Thermodynamic Study of Chlorobenzonitrile Isomers: A Survey on the

Article pubs.acs.org/JPCA

Thermodynamic Study of Chlorobenzonitrile Isomers: A Survey on the Polymorphism, Pseudosymmetry, and the Chloro···Cyano Interaction Inês M. Rocha, Tiago L. P. Galvaõ , Maria D. M. C. Ribeiro da Silva,* and Manuel A. V. Ribeiro da Silva Centro de Investigaçaõ em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal S Supporting Information *

ABSTRACT: The relationships among structural and thermodynamic properties of 2-, 3-, and 4-chlorobenzonitrile were investigated, in the present work, using several experimental techniques (Knudsen effusion, differential scanning calorimetry, and combustion calorimetry) and computational studies. The CN···Cl intermolecular interactions are weaker in 2chlorobenzonitrile, reflecting a lower enthalpy of sublimation. The two polymorphic forms of 4-chlorobenzonitrile were observed by differential scanning calorimetry and interpreted in terms of the strength of CN···Cl intermolecular interactions. The entropic differentiation due to the pseudosymmetry observed in the crystalline packing of 2-chlorobenzonitrile was evaluated. Using adequate working reactions and the respective standard molar enthalpies of formation, in the gaseous phase, the halogen−cyano intramolecular interaction was also evaluated. The theoretically estimated gas-phase enthalpies of formation were calculated using high-level ab initio molecular orbital calculations at the G3MP2B3 and MP2/cc-pVTZ levels of theory. The computed values support very well the experimental results obtained in this work.

1. INTRODUCTION Conjugated π systems containing cyano groups and metal complexes with −CC− fragments,1−4 resulting from reactions of the cyano group, play an important role in a wide range of optical and conducting materials used for industrial applications.5−11 The chlorobenzonitrile isomers are specifically involved in nucleophilic substitution reactions resulting in the synthesis of monomers of aromatic polyamides12,13 that are precursors to well-known polymers used in textiles and sportswear. Beyond their technological application, these halogenated benzonitriles are fundamental precursors of molecules used in medical treatment approaches applied to the cure of diseases that affect millions of people in the world. In fact, these molecules are intermediates involved in the synthesis of inhibitors of enzymes related with Chagas,14 Parkinson,15 and Alzheimer16 diseases, as well as other inhibitors used in cancer treatment17,18 and in the replication of the HIV virus.19 The molecular structures of the chlorobenzonitrile isomers were studied by other authors using microwave spectroscopy.20−22 In a recent study, other spectroscopic techniques (UV and FT-IR) were combined with theoretical calculations, making it possible to relate the structural parameters of the molecule to electronic interactions between atoms evaluated by a natural bond orbital (NBO) analysis.23 The intermolecular interactions between halogen···halogen24−28 and halogen··· cyano group29−32 have been investigated due to their © 2014 American Chemical Society

importance in materials properties, in particular the mechanical response of polymers (bending, hardness, and flowability). Some studies confirm the impact of the halogen bonds and substituent effects on the aromatic stacking interactions.33,34 Britton detected independent molecular structures with small differences in the bond lengths and bond angles in the packing of chlorobenzonitrile derivatives.35,36 This phenomenon, called pseudosymmetry,37 may be related to the shift of the layers in a crystal lattice to improve the layer-to-layer interactions.38,39 In the specific case of the 2-chlorobenzonitrile, each molecular structure can be converted into another structure through a symmetry operation.36 In the study of the crystal structure of 4-chlorobenzonitrile30 two polymorphic forms were observed: form I, obtained by sublimation, where the molecules are rearranged in linear chains of molecules linked by CN···Cl contacts, and form II, obtained by recrystallization from ethanol, where the molecules are organized in a zigzag conformation through CN···Cl contacts. Apart from the intermolecular interactions, structural changes at the molecular level, such as symmetry40 and packing, are reflected in the thermodynamic properties as previously referred observed for aromatic derivatives.41,42 Received: October 14, 2013 Revised: January 31, 2014 Published: January 31, 2014 1502

