Determination of the apparent molar refraction and partial molar volume at infinite dilution of thiophene-, pyrrole- and furan-2-carboxaldehyde phenylhydrazone derivatives in acetonitrile at 293.15 K

Article abstract of DOI:10.1007/s10953-006-9093-2

High-precision densitometry and high-accuracy refractometry measurements of extremely dilute solutions of the thiophene-2- (TCPH), pyrrole-2- (PCPH) and furan-2-carboxaldehyde-phenylhydrazone (FCPH) compounds in acetonitrile have been obtained at 293.15 K

Full text of DOI:10.1007/s10953-006-9093-2

J Solution Chem (2007) 36:1–11
DOI 10.1007/s10953-006-9093-2
ORIGINAL PAPER
Determination of the Apparent Molar Refraction and
Partial Molar Volume at Infinite Dilution of Thiophene-,
Pyrrole- and Furan-2-Carboxaldehyde Phenylhydrazone
Derivatives in Acetonitrile at 293.15 K
Ysa´ıas J. Alvarado · Jose´ Caldera-Luzardo ·
Gladys Ferrer-Amado · Victoria Mancilla-Labarca ·
Elba Michelena
Received: 22 March 2006 / Accepted: 14 June 2006 /
Published online: 7 December 2006
ꢀ Springer Science+Business Media, LLC 2006
C
Abstract High-precision densitometry and high-accuracy refractometry measurements of
extremely dilute solutions of the thiophene-2- (TCPH), pyrrole-2- (PCPH) and furan-2-
carboxaldehyde-phenylhydrazone (FCPH) compounds in acetonitrile have been obtained
at 293.15 K. The partial molar volumes, V2∞, of each compound at infinite dilution were
determined. The apparent molar refraction of these solutes at infinite dilution at 293.15
K has been experimentally determined within the Kohner-Geffcken-Grunwald-Haley ap-
proximation. The volumetric and refractometric results were interpreted in terms of the
Pauling electronegativity and intrinsic molar volume of the heteroatom, and the aromatic-
ity of the heterocyclic rings. The experimental results indicate that solute-solute inter-
actions are negligible within the concentration range studied. Theoretical calculations at
the DFT-B3LYP/6−311++G(3d,3p) level of molecular volumes support the interpretation
that the volumetric contribution from the solute-solvent interactions to the limiting par-
tial molar volumes of solutes are very small and thus solute molecules are isolated in this
medium.
Keywords Phenylhydrazone derivatives · Apparent molar refraction · Intermolecular
forces · Limiting partial molar volume · Pauling electronegativity
1 Introduction
The phenylhydrazone derivatives have been shown to have remarkable stability, high ten-
dency to form non-centrosymmetry crystal packing and exceptional electronic and chemical
properties needed for designing of new electronic devices with technological applications
[1]. Although these materials have analytical, inorganic, organic, biological and optical util-
ity [1–3] studies on their fundamental molecular properties are very limited. In fact, the
Y. J. Alvarado ( ) · J. Caldera-Luzardo ( ) · G. Ferrer-Amado · V. Mancilla-Labarca · E. Michelena
ꢀ
ꢀ
Laboratorio de Electro´nica Molecular (L.E.M), Departamento de Qu´ımica, Facultad Experimental de
Ciencias, La Universidad del Zulia (LUZ), Grano de Oro, Maracaibo, Estado Zulia, Repu´blica
Bolivariana de Venezuela
e-mails: yalvaradofec@yahoo.com; josecldr@yahoo.com
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J Solution Chem (2007) 36:1–11
Fig. 1
H
H
N
N
X
X= N-H (PCPH); O (FCPH) and S (TCPH)
molecular electronic polarizabilties of these molecular systems both in the gas phase and
solution are unknown, possibly due to their poor solubility in common organic solvents and
their low volatility [3]. Based on these considerations and continuing with our interest in
this field [4], we report in this work the experimental determination of the apparent molar
refractions and partial molar volumes of the thiophene, pyrrole and furan-2-carboxaldehyde
phenylhydrazone derivatives (henceforth referred as TCPH, PCPH and FCPH, respectively;
see Fig. 1) in dilute acetonitrile solutions using high-precision densitometry [4], high ac-
curacy refractometry [4] and a model proposed by Kohner-Geffcken-Grunwald-Haley [5].
