A Combined Experimental and Computational Thermodynamic


widely known as TNT),(4, 5) synthetic pesticides,(6-8) dyes, and pharmaceuticals,(9, 10) which are afterward introduced in the environment by huma...
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J. Phys. Chem. B 2010, 114, 12914–12925

A Combined Experimental and Computational Thermodynamic Study of Difluoronitrobenzene Isomers Manuel A. V. Ribeiro da Silva,* Manuel J. S. Monte, Ana I. M. C. Lobo Ferreira, ´ lvaro Cimas Juliana A. S. A. Oliveira, and A Centro de InVestigac¸a˜o em Quı´mica, Department of Chemistry and Biochemistry, Faculty of Science, UniVersity of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: August 30, 2010

This work reports the experimental and computational thermochemical study performed on three difluorinated nitrobenzene isomers: 2,4-difluoronitrobenzene (2,4-DFNB), 2,5-difluoronitrobenzene (2,5-DFNB), and 3,4difluoronitrobenzene (3,4-DFNB). The standard (p° ) 0.1 MPa) molar enthalpies of formation in the liquid phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T ) 298.15 K, measured by rotating bomb combustion calorimetry. A static method was used to perform the vapor pressure study of the referred compounds allowing the construction of the phase diagrams and determination of the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation, and fusion for two of the isomers (2,4-DFNB and 3,4-DFNB). For 2,5-difluoronitrobenzene, only liquid vapor pressures were measured enabling the determination of the standard molar enthalpies of vaporization. Combining the thermodynamic parameters of the compounds studied, the following standard (p° ) 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T ) 298.15 K, were derived: ∆fH°m(2,4DFNB, g) ) -(296.3 ( 1.8) kJ · mol-1, ∆fH°m(2,5-DFNB, g) ) -(288.2 ( 2.1) kJ · mol-1, and ∆fH°m(3,4DFNB, g) ) -(302.4 ( 2.1) kJ · mol-1. Using the empirical scheme developed by Cox, several approaches were evaluated in order to identify the best method for estimating the standard molar gas phase enthalpies of formation of these compounds. The estimated values were compared to the ones obtained experimentally, and the approach providing the best comparison with experiment was used to estimate the thermodynamic behavior of the other difluorinated nitrobenzene isomers not included in this study. Additionally, the enthalpies of formation of these compounds along with the enthalpies of formation of the other isomers not studied experimentally, i.e., 2,3-DFNB, 2,6-DFNB, and 3,5-DFNB, were estimated using the composite G3MP2B3 approach together with adequate gas-phase working reactions. Furthermore, we also used this computational approach to calculate the gas-phase basicities, proton and electron affinities, and, finally, adiabatic ionization enthalpies. 1. Introduction Nitroaromatic compounds are known to be highly toxic recalcitrant substances, very hazardous to both the environment and human beings. The nitro group, which provides chemical and functional diversity in these molecules, is also responsible for the increased recalcitrance of these compounds to biodegradation because of the electron-withdrawing nature of the group and combined with the stability of the benzene ring makes these compounds resistant to oxidative degradation.1–3 Most compounds belonging to this family are not found in nature but are usually obtained by incomplete combustion of fossil fuel or nitration reactions and used as chemical intermediates in the synthesis of explosives (e.g., 2,4,6-trinitrotoluene, widely known as TNT),4,5 synthetic pesticides,6–8 dyes, and pharmaceuticals,9,10 which are afterward introduced in the environment by human activity.11 The vast number of applications and the indiscriminate use of these materials have resulted in serious environmental pollution, predominantly through soil and groundwater contamination. Recently, several studies have been carried out to * Corresponding author. Phone: +351 22 0402 521. Fax: +351 22 0402 522. E-mail: [email protected]

establish more effective ways to qualitatively and quantitatively remove these dangerous compounds from the environment.3,12,13 Nitroaromatic compounds are also very hazardous to human health.14–16 An important characteristic of nitroaromatic compounds is their ability to rapidly penetrate the skin. Besides their acute toxicity and mutagenicity, they are easily reduced to carcinogenic aromatic amines.1–3 The role of difluorinated nitrobenzene compounds in chemistry is lesser known, and it seems that these compounds do not have a distinctive importance in the chemical world. On the other hand, they are very commonly used as intermediates in the synthesis of various compounds that have significant pharmaceutical, industrial, and agricultural applications.17–20 Research to further characterize these compounds has been increasing in the past few years. The regioselective synthesis of benzodiazepine derivatives, for example, is achieved by the cyclization of difluoronitrobenzenes.21 Benzodiazepines belong to a group of drugs used in the symptomatic treatment of anxiety and insomnia, and are considered as tranquilizers or sleep inducers (e.g., Diazepam, trade name Valium). In order to better understand the relative reactivity and the relationship between the energetics and structural properties of these compounds, we have examined the thermochemical

10.1021/jp1058885  2010 American Chemical Society Published on Web 09/22/2010

Thermodynamic Study of Difluoronitrobenzene Isomers properties of three difluoronitrobenzenes: 2,4-difluoronitrobenzene (2,4-DFNB); 2,5-difluoronitrobenzene (2,5-DFNB); and 3,4-difluoronitrobenzene (3,4-DFNB). This study is part of a broad research project that is being carried out in the University of Porto Chemical Research Center on the systematic study of the energetics of halogenated derivatives of various aromatic compounds such as anilines,22–24 nitroanilines,25–27 nitrobenzenes,28,29 phenols,30,31 etc. As a part of this project, we have previously published a thermochemical and thermodynamic study of the monofluorinated nitrobenzene isomers.32 2. Methods 2.1. Experimental Details. Materials and Purity Control. The compounds 2,4-difluoronitrobenzene [CAS 446-35-5]; 2,5difluoronitrobenzene [CAS 364-74-9]; and 3,4-difluoronitrobenzene [CAS 369-34-6] were purchased from Sigma-Aldrich Chemical Co., with an estimated minimum purity of 0.98 (mass fraction). The purification of these compounds, which are liquid at room temperature, was accomplished by successive fractional distillations under reduced pressure. The purity control of each difluoronitrobenzene isomer was carried out by gas chromatography, performed on an Agilent 4890D Gas Chromatograph equipped with an HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpolysiloxane (15 m × 0.530 mm i.d. × 1.5 µm film thickness), and using nitrogen as carrier gas. The temperature of the injector was set at 473 K, and the oven temperature was programmed as follows: 323 K (1 min), ramp at 10 K · min-1, 473 K (20 min). No impurities greater than 10-3 in mass fraction were detected in the samples of the difluoronitrobenzene isomers used for calorimetric and vapor pressure studies. Rotating Bomb Combustion Calorimetry Measurements. The massic energies of combustion of the compounds studied were determined by rotating bomb combustion calorimetry, from which the standard molar enthalpies of combustion were derived. An isoperibol rotating-bomb combustion calorimeter was used, following the design described in detail by Stig Sunner.33 The apparatus, as well as the operating technique, have already been described in detail,34–36 so only a brief summary of these aspects is presented here. The combustion reaction takes place in a stainless steel twin valve platinum lined combustion bomb, whose internal fittings are also constructed in platinum. It has an internal volume of 0.258 dm3 and wall thickness of 1 cm. The combustion bomb is suspended from the lid of the calorimeter can by a mechanism that also allows its simultaneous axial and end-over-end rotation. This movement permits the solution placed in the bomb to wash all internal surfaces of the bomb, yielding a homogeneous final solution. A mass of approximately 5222.5 g of distilled water, weighed in a Perspex vessel with a Mettler PM 11-N balance, sensitivity ((1 × 10-1) g, is added to the calorimeter can. A correction was applied to the energy equivalent in each experiment, to account for the difference between the mass of water used and the reference mass of 5222.5 g. The calorimeter can is inserted in an isothermal jacket which consists of a thermostatic bath containing a cavity of exactly the same shape as the calorimeter can, but 1 cm larger in overall dimensions, enclosed by a hollow lid. The jacket and lid are filled with water maintained at a temperature of ca. 303.5 K to ((1 × 10-3) K using a temperature controller (Tronac PTC 41). Calorimeter temperatures were automatically collected at regular intervals of 10 s by a Hewlett-Packard (HP-2804A)