dx.doi.org/10.1021/jp410187q | J. Phys. Chem. A 2014, 118, 1502−1510

The Journal of Physical Chemistry A

Article

The benzoic acid NIST Standard Reference Material, sample 39j,43 was used without any further purification to calibrate the rotating-bomb combustion calorimeter. The corrections from the apparent mass IR air to the true mass were calculated using the specific densities of 1.432,36 1.278,44 and 1.433 g·cm−3 30 for 2-, 3-, and 4-chlorobenzonitrile, respectively. The relative atomic masses used in the calculation of all molar quantities throughout this paper were those recommended by IUPAC Commission,45 yielding 137.5666 g·mol−1 for the molar mass of the chlorobenzonitrile isomers. 2.2. Combustion Calorimetry. An isoperibol rotatingbomb combustion calorimeter, formerly used at the National Physical Laboratory, Teddington, U.K., was used to measure the standard massic energies of combustion of the chlorobenzonitrile isomers. The apparatus and operating technique have been previously described in literature.46−49 All the combustion measurements were performed in a twinvalve bomb lined with tantalum (internal volume of 0.329 dm−3), placed inside the calorimeter, with a mass of 3969.2 g of distillated water. Calorimetric temperatures were measured every 10 s, with a repeatability uncertainty within the bounds of ±10−4 K, using a quartz thermometer (Hewlett-Packard HP 2804A) interfaced to a microcomputer programmed to compute the adiabatic temperature change, by means of a version of the LABTERMO program.50 The ignition temperature was chosen so that the final temperature would be close to T = 298.15 K. The ignition energy was determined by the change of the potential difference on the discharge of a capacitor (1281 μF) across a platinum wire with a diameter of 0.05 mm. The massic energy of combustion of the cotton thread (empirical formula CH1.686O0.843), used as a fuse, is assigned to Δcu° = −16240 J·g−1,51 a value confirmed in our laboratory. The rotation of the bomb was started when the temperature had risen to 63% of its final value and then continued throughout the rest of the experiment, as recommended by Good et al.52 The calorimetric system was calibrated according to the procedure proposed by Coops et al.,51 with benzoic acid (NBS Standard Reference Material 39j), having a standard massic energy of combustion, under bomb conditions, of −26434 ± 3 J·g−1.43 The three chlorobenzonitrile isomers were burned in pellet form, sealed inside polyester bags made of Melinex, in an oxygen atmosphere (p = 3.04 MPa) and in the presence of 25.00 cm3 of an aqueous solution of As2O3 ≈ 0.0900 mol·dm−3, to reduce all the free chlorine, Cl2, produced during the combustion to hydrochloridric acid, HCl. The massic energy of combustion of dry Melinex used was Δcu° = −22902 ± 5 J· g−1,53 a value confirmed in our laboratory. The extent of oxidation of As2O3(aq) was determined by titration with a standardized iodine solution. The energetic contribution of the oxidation of As2O3 to As2O5, in aqueous solution, was calculated using the procedure proposed by Hu et al.,54 including the enthalpy of oxidation of As2O3 by Cl255 and the thermal effects of mixing of As2O5 with strong acids, such as HCl.56 The nitric acid formed during the combustion of the chlorobenzonitrile samples and from traces of atmospheric N2 remaining inside the bomb was analyzed by Devarda’s alloy method.57 An energetic contribution based on −59.7 kJ· mol−1 58 for the standard molar energy of formation of HNO3 was made. The H2PtCl6 formed was determined from the mass loss of the platinum crucible and its supporting ring, and the

The present work aims to explore the electronic nature of intra- and intermolecular interactions of 2-, 3-, and 4chlorobenzonitrile isomers, to contribute to the knowledge of the physical-chemical behavior of these relevant units components of conducting polymers and molecules with biological relevance and potential applications. The standard (p° = 0.1 MPa) molar enthalpies and entropies of sublimation of the chlorobenzonitrile isomers were derived from the application of Clausius−Clapeyron equation to the vapor pressures, measured at several temperatures, using Knudsen effusion. These thermodynamic parameters can be related with intermolecular interactions and the symmetry of the crystal structure. The different samples of 4-chlorobenzonitrile obtained by the different crystallization methods, to investigate its polymorphism, were characterized by differential scanning calorimetry to investigate their polymorphism.30 Through the combination of the standard molar enthalpies of sublimation (at T = 298.15 K) and the respective standard molar enthalpies of formation (in the condensed phase) derived from the standard massic energies of combustion (at the same temperature) measured by rotating-bomb combustion calorimetry, the standard molar enthalpies of formation (in the gaseous phase, at T = 298.15 K) of the studied isomers were calculated. The standard molar enthalpies of formation in the gas phase at T = 298.15 K were calculated by combining the standard molar enthalpies of sublimation also at T = 298.15 K with their respective condensed phase enthalpies of formation measured by rotating-bomb combustion calorimetry. This parameter allowed the determination of the energetic contribution, in the gaseous phase, due to the interaction between the chlorine atom and the cyano group for the studied isomers.

