Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers

J. Chem. Thermodynamics 41 (2009) 361–366

Contents lists available at ScienceDirect

J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct

Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers Manuel A.V. Ribeiro da Silva *, Ana I.M.C. Lobo Ferreira Centro de Investigação em Química, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 15 September 2008 Accepted 17 September 2008 Available online 23 September 2008 Keywords: Thermochemistry Energy of combustion Enthalpy of vaporization Enthalpy of formation Rotating bomb combustion calorimetry Calvet microcalorimetry Cox scheme Fluoroanisole isomers

a b s t r a c t The standard (p = 0.1 MPa) molar enthalpies of formation in the liquid phase of three isomers of fluoroanisole were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g) and HF  10H2O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization of these compounds, also at T = 298.15 K, were determined using Calvet microcalorimetry. 1

Dc U m ðlÞ=ðkJ  mol 2-Fluoroanisole 3- Fluoroanisole 4- Fluoroanisole

3629.7 ± 1.0 3617.1 ± 1.1 3622.4 ± 1.2

Þ

Df Hm ðlÞ=ðkJ  mol 301.9 ± 1.5 314.5 ± 1.6 309.2 ± 1.6

1

Þ

Dgl Hm =ðkJ  mol1 Þ 52.2 ± 1.1 48.1 ± 1.1 48.7 ± 1.1

The standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived from the former two experimental quantities. These values are also compared with estimates based on two different methodologies: one using the empirical scheme developed by Cox and the other one based on highlevel density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311++G(d,p) basis set. The computed values and the estimated values using the Cox method compare well with the experimental results obtained in this work. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Recently, we have reported experimental thermochemical studies of the monochloroanisole (monochloromethoxybenzene) [1] and dichloroanisole (dichloromethoxybenzene) [2] isomers. In view of the sparse knowledge and the great importance of the thermochemical data of the halogenated anisoles, this work presents the study of the monofluoroanisole (monofluoromethoxybenzene) isomers, figure 1, with the purpose of extending the thermochemical database for halogenated aromatic compounds and for a better understanding of the effect of substitutions of the halogen atom in the aromatic ring of benzene derivatives [3–6]. For understanding the function of complicated large biological molecules, it is important to know the thermochemical properties of their local units. From this point of view, anisole (methoxybenzene) and its derivatives have been extensively studied due to their properties as models of biological systems. Fluoroanisoles are widely used in the synthesis of fluorine substituted medicals com* Corresponding author. Tel.: +351 22 0402 521; fax: +351 22 0402 522. E-mail address: [email protected] (M.A.V. Ribeiro da Silva). 0021-9614/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2008.09.012

pounds with pharmacological activity [7–10] and agro agents [11]. The 4-fluoroanisole isomer was applied in the synthesis of poly(paraphenylene) a polymer marked by thermal and chemical stability, with potential applications in areas such as optoelectronics, batteries, and sensors [12]. The intramolecular isomerism plays a significant role in biologically relevant molecular systems. As a result, the conformational properties of anisole and monofluoroanisole isomers have been subject of many experimental and theoretical investigations over the past years. Some researchers groups have investigated the conformal properties of anisole molecule, by electron diffraction [13], microwave spectroscopy [14,15], high resolution spectroscopy [16], fluorescence spectroscopy [17], and high-level ab initio calculations and DFT calculations [15–19], and have conclude that anisole exists only as a single conformer with planar heavy atom skeleton, being this sterically unfavourable structure stabilized by the electron delocalization between the oxygen lone pairs and the electron system of the ring. The molecular geometry of 2-fluoroanisole has been studied using gas-phase electron diffraction, low-temperature matrix isolated FT-IR spectroscopy and quantum chemical methods [20,21], and it was demonstrated that the

362

M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira / J. Chem. Thermodynamics 41 (2009) 361–366

F O

FIGURE 1. Structural formula of monofluoroanisole isomers.