J. Phys. Chem. B, Vol. 114, No. 40, 2010 12915 quartz crystal thermometer, sensitivity ((1 × 10-4) K, interfaced to a PC programmed to collect data and to accurately determine the adiabatic temperature change, using a version of the LABTERMO program.37 Besides the data acquisition, this software also allows the control of the calorimeter. For the main period and for both the initial and final periods, at least 100 temperature readings were taken. For all combustion experiments, the ignition temperature was estimated so that the final temperature would be as close as possible to T ) 298.15 K. The electrical energy of ignition was determined from the change in potential across a capacitor (1400 µF) when discharged through a platinum wire (φ ) 0.05 mm, Goodfellow, mass fraction 0.9999). For each combustion experiment of the difluoronitrobenzene isomers, the rotation of the bomb was started when the temperature rise in the main period reached about 0.63 of its total value and was continued throughout to the end of the experiment. With this procedure, the frictional work associated with bomb rotation and stirring is automatically included in the temperature corrections for the work of water stirring and for the heat exchanged with the thermostatted jacket, as shown by Good et al.38 The combustion of benzoic acid (NIST Standard Reference Material 39j), with a reported massic energy of combustion of -(26434 ( 3) J · g-1 under bomb conditions,39 was used to determine the energy equivalent of the calorimeter and to conventionally calibrate the calorimetric system. Following the procedure suggested by Coops et al.,40 the calibration experiments were executed without bomb rotation and in the presence of oxygen, at p ) 3.04 MPa, and 1.00 cm3 of deionized water. The mean of a set of seven calibration experiments led to the value of the energy equivalent of the calorimeter: ε(calor) ) (25157.4 ( 1.1) J · K-1,30 where the uncertainty quoted is the standard deviation of the mean. The combustion experiments of the three difluoronitrobenzenes were carried out in oxygen, at p ) 3.04 MPa, in the presence of 10.00 cm3 of water, with bomb rotation. The liquid compounds were burnt enclosed in polyethylene bags, [∆cu° ) -(46282.4 ( 4.8) J · g-1],41 a value measured in our laboratory by combustion of polyethylene samples. All the necessary weighing was made on a Mettler Toledo AE 240 balance, with a sensitivity of ((1 × 10-5) g, and the necessary corrections from apparent mass to true mass were calculated. The specific densities,42 used to calculate the true mass from the apparent mass in air for 2,4-; 2,5-; and 3,4-DFNB were 1.451, 1.467, and 1.437 g · cm-3, respectively. The relative atomic masses used in the calculation of all molar quantities throughout this paper were those recommended by the IUPAC Commission in 2007,43 yielding 159.0903 g · mol-1 for the molar mass of the difluoronitrobenzene isomers. The nitric acid, formed from traces of N2 introduced in the bomb as an impurity of the O2 used for pressurization, was analyzed by the Devarda’s alloy method.44 The standard molar energy for the formation of 0.1 mol · dm-3 HNO3(aq) from O2(g), N2(g), and H2O(l), -59.7 kJ · mol-1, was used for the correction relative to the nitric acid formation.45 The necessary correction for the cotton thread, of empirical formula CH1.686O0.843, used as an ignition fuse was based on ∆cu° ) -16240 J · g-1,40 which has been confirmed in our laboratory. Corrections to the standard state, ∆UΣ, were calculated by the procedure implemented by Good and Scott,46 for fluorine containing compounds, which was based on the method developed by Hubbard et al.,47 and including the values for the solubility of carbon dioxide in hydrofluoric acid solutions, provided by Cox et al.48 The value for the pressure coefficient of massic energy, (∂u/∂p)T, for the

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compounds studied was assumed to be -0.2 J · g-1 · MPa-1 at T ) 298.15 K, a typical value for most organic compounds.49 Vapor Pressure Measurements. A static method was employed to accurately determine the vapor pressures of the three studied difluoronitrobenzene isomers. The vapor pressures of the condensed phases of these compounds were measured using a static apparatus equipped with a capacitance diaphragm gauge, previously tested and described in detail.50,51 Two different Baratron manometers from MKS Instruments, Inc., operating at self-controlled constant temperatures, were used: Baratron 631A01TBEH (Tgauge ) 423 K) for measuring pressures in the range 0.3-130 Pa and in the sample temperature range 253-413 K and Baratron 631A11TBFP (Tgauge ) 473 K) for measuring pressures in the range 3-1300 Pa and in the sample temperature range 253-463 K. The temperature of the condensed sample was measured using a platinum resistance thermometer Pt100 class 1/10 (in a four wire connection), which is in good thermal contact with the sample, calibrated by comparison with a SPRT (25 Ω; Tinsley, 5187A). The uncertainty of the temperature measurements is estimated to be less than ((1 × 10-2) K. To avoid condensation of the vapor, the tubing between the condensed sample and the pressure gauge is kept at a temperature higher than the temperature of the sample and lower than the temperature of the gauge. 2.2. Computational Details. All the calculations were carried out at the G3MP2B3 level of theory52 using the Gaussian 03 series of programs.53 Within this approach, the geometry optimization and frequency calculation are done at the B3LYP/ 6-31G(d) level of theory.54,55 Then, single-point calculations are performed at higher levels of theory: quadratic configuration interaction (QCISD(T)) and second-order Moller-Plesset (MP2) with, respectively, the 6-31G(d) and GTMP2Large basis sets. The energies (with added zero-point vibrational energies scaled by 0.96 as usual with the G3MP2B3 method), computed at T ) 0 K, were thermally corrected for T ) 298.15 K by introducing the pV terms and the vibrational, translational, and rotational terms computed at the B3LYP/6-31G(d) level of theory. The enthalpy of formation of the six isomers was estimated after consideration of the following gas-phase atomization and isodesmic reactions:

Ribeiro da Silva et al. TABLE 1: Typical Combustion Results at T ) 298.15 K (p° ) 0.1 MPa) for the Three Studied Difluoronitrobenzene Isomersa experiment