It is very important to mention that this refractometric method is particularly useful for
compounds with only moderate solubility in a given solvent. The changes induced by the
heteroatom of an aromatic five-membered ring moiety on the apparent molar refraction
and limiting molar volume at infinite dilution are discussed. Acetonitrile was chosen as the
solvent in this study because the phenylhydrazone derivatives are insufficiently soluble in
common aprotic solvents and thus this solvent has chromatographic, industrial, biological
and chemical importance. In this work we also carry out a comparative study between the
partial molecular volumes and apparent molar polarizabilities at infinite dilution and the
molecular volumes and electronic molar polarizabilitiy values obtained from fully optimized
geometries by using theoretical calculations at the DFT-B3LYP/6−311++G(3,3p) level.
2 Theory
In a two-component system, the apparent molar volume of the solute, V2,φ, can be calculated
from the measured densities of the solutions, ρ, through the relationship [6a]:
ꢀ
ꢁ
ꢀ
ꢁ
M2
ρ1
ρ − ρ1
ρ1C
V2,φ
=
− 103
(1)
where M2 is the molar mass of the solute, ρ and ρ1 are the densities of the solution and the
solvent, respectively, and C is the molar concentration of the solute. However, for extremely
dilute solutions, the V2,φ values are independent of the concentration. Therefore, by definition,
it can be assumed that the V2,φ values are equal to the partial molar volume of the solute at
infinite dilution, V2∞, and, as a consequence, V2∞ can be estimated by the method of least
squares as described in reference [6b]:
ꢀ
ꢁ
V2∞
M2 − ρ1
ρ = ρ1 +
C
(2)
103
Alternatively, V2∞ can be calculated as the average value of V2,φ determined within the
studied concentration range [7]. On the other hand, under the same experimental conditions,
Kohner [5a], Geffcken [5b], and Grunwald and Haley [5c] have demonstrated that the
Lorentz-Lorenz equation for a binary mixture in differential form, retaining only first-order
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J Solution Chem (2007) 36:1–11
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terms of the Taylor series, reduces to
ꢀ
ꢁ
ꢂ
ꢀ
ꢁ
ꢃ
2
1
n21 − 1
n21 + 2
θ = 6000
n1(n − n1) = R2,φ
−
V2,φ C = ꢀ2C
(3)
n21 + 2
where θ is the relative refraction, R2,φ is the apparent molar refraction of the solute and n and
n1 are the refractive indexes of the solution and solvent, respectively. However, for highly
dilute solutions, ꢀ2 is effectively constant [5c] and its value can be determined by the method
of least squares. Therefore, R2,φ and V2,φ can be assumed to be equal to R2,0 (the apparent
dynamic molar refraction of the solute at infinite dilution) and V2∞, respectively. The R2,0
value corresponds only to the electronic part of this property because the contributions from
the infrared-active modes (vibrational polarizability) are not considered here [4, 8]. In fact,
there is sufficient experimental evidence that justifies this assumption, which indicates this
contribution is only between 5 and 10% of the electronic component [9]. As a consequence,
it is appropriate to calculate this term as R2ν,0 = 0.10R2,0
.
3 Experimental section
3.1 General considerations
The phenylhydrazone derivatives (TCPH, PCPH and FCPH) used in this work were prepared
by free solvent condensation assisted by microwave irradiation of phenylhydrazine with the
appropriate carboxaldehydes, following the methods developed by our group [10].