2. EXPERIMENTAL PROCEDURES 2.1. Compound and Purity Control. The origin and control of purity of 2-, 3-, and 4-chlorobenzonitrile are summarized in Table 1. The 2- and 3-chlorobenzonitrile Table 1. Purification Details of the Studied Chlorobenzonitrile Isomers

a

compound

CAS Registry No.a

2-chlorobenzonitrile

873-32-5


766-84-7


623-03-0

provenance

mass fraction purityb after purification

Alfa Aesar, 98% SigmaAldrich, 99% SigmaAldrich, 99%

0.9994 0.9991 0.9993

Supplied by author. bGas−liquid chromatography.

isomers were purified by sucessive sublimations under reduced pressure, and 4-chlorobenzonitrile was recrystallized from hexane followed by sublimation. After that, it was also sublimated. Sublimated samples were used in the combustion, Knudsen effusion, and DSC measurements.4-Chlorobenzonitrile recrystallized from ethanol was also used in DSC measurements. The degree of purity of each isomer was checked by gas chromatography before and after the purification, using an Agilent 4890D gas chromatograph Agilent HP-4890 apparatus with an HP-5 column, 5% diphenyl, and 95% dimethylpolysiloxane, under nitrogen pressure as a carrier gas. 1503



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Table 2. Derived Standard (p° = 0.1 MPa) Molar Energies of Combustion, ΔcUm ° , Standard Molar Enthalpies of Combustion, ΔcH°m, and Standard Molar Enthalpies of Formation, in the Condensed Phase, ΔfH°m(cr), of the Studied Compounds, at T = 298.15 K compound

−⟨Δcu°⟩(cr)/J·g−1

−ΔcU°m(cr)/kJ·mol−1

−ΔcH°m(cr)/kJ·mol−1

ΔfH°m(cr)/ kJ·mol−1

2-chlorobenzonitrile 3-chlorobenzonitrile 4-chlorobenzonitrile

25235.4 ± 1.1 25192.1 ± 1.7 25173.6 ± 2.0

3471.5 ± 0.9 3465.6 ± 1.0 3463.0 ± 1.0

3472.1 ± 0.9 3466.2 ± 1.0 3463.6 ± 1.0

122.2 ± 1.3 116.3 ± 1.4 113.7 ± 1.4

energy correction was based on ΔfH°m(H2PtCl6,aq) = −(676.1 ± 0.1) kJ·mol−1.58 The energy correction for the carbon soot found at the end of some measurements was based on the weighing the crucible before and after calcinations, considering a value for the standard massic energy of combustion of carbon of Δcu° = −33 kJ·g−1.51 The standard state corrections, ΔUΣ, and the heat capacities of the bomb contents, εi and εf, were calculated by the procedure given by Hubbard et al.,59 using the solubility constants and energies of solution of CO2 and O2, as given by Hu et al.51 The pressure coefficient of massic energy, (∂u/∂p)T, at T = 298.15 K was assumed to be −0.2 J·g−1·MPa−1,60 a typical value for most organic compounds. 2.3. Knudsen Effusion Measurements. The vapor pressures of the crystals of the three isomers were measured by the mass-loss Knudsen effusion technique. A detailed description of the apparatus, operating technique, and results obtained for two test substances (benzoic acid and ferrocene) has already been reported.61 The Knudsen apparatus enables the simultaneous operation of three cells with different effusion orifices, each cell containing a small amount of compound compressed to form a smooth surface. The measurement of the mass sublimed from the effusion cell, Δm, during a time period, t, by weighing the cell to ±0.01 mg (Mettler AE 163 balance), before and after the measurement, enabled the determination of the vapor pressure, p, at the temperature, T, by using the Knudsen eq 1, where A0 is the area of the effusion orifice, R is the gas constant (R = 8.314472 J· K−1·mol−1), M is the molar mass of the effusion vapor, and w0 is the Clausing probability factor. p=

Δm ⎛⎜ 2πRT ⎞⎟1/2 · A 0w0t ⎝ M ⎠

compound and a sample obtained by recrystallization from ethanol.