2,4-DFNB

2,5-DFNB

3,4-DFNB

m(cpd)/g m′(fuse)/g m′′(polyethylene)/g Ti/K Tf/K ∆Tad/K εi/J · K-1 εf/J · K-1 ε(calor)corr/J · K-1 ∆m(H2O)/g -∆U(IBP)b/J ∆U(fuse)/J ∆U(polyethylene)/J ∆U(HNO3)/J ∆U(ign)/J ∆U∑/J -∆cu°/J · g-1

0.72708 0.00297 0.17181 297.3051 298.1488 0.82641 51.68 53.06 25158.7 0.3 20832.88 48.23 7951.55 35.58 1.20 17.84 17576.72

0.76259 0.00278 0.16309 297.2978 298.1517 0.83636 51.71 53.06 25154.1 -0.8 21079.90 45.15 7547.98 33.79 1.21 21.04 17613.58

0.64599 0.00283 0.16795 297.4036 298.1821 0.76195 51.56 52.87 25160.3 0.7 19209.02 45.96 7773.16 30.33 1.18 13.51 17563.83

a m(cpd), m′(fuse), and m′′(polyethylene) are, respectively, the mass of compound burnt, the mass of fuse (cotton), and the mass of polyethylene used in each experiment; Ti is the initial temperature rise; Tf is the final temperature rise; ∆Tad is the corrected temperature rise; εi and εf are the energy equivalent of contents in the initial and final states, respectively; ε(calor)corr is the energy equivalent of the calorimeter; ∆m(H2O) is the deviation of mass of water added to the calorimeter from 5222.5 g; ∆U(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; ∆U(fuse) is the combustion energy of the fuse (cotton); ∆U(polyethylene) is the energy of combustion of polyethylene; ∆U(HNO3) is the energy correction for the nitric acid formation; ∆U(ign) is the electrical energy for ignition; ∆U∑ is the standard state correction; ∆cu° is the standard massic energy of combustion. b ∆U(IBP) includes ∆U(ign).

calculations were also extended to anionic, cationic, and radical species of the six isomers. 3. Results and Discussion Experimental Enthalpies of Formation, in the Liquid Phase. Detailed results for a typical combustion experiment of each compound are given in Table 1. Equation 4, shown below, was used to determine the values of the energy associated with the isothermal bomb process, ∆U(IBP):

∆U(IBP) ) -{ε(calor) + ∆m(H2O)cp(H2O,l)}∆Tad + (Ti - 298.15 K)εi + (298.15 K - Ti - ∆Tad)εf + ∆Uign (4)

These reactions have been chosen on the basis of the available experimental thermochemical data for the compounds used there. The same computational approach was also used to calculate the proton and electron affinities, adiabatic ionization enthalpies, and gas-phase basicities. For that purpose, the G3MP2B3

where ∆m(H2O) is the deviation of the mass of water added to the calorimeter from 5222.5 g, the mass assigned to ε(calor), cp(H2O,l) is the heat capacity of liquid water, εi and εf are, respectively, the energy equivalents of the bomb contents in the initial and final states, ∆Tad is the calorimeter temperature change corrected for heat exchange, work of stirring, and the frictional work of bomb rotation, and ∆Uign is the electrical energy of ignition. ∆UΣ, the energy correction to the standard state (Washburn correction), was derived as recommended by Good and Scott for fluorine containing compounds.46 The remaining quantities are as previously defined.47,49 The detailed results for all the combustion experiments of each compound and the respective mean value of the standard massic energies of combustion, ∆cu°, are presented in the Supporting Information (Tables S1-S3).

Thermodynamic Study of Difluoronitrobenzene Isomers TABLE 2: Individual Values of the Standard (p° ) 0.1 MPa) Massic Energies of Combustion, ∆cu°, for the Three Studied Difluoronitrobenzenes, at T ) 298.15 K 2,4-DFNB 17576.72 17572.89 17580.85 17580.00 17578.64 17571.70 17576.8 ( 1.5 a

2,5-DFNB -∆cu°/J · g 17613.58 17637.76 17618.45 17634.60 17626.88 17627.65

3,4-DFNB

-1

-〈∆cu°〉a/(J · g-1) 17626.5 ( 3.8

17563.83 17550.75 17563.16 17542.56 17565.68 17556.88 17557.1 ( 3.7

Mean value and standard deviation of the mean.

TABLE 3: Derived Standard (p° ) 0.1 MPa) Molar Values in the Condensed Phase, at T ) 298.15 Ka compound

-∆cU°m(1)a/ kJ · mol-1

-∆cH°m(1)/ kJ · mol-1

-∆fH°m(1)/ kJ · mol-1

2,4-DFNB 2,5-DFNB 3,4-DFNB

2796.3 ( 0.9 2804.2 ( 1.4 2793.2 ( 1.4

2793.2 ( 0.9 2801.1 ( 1.4 2790.1 ( 1.4

354.8 ( 1.8 346.9 ( 2.1 357.9 ( 2.1

a The uncertainties are twice the overall standard deviation of the mean, and include the contributions from the calibration with benzoic acid and from the energy of combustion of auxiliary materials.

The individual values of the standard massic energy of combustion, ∆cu°, for all the combustion experiments of each compound, are presented in Table 2, along with the respective mean values, 〈∆cu0〉, and their standard deviations of the mean. The values of ∆cu° refer to the idealized combustion reaction of difluoronitrobenzenes, in which the only fluorine-containing product in the final state is HF · 10H2O(l), according to eq 5:

C6H3O2NF2(l) + 5.25O2(g) + 19.5H2O(l) f 6CO2(g) + 0.5N2(g) + 2HF · 10H2O(l) (5) Table 3 lists the derived values of the standard molar energies and enthalpies of combustion, ∆cU°m(l) and ∆cH°m(l), as well as the standard molar enthalpies of formation, ∆fH°m(l), in the liquid phase, at T ) 298.15 K, for the three isomers, which were derived from the values of ∆cH°m(l) and from the standard molar enthalpies formation, at T ) 298.15 K, of the following compounds: ∆fH°m(CO2,g) ) -(393.51 ( 0.13) kJ · mol-1;56 ∆fH°m(H2O,l) ) -(285.830 ( 0.040) kJ · mol-1;56 and ∆fH°m(HF · 10H2O,l) ) -(322.034 ( 0.650) kJ · mol-1.57 The uncertainties ascribed to the standard molar energies of combustion are, in each case, twice the overall standard deviation of the mean and contain the contributions from the calibration with benzoic acid and from the energy of combustion of polyethylene used as combustion auxiliaries.58,59 Vapor Pressures and Enthalpies of Phase Transitions. The measured vapor pressures for the liquid (both stable and supercooled) and for the crystalline phases of 2,4-DFNB and 3,4-DFNB are presented in Tables 4 and 6. Table 5 presents the measured vapor pressures for the liquid phase of 2,5-DFNB. As the temperature of fusion of this compound is lower than the lowest working temperature of the static apparatus, it was not possible to measure its sublimation vapor pressures. The experimental results of the solid and liquid vapor pressures were independently fitted by a truncated form of the Clarke and Glew (eq 6)60