preferred conformation is a planar conformer with anti orientation of the methyl group with respect to fluorine, u = 180°, while the minor conformer is non-planar with the CH3 group rotated toward the fluorine atom by u  60°. Lister et al. [22–24] have done some studies using microwave spectroscopy aiming the understanding of the molecular conformation of 3- and 4-fluoroanisole, but the molecular conformation of the 3- and 4-fluoroanisoles, was only better understood more recently, with the work developed by Oberhammer et al. [25,26], using gas-phase electron diffraction and quantum chemical methods. They have demonstrated that for fluoroanisoles with the fluorine atoms in the meta and para positions the planar conformer is the stable molecular structure like anisole itself, and that the 3-fluoroanisole exists as a mixture of two conformers, syn and anti form, in approximately equal abundance, the anti conformation being the most stable. So, with those studies, it was verified that the orientation of the methoxy group in anisoles, in the ring plane or perpendicular to it, depends mainly on a delicate balance between two opposing effects: the orbital interactions between the oxygen lone pairs and the benzene ring and the steric effects between the methyl group and the ortho atoms in the benzene ring. To the best of our knowledge both the enthalpies of combustion, in the liquid state, and the enthalpies of vaporization of the 2-, 3-, and 4-fluoroanisole isomers have never been reported. For 3-fluoroanisole and 3,4-difluoroanisole, have been recently published the gas-phase enthalpies of formation obtained by theoretical calculations, as well as the standard entropies, S(T), heat capacities, C p ðTÞ and enthalpies, [H(T)  H(0)] [26]. The proton affinities (PA) of the three monofluoroanisole isomers are known having been determined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry [27]. Hence, this paper reports the standard (p = 0.1 MPa) molar energies of combustion of the three liquid isomers of monofluoroanisole, measured by rotating bomb combustion calorimetry, as well as their standard molar enthalpies of vaporization, at the T = 298.15 K, measured by Calvet microcalorimetry, and the derived values for the standard molar enthalpies of formation, in the gas phase, at T = 298.15 K. In addition to the experimental work, we have calculated the gas-phase enthalpies of formation for the three monofluoroanisoles by applying the empirical method suggested by Cox [28] and by computational thermochemistry using the density functional theory at the B3LYP/6-311++G(d,p) level of theory. 2. Experimental 2.1. Materials and purity control The 2-fluoroanisole [CAS 321-28-8], 3-fluoroanisole [CAS 45649-5], and 4-fluoroanisole [CAS 459-60-9] were purchased from Sigma–Aldrich Chemical Co., with an assessed minimum purity of 0.98 (mass fraction). The three compounds, which are liquid at room temperature were purified by successive fractional distillations under reduced pressure and stored under nitrogen atmosphere. The final purity of each isomer (mass fraction purity greater than 0.9995) was checked 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 lm film thickness), and with 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  min1, 423 K (5 min). Determination of purities for the three isomers showed that the mass fractions of impurities were <103. The specific densities [29] for 2-, 3-, and 4-fluoroanisole were taken as (1.124, 1.104, and 1.114) g  cm3, respectively. The relative atomic masses used in the calculation of all molar quantities throughout this paper were those recommended by the IUPAC Commission in 2005 [30]; using those values, the molar mass for the 2-, 3-, and 4-fluoroanisole isomers is 126.1289 g  mol1. The benzoic acid used in the calibration of the rotating bomb calorimeter was the NIST Standard Reference Material, sample 39j [31], while n-undecane (Aldrich, mass fraction purity >0.999) was used to calibrate the high-temperature Calvet microcalorimeter. 4-Fluorobenzoic acid, used as a reference substance for rotating-bomb combustion calorimetry of organic fluorine compounds, was supplied by Sigma–Aldrich Chemical Co., with a mass fraction purity of 0.99 and further purified by zone melting. 2.2. Combustion calorimetry measurements The combustion experiments were performed with an isoperibol rotating-bomb calorimeter, originally constructed at the University of Lund according to the design of Professor Stig Sunner [32]. Both the apparatus and the operating technique have been described [33–35] so only a brief description of the apparatus will be given here. The stainless steel combustion bomb, internal volume of 0.258 dm3 and wall thickness of 1 cm, is a twin-valve bomb platinum-lined with all the internal fittings machined from platinum. The bomb is suspended from the lid of the calorimeter can, to which a mass of nearly 5222.5 g of water is added. A Mettler PM 11-N balance, sensitivity ±(1  101) g, was used to weigh the amount of distilled water added to the calorimeter from a weighed Perspex vessel and, for each experiment of calibration or of combustion of the studied compounds, a correction to the energy equivalent was made for the difference between the mass of water used and the reference mass of 5222.5 g. Calorimeter temperatures were measured within the bounds of ±(1  104) K, at time intervals of 10 s, using a Hewlett–Packard (HP-2804A) quartz crystal thermometer interfaced to a PC programmed to compute the adiabatic temperature change. At least 100 temperature readings were taken for the main period and for both the fore and after periods. Data acquisition and control of the calorimeter were performed using the program LABTERMO [36]. For all combustion experiments, the ignition temperature was chosen so that the final temperature would be close to T = 298.15 K. The electrical energy for ignition was determined from the change in potential across a condenser (1400 lF) when discharged through a platinum wire (/ = 0.05 mm, Goodfellow, mass fraction 0.9999). The rotating mechanism of the combustion bomb allows its simultaneous axial and end-over-end rotation, causing the deionised water placed in the bomb to wash all internal surfaces of the bomb, yielding a homogeneous final solution. For each combustion experiment of the fluoroanisole isomers, the rotation of the bomb was started when the temperature rise of the main period reached about 0.63 of its total value and then continued throughout the experiment. It has been shown that by adopting this procedure, the frictional work due to the rotation of the bomb is automatically accounted in the temperature corrections for the work of water stirring and for the heat exchanged with the surrounding isothermal jacket [37]. This one 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 were filled with water maintained at a temperature ca. 303.5 to 104 K using a temperature controller (Tronac PTC 41).