3. COMPUTATIONAL DETAILS The computational calculations were performed using the Gaussian 03 software package.63 The G3(MP2)//B3LYP composite method was used throughout this work.64 This is a variation of the G3(MP2) theory,65 which uses the B3LYP density functional method66,67 with the 6-31G(d) basis set for geometries and zero-point vibrational energies. Introduction of high-order corrections to the QCISD(T)/6-31G(d) energy is done in a way that follows the Gaussian-3 philosophy, albeit using a second-order Moller−Plesset perturbation instead of MP4, as in the original G3 method.68 The energies computed at T = 0 K were thermally corrected for T = 298.15 K by introducing the vibrational, translational, rotational, and PV terms. The vibrational term is based on the vibrational frequencies calculated at the B3LYP/6-31G(d) level. Secondorder Møller−Plesset perturbation theory69 with the cc-pVTZ basis set and the B3LYP/6-311++G(2d,2p) method were also used for the computation of enthalpies and entropies, in the gaseous phase, of the chlorobenzonitrile isomers. 4. RESULTS 4.1. Experimental Enthalpies of Formation, in the Condensed Phase. Detailed results of each combustion experiment performed for the studied compounds studied are presented in Tables S1, S2, and S3 in the Supporting Information. The energy equivalent of the calorimeter used in this work was ε(calor) = 20361.4 ± 0.6 J·K−1, where the uncertainty quoted is the standard deviation of the mean of six calibration experiments. The energy of the isothermal bomb process, ΔU(IBP), is calculated through eq 2, correcting the energy equivalent, ε(calor), for the deviation of the mass of water used from 3969.2 g, Δm(H2O), where ΔTad is the calorimeter temperature change corrected for the heat exchange, the work of stirring, and the frictional work of the bomb rotation, whereas the remaining terms are defined in the literature.70,60

(1)

2.4. Differential Scanning Calorimetry (DSC) Measurements. The temperatures and the standard molar enthalpies of fusion of the solid−solid phase transitions of the 2-, 3-, and 4chlorobenzonitrile isomers were measured in a power compensation differential scanning calorimeter, model SETARAM DSC 141, with a heating rate of 3.3 × 10−2 K·s−1, where the samples were sealed in aluminum crucibles. The temperature and the power scale were calibrated by measuring, respectively, the melting temperature and enthalpy of the following reference materials:62 o-terphenyl [CAS 84-15-1], benzoic acid [CAS 65-85-0], indium [CAS 7440-74-6], triphenylene [CAS 217-59-4], tin [CAS 7440-31-5], perylene [CAS 198-55-0], lead [CAS 7439-92-1], and zinc [CAS 744066-6] (CAS Registry Numbers provided by the author). At least four experiments leading to consistent results were performed, using the same experimental procedure, for the chlorobenzonitrile isomers and the calibration runs. Two different samples of 4-chlorobenzonitrile were studied: a sample of the sublimated

ΔU (IBP) = − {ε(calor) + Δm(H 2O) cp(H 2O, l)}ΔTad + (Ti − 298.15 K)εi + (298.15 K − Ti − ΔTad)εf + ΔUign

(2)

The individual values of the standard massic energies of combustion for the measurements of the three chlorinated isomers correspond to the reaction described by eq 3, yielding HCl·600H2O(l) as the only chlorine containing product in the final state. C7H4NCl(cr) + 7.75O2 (g) + 598.5H 2O(l) → 7CO2 (g) + 0.5N2(g) + [HCl · 600H 2O](l) 1504

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Table 3. Standard (p° = 0.1 MPa) Molar Enthalpies, Δ1crHm ° , at the Temperature of Fusion, Tfus, and the Derived Molar Enthalpies, Δ1crH°m(298.15K), and Entropies of Fusion, Δ1crS°m(298.15K), at T = 298.15 K, for the Chlorobenzonitrile Isomers compound

Tfus/K

Δ1crHm ° (Tfus)/kJ·mol−1

Δ1crHm ° (298.15K)/kJ·mol−1

Δ1crSm ° (298.15K)/J·K−1·mol−1


317.5 ± 0.6 314.1 ± 0.1 363.5 ± 0.2

17.6 ± 0.4 18.7 ± 0.4 18.5 ± 0.2

17.0 ± 0.4 18.2 ± 0.4 16.5 ± 0.8

53.4 ± 1.4 57.8 ± 1.4 44.9 ± 2.3

Table 4. Standard (p° = 0.1 MPa) Molar Enthalpies, ΔgcrHm ° , and Entropies, ΔgcrSm ° , of Sublimation and the Vapor Pressure, p, at the Mean Temperature of the Experiments, ⟨T⟩, for the Studied Compounds compound