J. Phys. Chem. B, Vol. 114, No. 40, 2010 12917 TABLE 4: Vapor Pressures of 2,4-Difluoronitrobenzenea T/K

p/Pa

260.38 261.42 263.32 264.28 265.27

0.60 0.68 0.88 0.99 1.12

∆p/Pa 0.00 0.00 0.01 0.00 0.00

T/K

p/Pa

∆p/Pa

T/K

Crystalline Phase 266.25 1.26 -0.01 273.17 267.23 1.42 -0.01 274.17 269.24 1.82 -0.01 275.23 271.20 2.33 0.01 272.19 2.60 -0.01

Liquid Phase 262.38b 1.14 -0.02 295.02 25.01 0.01 265.29b 1.57 -0.02 298.03 31.66 -0.17 b 2.17 0.01 300.99 39.98 -0.15 268.24 0.04 304.02 50.95 0.37 271.21b 2.95 b 3.92 0.01 306.98 62.94 -0.12 274.18 b 5.23 0.03 309.99 78.83 0.35 277.16 0.02 313.00 96.47 -0.69 280.11b 6.88 283.08 9.08 0.08 315.97 119.6 0.30 286.07 11.82 0.08 318.96 145.5 -0.60 289.03 15.19 0.02 321.93 177.5 -0.20 292.04 19.60 0.04 324.92 213.3 -2.10

p/Pa

∆p/Pa

2.95 0.02 3.29 -0.01 3.76 0.02

327.89 257.7 330.88 310.5 333.86 371.6 336.82 442.9 339.79 526.3 342.76 621.1 345.74 733.2 348.71 861.8 351.69 1011 354.67 1177

-2.00 -1.60 -1.60 -1.10 -0.20 -0.80 1.00 3.50 8.00 8.00

a ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew eq 6 with parameters given in Table 7. b Supercooled liquid.

TABLE 5: Vapor Pressures of 2,5-Difluoronitrobenzenea T/K

p/Pa

∆p/Pa

261.44 1.01 -0.03 264.33 1.43 0.01 267.25 1.95 0.02 270.27 2.63 0.00 273.21 3.59 0.06 276.17 4.72 0.01 279.13 6.27 0.03 282.12 8.23 0.01 285.09 10.74 0.00 288.06 13.87 -0.07 291.05 17.99 -0.01

T/K

p/Pa

∆p/Pa

Liquid Phase 294.05 23.06 -0.05 297.04 29.55 0.09 300.02 37.09 -0.21 303.00 47.16 0.20 306.00 59.23 0.34 309.00 73.18 -0.24 311.98 90.43 -0.50 314.97 111.6 -0.50 317.96 138.0 0.50 320.93 167.3 -0.40 323.92 203.9 0.10

T/K

p/Pa

326.88 244.6 329.88 297.2 332.81 352.6 335.84 423.3 338.82 503.0 341.77 597.4 344.76 702.1 347.77 831.1 350.70 968.5 353.72 1136

∆p/Pa -1.50 0.70 -1.70 -0.90 -1.20 1.50 -1.20 3.30 1.50 6.00

a ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew eq 6 with parameters given in Table 7.

TABLE 6: Vapor Pressures of 3,4-Difluoronitrobenzenea T/K

p/Pa

255.41 256.45 257.44

0.85 0.98 1.12

∆p/Pa

T/K

p/Pa

∆p/Pa

T/K

Crystalline Phase 0.00 259.47 1.47 0.00 263.37 0.00 261.45 1.92 0.00 264.33 0.00 262.39 2.18 0.01 265.30

258.43b 1.67 -0.02 0.00 261.43b 2.31 264.34b 3.09 -0.02 b 4.18 0.03 267.27 270.24 5.57 0.03 273.21 7.37 0.04 276.18 9.66 0.04 279.15 12.58 0.04 282.13 16.28 0.02 285.09 20.87 -0.04

Liquid Phase 288.07 26.67 -0.08 291.06 33.94 -0.11 294.06 42.96 -0.16 297.05 54.25 0.02 300.03 67.99 0.21 303.02 83.94 -0.39 305.99 105.0 0.80 309.00 127.5 -1.00 311.97 158.0 0.80 314.99 190.7 -1.30

p/Pa

∆p/Pa

2.46 0.00 2.79 0.00 3.15 -0.01

317.96 233.9 320.97 279.5 323.93 339.2 326.92 401.8 329.89 482.7 332.86 570.8 335.83 676.2 338.82 796.4 341.80 937.1 344.76 1097

1.20 -1.90 1.40 -2.80 0.80 -1.00 0.30 -0.30 2.00 4.00

a ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew eq 6 with parameters given in Table 7. b Supercooled liquid.

∆gcdG°m(θ) p 1 1 R ln )+ ∆gcdC°p,m(θ) + ∆gcdH°m(θ) p° θ θ T θ T - 1 + ln (6) T θ

( )

( ) ( )] [( )

where p is the vapor pressure at the temperature T, p° is a selected reference pressure (in this study, p° ) 105 Pa), θ is a selected reference temperature (in this study, θ ) 298.15 K), R g G°m is the molar gas constant (R ) 8.31447 J · K-1 · mol-1), ∆cd is the difference in molar Gibbs energy between the gaseous and the crystalline or liquid phases (condensed phases) at the selected reference pressure, and ∆gcdH°m is the difference in molar g C°p,m enthalpy between the gaseous and condensed phases. ∆cd

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Ribeiro da Silva et al.

TABLE 7: Parameters of the Clarke and Glew Equation (eq 6), Derived from Static Vapor Pressure Results for the Difluoronitrobenzene Isomers phase

∆T/K

2,4-DFNB crystalline

260.38-275.23

liquidf

262.38-354.67

2,5-DFNB liquid

261.44-353.72

3,4-DFNB crystalline

255.41-265.30

liquidf

258.43-344.76

θ/K b

298.15 267.81d 282.59e 298.15 308.53d 282.59e 298.15 307.58d 298.15b 260.36d 268.04e 298.15 301.60d 268.04e

g G°m(θ)/ ∆cr,1 kJ · mol -1

g ∆cr,1 H°m(θ)/ kJ · mol-1

g ∆cr,1 S°m(θ)/ J · K -1 · mol-1

19.18 ( 0.02 24.68 ( 0.01 21.99 ( 0.01 19.94 ( 0.01 18.61 ( 0.01 21.99 ( 0.01 19.93 ( 0.01 18.72 ( 0.01 16.50 ( 0.02 23.83 ( 0.01 22.32 ( 0.01 18.44 ( 0.01 18.01 ( 0.01 22.32 ( 0.01

72.8 ( 0.4 73.6 ( 0.2 73.2 ( 0.2 58.53 ( 0.04 57.51 ( 0.03 60.07 ( 0.03 58.74 ( 0.04 57.81 ( 0.04 73.8 ( 0.4 74.8 ( 0.2 74.6 ( 0.2 55.49 ( 0.03 55.19 ( 0.03 58.14 ( 0.03

179.7 ( 0.6 182.6 ( 0.7 181.2 ( 0.7 129.4 ( 0.1 126.1 ( 0.1 134.8 ( 0.1 130.2 ( 0.1 127.1 ( 0.1 192.2 ( 0.5 195.8 ( 0.6 195.0 ( 0.6 124.3 ( 0.1 123.3 ( 0.1 133.6 ( 0.1

p(θ)/Pa 43.63 1.54 8.61 32.11 70.69 8.61 32.24 66.21 128.6 1.66 4.48 58.81 76.01 4.48

