Solvatochromism of heteroaromatic compounds: VI. Comparison of the empirical and theoretical approaches to description of solvatochromism in nonspecific solvation of nitropyrazoles

Article abstract of DOI:10.1023/A:1012358212623

The effect of nonspecific solvation on the long-wave absorption band in the UV spectra of isomeric nitropyrazoles was studied. The aprotic inert and aprotic protophilic solvents exert different spectroscopic effects. In the former solvents, the solvatochr

Full text of DOI:10.1023/A:1012358212623

Russian Journal of General Chemistry, Vol. 71, No. 1, 2001, pp. 137 142. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 1, 2001,
pp. 150 156.
Original Russian Text Copyright
2001 by Vokin, Shulunova, Aksamentova, Es’kova, Elokhina, Lopyrev, Turchaninov.
Solvatochromism of Heteroaromatic Compounds:
VI.1 Comparison of the Empirical and Theoretical Approaches
to Description of Solvatochromism in Nonspecific Solvation
of Nitropyrazoles
A. I. Vokin, A. M. Shulunova, T. N. Aksamentova, L. A. Es’kova,
V. N. Elokhina, V. A. Lopyrev, and V. K. Turchaninov
Favorskii Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received August 16, 1999
Abstract The effect of nonspecific solvation on the long-wave absorption band in the UV spectra of
isomeric nitropyrazoles was studied. The aprotic inert and aprotic protophilic solvents exert different spec-
troscopic effects. In the former solvents, the solvatochromism is mainly governed by orientation interactions,
and in the latter solvents, along with the orientation interactions, the UV spectrum is appreciably influenced
by the induction, polarization, and dispersion interactions, which is due to the presence of readily polarizable
nonbonding electron pairs and/or a -electron systen in molecules of such solvents.
*
The principal origin of solvatochromism was eluci-
dated long ago. However, the views on its specific
features are still very approximate, and the existing
theories are insufficiently complete to describe with
fair accuracy the actual solvent effects. Therefore,
empirical approaches are widely used. In particular,
relatively recently Kamlet, Taft, et al. developed
a method of solvatochromic comparison. Using this
method, they obtained a set of empirical parameters
of solvents taking into account both specific and non-
specific interactions [2, 3]. These parameters were
successfully used to describe regular trends in many
chemical phenomena [3]. In our previous studies
[1, 4 7], we used these parameters when studying
the effect of solvation on the UV spectra of neutral
molecules and anions of a number of nitrogen-contain-
ing five-membered heterocycles. These studies fur-
nished interesting information on the rotation isomer-
ism of 1-ethyl-2-formylimidazole [1], tautomerism of
3(5)-nitropyrazole [6], and structure of solvation com-
plexes of NH-containing pyrazoles and pyrroles [4 7].
The fruitfulness of using the Kamlet Taft solvato-
chromism parameters calls for detailed elucidation of
their physical sense.
=
+ c
0
.
(1)
max
In this equation,
is a constant characterizing
the spectrum of a mo0lecule in a fully inert solvent
*
*
(
= 0); c = A *m, where
is the polarity parameter
m
of the compound under consideration [3]. Introduction
of this parameter into the solvatochromism equation
is based on the general knowledge; it corresponds to
the ability of a solvent to enter electrostatic and/or
electrodynamic interactions with the solvation shell
molecules. Such interactions can be described theo-
retically using the functions of universal interactions
(see, e.g., [9]):
=
+ c1 f1 + pf2,
gas
(2)
max
where f1 = [(2n2 + 1)/(n2 + 2)]{[(
1)/( + 2)]
[(n2 1)/(n2 + 2)]} and f2 = (n2 1)/(n2 + 2). The pa-
rameters of this equation have a clear physical sense:
=
max in the gas phase, c1 = 2 (
ecos )/a3,
gas
where
g
g
is the dipole moment in the ground (g)
g, e
and excited (e) states; is the angle between the vec-
tors of the moments; a is the Onsager radius; the pa-
rameter p is related to the induction, dispersion,
and polarization interactions [9]. Equation (2) includes
the function of the dipole moment of the solute in
the ground state as the factor influencing its spectro-
scopic features in a wide range of solvents of different
nature. In this work, using isomeric nitropyrazoles
I V as example, we studied empirically the correla-
In terms of the Kamlet Taft approach, in the ab-
sence of specific solvation the position of the UV
absorption maximum ( max) of a compound depends
*
only on the polarity parameter
of the medium [8]:
1
For communication V, see [1].