3.1.1 Experimental determination of the partial molar volumes at infinite dilution
For the determination of the volumetric quantities, the densities of solutions, ρ, as well
as the density of solvent, ρ1, were measured at 293.15 K using a variable-temperature
Anton Parr DMA-5000 densitometer that was calibrated before each series of measurements
with bi-distilled/degassed water and dry air, with their densities taken from the literature
[4, 11]. Additionally, benzene (>99.8%, Merck, spectroscopic grade) was used as a test
liquid and the density measured in this work (0.878914 g·cm−3) shows excellent concor-
dance with published data (0.879000, 0.878800 and 0.878660 g·cm−3) [12]. Each density
value was determined by measuring the period of oscillation of a vibrating sample cell with
a sample volume of 1 mL. The concentration range used was 1.09 × 10−3 to 67 × 10−3
mol·dm−3 in all cases. Although, the solvent acetonitrile is known to be a good coordi-
nating compound, a highly dipolar solvent, and weak hydrogen-bond acceptors and donors
[4, 13], this dilute range was chosen to avoid experimental problems (precipitation) associated
with the low solubility of these materials in this aprotic solvent. For acetonitrile (>99.9%,
Merck, spectroscopic grade), the determined value of density ρ1 (0.782011 g·cm−3) at 293.15
K in this work was found to agree with values previously reported in the literature (0.782000,
0.781983, 0.782100 g·cm−3) [4, 13f,g]. Differences of 5 × 10−6 g·cm−3 (TCPH), 8 × 10−6
g·cm−3 (FCPH) and 3 × 10−6 g·cm−3 (PCPH) were observed between the values of the
experimental densities, ρ, of the solutions compared with the values of density ρT calculated
as ρT = ρ0 + A(Q2 − Q02), where ρ0 is the acetonitrile density; Q and Q0 are the periods
of oscillation of solution and acetonitrile, respectively; and A is an empirical constant deter-
mined by measurements of the periods of oscillations of pure water and air. Each reported
value is an average of five measurements. The reproducibility of each measurement was
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within 3 × 10−6 g·cm−3. Apparent molar volumes, V2,φ, of each synthesized compound
were calculated from the measured densities of the dilute solutions using Eq. (1). The lim-
iting partial molar volumes, V2∞, of the solutes were determined from the following two
procedures: 1) as the average value of V2,φ determined within the investigated concentration
range, and 2) from Eq. (2) using the method of least squares. A comparative analysis between
these experimental results was also performed in this work. The partial molecular volumes
at infinite dilution for each solute were estimated by the equation v2∞ = V2∞/NA.
3.1.2 Experimental determination of the apparent molar refraction at infinite dilution
Refractive indexes of both the solvent and solutions were measured with a Bellingham
and Stanley High Accuracy ABBE Refractometer 60/LR with a CCD camera at 293.15 K.
Because the lowest absorption of each compound occurs at about 470 nm, a sodium (λ =
589.3 nm, line D) lamp was employed in our experiments. The concentration range used for
these experiments was 1.09 × 10−3 to 67 × 10−3 mol·dm−3 in all cases. For acetonitrile, the
value of n1 (1.343695) agreed with values previously reported in the literature (1.343688) [4].
ꢄ
ꢅ
n21−1
Therefore, the value used here for the
term in the Eq. (3) was 0.211671. The procedure
n21+2
followed allowed us to determine that the reproducibility of the experimental refraction
index values is better than 1.5 × 10−5. The determination of the dynamic apparent molar
refraction, R2,0, of each solute at infinite dilution in acetonitrile was carried out within the
Kohner-Geffcken-Grunwald-Haley approximation, Eq. (3) [5]. The corresponding electronic
and vibrational components of the dynamic apparent molar mean polarizability of each solute
were derived from R2,0 by means of Eqs. (4) and (5), respectively [14],
ꢀ
ꢀ
ꢁ
ꢁ
3
α¯me = NAα¯e =
R2,0
(4)
(5)
4π
3
α¯mν = NAα¯ν
=
(0.1)R2,0
4π
where NA is Avogrado’s number, α¯e and α¯v are the dynamic molecular mean electronic
and vibrational polarizabilities, α¯me and α¯mv are dynamic molar electronic and vibrational
polarizabilities of the solutes, respectively. Finally, the total molar polarizability of each
solute was estimated as being α¯me+v = α¯me + α¯mv . The analyses of the experimental data were
performed using the MicrocalTM Origin software package.