R2

∆cr,1 g C° p,m(θ)/ J · K -1 · mol-1 c

sa

0.9999

26.7

0.005

1.0000

98.7 ( 2.8g

0.007

1.0000

98.7 ( 2.9g

0.007

1.0000

26.7c

0.003

1.0000

87.9 ( 2.3g

0.005

n s is the standard deviation of the fit defined as s ) [(∑i)1 (ln p - ln pcalc)i2)/(n - m)]1/2, where n is the number of experimental points used in the fit and m is the number of adjustable parameters in the Clarke and Glew equation. b Given that, at T ) 298.15 K, the compound studied is in the liquid phase, the thermodynamic parameters reported for the crystalline phase are considered virtual. c Estimated value. d Mean temperature. e Calculated temperature of triple point. f Including supercooled liquid. g Adjustable parameter. a

is the difference between the heat capacities of the perfect gas and that of the condensed phase. The experimental results derived from the fittings of eq 6 to the experimental (T,p) results are presented in Table 7. For the sublimation data, the value of g C°p,m ) -26.7 J · K-1 · mol-1 used in these fittings was ∆cr estimated from eq 7.61 o ∆gcrC°p,m(θ)/(J · K-1 · mol-1) ) -{0.9 + 0.176Cp,m (g)} (7)

This equation is a rearrangement of eq 8 suggested by Chickos et al.62,63 for estimation of {C°p,m(g) - C°p,m(cr)}, at T ) 298.15 K, from a known value of C°p,m(cr):

∆gcrC°p,m(θ)/(J · K-1 · mol-1) ) -{0.75 + 0.15C°p,m(cr)} (8)

TABLE 8: Standard (p° ) 0.1 MPa) Molar Enthalpies of Fusion, at T ) 298.15 K and at the Temperature of the Triple Point compound

θ/K

1 ∆cr H°p,m(θ)/kJ · mol-1

2,4-DFNB

298.15 282.59a 298.15 268.04a

14.3 ( 0.3 13.1 ( 0.3 18.3 ( 0.2 16.4 ( 0.2

3,4-DFNB a

Triple point.

TABLE 9: Standard (p° ) 0.1 MPa) Molar Enthalpies of Formation, in Condensed and Gaseous Phases, and Standard Molar Enthalpies of Vaporization, at T ) 298.15 Ka compound

-∆fH°m(l)/ kJ · mol-1

∆lgH°m(298.15 K)/ kJ · mol-1

-∆fH°m(g)/ kJ · mol-1

2,4-DFNB 2,5-DFNB 3,4-DFNB

354.8 ( 1.8 346.9 ( 2.1 357.9 ( 2.1

58.5 ( 0.2 58.7 ( 0.2 55.5 ( 0.2

296.3 ( 1.8 288.2 ( 2.1 302.4 ( 2.1

a

63

According to the procedure followed by Roux et al., the uncertainty for the correction of the experimental enthalpy of sublimation at the mean temperature to another temperature may be estimated as being equal to one-third the magnitude of the total temperature adjustment. These uncertainties were taken into account in the calculated values of the enthalpies of sublimation at T ) 298.15 K and at the temperature of the triple point presented in Table 7. For the compounds covered in this study, the value of C°p,m(g) ) 146.72 J · K-1 · mol-1 was inserted into eq 7, following estimation using a second-order group additivity approach developed by Benson et al.:64 C°p,m(DNFB) ) {(3 × CB-(H)(CB)2) + (2 × CB-(F)(CB)2) + CB-(NO2)(CB)2}

(9)

using the following data provided by Domalski and Hearing:65 C°p,m[CB-(H)(CB)2,g] ) 13.61 J · K-1 · mol-1, C°p,m[CB-(F)(CB)2,g] ) 26.10 J · K-1 · mol-1, and Ribeiro da Silva et al.:26 C°p,m[CB-(NO2)(CB)2,g] ) 53.69 J · K-1 · mol-1. The results of ∆g1C°p,m, also presented in Table 7, were obtained directly from the regression of the fitting of the liquid vapor pressure-temperature data to eq 6. This table also contains the

The standard enthalpies of vaporization were calculated from the fitting of the liquid vapor pressure results to eq 6, presented in Table 7. The uncertainties were estimated to be about 5 times greater than those presented in Table 7.

Figure 1. Structural formula of the difluoronitrobenzene isomers.

experimental temperature intervals and the differences in molar standard entropy between the gaseous and condensed phases, g S°m(θ), at three different temperatures (298.15 K, mean ∆cr,1 experimental temperature, and triple point); the (p,T) coordinates of the triple point are also registered. As the mathematical uncertainties of the enthalpies of vaporization derived from the fitting of eq 6 look rather small (about 5 times smaller than the uncertainties derived for the enthalpy of vaporization at the mean temperature when using the Clausius-Clapeyron equation), the uncertainties of the enthalpies of vaporization presented in Table 9 were estimated to be 5 times greater than the values derived from the fitting of eq 6. Figures 2-4 represent the phase diagrams (ln p versus 1000/ T) at low pressures of the isomers studied. As mentioned before, it was not possible to perform the vapor pressure study of the

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Figure 2. Phase diagram of 2,4-difluoronitrobenzene at low pressures. O, stable liquid vapor pressures; b, supercooled liquid vapor pressures; 0, sublimation vapor pressures.

Figure 3. Liquid vapor pressures of 2,5-difluoronitrobenzene at low pressures.

crystalline phase of the 2,5-DFNB isomer, so only liquid vapor pressures are presented in Figure 3. Table 8 presents the enthalpies of fusion of 2,4- and 3,4difluoronitrobenzene derived as the differences between the molar enthalpies of sublimation and of vaporization, at T ) 298.15 K and at the temperature of the triple point. Experimental Enthalpies of Formation, in the Gaseous Phase. Table 9 summarizes the derived standard molar enthalpies of formation in the gaseous phase, ∆fH°m(g), at T ) 298.15 K, for the three difluoronitrobenzene isomers. When comparing these experimental results with the ones obtained for the monofluoronitrobenzene isomers,32 it is possible to conclude that the insertion of two fluorine atoms as substituents in the nitrobenzene ring introduced a stabilizing effect more significant than that observed in the case of the monofluorinated isomers.

From the difluorinated isomers studied, the 3,4-isomer is enthalpically more stable when compared to 2,4- and 2,5difluoronitrobenzene, as shown in the scheme in Figure 5. The decrease in enthalpic stability of these two isomers is explained predominantly by the strong steric repulsion between the nitro group and fluorine atom substituted on adjacent carbons. This enthalpic effect has already been observed in the ortho-halonitrobenzenes28,29 and ortho-halonitroanilines derivatives25–27 studied in this laboratory, as well as in the dihalogenated derivatives of the families of compounds previously mentioned. The electron withdrawing potential of the nitro group is maximized when it is coplanar with the aromatic π ring in the molecule, so the potential for internal rotation of the nitro group in nitroaromatic compounds can be interpreted as a measure of the strength of π-interaction in the molecule. The optimized

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Figure 4. Phase diagram of 3,4-difluoronitrobenzene at low pressures. O, stable liquid vapor pressures; b, supercooled liquid vapor pressures; 0, sublimation vapor pressures.