⟨T⟩/K

p(⟨T⟩)/Pa

ΔgcrHm ° (⟨T⟩)/kJ·mol−1

ΔgcrSm ° (⟨T⟩,p(⟨T⟩))/J·K−1·mol−1


273.25 268.04 271.14

0.288 0.503 0.346

72.9 ± 0.8 77.4 ± 0.5 77.3 ± 0.4

266.8 ± 2.9 288.8 ± 1.9 285.1 ± 1.5

several temperatures, as described in section 2.3. The experimental results, obtained for each effusion cell, together with the residuals of the Clausius−Clapeyron equation, 102·Δln(p/Pa), derived from least-squares adjustments are summarized in Table S6 of the Supporting Information. The Clausing probability factor, w0, was calculated according to eq S4 (Supporting Information), where the areas and respective values of each effusion orifice are presented in Table S5 of the Supporting Information. The standard molar enthalpies of sublimation at the mean temperature of the experimental temperature range, ΔgcrH°m(⟨T⟩), were derived from the integrated form of the Clausius−Clapeyron equation, ln(p/Pa) = a − b·(T/K)−1, where a is a constant and b = ΔgcrHm ° (⟨T⟩)/R. The equations obtained for each isomer, as a result of the combination of the vapor pressures of the three different orifices, were reported in eqs 6−8:

The mean value of the standard massic energies of combustion, ⟨Δcu°⟩, for all combustion experiments of each compound and the standard deviation of the mean value, the derived standard molar energies, ΔcU°m, and enthalpies of combustion and the standard molar enthalpies of formation, in the condensed phase, ΔcHm ° , at T = 298.15 K, of the three isomers, are listed in Table 2. The uncertainty assigned to the standard molar energy of combustion corresponds to twice the overall standard deviation of the mean and includes the contributions from the calibration with benzoic acid and the Melinex. ΔfH°m(cr) was derived from ΔcH°m(cr), using the following standard molar enthalpies of formation, at T = 298.15 K: ΔfHm ° (CO2,g)= −393.51 ± 0.13 kJ·mol−1,71 ΔfHm ° (H2O,l)= −285.830 ± 0.040 kJ·mol−1,71 and ΔfHm ° (HCl·600H2O,l) = −166.540 ± 0.005 kJ·mol−1.58,71 4.2. Experimental Enthalpies and Entropies of Phase Transition. For each isomer, the standard molar enthalpy of fusion, Δ1crH(Tfus), and the respective temperature of fusion, Tfus, were obtained from DSC measurements, by the integration of the area of the melting peak of several experiments. For each isomer, the standard molar enthalpy of fusion, at T = 298.15 K, Table 3, was calculated according eq 4, where the detailed results are presented in Table S7 of the Supporting Information. Δcrl H(T )

=

Δcrl H(Tfus)

+

Δcrl C °p ,m(T

− Tfus)

(4)

The calculated standard molar entropy of fusion, at T = 298.15 K, presented in Table 3 was derived for each compound by using eq 5. Δ1cr S(T ) =

⎛ T ⎞ Δ1cr H(Tfus) + Δ1cr C °p ,m ln⎜ ⎟ Tfus ⎝ Tfus ⎠

(5)

2‐chlorobenzonitrile: ln(p) 8764 ± 92 = (30.83 ± 0.34) − T

(6)


(7)


(8)

Table 4 presents the values of the thermodynamic properties derived from the Clausius−Clapyeron equation, ΔgcrHm ° and ΔgcrS°m, as well as the vapor pressure, p, at the mean temperature, ⟨T⟩. The standard molar enthalpies of sublimation, at T = 298.15 K, were derived by using eq 9, where the heat capacity for the three compounds is estimated by using the Domalski and Hearing method:72 ΔgcrC°p,m = −(22.9 ± 11.3) J·K−1·mol−1. The parameters, used in eq S3 (Supporting Information) to calculate Cp,m ° (cr), are presented in Table S4 of the Supporting Information.

In both equations, Δ1crCp,m ° = Cp,m ° (1) − Cp,m ° (cr) = 30.1 ± 11.3 J·K−1·mol−1, where the heat capacities in the liquid and in the crystalline phases were estimated by application of the Domalski and Hearing method,72 an extension of the secondorder group additivity method, formerly developed by Benson and co-workers.73 The heat capacity of the studied compounds, in the liquid phase, was estimated as being 177.8 ± 8.0 J·K−1· mol−1, and 147.7 ± 8.0 J·K−1·mol−1 for the crystalline phase. The parameters used in the equations (eqs S1 and S2 in the Supporting Information) are presented in Table S4 of the Supporting Information. [CB−(CN)(CB)2]cr = 33.55 J·K−1· mol−1 was obtained in a previously work.74 The DSC study of the chlorobenzonitrile isomers was complemented by the measurement of the vapor pressure at