3.1.3 Theoretical determination of the molar volume and electronic molar polarizability
The molecular geometries of TCPH, PCPH and FCPH were fully optimized under the
constrained Cs symmetry by using gradient techniques with the density functional theory
(DFT), using the B3LYP method and the 6−311++G(3d,3p) basis set. Numeric B3LYP
methods were employed to calculate the static electronic dipole polarizability, α¯Te . It is
important to note that the B3LYP DFT function is able to account accurately for the exchange
and electron correlation effects in the static mean molecular polarizability and molecular
geometries [4]. In fact, the geometric parameters obtained in this work for TCPH show
excellent concordance with the experimental data available in the literature for the thiophene
phenylhydrazone analog (see Table S1 of the supplementary data). The electronic mean
molar polarizability was estimated as being α¯Te ,m = NAα¯Te . All calculations were carried
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J Solution Chem (2007) 36:1–11
5
out using the Gaussian 2003 quantum chemistry package in a PCX86 under Linux [15a].
The molecular volumes for the TCPH, PCPH and FCPH compounds in the gas phase, VT,m
,
were calculated using fully optimized geometries at the B3LYP/6−311++G(3d,3p) level
with the Monte-Carlo integration method as implemented in this program. This method
estimates the greatest dimension of the molecule based on tabulated values of atomic radii
(e,g., van der Waals radii) scaled up by 20%.
First, select a rectangular box that encloses the greatest dimension of the molecule.
Calculate the electron density of the outer surface of the box. Then revise this calculation
if the estimated electron densities of the grid points are greater than the threshold criterion
(0.001 au, 1 au = 6.748 e·Å−3) and increase the size of the box, and again compute the
electron density of the outer surface of the box if any of the grid points have an electron
density greater than 0.001 au. Then calculate the volume of the box (Vbox) and generate N
independent random points inside the box, based on the number of points per unit volume.
And then, evaluate the electron density of the sample points. Calculate the number of “hits”
(Nocc) with electron densities greater than 0.001 au. Finally, calculate the molecular volume
ꢄ
ꢅ
Vb∗ox NOCC
N
using the equation V2Tm
=
[15b].
4 Results and discussion
The measured densities, ρ, of the solutions in acetonitrile at 293.15 K are presented in Table 1.
These data were used in conjunction with Eq. (1) to yield the corresponding apparent molar
volume, V2,φ, at a molarity of C. Table 1 reports the experimental V2,φ values obtained for
the examined solutions. The V2,φ values are shown in Fig. 2 as a function of the molarity
of the solutions. Because the solutions used in our study were extremely dilute, in all cases
considered the obtained V2,φ values were practically constant (99% confidence interval) over
the examined concentration range. The limiting partial molar volumes V2∞ calculated as the
average of the V2,φ values (method 1) showed excellent agreement (within experimental error)
with the V2∞ values obtained by the least-square fitting method based on Eq. (2) (method 2),
and the results are also shown in Table 1. It is very important to note that these experimental
results suggest that the TCPH, FCPH and PCPH molecules have very low tendencies to form
dipolar clusters by self-association in the acetonitrile solvent and thus the microenvironment
of each solute in this solvent remains constant through the concentration range studied.