M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira / J. Chem. Thermodynamics 41 (2009) 361–366

Benzoic acid (NIST Standard Reference Material 39j) was used for calibration of the calorimeter. Its mass-related energy of combustion is of (26434 ± 3) J  g1 [38], under certificate conditions. Calibration experiments were carried out in oxygen, at a pressure of 3.04 MPa, with 1.00 cm3 of deionised water added to the bomb, according to the procedure in the conventional way suggested by Coops et al. [39], without bomb rotation. The obtained value of energy equivalent of the calorimeter was e(calor) = (25157.4 ± 1.1) J  K1, (0.0044%), as the mean of seven calibration experiments, where the uncertainty quoted is the standard deviation of the mean. For bomb combustion calorimetry of organic fluorine compounds, in which the atomic ratio of hydrogen to fluorine is equal to, or greater, than unity, 4-fluorobenzoic acid was recommend as a test material [40], initially proposed by Good et al. [37], where the combustion reaction of the former compound yields HF  10H2O(l) as the sole fluorine-containing product in the final state. Hence, the accuracies of the experimental procedure and of the calorimeter were checked in our laboratory by measuring the energy of combustion of 4-fluorobenzoic acid in the pellet form under oxygen, at p = 3.04 MPa, in the presence of 10.00 cm3 of deionised water placed in the bomb. The standard massic energy of combustion obtained for 4-fluorobenzoic acid as the mean of six independent experiments was Dc u ¼ ð21865:1  2:0Þ J  g1 [3], in good agreement with the recommended value: Dc u ¼ ð21 860  4Þ J  g1 [40]. The liquid isomers of fluoroanisoles were burnt enclosed in sealed polyester bags made of Melinex (0.025 mm thickness) using the technique described by Skinner and Snelson [41]. The combustion experiments of the three isomers of fluoroanisole were also carried out in oxygen, at p = 3.04 MPa, in the presence of 10.00 cm3 of deionised water to produce an acid of uniform, well-defined concentration. All the necessary weighings for the combustion experiments were made on a Mettler Toledo AE 240 balance, sensitivity of ±(1  105) g, and corrections from apparent mass to true mass were introduced. The HNO3 formed from traces of atmospheric N2 remaining inside the bomb was analyzed by the Devarda’s alloy method [42] and corrections for the nitric acid formed were based on Df U m (HNO3, aq, 0.1 mol  dm3) = 59.7 kJ  mol1 [43], from 1/2 N2(g), 5/4 O2(g), and 1/2 H2O(l). For the cotton thread fuse, whose empirical formula is CH1.686O0.843, Dc u ¼ 16 240 J  g1 [39] was used, and for dry Melinex, Dc u ¼ ð22 902  5Þ J  g1 [41]. The values of the standard massic energies of combustion of cotton thread fuse and Melinex have been previously confirmed in our Laboratory. The mass of Melinex used in each experiment was corrected for the mass fraction of water (0.0032) and the mass of carbon dioxide produced in its combustion was calculated using the factor previously reported [41]. An estimated pressure coefficient of massic energy: (ou/op)T = 0.2 J  g1  MPa1, at T = 298.15 K, a typical value for most organic compounds [44], was used for all monofluoroanisole isomers. Corrections to the standard state were made by the procedure given by Good and Scott [45] for fluorine containing compounds, based on the method developed by Hubbard et al. [46], including the values for the solubility of carbon dioxide in hydrofluoric acid solutions, as given by Cox et al. [47]. 2.3. Calvet drop microcalorimetry measurements The enthalpies of vaporization of the 2-, 3-, and 4-fluoroanisole were measured using a similar technique of the drop-method described for the sublimation of solid compounds by Skinner et al. [48], previously tested in our laboratory for liquid vaporization [49]. For these measurements a High-Temperature Calvet Microcalorimeter (Setaram HT 1000) was used with the vacuum pro-

