4 2.0 + 0.3) along with the decrease in
0
calculated by substituting Kox in eqn (2).
peak currents, we are unable to calculate Kred
.
This indicates an electrochemical change coupled with
chemical reaction in the experimental molecules with addition
of acetone. Positive shifting of the potentials is suggestive of
associated electron withdrawing effects. Reduction wave is indi-
cative of the gain of an electron by the system, which is possible
À
Á
1 ꢁ A2
1
Dc
A2 ¼
þ
(1)
(2)
Kox ½Compð1 ꢁ 5Þꢂ Df
(aE ꢁ fE ) = 0.0295 log(Kox/Kred
where A = Ip/I0p; Ip and I0p are the peak currents in the presence according to Scheme S2, ESI.† In this scheme, an electron is
)
1/2
1/2
and absence of acetone, and Dc and Df are the diffusion transferred to carbonyl oxygen leading to the formation of a
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4. c. Computational study
It is evident from the results of the NMR experiment that the
acetone molecule associates with the 3-methyl proton of the
pyrazole nucleus. Therefore, we placed the acetone molecule
hydrogen-bonded to that methyl proton of pyrazole. The other
structures of the complex were also investigated but they were
found to be considerably less stable (Fig. 5). The optimised
structures of all of the complexes are shown in Fig. 1 and
selected geometrical parameters are collected in Table 2.
Inspecting the resulting structures, we found that the initial
planar H-bonded structures passed upon optimisation in vacuum
as well as in acetone to stacked structures, but the hydrogen bonds
(from methyl) C–HꢀꢀꢀO (from acetone) remained intact. The
resulting stabilisation energies as well as the shift in the C–H
stretching frequencies are summarised in Table 2 and the corre-
sponding correlation between the stabilisation energies and
C–HꢀꢀꢀO are visualised in Fig. 6 and 7. The formation of
hydrogen-bonded complexes is accompanied by red as well as
blue shifts of C–H stretching frequencies. Table 2 also shows the
stabilisation energies of all the complexes investigated in acetone
(continuous solvent) as well as in vacuum (also Table S1 in the
ESI†). The stabilisation energies in acetone are reduced on average
by 20%. The same table shows the changes in C–H bond lengths
upon complex formation; evidently, these changes are rather small
and do not correlate with the strength of the H-bond. This is
probably caused by the fact that the total stabilisation energy is
determined not only by H-bonding but also by C–Hꢀꢀꢀp interaction
(Fig. 1) where the dispersion energy shown in Table 3 plays an
important role. It should also be noted that BSSE corrections were
important not only in vacuum but also in an acetone environment.
We have calculated the NBO charges as well as charge transfer for
Fig. 5 Interaction energies at various possible binding sites of the
compound 5 with the acetone molecule.
radical anion. The carbon radical undergoes resonance with the
p electrons of the pyrazole system and the anion radical is now
created at position 5. The nitrogen atom at position 2 of the
pyrazole ring system now transfers one of its electrons to carbon-
5 of the pyrazole ring and forms a cross-linked bond. Thus a
radical is created at position 3 and now hydrogen atoms of the
methyl group system at position 3 do not undergo any bond
resonance to interact with the oxygen atom of the carbonyl group
of acetone.
Table 2 DC–H distance and stretching frequencies (n C–H) vs. interaction energy (DE) (in acetone solvent)
Compound no.
DC–H1 (Å)
DC–H2 (Å)
DC–H3 (Å)
nC–H (cmꢁ1
)
DE (kcal molꢁ1
)
1
2
3
4
5
0.000 (0.001)
ꢁ0.002 (ꢁ0.001)
0.002 (ꢁ0.001)
0.000 (0)
ꢁ0.001 (ꢁ0.001)
0.000 (0.001)
0.000 (0.001)
0.000 (0)
0.000 (0.001)
0.000 (0)
+14a (+19a)
ꢁ5.1790 (ꢁ7.864)
ꢁ4.0780 (ꢁ4.5058)
ꢁ6.4844 (ꢁ6.7296)
ꢁ4.6985 (ꢁ6.760)
ꢁ6.5568 (ꢁ8.3693)
+15a, ꢁ13a (+8s)
ꢁ26a (ꢁ15a)
ꢁ0.001 (ꢁ0.001)
ꢁ0.001 (0)
+5a (ꢁ4a, +18a)
+6a, ꢁ27s (ꢁ21a)
0.003 (0.003)
0.001 (0.000)
0.001 (0)
s = symmetry; a = asymmetry; ‘+’ = blue shift; and ‘ꢁ’ = red shift. Values mentioned within parentheses are obtained from the calculations
performed in vacuum. DC–H1 corresponds to the hydrogen bonded methyl C–H to the carbonyl group of acetone.
Fig. 6 The interaction energy vs. C–Hꢀ ꢀ ꢀO distances in acetone solvent.
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Fig. 7 The interaction energy vs. C–H stretching frequency shift in
acetone solvent.
C–H and CQO groups (see Table 3). We believe that this informa-
tion supports our arguments.
Fig. 8 Potential energy surface scan of the –CH3 rotation. The calcula-
tions were carried out using system 4. The minima at 01 and 1201
correspond to the structures where the hydrogens of the methyl group
make simultaneous interactions, intermolecularly with OQCo of acetone
and intramolecularly with the ‘N’ atom.