In contrast, the acetone phenylhydrazone derivative showed significant self-association by
intermolecular hydrogen bonding of the NH proton to the iminic nitrogen in solvents of low
polarity [16]. However, in a solvent such as acetonitrile that is known to be a weak hydrogen-
bond donor and have a large dipole moment, solute-solvent interactions (dipole-dipole and
induced dipole-dipole interactions) appear to become relevant and compete efficiently with
the self-association of TCPH, FCPH and PCPH molecules. As a consequence, solute-solute
interactions are minimized in the very dilute region studied here. On the other hand, their
low solubility in solvents of low polarity means that the attractive forces between solute
molecules are more effective in these cases than the solute-solvent induced dipole-dipole
and dispersion interactions.
As seen from the data given in Table 1, the V2∞ values follow the following decreasing
order: TCPH > PCPH > FCPH. It is noteworthy that the partial molar volume decreases with
increasing Pauling electronegativity [17] of the heteroatom of the five-membered heterocyclic
ring in the respective solute and with decreasing functional group contribution to the van der
Waals molar volume {S(10.8 cm3·mol−1), N-H(8.08 cm3·mol−1) and O(5.20 cm3·mol−1)}
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Table 1 Measured values of the TCPH-, PCPH- and FCPH-acetonitrile solutions: concentration (C),
density (ρ), partial molar volume (V2,φ), refractive index (n), refraction relative per cm3 (θ), limiting partial
molar volume (V2∞) and limiting partial molar refraction (R2,0) at 293.15 K
103 × C/
mol·dm−3
ρ/g·cm−3
V2,φ/cm3·mol−1a
n
θ
TCPH
2.24
3.59
0.782173
0.782270
0.782365
0.783168
0.784326
0.785990
0.786888
165.83
166.05
165.92
165.84
165.80
165.80
165.78
1.343828
1.343908
1.343999
1.344648
1.345599
1.346969
1.347707
0.074042
0.118578
0.169238
0.530539
1.059966
1.822651
2.233499
4.90
16.00
32.00
55.01
67.40
V2∞ = 165.86 0.10 cm3·mol−1b
ꢀ2 = 33.1035 cm3·mol−1
V2∞ = 165.78 0.01 cm3·mol−1c
αe = 2.704×
R2,0 = 68.21 0.05 cm3·mol−1
10−23 cm3d
PCPH
1.09
2.24
3.35
4.49
16.38
33.57
50.27
67.30
0.782078
0.782149
0.782218
0.782288
0.783024
0.784088
0.785121
0.786175
158.10
157.71
157.47
157.62
157.48
157.45
157.46
157.45
1.343761
1.343830
1.343897
1.343966
1.344684
1.345723
1.346761
1.347760
0.036743
0.075155
0.112454
0.150867
0.550581
1.128997
1.706856
2.263004
V2∞ = 157.60 0.24 cm3·mol−1b
ꢀ2 = 33.7304 cm3·mol−1
V2∞ = 157.44 0.01 cm3·mol−1c
αe = 2.660×
R2,0 = 67.09 0.03 cm3-mol−1
10−23 cm3d
FCPH
1.09
2.28
3.40
4.60
17.00
33.00
50.30
70.30
0.782083
0.782162
0.782236
0.782315
0.783138
0.784199
0.785346
0.786272
153.38
153.16
153.22
153.34
153.07
153.06
153.06
153.06
1.343751
1.343811
1.343869
1.343930
1.344564
1.345381
1.346265
1.347287
0.031175
0.064578
0.096867
0.130826
0.483776
0.938604
1.430731
1.999863
V2∞ = 153.17 0.13 cm3·mol−1b
ꢀ2 = 28.4441 cm3·mol−1
V2∞ = 153.16 0.12 cm3·mol−1c
αe = 2.413×
R2,0 = 60.87 0.08 cm3·mol−1
10−23 cm3d
aDetermined using Eq. (1).
ꢆ
n
(V
2,φ
)
i
b
∞
i=1
¯
Estimated as V2
=
σ (method 1).
cDetermined using Eq. (2) n(method 2).
d The estimated random errors in this property lie beyond the last digit quoted.