363

moted by a rotary vacuum pump and a vapor diffusion pump. Both apparatus and technique have been recently described [50], as well as the experimental results obtained during its testing by measuring reference compounds (benzoic acid, phenanthrene, anthracene, and ferrocene). Samples of (5 to 8) mg of liquid fluoroanisole contained in thin glass capillary tubes sealed at one end, and blank reference capillaries were simultaneously dropped at room temperature into the hot reaction vessels, held at a suitable predefined temperature T, for the study of the title compounds. First, an endothermic peak due to the thermo stability of the sample was observed and when the signal returned to the baseline both calorimetric cells were simultaneously evacuated by a vacuum system and the measuring curve, corresponding to the vaporization of the compound, was obtained. The thermal corrections for the differences in the mass of both capillary tubes and different sensibilities of the two measuring cells were obtained by making individual blank correction experiments, dropping empty tubes of nearly equal mass into each of the twin cells [50]. The samples of compounds and the glass capillary tubes were weighed on a Mettler CH-8608 analytical balance with a sensitivity of ±(1  106) g. The observed standard molar enthalpies of vaporization,  Dg;T 1298:15K Hm , have been corrected to T = 298.15 K using the corrective term DT298:15K Hm ðgÞ, which represents the molar enthalpic correction for the respective heat capacity of the gaseous phase, calculated by computational thermochemistry. The n-undecane, with a reported standard molar enthalpy of 1 [40], at T = 298.15 K, vaporization of ð56:580  0:566Þ kJ  mol was used to calibrate the Calvet microcalorimeter. The calibrations constants, k, of the calorimeter were k(T = 335 K) = (0.9982 ± 0.0029) for the vaporization experiments of the 2- and 4-fluoroanisole and k(T = 329 K) = (0.9967 ± 0.0047) for the vaporization experiments of the 3-fluoroanisole, where the quoted uncertainty is the standard deviation of the mean of five independent experiments. 2.4. Theoretical calculations Full geometry optimizations for the three monofluoroanisoles, anisole, benzene, and fluorobenzene were performed using density functional theory (DFT) with the hybrid exchange correlation functional (B3LYP) [51,52] and the Pople’s split-valence 6311++G(d,p) [53] extended basis set. Fundamental vibrational frequencies calculations have been also carried out at the same level of theory, in order to obtain the calculated enthalpy at T = 298.15 K. The scaling factors of 0.9887 and 0.9688 were used for the calculation of the zero-point vibrational energies and fundamental vibrational frequencies, respectively [54]. All theoretical calculations were performed using the Gaussian 03 software package [55].

3. Results 3.1. Experimental enthalpies of formation Detailed results for a typical combustion experiment of each isomer of fluoroanisole are given in table 1, where Dm(H2O) is the deviation of the mass of water added to the calorimeter from 5222.5 g, the mass assigned to e(calor), and DUR is the energy correction to the standard state (Washburn correction) derived as recommended in the literature for compounds containing fluorine [45]. The remaining quantities are as previously defined [44,46]. The values of the energy associated to the isothermal bomb process, DU(IBP), were calculated using the expression

364

M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira / J. Chem. Thermodynamics 41 (2009) 361–366

TABLE 1 Typical combustion results at T = 298.15 K (p° = 0.1 MPa), for the monofluoroanisole isomers Experiment