A detailed analysis of the optimised structure provides more
information on the stability of the complex. In all of the
compounds, the free rotation of the methyl group is prevented
by intermolecular hydrogen bonding from the nitrogen atom at
position 2 of the pyrazole ring and also by the oxygen atom of
the acetate group. We proved this by analysing the potential
energy surface (PES) scan of the torsion angle involving methyl
C–H as shown in Fig. 8. From this we note that the minima at 01
and 1201 correspond to the structures where the hydrogens of
the methyl group simultaneously interact with OQC of acetone
(intermolecularly) and with the nitrogen atom of the system
(intramolecularly).
The added acetone molecule stabilises the complex not only
by its hydrogen bonding with the methyl group but also by the
C–Hꢀ ꢀ ꢀp interaction with the pyrazole ring; apart from that, the
phenyl and methoxy groups of the pyrazole are vital to this
stabilisation effect. Therefore, the hydrogen-bond angle C–Hꢀ ꢀ ꢀO
is not linear and varies from 110 to 150 degrees in different
complexes.
In compound 3, the phenyl group of the pyrazole group
shows intermolecular C–Hꢀ ꢀ ꢀp interaction with the ‘Z’-group
(Table 1) and the methyl group of acetate also forms hydrogen
bonds with acetone; in the case of compound 1, however, due
to the increment of the ether chain by a methylene group, the
intermolecular C–Hꢀ ꢀ ꢀp interaction of the phenyl group is
absent. Apart from that, we also noted that unlike in compound
3, the freely rotatable methoxy group is away from the acetone
molecule. In the case of compound 4, the acetone molecule has
bond with CQO of the ‘Z’ substituent group. In compound 2,
there is an increment in the ether chain; nevertheless, the
methyl group of the ‘Z’ substituent still exhibits a C–Hꢀ ꢀ ꢀp
interaction with the phenyl group. After complexation with
acetone, the pyrazole and phenyl rings are not in the same
plane (please check the RMSD value from Table 3).
In compound 5, the acetone molecule is more stabilised by
the methoxy and phenyl groups of the pyrazole, and the phenyl
group is not involved in intermolecular C–Hꢀ ꢀ ꢀp interaction or
in the hydrogen bond effect with the ‘Z’ substituent. These
effects are reflected in the results, namely in the increase in
interaction energy (Fig. 6 and 7) and the C–H bond distance
(Table 2) and in the decrease in the hydrogen-bond (C–Hꢀ ꢀ ꢀO)
distance. It is also evident from the RMSD value (Table 3) that
the planarity between the pyrazole and phenyl is perturbed in
the presence of the acetone molecule.
The optimised structures of all the complexes in vacuum
were structurally similar to the corresponding ones in the
implicit acetone solvent. However, as shown in the ESI†
(Fig. S1 and S2 and Table S1), there is a slight variation in
frequencies (red and blue shifts) and stabilisation energies.
5. Concluding remarks
a C–Hꢀ ꢀ ꢀp interaction effect both with the pyrazole and phenyl The hydrogen atoms of the 3-methyl group in 3-methyl-1-
groups. The C–H of the phenyl group forms a week hydrogen phenyl-pyrazole-5-one were found to interact with the carbonyl
Table 3 RMSD value of compounds 1–5
Compound no.
Hꢀ ꢀ ꢀO distance in (Å)
Dispersion E (kcal molꢁ1
)
BSSE (kcal molꢁ1
)
Dq(C–H) (e)
Dq(OQC) (e)
RMSD (Å)
1
2
3
4
5
2.5 (2.6)
2.8 (2.9)
2.6 (2.5)
2.5 (2.6)
2.7 (2.7)
ꢁ8.6 (ꢁ9.2)
ꢁ6.2 (ꢁ6.1)
ꢁ10.0 (ꢁ9.7)
ꢁ7.5 (ꢁ8.4)
ꢁ9.6 (ꢁ9.8)
ꢁ0.8 (0.7)
ꢁ0.9 (0.3)
ꢁ1.8 (0.6)
ꢁ1.1 (0.4)
ꢁ1.6 (0.5)
ꢁ0.0038
ꢁ0.0035
ꢁ0.0029
ꢁ0.0010
ꢁ0.0110
+0.0044
+0.0011
+0.0036
+0.0036
+0.0042
0.26
0.19
0.27
0.34
0.36
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1H NMR titration and cyclic voltammetric studies. A low-
temperature NMR of the compounds investigated was performed
in acetone, with the chemical shift of the 3-methyl proton being
about 1.6 ppm. These observations have led to the conclusion
that the 3-methyl protons interact with acetone and form a weak
C–Hꢀ ꢀ ꢀO bond. The structures of the complexes studied were
determined theoretically, and besides the formation of weak
C–Hꢀ ꢀ ꢀO hydrogen bonds, an important role of C–Hꢀ ꢀ ꢀp inter-
action was shown. Thus, this weak non-covalent C–Hꢀ ꢀ ꢀO inter-
action between methyl pyrazole ester and acetone helps to
understand the role of polar aprotic solvent in bimolecular
nucleophilic substitution reactions. Such types of studies can
be used as an important tool for the study of the mechanism of
such non-covalent interactions in the near future.
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Acknowledgements
We acknowledge UGC India Grant F.37-54/2009 (SR) for its
financial support of this work. The Department of Chemistry
of Banaras Hindu University is acknowledged for departmental
facilities. For the calculation of the association constant, authors
are highly thankful to Dr Vellaichamy Ganesan. One of the author
Ramachandran Gnanasekaran, also want to acknowledge the
Institute of Organic Chemistry and Biochemistry, Academy of
Science of the Czech Republic (research project RVO 61388963)
and the support from Czech Science Foundation [P208/12/G016].
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