Δcrg Hm° (298.15K ) = Δcrg Hm° (⟨T ⟩) + Δcrg C p°,m (298.15 − ⟨T ⟩) 1505

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Table 5. Standard (p° = 0.1 MPa) Molar Enthalpies, ΔgcrHm ° , and Entropies, ΔgcrSm ° , of Sublimation and the Pressure, p, at T = 298.15 K, for the Studied Compounds compound

p(298.15K)/Pa

ΔgcrHm ° /kJ·mol−1

ΔgcrSm ° /J·K−1·mol−1

ΔgcrGm ° /kJ·mol−1


4.2 16.4 7.6

72.3 ± 0.8 76.7 ± 0.5 76.7 ± 0.4

158.7 ± 2.9 184.9 ± 1.9 178.4 ± 1.5

25.0 ± 1.2 21.6 ± 0.8 23.5 ± 0.6

Table 6. Standard (p° = 0.1 MPa) Molar Enthalpies of Formation, and Enthalpic Interaction between the Halogen Atom and the Cyano Group, in the Gaseous Phase, at T = 298.15 K of the Chlorobenzonitrile Isomers Determined Experimental and Computationallya ΔrH/kJ·mol−1 compound 2-chlorobenzonitrile 3-chlorobenzonitrile 4-chlorobenzonitrile a

−1

ΔfHm ° (g)/kJ·mol 194.5 ± 1.5 193.0 ± 1.5 190.0 ± 1.5

exp

G3B3(MP2)

MP2/cc-pVTZ

B3LYP/6-311++G(2d,2p)

9.4 ± 2.9 7.9 ± 2.9 5.3 ± 2.9

9.1 [0.3] 5.6 [2.3] 3.9 [1.4]

7.9 [1.5] 5.3 [2.6] 3.4 [1.9]

12.6 [−3.2] 6.5 [1.4] 3.9 [1.4]

Enthalpic differences between the experimental and computed values are given in parentheses.

depending on the process of crystallization used to obtain the samples used in the experiments. Figure 1 presents the DSC thermograms of the different polymorphs of 4-chlorobenzonitrile, obtained by sublimation

The values of the standard molar enthalpies, entropies, and Gibbs energies of sublimation, at T = 298.15 K, and the vapor pressure, at the same temperature, are listed in Table 5.

5. DISCUSSION 5.1. Energetic Evaluation of the Interactions of Substituents in the Gaseous Phase. The standard molar enthalpies of formation, in the gaseous phase, of the chlorobenzonitrile isomers, in the gaseous phase, presented in Table 6, were calculated by combining the enthalpy of formation, in the condensed phase (Table 2), with the standard molar enthalpy of sublimation, at T = 298.15 K (Table 5). The enthalpy of the several gaseous reactions, ΔrH, represented in Scheme 1, is calculated using the enthalpies of Scheme 1. General Isodesmic Gas-Phase Reaction Used for the Evaluation of the Interaction between Cl Atom and CN Group in the Three Isomers

Figure 1. Experimental DSC heating curves of crystalline 4chlorobenzonitrile: (1) sublimated; (2) recrystallized from ethanol; (3) samples (1) and (2) reheated.

formation, in the gaseous phase, of the species involved and reflects the interaction between the chlorine atom and the cyano group in the respective chlorobenzonitrile isomers. According to Table 6, the chlorination of benzonitrile in different positions has an enthalpic destabilizing effect, the largest destabilization found for the 2-chlorobenzonitrile, which is in agreement with the results obtained for the fluorobenzonitrile74 and bromobenzonitrile75 isomers. The values of ΔrH derived from the enthalpies of formation calculated from theoretical methods do not differ significantly from the ΔrH derived from the experimental values. However, the B3LYP/6311++G(2d,2p) method shows a high large electrostatic repulsion in 2-chlorobenzonitrile, predicting a larger value of ΔrH (3.2 kJ·mol−1) than the one obtained from the experimental values, ΔrH(exp.). 5.2. Intermolecular Interactions and the Crystal Lattice. The heating of each compound during the DSC measurements started at T = 298.15 K, and no phase transition was observed before the melting peak for 2- and 3chlorobenzonitrile. However, in 4-chlorobenzonitrile two phase transitions were observed before the melting peak,