2-Fluoroanisole

3-Fluoroanisole

4-Fluoroanisole

m(cpd)/g m0 (fuse)/g m00 (Melinex)/g Ti/K Tf/K DTad/K ei/(J  K1) ef/(J  K1) e(calor)corr./(J  K1) Dm(H2O)/g DU(IBP)a/J DU(fuse)/J DU(Melinex)/J DU(HNO3)/J DU(ign)/J DUR/J Dcu/(J  g1)

0.74719 0.00311 0.04006 297.2354 298.1448 0.89210 51.71 53.18 25162.8 1.3 22492.54 50.51 917.45 2.39 1.29 20.93 28776.16

0.74570 0.00247 0.04001 297.3258 298.2287 0.88751 51.70 53.18 25152.8 1.1 22368.34 40.11 916.31 5.37 1.00 20.92 28678.60

0.71748 0.00304 0.03818 297.2949 298.1660 0.85526 51.65 53.08 25155.3 0.5 21557.19 49.37 874.40 4.42 1.31 21.27 28722.38

C7 H7 OFðlÞ þ 8O2 ðgÞ þ 7H2 OðlÞ ! 7CO2 ðgÞ þ HF  10H2 OðlÞ:

m(cpd) is the mass of compound burnt in each experiment; m0 (fuse) is the mass of fuse (cotton) used in each experiment; m00 (Melinex) is the mass of Melinex used in each experiment; Ti is the initial temperature rise; Tf is the final temperature rise; DTad is the corrected temperature rise; ei is the energy equivalent of the contents in the initial state; ef is the energy equivalent of the contents in the final state; e(calor)corr. is the corrected energy equivalent of the calorimeter for the amount of water used; Dm(H2O) is the deviation of mass of water added to the calorimeter from 5222.5 g; DU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; DU(fuse) is the energy of combustion of the fuse (cotton); DU(Melinex) is the energy of combustion of Melinex; DU(HNO3) is the energy correction for the nitric acid formation; DU(ign) is the electrical energy for ignition; DUR is the standard state correction; and Dc u is the standard massic energy of combustion. a DU(IBP) includes DU(ign).

DUðIBPÞ ¼ feðcalorÞcorr þ DmðH2 OÞcp ðH2 O; lÞgDT ad þ ðT i  298:15 KÞei þ ð298:15 K  T i  DT ad Þef þ DU ign ; ð1Þ where DTad is the calorimeter temperature change corrected for the heat exchange, work of stirring and the frictional work of bomb rotation and, e(calor)corr = e(calor) + cp(H2O, l)  Dm(H2O, l). Detailed results for each combustion calorimetric experiment performed for each compound are given as Supplementary Information (tables S1–S3). TABLE 2 Individual values of the standard (p = 0.1 MPa) massic energies of combustion, Dc u , of the monofluorine substituted anisoles at T = 298.15 K 2-Fluoroanisole

3-Fluoroanisole

4-Fluoroanisole

28776.16 28775.77 28775.24 28784.38 28779.71 28773.81

Dc u =ðJ  g1 Þ 28678.60 28676.78 28681.82 28670.59 28672.51 28687.11

28722.38 28725.88 28713.24 28714.36 28713.52 28728.75

hDc u i=ðJ  g1 Þa 28677.9 ± 2.5

28777.5 ± 1.6 a

The individual values of Dc u together with the mean value, hDc u i, and its standard deviation of the mean, are listed, for each compound, in table 2. The values of the standard massic energies of combustion, Dc u , refer to the idealized combustion reaction of fluoroanisole, yielding HF  10H2O(l) as the only fluorine-containing product in the final state, according to Eq. (2)

28719.7 ± 2.8

Mean value and standard deviation of the mean.