1070-3632/01/7101-0137$25.00 2001 MAIK Nauka/Interperiodica

138
VOKIN et al.
Table 1. Dipole moments of isomeric nitropyrazoles in various media
Aprotic inert media, 25, D
Aprotic protophilic media, 25, D
Comp.
no.
Free molecule,
(6 31G*), D
hexane
CCl4
xylene
benzene
dioxane
I
3.71
3.64
5.75
4.35
4.53
4.07
3.64
3.63
6.26b
4.61
4.63b
4.04
3.60
6.20b
4.83
4.89b
4.19
4.78
7.26
5.69
5.47
4.77
a
a
II
III
IV
V
a
4.62
4.62
4.65
4.16
a
b
The attainable concentration is insufficient for measuring the dipole moment. Data of T.N. Aksamentova, I.G. Krivoruchek,
V.N. Elokhina, A.I. Vokin, V.A. Lopyrev, and V.K. Turchaninov will be published.
Table 2. Solvatochromism of the long-wave absorption band in the UV spectra of isomeric nitropyrazoles I V in aprotic
inert solvents
1
Solvent parametersa
max, cm
Solvent
*
f1
f2
I
II
III
IV
V
Hexane
Pentane
Heptane
Cyclohexane
Octane
0.08
0.08
0.02
0.00
0.01
0.03
0.28
0.39
0.58
0.62
0.81
0.82
0.95
0.00
0.00
0.00
0.00
0.01
0.00
0.02
0.23
0.22
0.24
0.26
0.24
0.25
0.28
38200
38250
38150
38100
38100
38100
37900
37700
37500
37500
37300
37300
37250
39700
39700
39500
39550
39600
37800
37800
37700
37650
37700
37650
37100
36850
36500
36550
36100
36100
36000
36500
36400
36400
36200
36300
36300
35900
35600
35300
35300
35000
35000
34900
34650
34700
34550
34500
34600
34500
34000
33900
33500
33500
33350
33350
33250
Decane
Tetrachloromethane
1-Chlorobutane
Chloroform
Pentachloroethane
1,2-Dichloroethane
Dichloromethane
1,1,2,2-Tetrachloroethane
38800
38350
0.37
0.25
0.63
0.60
0.53
0.27
0.30
0.27
0.26
0.29
37800
37350
37400
37050
a
2
2
2
2
*
Here and in Tables 3 and 4,
was taken from [2, 3], f = [(2n + 1)/(n + 2)]{[(
1)/( + 2)]
[(n
1)/(n + 2)]},
1
2
2
f
= (n
1)/(n + 2),
and n are taken from [10 13].
2
*
tion between the regression coefficient c in Eq. (1),
the quantity g, and the parameters c1 and p. As non-
specifically solvating media we chose aprotic inert
and aprotic protophilic solvents uncapable of hydro-
gen bonding with pyrazoles I V.
(
< 0.6) having no own dipole moment (Table 1).
The results allow the solutes to be subdivided in
two groups. In the first group (compounds II IV,
Table 1)
inert to aprotic protophilic solvents; this trend be-
comes more pronounced as the dipole moment of
the solute increases. In the second group (pyrazoles
somewhat increases in going from aprotic
g
R3
R4
R2
I and V, Table 1)
is independent of the solvent.
g
N
Among the nitropyrazoles studied, 1-nitropyrazole has
the lowest dipole moment, and 1-methyl-3-nitropyr-
azole, the highest dipole moment, which is consistent
with the results of ab initio quantum-chemical calcu-
N
R1
R1 = NO2 (I), Me (II IV), CH=CH2 (V); R2 = H (I, III),
NO2 (II), Me (IV, V); R3 = H (I, II), NO2 (III V); R4 =
H (I III), Me (IV, V).
lations of
of the free molecules.
g
Solvatochromism of the long-wave absorption
Dipole moments of isomeric nitropyrazoles were
band in the UV spectra of isomeric nitropyrazoles.
measured in nonpolar and moderately polar solvents The position of this band was determined in 35 sol-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

SOLVATOCHROMISM OF HETEROAROMATIC COMPOUNDS: VI.