Figure 5. Enthalpic effect of the introduction of two fluorine atoms in the 2,4; 2,5; and 3,4 positions of the aromatic ring of nitrobenzene and isomerization energies (all values are in kJ · mol-1).

geometries of aromatic molecules tend to reflect the role played by resonance in these systems. However, in this case, because of the proximity between the fluorine atom and the closest oxygen of the nitro group, the inductive and resonance stabilizing effects of the nitro group are counteracted by the magnitude of the steric repulsion effect present in the 2,4- and 2,5-difluorinated isomers, which suggests that the extent of the conjugation of the NO2 group with the aromatic ring is comparatively small and that steric repulsion of the ortho substituents in the nitrobenzene plays a significant role in the geometrical structure of the molecules.67 As a result, an induced twisting of the nitro group from the plane of the benzene ring to a nonplanar arrangement occurs, resulting in less conjugation between the benzene and the nitro group’s π orbitals, which accounts for the characteristic decrease in enthalpic stability of these two compounds. Even though the 2,4- and 2,5-DFNB isomers both have the nitro group and fluorine atom substituted on adjacent carbons,

the first one is slightly more stable than the latter. This difference in enthalpic stability is related to the position of the second fluorine atom in the aromatic ring. According to what was previously observed for the monofluorinated isomers,32 the introduction of a fluorine atom in the nitrobenzene ring is enthalpically more favorable in the para position, as opposed to the meta position. Then, we can interpret the difference in enthalpic stability as a reproduction of this behavior for the introduction of the second fluorine atom which justifies the additional enthalpic stability of the 2,4-DFNB isomer when compared to the 2,5-DFNB isomer. The results obtained can also be interpreted from the point of view of the interaction between the two fluorine atoms in the aromatic ring. The standard molar enthalpies of formation, in the gaseous phase, of the three difluorobenzene isomers are available in the literature: ∆fH°m(o-difluorobenzene,g) ) -(293.7 ( 0.9) kJ · mol-1,66 ∆fH°m(m-difluorobenzene,g) ) -(309.2 ( 1.0) kJ · mol-1,66 ∆fH°m(p-difluorobenzene,g) )

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Figure 6. Empirical schemes for estimation of ∆fH°m(g) by the Cox scheme.

-(306.7 ( 1.0) kJ · mol-1.66 When analyzing their relative stability, the meta isomer is slightly more stable then the para isomer, which is compatible with what was observed for the 2,4- and 2,5-DFNB isomers, taking into account the relative positions of their fluorine atoms. As previously mentioned, as a consequence of the steric repulsions suggested in the 2,4- and 2,5-difluorinated compounds, a deviation from the planarity is to be expected in the structural analysis of these compounds, as a result of the torsion of the nitro group relatively to the benzene ring. The literature has no structural studies of these particular compounds, which could support this supposition. However, Dorofeeva et al.68 have performed molecular structure studies on 3,5-difluoronitrobenzene (3,5-DFNB) and 2,6-difluoronitrobenzene (2,6-DFNB), by gas phase electron diffraction and theoretical calculations. In their studies, they confirmed that, while the 3,5-DFNB gaseous molecule is planar, the 2,6-DFNB gaseous molecule is not. They detected, as would be expected, rotation of the nitro group from planarity, reporting a dihedral angle of 53.8°, between the planes of the benzene ring and that of the nitro group. This is a greater angle than previously reported for 2-fluoronitrobenzene (31.7° by Correll et al.69 and 37.6° by Shishkov et al.70). Computational gas-phase molecular structures of the six difluoronitrobenzene isomers performed in the present work (see the section GasPhase Molecular Structures, further along this paper) confirmed this result. Enthalpies of Formation Estimated by the Cox Scheme. In order to further discuss and interpret the results obtained experimentally, the enthalpies of formation in the gaseous state were also estimated by the method suggested by Cox.71 His empirical scheme is based on the transferability of enthalpic group contributions in benzene derivatives, assuming that each group, when bounded to the benzene ring, produces a characteristic enthalpic increment in the enthalpy of formation in the

gaseous phase. The addition of a correction term of +4 kJ · mol-1 is necessary whenever a pair of substituents are in ortho positions and when three substituents are attached to consecutive carbon atoms of the aromatic ring. Using the methodology developed by Cox, it was possible to estimate ∆fH°m(g) for the difluoronitrobenzene isomers according to the four different approaches, presented in Figure 6, taking into account the following literature values of the gasphase standard molar enthalpies of formation: ∆fH°m(benzene,g) )(82.6 ( 0.7)kJ · mol-1,66 ∆fH°(fluorobenzene,g))-(115.9 ( 1.4) m kJ · mol-1,66 ∆fH°m(nitrobenzene,g) ) (67.5 ( 0.5) kJ · mol-1,66 ∆fH°(o-difluorobenzene,g) ) -(293.7 ( 0.9) kJ · mol-1,66 ∆fH°(mm m difluorobenzene,g) ) -(309.2 ( 1.0) kJ · mol-1,66 ∆fH°m(pdifluorobenzene,g) ) -(306.7 ( 1.0) kJ · mol-1,66 ∆fH°m(ofluoronitrobenzene,g) ) -(102.4 ( 1.5) kJ · mol-1,32 ∆fH°m (m-fluoronitrobenzene,g) ) -(128.0 ( 1.7) kJ · mol-1,32 and ∆fH°m(p-fluoronitrobenzene,g) ) -(133.9 ( 1.4) kJ · mol-1.32 In previous studies made in our research group, it was established that the correction of +4 kJ · mol-1 suggested by Cox is not sufficient to account for the destabilization effects observed as a result of the steric interaction between a halogen atom and the nitro group as ortho substituents in a benzene ring and that a larger enthalpic correction is needed. Ribeiro da Silva et al.25 suggested an additional correction term of ∼+22 kJ · mol-1 in this case, in addition to the +4 kJ · mol-1 correction recommended by Cox. This correction has been proven adequate, when applied to other compounds exhibiting the same substitution pattern.26–29,32 Taking into consideration all the necessary corrections, the estimated values were calculated for each of the proposed approaches, as presented in Figure 6. The results are compiled in Table 10 along with the experimental results and the deviation ∆ between them.

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TABLE 10: Experimental and Estimated Gas-Phase Enthalpies of Formation of the Three Studied Difluoronitrobenzene Isomers -∆fH°m(g) Cox/kJ · mol-1 compound

-∆fH°m(g) Exp/kJ · mol-1

2,4-DFNB 2,5-DFNB 3,4-DFNB

296.3 ( 1.8 288.2 ( 2.1 302.4 ( 2.1

a

I

II

III

∆a/kJ · mol-1 IV

I

II

III

IV

303.5 ( 2.3 298.3 ( 1.3 300.9 ( 2.2 303.8 ( 2.1 7.2 ( 2.9 2.0 ( 2.2 4.6 ( 2.8 7.5 ( 2.8 303.5 ( 2.3 295.8 ( 1.3 300.9 ( 2.2 297.9 ( 2.3 15.3 ( 3.1 7.6 ( 2.5 12.7 ( 3.0 9.7 ( 3.1 325.5 ( 2.3 308.8 ( 1.2 322.5 ( 2.3 325.4 ( 2.3 23.1 ( 3.1 6.4 ( 2.4 20.1 ( 3.1 23.0 ( 3.1

Difference between the experimental and the estimated values.