ð2Þ

Table 3 lists the derived values of the standard molar energies and enthalpies of combustion, Dc U m ðlÞ, and Dc Hm ðlÞ, as well as the standard molar enthalpies of formation, Df Hm ðlÞ, for the three isomers of fluoroanisole in the condensed phase, at T = 298.15 K, which were derived from the values of Dc Hm ðlÞ and from the standard molar enthalpies formation, at T = 298.15 K, of the following compounds: 1 Df Hm ðCO2 ; gÞ ¼ ð393:51  0:13Þ kJ  mol [56]; Df Hm ðH2 O; lÞ ¼ 1 [56]; and Df Hm ðHF  10H2 O; lÞ ¼ ð285:830  0:040Þ kJ  mol 1 ð322:034  0:650Þ kJ  mol [57]. The uncertainties assigned to the standard molar energies of combustion correspond, in each case, to 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 Melinex used as combustion auxiliary [58,59]. Results of the measurements of the enthalpies of vaporization  Dg;T 1298:15K Hm for the three monofluoroanisoles obtained by Calvet microcalorimetry are given in table 4, with the respective uncertainties, taken as the standard deviations of the mean of five individual results. The uncertainties quoted for the standard molar enthalpies of vaporization, Dgl Hm ðT ¼ 298:15 KÞ, at T = 298.15 K, are twice the overall standard deviation of the mean which include the uncertainties in calibration with n-undecane [58,59]. The values of the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, for the three fluoroanisoles, derived from the respective values of standard molar enthalpies of formation in the liquid phase (table 3), and the standard molar enthalpies of vaporization (table 4), are summarized in table 5. 3.2. Enthalpies of formation estimated with the Cox scheme Cox [28] suggested a method to estimate the standard molar enthalpies of formation of gaseous aromatic compounds based on the transferability of group enthalpic contributions, by assuming that each group, when substituted into a benzene ring, produces a characteristic increment in Df Hm ðgÞ and that each ortho-pair of substituents leads to an enthalpy increment of 4 kJ  mol1. So, from the values of Df Hm ðgÞ, at T = 298.15 K, available in the literature [60] for benzene and for fluorobenzene, respectively, {(82.6 ± 0.7) and (115.9 ± 1.4)} kJ  mol1,

TABLE 3 Derived standard (p = 0.1 MPa) molar values of the monofluoroanisole isomers, in the liquid phase, at T = 298.15 K Compound

Dc U m ðlÞ= 1 ðkJ  mol Þ

Dc Hm ðlÞ= 1 ðkJ  mol Þ

Df Hm ðlÞ= 1 ðkJ  mol Þ

2-Fluoroanisole 3-Fluoroanisole 4-Fluoroanisole

3629.7 ± 1.0 3617.1 ± 1.1 3622.4 ± 1.2

3632.2 ± 1.0 3619.6 ± 1.1 3624.9 ± 1.2

301.9 ± 1.5 314.5 ± 1.6 309.2 ± 1.6

TABLE 4 Standard (p = 0.1 MPa) molar enthalpies of sublimation, Dg1 Hm , at T = 298.15 K determined by Calvet microcalorimetry for the different monofluoroanisoles Compound

Number of experiments

T/K

1  Dg;T Þ 1298:15K Hm =ðkJ  mol

DT298:15K Hm ðgÞ=ðkJ  mol1 Þ

Dgl Hm ð298:15 KÞ=ðkJ  mol1 Þ

2-Fluoroanisole 3-Fluoroanisole 4-Fluoroanisole

5 5 5

334.9 329.4 334.9

57.4 ± 0.1 52.6 ± 0.1 54.0 ± 0.1

5.2 4.5 5.3

52.2 ± 1.1 48.1 ± 1.1 48.7 ± 1.1

365

M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira / J. Chem. Thermodynamics 41 (2009) 361–366 TABLE 5 Experimental and estimated (Cox scheme and B3LYP/6-311++G(d,p) calculations) gas-phase enthalpies of formation of the three monofluoroanisole isomers Compound

2-Fluoroanisole 3-Fluoroanisole 4-Fluoroanisole a b

Df Hm ðgÞ=ðkJ  mol

1

1

Da/(kJ  mol

Þ

Experimental

Cox Scheme

Calculated

249.7 ± 1.9 266.4 ± 1.9 260.5 ± 1.9

262.4 ± 1.8 266.4 ± 1.8 266.4 ± 1.8

252.1 266.6 261.8

b

)

Cox Scheme

Calculatedb

12.7 ± 2.6 0.0 ± 2.6 5.9 ± 2.6

2.4 0.2 1.3

Difference between the experimental and the estimated values. B3LYP/6-311++G(d,p).