139
Table 3. Solvatochromism of the long-wave absorption band in the UV spectra of isomeric nitropyrazoles I V in aprotic
protophilic solvents
1
Solvent parameters
max, cm
Solvent
*
f1
f2
I
II
III
IV
V
Triethylamine
0.14
0.24
0.27
0.55
0.55
0.58
0.70
0.75
0.83
0.83
0.88
0.92
1.00
0.07
0.21
0.38
0.24
0.24
0.22
37900
37850
37800
37450
37500
37400
37350
37100
37000
36800
36200
36300
36200
36000
35900
35700
35650
35600
35500
35250
36100
35900
35800
35300
35400
35300
34300
34250
34200
33650
Dibutyl ether
Diethyl ether
Dioxane
Ethyl acetate
THF
Ethyl chloroacetate
Acetonitrile
Propylene carbonate
Tetramethylurea
DMF
38650
38500
37750
37800
37700
37150
37100
36850
37050
36800
36700
36500
0.49
0.55
0.65
0.86
0.88
0.79
0.84
0.81
0.84
0.23
0.25
0.25
0.21
0.25
0.27
0.26
0.28
0.28
33500
34900
34650
34700
34500
34500
34200
33400
33250
33250
33150
33100
32950
37300
37250
37200
37150
36950
1-Methyl-2-pyrrolidone
DMSO
vents. Data on
in aprotic inert and aprotic proto-
the dipole moment in the ground state and the regres-
sion coefficient c. This hypothesis is based on the fact
philic solvents armealxisted in Tables 2 and 3, respective-
ly. Supplementary data are given in Table 4. Because
N-substituted nitropyrazoles cannot form hydrogen-
bonded complexes with aprotic protophilic solvents,
we described the solvatochromism quantitatively in
terms of the nonspecific solvation model [Eq. (1)].
The coefficients of Eq. (1) are listed in Table 5 which
shows that only for 1-nitropyrazole and 1-vinyl-3,5-
dimethyl-4-nitropyrazole the solvatochromism can be
described by a single equation valid for all the aprotic
media. For compounds II IV, equations for the inert
and protophilic solvents have different regression
coefficients. To explain this fact, we considered three
hypotheses.
that
of pyrazoles I and V is independent of the sol-
g
vent, whereas for II IV such a dependence was ob-
served experimentally (Table 1). As already noted,
the solvatochromism of the first two compounds
can be described by a common equation, whereas in
the case of II IV there are two specific cases (Ta-
ble 5). Indeed, c does depend on g, showing a non-
linear correlation (Fig. 1). However, the correlation
includes both inert and protophilic solvents. Thus,
the second hypothesis does not explain the specific
Table 4. Supplementary data on solvatochromism of
nitropyrazoles I, IV, and V
The first hypothesis is based on considering the long-
wave absorption band of isomeric nitropyrazoles, ac-
cording to [14], as a sum of two (strong and weak)
components. Both components are due to transitions
with intramolecular charge transfer [14]. Therefore,
the effect of the medium on the position of the overall
maximum can be partially determined by the weak
component. However, in this case extrapolation to
the gas phase ( gas, Table 5) by Eq. (1) for all the five
compounds should give different results depending
on the solvent type (inert or protophilic). Actually,
Solvent parameters
,
1
Comp.
no.
max
cm
Solvent
*
f1
f2
I
Dodecane
Diisopropyl
ether
0.08 0.00 0.25 38050
0.27 0.33 0.23 37900
1-Bromoheptane 0.48 0.62 0.26 37550
Sulfolane
Diisopropyl
ether
1,2-Dimethoxy-
ethane
Toluene
Benzene
Acetone
0.98 0.83 0.28 37100
0.27 0.33 0.23 35750
IV
0.53 0.49 0.23 35150
the difference in the estimated
values exceeds
gas
1
the experimental error (50 cm ) only for 1,3,5-tri-
methyl-4-nitropyrazole. That is, only in IV the com-
plex nature of the long-wave absorption band plays
a certain role.