[18]. On the basis of these observations, and considering that the phenylhydrazonyl unit
is present in all molecular systems studied here, it is possible to argue that the intrinsic
electronic and volumetric properties of each heteroatom in the respective heterocyclic moiety
play fundamental roles in the local organization of the solvent around the solute molecules.
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167
166
165
164
163
162
161
160
159
158
157
156
155
154
153
152
0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
C/mol dm-3
Fig. 2 The apparent molar volumes V2,φ of the FCPH (ꢀ), PCPH (•) and TCPH (ꢁ) compounds versus the
concentration in acetonitrile at 293.15 K
With the purpose of obtaining additional information on changes in the electronic en-
vironment experienced by solutes as the solution becomes more concentrated, we decided
to carry out the experimental determination of the apparent electronic molar refraction R2,0
of the TCPH, FCPH and PCPH compounds, in acetonitrile as the solvent, in the extremely
dilute region. This was done by measuring the refractive index values of both solutions,
n, and solvent, n1, at 293.15 K, using the Na D line (589.3 nm) and High Accuracy Re-
fractometry (HAR) [4] and by using the Kohner-Geffcken-Grunwald-Haley approach [5].
However, prior to the refractometric measurements, the molecular interactions in solution
were assessed using UV-vis spectroscopy following the procedure proposed by Lee and co-
workers [19]. It was found for each system (see Figs. S2–S4 of the supplementary data) that
the lowest wavelength absorption occurs at about 430 nm without an absorption shoulder
and that an excellent linear correlation occurs between the absorbance and concentration
for the first excited singlet of the solute. This information coupled with the absence of a
discernible change in band shape suggest that a constant electronic environment (cybotatic
region) occurs for solute molecules over the concentration range studied here. This finding
is consistent with the volumetric and refractometric results (see below).
Based on these results, we carried out the experimental determination of the apparent
molar refraction of the TCPH, FCPH and PCPH compounds in an off-resonance region in
acetonitrile. The excellent linearity (coefficient correlation, r2 ≈ 1) in the refractometric
data (θ versus C plots) also indicates that no changes occur in the solute-solvent interactions
over the concentration range studied and therefore the electronic molecular polarizations are
additive in all cases. Thus, the concentration-dependent experimental results of the relevant
terms of Eq. (3) and the corresponding apparent dynamic electronic molar refraction, R2,0
,
of the TCPH, FCPH and PCPH compounds, are reported in Table 1. As can be seen in this
table, the dynamic value of the apparent molar refraction at 589.3 nm determined for these
solutes follows the same trend as observed for the partial molar volume at infinite dilution,
i.e., the apparent molar refraction decreases with decreasing partial molar volume at infinite
dilution.
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The corresponding electronic, α¯me , and vibrational, α¯mv , components of the molar mean
polarizabilities deduced from Eqs. (4) and (5) were estimated as being (for α¯me ) 16.28
cm3·mol−1 (TCPH), 15.98 cm3·mol−1 (PCPH), 14.52 cm3·mol−1 (FCPH), and (for α¯mv )
1.63 cm3·mol−1 (TCPH), 1.60 cm3·mol−1 (PCPH), 1.45 cm3·mol−1 (FCPH), respectively.