(1) and by recrystallization in ethanol (2). Phase transition I occurs at ⟨Ttrans(I)⟩ = 341.8 ± 0.4 K, whereas phase transition II was detected at ⟨Ttrans(II)⟩ = 344.1 ± 0.2 K. Phase transition III corresponds to the melting of 4chlorobenzonitrile, which exhibits a pronounced sharp endothermic peak at T = 363.5 5 ± 0.2 K. Previously melted samples of sublimated and recrystallized 4-chlorobenzonitrile were reheated, and the thermograms obtained for both presented only the melting peak of the compound, suggesting the irreversibility of phase transitions I and II. In Table S7 (Supporting Information), experiments 1 and 2 for 4-chlorobenzonitrile correspond to the samples recrystallized in ethanol, presenting peaks II and III. Experiments 4 and 5 are related to the sublimed samples, showing peaks I and III. Experiments 3 and 6 correspond to the reheated samples from experiments 2 and 5. The calculated values of the standard molar enthalpies of fusion, at T = 298.15 K, of 2- and 3-chlorobenzonitrile, presented in Table 3, are similar, considering the associated 1506



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Figure 2. Minimum unit cell of the crystal of 2-chlorobenzonitrile,36 with the four independent structures (molecules A, B, C, and D). Dashed lines represent CN···Cl short contacts.

uncertainty, and are smaller than the value obtained for 4chlorobenzonitrile. The standard molar enthalpy of sublimation of 2-chlorobenzonitrile is similar to that of 3-chlorobenzonitrile and larger than the value obtained for 4-chlorobenzonitrile. It should be noted that for the combustion calorimetric and vapor pressure study of 4-chlorobenzonitrile, the sample was newly sublimed and that the measurements were carried out in a temperature range below the phase transition observed by DSC. The unit cell (Figure 2) of the crystallographic structure of 2chlorobenzonitrile, determined by Britton,36 presents four independent structures, with small differences in the bond lengths (Figure 2). Each molecule is linked with two other molecules by CN···Cl intermolecular interaction bonds, larger than the van der Waals distance (0.330 nm36). The standard molar enthalpy of sublimation of 2chlorobenzonitrile (Table 5) has the smallest value of the three isomers, reflecting the weakness of the intermolecular interactions. The enthalpy of sublimation of 2-chlorobenzonitrile is 4.4 kJ·mol−1 smaller than that for 4-chlorobenzonitrile, which is evidence of the trend already described for other halogenated derivatives.76−78 This difference is a possible result of the distance of the intermolecular interactions of both isomers. 4-Chlorobenzonitrile, in the crystal, presents the same type and number of interactions per molecule, as do the molecules of 2-chlorobenzonitrile, differing only in the distance between the atoms of the intermolecular interaction between the two atoms. According to Table 7, the mean distance of the CN···Cl interactions is 0.3429 nm, which is larger than the distance of same interaction in both polymorphs of 4chlorobenzonitrile. This change in the bond lengths may justify

Table 7. CN···Cl Intermolecular Distances, d(CN···Cl), in the Crystal Packing of 2-Chlorobenzonitrile and 4Chlorobenzonitrile compound 2-chlorobenzonitrile

4-chlorobenzonitrile polymorph of phase transition I polymorph of phase transition II

d(CN···Cl)/nm 0.338036 0.339436 0.346636 0.347736 0.335030 0.336130 0.337030

the reason for the difference of 4.4 kJ·mol−1 in the enthalpies of sublimation. Another hypothesis to be considered is related to the stacking interactions of the aromatic rings.79,80 of the chlorobenzonitrile isomers. In the crystalline structure of 4chlorobenzonitrile,30 the molecular layers are perfectly overlapped with each other (sandwich stacking79), maximizing the dispersive interactions, whereas the molecules of 2-chlorobenzonitrile are not organized in the same manner.36 Theoretical studies reported in of Table 8, performed by Wheeler,79 predicted the relative interaction energy,Erel int, of dimers of p-C6H5X··· p-C6H5Y molecules, stacked in the sandwich mode, considering the benzene dimer as the reference. The values of the relative interaction energy, Erel int, of several dimers with of benzonitrile derivatives are between 3.47 and 7.02 kJ·mol−1. This is in agreement with the difference reported previously for the enthalpies of sublimation of 4chlorobenzonitrile and 2-chlorobenzonitrile, 4.4 ± 3.5 kJ·mol−1. 1507



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Table 8. Relative Interaction Energy, Erel int, of Dimers of pC6H5X··· p-C6H5Y, Calculated at the CCSD(T)/AVTZ Level of Theory79 X