0.54 0.03 0.29 35150
0.59 0.01 0.30 35100
0.71 0.79 0.22 35000
0.98 0.83 0.28 34500
0.43 0.01 0.29 33700
0.59 0.01 0.30 33550
Sulfolane
p-Xylene
Benzene
V
The second hypothesis suggests a direct correlation,
different for inert and protophilic solvents, between
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

140
VOKIN et al.
Table 5. Quantitative characteristics of solvatochromism of nitropyrazoles I V
25
a
,
,
c,
cm
,
c1,
cm
p,
cm
nb
0
cm
gas
cm
Solvent
r
s
n
R
s
1
1
1
1
1
D
I
Inert
Protophilic 3.63f 38080 40 1040 50 0.984 60 14 39200
3.64c 38120 20 1000 30 0.994 40 15 39200 1210 50
830 100
4530 170 0.994 40 14d,e
5090 460 0.977 90 14d
Inert and
38110 20 1050 30 0.990 60 29
protophilic
II Inert
Protophilic 6.26f 39300 70 2850 100 0.994 80 12 42400 2610 280 12900 107 0.987 180 12d
5.75c 39510 30 2640 60 0.998 70 11 42400 3170 220 12200 700 0.988 170 11d
III Inert
Protophilic 4.62f 37430 40 2100 50 0.997 50 13 39700 1810 120
4.35c 37660 20 1870 40 0.997 60 13 39700 2290 120
8600 400 0.991 100 13d
9900 500 0.992 100 13d
IV Inert
4.53c 36310 30 1640 50 0.995 70 13g 38100 1910 100
7600 300 0.992 80 12d,e
9400 700 0.954 190 17d
Protophilic 4.63f 36340 50 1970 80 0.987 90 17 38100h 1320 150
V
Inert
Protophilic 4.04f 34530 50 1580 80 0.987 80 13 36200 1080 120
4.07c 34520 30 1490 70 0.989 90 13 36200 1730 140
7300 500 0.979 120 13d
8400 600 0.972 150 13d
Inert and
34520 30 1540 50 0.990 80 26
protophilic
a
b
*
(
)
for the gas phase, estimated at
=
1.1 by Eq. (1).
Taking into account the estimated
for the gas
gas
max
max
c
d
e
f
g
phase.
cyclohexane.
In tetrachloromethane.
Without 1-chlorobutane and dioxane. Without 1-bromoheptane. In benzene.
Without
h
The value was obtained for aprotic inert media.
features of solvatochromism of I V either. Further-
more, within a narrow group of protophilic solvents
(xylene, benzene, dioxane) the measured dipole mo-
ment of II IV is not always constant (Table 1). Hence
follows that the regression coefficient c depends on
a certain function which remains approximately con-
stant within a set of related solvents. According to
the theory, such a function can be the difference be-
tween the vectors of the dipole moments of the mol-
ecule in the ground and electronically excited states
[Eq. (2)] this is our third hypothesis.
To describe the solvatochromism of isomeric nitro-
pyrazoles with functions of universal interactions, we
used the generalized solvent parameters and n from
[10 13]. The values of f1 and f2 are given in Ta-
bles 2 4, and the parameters of Eq. (2), in Table 5.
The data thus obtained allow us to examine the cor-
relation between c and c1 for each type of solvents
separately. The correlation is shown in Fig. 2. In inert
solvents c linearly correlates with c1 (Fig. 2, curve 1),
with the straight line passing close to the origin.
Hence, in inert media variation of c is governed by
orientation interactions. In going from inert to proto-
philic solvents, the correlation between c and c1 be-
comes nonlinear (Fig. 2, curve 2), though c still tends
to increase with increasing c1. As seen from Table 5
(parameter p), in protophilic solvents a stronger effect
on the spectrum is exerted by induction, polarization,
and dispersion interactions, which is due to the pres-
ence in such solvents of readily polarizable nonbond-
Fig. 1. Correlation between the regression coefficient
c in Eq. (1) and the dipole moment of nitropyrazoles in
the ground state : (squares) data for inert media (CCl
g
4
as example); (circles) data for protophilic media (ben-
zene as example).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

SOLVATOCHROMISM OF HETEROAROMATIC COMPOUNDS: VI.
ing electron pairs and/or -electron system. Thus,
141
the third hypothesis also fails to describe the whole
set of the experimental results.