Therefore, the value of total molar mean polarizability α¯me+ν for each compound is 17.95
cm3·mol−1 (TCPH), 17.58 cm3·mol−1 (PCPH) and 15.97 cm3·mol−1 (FCPH). In this con-
text, the static electronic molar polarizabilities, α¯Te ,m, determined theoretically at the DFT
level in this work for TCPH (18.29 cm3·mol−1), PCPH (17.27 cm3·mol−1) and FCPH (16.88
cm3·mol−1) in the gas phase, are higher than our apparent electronic molar polarizabilities,
α¯me , in acetonitrile. The differences are mainly due to the dielectric solvent effects not con-
sidered in the calculations. In fact, the values of molar electronic polarizability for TCPH
(14.21 cm3·mol−1), PCPH (13.45 cm3·mol−1) and FCPH (12.76 cm3·mol−1), as estimated
using the optimized atomic hybrid polarizabilities method proposed by Miller [20], give a
deviation of about 11.5% when compared with the experimental data. Similar differences
were obtained by using the empirical method proposed by Vogel [21]. This difference could
be the result of solvent-solute interactions. However, the theoretical trends reproduce the
experimentally observed trend: TCPH > PCPH > FCPH. It is noteworthy that the theoret-
ical and experimental tendencies for the electronic polarizability of these phenylhydrazone
derivatives follow the same order as the electronic polarizability and aromaticity reported
previously for heteroaromatic rings (thiophene > pyrrole > furan) and electronegativity of
the heteroatom [17]. At this point, it is important to mention that Lazzaretti and Tossell [17a]
and Soscu´n and Hinchliffe [17b] proposed that the electronic polarizability is both an index
of aromaticity and a measure of electron delocalization for thiophene, pyrrole and furan
compounds. Then, the differences in molar refraction for these phenylhydrazone derivatives
in acetonitrile solvent can be related to aromaticity and electronegativity differences among
the thiophene, pyrrole and furan moieties.
From of the volumetric, refractometric and spectroscopic results obtained here for the
TCPH, PCPH and FCPH compounds, it is reasonable to believe that the solute molecules
are isolated in the observed concentration range. In fact, a comparison of the experimental
values of the partial molecular volumes at infinite dilution, ν2∞ (2.77 × 10−22, 2.61 × 10−22
,
and 2.54 × 10−22 cm3·molecule−1 for TCPH, PCPH and FCPH, respectively), with re-
spect to our theoretical values for the molecular volumes, V2T , estimated at the level
B3LYP/6−311++G(3d,3p) (2.59 × 10−22 (TCPH), 2.46 × 10−22 (PCPH) and 2.26 ×
10−22 cm3·molecule−1 (FCPH)), gives a variation of 7%, 6.1% and 12%, respectively. Then,
on the basis of these results and the solvation model proposed by Terasawa et al. [22], it is
possible to also argue that the volumetric contribution from solute-solvent interactions to the
molar volumes at infinite dilution is very small in each case and that solute molecules are
isolated in this medium.
5 Conclusions
The experimental and theoretical results point out, in fact, that the phenylhydrazone derivates
of 2-carboxaldehyde thiophene (TCPH), pyrrole (PCPH) and furan (FCPH) do not form
stable complexes with the solvent acetonitrile and do not exhibit significant solute-solute
association, and that the volumetric contribution from solute-solvent interactions to the
partial molar volume is negligible within the studied concentration range. The order of the
partial molar volume, V2∞, and the apparent molar refraction, R2,0, at infinite dilution of
these compounds in acetonitrile at 293.15 K (TCPH > PCPH > FCPH) is parallel to the
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J Solution Chem (2007) 36:1–11
9
order of the aromaticity of the five-membered heterocyclics (thiophene > pyrrole > furan),
the heteroatom volumetric contribution to the van der Waals molar volume and the Pauling
electronegativity of the heteroatom.
Acknowledgments
The authors thank the Fondo Nacional de Ciencia, Tecnolog´ıa e Innovacio´n (FONACIT-
Venezuela, Proyect: Apoyo a Grupos No. G-2005000403) and the Consejo de Desarrollo
Cient´ıfico y Human´ıstico (CONDES-LUZ) for partial financial support of this work and
the CeCalcULA (Centro Nacional de Ca´lculo Cient´ıfco de la Universidad de los Andes,
Me´rida-Venezuela). We also are grateful to the referees for providing valuable suggestions
and helpful comments concerning this work.
Electronic supplementary material
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