Y

−1 −Erel int/kJ·mol

F CN CN CN CN

F CN F CH3 NH2

8.45 3.47 6.10 6.07 7.02

sublimation was also evaluated, specifically the pseudosymmetry phenomena detected in crystallographic data of 2chlorobenzonitrile. The presence of a solid−solid phase transition was evaluated by DSC: whereas the isomers 2- and 3-chlorobenzonitrile present only the peak corresponding to the melting of the compound, in 4-chlorobenzonitrile two different phase transitions were detected before the melting peak in 4chlorobenzonitrile, each one corresponding to a different sample obtained by two different crystallization methods (sublimation and recrystallization in ethanol). This work was also complemented by the derivation of the standard molar enthalpies of formation (in the gaseous phase) for each compound, through the combination of the enthalpies of sublimation with the standard molar enthalpy of formation (in the condensed phase), by combustion calorimetry, and by high level quantum mechanical calculations. This parameter contributes to the analysis on the energetic contribution of the intramolecular interaction between the chlorine atom and the cyano group.

5.3. Comments on Entropy, Symmetry, and Pseudosymmetry. The standard molar entropy of fusion (Table 3) of 4-chlorobenzonitrile is smaller than that observed in the other isomers, suggesting a similar organization in the solid and the liquid phases. Removing the influence of symmetry81−83 in 4-chlorobenzonitrile, (R·ln(σsym = 2)), the entropy of sublimation increases from 178.4 to 184.1 J·K−1·mol−1, a value identical to the entropy of sublimation of 3-chlorobenzonitrile (184.9 J·K−1· mol−1). In contrast with other benzene derivatives,75,84,85 the lowest entropy of sublimation was obtained for the 2-chlorobenzonitrile isomer instead of the 4-chlorobenzonitrile isomer, the isomer with the highest symmetry. Although 2-chlorobenzonitrile is not a symmetric molecule, Britton detected six pseudoconversions of the four independent structures, as Figure 3 represents in shading. As each pseudoconversion

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ASSOCIATED CONTENT

S Supporting Information *

All the rotative-bomb combustion calorimetry experiments for 2-, 3-, and 4-chlorobenzonitrile isomers (Tables S1−S4) as well as the areas of the orifices of the cells (Table S5) used and the vapor pressure obtained by mass-loss Knudsen effusion method for each studied compound (Table S6). Tables S7 has all the details of DSC experiments. Cartesian coordinates of the optimized structures for the three chlorine benzonitrile isomers at G3MP2B3 and MP2/cc-pVTZ levels of theory, as well as the computed enthalpies for the studied compounds, are listed in Tables S8−S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. D. M. C. Ribeiro da Silva: tel, +351 22 0402 538; fax, +351 22 0402 659; e-mail address, [email protected].

Figure 3. Symmetry operations in the unit cell of 2-chlorobenzonitrile.

Notes

contributes with a symmetry of 2, the number of positions that the molecule of 2-chlorobenzonitrile can adopt, σsim, will be 12 (6 × 2, Figure 3). Removing the entropy contribution, R·ln(σsim = 12), the entropy of sublimation increases from 158.1 to 178.8 J·mol−1·K−1, a value closer to that of the asymmetric isomer, 3chlorobenzonitrile (184.9 J·mol−1·K−1).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and to FEDER for financial support ́ to Centro de Investigaçaõ em Quimica, University of Porto (strategic project PEst-C/QUI/UI0081/2013). I.M.R. and T.L.P.G. thank FCT and European Social Fund (ESF) under the Community Support Framework (CSF) for the award of Ph.D. fellowships (SFRH/BD/61915/2009 and SFRH/BD/ 62231/2009, respectively).

6. CONCLUSIONS The present study uses the results obtained by experimental techniques to explore the intermolecular interactions of these isomers, the polymorphism presented by 4-chlorobenzonitrile, and the specific case of pseudosymmetry in 2-chlorobenzonitrile. A vapor pressure study, performed by the mass-loss Knudsen effusion technique, allowed the standard molar enthalpies and entropies of sublimation of the studied isomers studied to be obtained. The strength of the intermolecular interactions was analyzed through the values of the enthalpies of sublimation: the 2-chlorobenzonitrile presents the lowest value of this set, whereas 4- and 3-chlorobenzonitrile isomers present a similar value. The observed enthalpic differentiation is mainly due to the chlorine···cyano intermolecular distances and the type of packing. The symmetry contribution in the entropies of

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Thermodynamic Study of Chlorobenzonitrile Isomers: A Survey on the

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