It should be emphasized that in all cases, when us-
ing the functions of universal interactions, we had to
exclude dioxane and 1-chlorobutane from the sample
of solvents. The specific feature of dioxane may be
due to the fact that the microscopic characteristics
of the solvation shell formed by its molecules differ
from the macroscopic characteristics of the solvent in
the bulk. Within the framework of the Kamlet Taft
approach, these differences are taken into account au-
tomatically and are reflected in *. At the same time,
the solvatochromism theory based on the Onsager
Batcher model fails to give the required specification.
As for 1-chlorobutane, one of generalized parameters
of this compound may have been determined insuffi-
ciently accurately. When describing the solvatochrom-
ism of 1,3,5-trimethyl-4-nitropyrazole, we excluded
Fig. 2. Correlation between the regression coefficient c
in Eq. (1) and parameter c in Eq. (2): (1) inert and (2)
1
protophilic solvents.
spectrophotometer at 22 25 C. The dielectric permit-
tivities of the solutions were measured with an Sh2-5
device (OKBA Joint-Stock Company, Angarsk) at
1 MHz. The dipole moments were calculated by
the Higasi formula [15]. The substances were purified
by recrystallization from organic solvents followed by
double vacuum sublimation. Ab initio calculations of
the dipole moments of nitropyrazoles in the 6 31G*
basis were performed using the Gaussian 94 program
package [16].
from consideration the values of
corresponding
max
to cyclohexane and 1-bromoheptane (Table 5), which
is due to the fact that the long-wave band consists
of components and the experimental error is therefore
large. For example, instead of the
value of
max
1
36200 cm , reported for the solution in cyclohex-
ane in [6], after repeated measurements we obtained
1
the average value of 36300 cm , which fits the sug-
1-Nitropyrazole (I) was prepared by nitration of
pyrazole by the procedure described in [17]. Pyrazoles
IV and V were prepared according to [18].
gested correlation much better. When using Eq. (2),
the values of
(required to extend the variation
gas
range of max) and f2 were calculated by Eq. (1).
1-Methyl-3-nitropyrazole (II). A 2-g portion of
3-nitropyrazole was dissolved in 30 ml of 1 N NaOH.
The solution was heated to 30 C, and 1.64 ml of di-
methyl sulfate was added dropwise. The mixture was
kept for 1 h at 30 C. After cooling, the precipitate was
filtered off, washed with cold water, and dried. Yield
of II 0.83 g (39%), mp 86 87 C (cf. 85 86 C [19]).
Thus, we failed to gain a deep insight into the ef-
fect of various types of solute solvent interactions on
the spectrum. This is due to the use of related objects
and to large errors arising from the use of the func-
tions of universal interactions. Nevertheless, we
should ascertain that the regression coefficient at
*
the solvatochromic parameter
is determined by
the whole set of electrostatic and electrodynamic inter-
actions between the solvent and solute (in the ground
and electronically excited states) molecules. This evi-
dently follows from the two-parameter equation
1-Methyl-4-nitropyrazole (III). A 5.65-g portion
of 4-nitropyrazole was dissolved in 60 ml of 1 N
NaOH. The solution was heated to 30 C, and 4.8 ml
of dimethyl sulfate was slowly added with stirring.
After keeping for 1 h at 30 C, the mixture was cooled,
and the precipitate was filtered off, washed with cold
water, dried at 85 C, and recrystallized from ethanol.
Yield of III 4.83 g (76%), mp 92 93% (cf. 91 92 C
[20]).
c = (96 78) + (0.11 0.05)c1 + (0.20 0.01)p;
R 0.987, s 70, n 10.
(3)
To the same c, there can correspond two pairs of
c1 and p (Table 5).
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001