Approaches I, III, and IV do not reproduce the experimental results very well, leading to deviations that exceed the (10 kJ · mol-1 limit of acceptance for agreement between experimental and estimated values, as indicated by Cox for his scheme.71 As can be observed, the estimated values show in general a greater enthalpic stability when compared to the experimental results. Approach I is based on the sum of the individual contributions of each substituent in the aromatic ring, obtained from the corresponding monosubstituted benzene derivatives. This approach neglects consideration of some of the steric, inductive, and mesomeric interactions between the substituents in the ring, reasons that justify the disparity between estimated and experimental values. Both approaches III and IV include consideration of the interaction between a fluoride atom and the nitro group, since estimates of the enthalpic contribution of this interaction are included in the enthalpies of formation of each monofluoronitrobenzene isomer. This approach led to better results than those obtained with approach I but still not within limits of agreement. The approach that best applies to the estimation of the enthalpies of formation in the gaseous phase of the compounds studied is approach II, which uses the difluorobenzene derivatives, which includes the interaction between two fluorine atoms substituted on the aromatic ring, along with the additional correction of ∼ +22 kJ · mol-1 necessary to account for the interaction of a fluorine atom and an ortho substituted nitro group. The results estimated by this approach are in good agreement with the experimental values, yielding deviations that are well within the limit of acceptance previously mentioned.71 After determining that approach II is the best one, we applied it to the estimation of the standard molar enthalpy of formation of the other difluorinated isomers not covered by the study: 2,3difluoronitrobenzene (2,3-DFNB); 2,6-difluoronitrobenzene (2,6DFNB); and 3,5-difluoronitrobenzene (3,5-DFNB). A compilation of the estimates for all difluoronitrobenzene isomers can be found in Table 11. These estimates show that the most stable isomer should clearly be 3,5-DFNB, where all substituents are attached to alternate carbons, minimizing steric repulsions. Also, 2,6-DFNB should be the least stable isomer, especially due to strong steric repulsions, since the nitro group is flanked by two fluorine atoms in both adjacent positions, forcing the twisting of the nitro group in an accentuated angle, as mentioned earlier. These conclusions are compatible with what was previously reasoned for the isomers studied experimentally. Gas Phase Molecular Structures. In Figure 7, we present the optimized geometries of the six different difluoronitrobenzene isomers calculated at the G3(MP2)//B3LYP level of theory. Bond distances and angles are included. All the isomers studied containing a fluorine atom in an ortho position with respect to the nitro group are not planar. The isomers which present the major deviation from planarity are those which have both fluorine atoms in the ortho position with

TABLE 11: Comparison between the Experimental, Estimated, and Computed G3MP2B3 Gas-Phase Enthalpies of Formation of the Six Isomers of Difluoronitrobenzene, at T ) 298.15 Ka -∆fH°m(g)/kJ · mol-1 G3MP2B3

compound

estimated Cox scheme

atomization reaction (eq 4)

eq 5

2,3-DFNB 2,4-DFNB 2,5-DFNB 2,6-DFNB 3,4-DFNB 3,5-DFNB

278.8 ( 1.2 298.3 ( 1.3 295.8 ( 1.3 268.3 ( 1.3 308.8 ( 1.2 324.3 ( 1.3

274.1 291.9(4.4) 286.1(2.1) 273.6 302.0(0.4) 313.4

272.4 290.6(5.7) 285.0(3.2) 272.3 300.3(2.1) 312.0

eq 6

experimental value

275.0 295.6(0.7) 296.3 ( 1.8 287.1(1.1) 288.2 ( 2.1 279.3 303.0(-0.6) 302.4 ( 2.1 320.9

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

respect to the nitro group (2,6-DFNB, φ(C-N) ) 43.3°), and one in the ortho position and the other in the meta position with respect to the nitro group (2,3-DFNB, φ(C-N) ) 11.7°). This effect is due to the steric interaction between the fluorine atoms and with the nitro group. Two other isomers, i.e., 2,4DFNB and 2,5-DFNB, are not planar but are very close to being planar, having dihedral angles of 6.0 and 5.6°, respectively. Finally, the other two isomers which do not have a fluorine atom in the ortho position, i.e., 3,4-DFNB and 3,5-DFNB, are planar. These results are in very good agreement with the observations of Dorofeeva et al.68 As it is shown in Figure 7, the ortho C-F distances are shorter in the 2,X-DFNB (6 > X >2) isomers than in the 3,X-DFNB (6 > X >3) ones. The ortho 2,3-DFNB has the shortest C-F distance. In 2-fluoronitrobenzene,32 this fact can be attributed to the polar effect of the nitro group. Gas-Phase Theoretical Enthalpies of Formation. The gasphase enthalpies of formation of the six isomers studied were estimated using the reactions described by eqs 4-6 and the experimental enthalpies of formation in the gaseous phase of the other atoms and molecules involved. The values of ∆fH°m(g) were as follows: carbon, 716.7 kJ · mol-1;72 hydrogen, 218.0 kJ · mol-1;72 oxygen, 249.2 kJ · mol-1;72 nitrogen, 472.7 kJ · mol-1;72 fluorine, 79.4 kJ · mol-1;72 benzene, 82.6 kJ · mol-1;66 nitrobenzene, 67.5 kJ · mol-1;66 1,2-difluorobenzene, -293.7 kJ · mol-1;66 aniline, 87.1 kJ · mol-1;66 2-fluoronitrobenzene, -102.4 kJ · mol-1;32 2-fluoroaniline, -107.2 kJ · mol-1;66 1,3difluorobenzene, -309.2 kJ · mol-1;66 3-fluoroaniline, -115.6 kJ · mol-1;66 1,4-difluorobenzene, -306.7 kJ · mol-1;66 4-fluoroaniline, -190.6 kJ · mol-1;66 3-fluoronitrobenzene, -128.0 kJ · mol-1;32 and 4-fluoronitrobenzene, -133.9 kJ · mol-1.32 In Table 11, we report the experimental enthalpies of formation along with the calculated ones. From the table, we can conclude that the agreement between the experimental and G3MP2B3 calculated values are very good. The maximum deviations from the experimental results are those corresponding

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Figure 7. Front and side views of the B3LYP/6-31G(d) optimized geometries of the six different isomers of difluoronitrobenzene. Distances are given in Å and angles in degrees. The atom numbering is the same for all six isomers.

to 2,3-DFNB, 5.7 and 4.4 kJ · mol-1. These deviations are within the uncertainty associated with the experimental and calculated values.52 At the G3MP2B3 level, the most stable isomer is the 3,5DFNB followed by the 3,4-DFNB which lies 11.3 kJ · mol-1 higher. Finally, the less stable one is the 2,6-DFNB which lies more than 30 kJ · mol-1 higher. Other Gas-Phase Thermodynamic Properties. We have also computed other thermodynamic properties for all the six possible isomers of difluoronitrobenzene using the G3MP2B3 approach. The calculated values of gas-phase basicity (∆Gbasicity), proton (PA) and electron affinities (EA), and adiabatic ionization enthalpies are registered in Table 12. The calculated gas-phase basicities allow us to propose the following order in basicity: 2,4-DFNB > 2,6-DFNB > 3,4-DFNB > 2,3-DFNB > 2,5-DFNB > 3,5-DFNB. Proton affinities follow the same pattern. In all cases, both oxygen atoms have similar basicities. No experimental or computational data have been found in the literature for comparison with our results on gasphase basicity and proton affinity. From Table 12, it can be seen that the ionization enthalpies of all six isomers are similar. 2,5-DFNB is the compound for which the electron is removed easier. Finally, with respect to

TABLE 12: G3MP2B3 Computed Gas-Phase Basicities, ∆Gbasicity, Proton Affinities (PA), Electron Affinities (EA), and Adiabatic Ionization Enthalpies (IE), at T ) 298.15 K, for the Six Isomers of Difluoronitrobenzenea compound

∆Gbasicity

PA

EA

IE

2,3-DFNB

770.6 (O1) 769.7 (O2) 781.2 (O1) 780.4 (O2) 769.6 (O1) 768.4 (O2) 774.2 (O1) 774.2 (O2) 771.3 (O1) 771.1 (O2) 758.9 (O1) 758.9 (O2)

771.6 (O1) 771.0 (O2) 783.5 (O1) 782.6 (O2) 773.0 (O1) 772.1 (O2) 772.1 (O1) 772.1 (O2) 770.3 (O1) 770.1 (O2) 758.0 (O1) 758.0 (O2)

109.4

962.8

96.2

963.5

111.9

948.8

81.2

965.4

109.7

971.7

124.3

971.3

2,4-DFNB 2,5-DFNB 2,6-DFNB 3,4-DFNB 3,5-DFNB a

All values are given in kJ · mol-1.

the electron affinities, there are larger differences between the six isomers. 3,5-DFNB has the largest electron affinity followed by 2,5-DFNB > 3,4-DFNB ≈ 2,3-DFNB > 2,4-DFNB > 2,6DFNB. No computational or experimental values regarding these properties have been found in the literature.

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4. Final Remarks The G3MP2B3 approach was used to estimate the gas-phase enthalpies of formation of all the isomers of difluoronitrobenzene, at T ) 298.15 K, and considering several appropriate working reactions. All computed values are in agreement with the experimental data reported here; all are in the range of the experimental and computational uncertainties. Other thermodynamic properties were also calculated by means of this composite method. Acknowledgment. Thanks are due to Fundac¸a˜o para a Cieˆncia e Tecnologia (FCT), Lisbon, Portugal, and to FEDER for financial support to Centro de Investigac¸a˜o em Quı´mica, University of Porto. A.I.M.C.L.F. thanks FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the postdoctoral fellowship (SFRH/BPD/ 27053/2006). Supporting Information Available: Details of all the combustion calorimetry experiments for the three difluoronitrobenzenes studied, as well as the fully optimized structures and the calculated energies of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ju, K.-S.; Parales, R. E. Microbiol. Mol. Biol. ReV. 2010, 74, 250– 272. (2) Cronin, M. T. D.; Gregory, B. W.; Schultz, T. W. Chem. Res. Toxicol. 1998, 11, 902–908. (3) Snellinx, Z.; Nepovı´m, A.; Taghavi, S.; Vangronsveld, J.; Vanek, T.; Van der Lelie, D. EnViron. Sci. Pollut. Res. 2002, 9, 48–61. (4) Dennis, W. H., Jr.; Rosenblatt, D. H.; Blucher, W. G.; Coon, C. L. J. Chem. Eng. Data 1975, 20, 202–203. (5) Dorey, R. C.; Carper, W. R. J. Chem. Eng. Data 1984, 29, 93–97. (6) Sipyagina, A. M.; Enshovb, V. S.; Kashtanovb, S. A.; Batemana, C. P.; Mullena, B. D.; Tana, Y.-T.; Thrashera, J. S. J. Fluorine Chem. 2004, 125, 1305–1316. (7) Tas, D. O.; Pavlostathis, S. G. J. Agric. Food Chem. 2007, 55, 5390–5398. (8) Van Alfen, N. K.; Kosuge, T. J. Agric. Food Chem. 1976, 24, 584– 588. (9) Pinheiro, H. M.; Touraud, E.; Thomas, O. Dyes Pigm. 2004, 61, 121–139. (10) Srinivasa, G. R.; Abiraj, K.; Gowda, D C. Aust. J. Chem. 2004, 57, 609–610. (11) Kulkarni, M.; Chaudhari, J. EnViron. Manage. 2007, 85, 496–512. (12) Cho, Y.-S.; Lee, B.-U.; Oh, K.-H. J. Chem. Technol. Biotechnol. 2008, 83, 1211–1217. (13) Doty, S. L. New Phytol. 2008, 179, 318–333. (14) Bronaugh, R. L.; Maibach, H. I. J. InVest. Dermatol. 1985, 84, 180–183. (15) Letzel, S.; Go¨en, Th.; Bader, M.; Angerer, J.; Kraus, T. Occup. EnViron. Med. 2003, 60, 483–488. (16) Tchounwou, P. B.; Newsome, C.; Glass, K.; Centeno, J. A.; Leszczynski, J.; Bryant, J.; Okoh, J.; Ishaque, A.; Brower, M. ReV. EnViron. Health 2003, 18, 203–229. (17) Baek, J.-B.; Harris, F. W. J. Polym. Sci., Part A 2005, 43, 801– 814. (18) Suzuki, Y.; Toyota, T.; Miyashita, A.; Sato, M. Chem. Pharm. Bull. 2006, 54, 1653–1658. (19) Kotovskaya, S. K.; Zhumabaeva, G. A.; Perova, N. M.; Baskakova, Z. M.; Charushin, V. N.; Chupakhin, O. N.; Belanov, E. F.; Bormotov, N. I.; Balakhnin, S. M.; Serova, O. A. Pharm. Chem. J. 2007, 41, 62–68. (20) Krcho`a´k, V.; Smith, J.; Va´gner, J. Tetrahedron Lett. 2001, 42, 1627– 1630. (21) Parker, K. A.; Coburn, C. A. J. Org. Chem. 1992, 57, 97–100. (22) Ribeiro da Silva, M. A. V.; Lobo Ferreira, A. I. M. C.; Gomes, J. R. B. J. Phys. Chem. B 2007, 111, 2052–2061. (23) Ribeiro da Silva, M. A. V.; Lobo Ferreira, A. I. M. C.; Gomes, J. R. B. Bull. Chem. Soc. Jpn. 2006, 79, 1852–1859. (24) Ribeiro da Silva, M. A. V.; Lobo Ferreira, A. I. M. C.; Gomes, J. R. B. J. Phys. Chem. B 2005, 109, 13356–13362. (25) Ribeiro da Silva, M. A. V.; Lima, L. M. S. S.; Moreno, A. R. G.; Lobo Ferreira, A. I. M. C.; Gomes, J. R. B. J. Chem. Thermodyn. 2008, 40, 155–165.

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