F

=

+

OCH3

− F

OCH3

2-, 3- and 4-Fluoroanisole

Fluorobenzene

Anisole

FIGURE 2. Empirical scheme for

the calculated enthalpic increment of the substitution of a fluorine atom in a benzene ring is –(198.5 ± 1.6) kJ  mol1, which together with the enthalpy of formation of anisole, (67.9 ± 0.8) kJ  mol1 [60], allows the estimation of the standard molar enthalpies of formation of the three monofluoroanisole isomers, taking into account the scheme presented in figure 2. The values estimated by following the methodology suggested by Cox are registered in table 5. 3.3. Computed enthalpies of formation In the case of the computations with the B3LYP/6-311++G(d,p) methodology, the estimates of the theoretical gas phase enthalpies of formation for the three monofluoroanisoles are based on the computed enthalpy for the homodesmic reaction presented in figure 2, and on the experimental standard molar enthalpies of formation in the gas-phase of benzene, anisole and fluorobenzene registered above, all taken from the compilation due to Pedley [60]. The enthalpies of formation calculated for the three fluoroanisole isomers are collected in table 5 and are compared directly with the experimental results. The results show clearly that the computational estimates are in good agreement with the experimental results, involving an absolute deviations no larger than 2.4 kJ  mol1.

Df Hm ðgÞ

Benzene

estimation.

4. Discussion It can be seen from the scheme of figure 3 that the insertion of a fluorine atom in the aromatic ring of the anisole induces an enthalpic effect of stabilization higher in meta > para > ortho position. Therefore, the 3-fluoroanisole is the most stable isomer, with an enthalpic increment of substitution of a fluorine atom in the benzene ring of anisole similar to the observed for the introduction of a fluorine atom in the aromatic ring of the benzene: (198.5 ± 1.6) kJ  mol1 (as registered above). The 2-fluoroanisole is the least stable isomer due primarily to steric effects caused by the proximity between the fluorine atom and the –OCH3 group. The introduction of one fluorine atom in the ortho position of the aromatic anisole ring yields to a less significant enthalpic effect of stabilization than the observed for 3- and 4-fluoroanisole isomers. The methoxy group is a medium p electron donor, reaching its maximum when it is coplanar with the aromatic ring. In the case of the 2-fluoroanisole, the global effect of stabilization due to the methoxy group, adopting a perpendicular conformation yielding to less steric effects, would be lower than the effect of conjugation between the lone electron pairs of the oxygen atom and the p electron system of the benzene ring, in a planar conformation [20]. Taking into account the optimization calculations performed

−181.8 ± 2.1 F OCH 3

−249.7 ± 1.9

−10.8 ± 2.7

F

−198.5 ± 2.1

−16.7 ± 2.7

OCH3

−67.9 ± 0.8 [60]

OCH3

−266.4 ± 1.9 −192.6 ± 2.1

F

5.9 ± 2.7

OCH3

−260.5 ± 1.9 FIGURE 3. Enthalpic effect of substitution of a fluorine atom in the ortho, meta and para position of the aromatic ring of anisole and isomerization energies (all values are in 1 kJ  mol ).

366

M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira / J. Chem. Thermodynamics 41 (2009) 361–366

at the B3LYP/6-311++G(d,p) (tables S7–S9 of the Supplementary Information), the methoxy group was found to be coplanar with the aromatic benzene ring in the most stable conformations for all the three monofluoroanisole isomers, with the methoxy group in anti orientation with respect to fluorine atom for the 2- and 3fluoroanisole. The gas phase enthalpies of formation obtained by computational thermochemistry for the three fluoroanisole isomers, c.f. table 5, are in good agreement with the experimental results. The values given by theoretical calculations also follow the order of stability of meta > para > ortho fluoroanisole isomers, in agreement with the experimental results. The Cox scheme [28] was found to reproduce well the experimental enthalpies of formation in the gas phase for 3- and 4-fluoroanisole, failing for 2-fluoroanisole where is predicted a lower effect of destabilization due to the steric effects between the fluorine atom and methoxy group attached in ortho position of the benzene ring. In the former case, the difference between experi1 mental and estimated values is D ¼ ð12:7  2:6Þ kJ  mol , which 1 is a large difference if one considers the value of 10 kJ  mol as the usually accepted limit for a reasonable agreement between experimental and estimated values [28]. Acknowledgments Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Química, 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 post-doctoral fellowship (SFRH/BPD/27053/2006). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2008.09.012. References [1] M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira, J. Chem. Thermodyn. 40 (2008) 362–368. [2] M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira, J. Chem. Thermodyn. 40 (2008) 924–930. [3] M.A.V. Ribeiro da Silva, A.I.M.C.L. Ferreira, J.R.B. Gomes, J. Phys. Chem. B 111 (2007) 2052–2061. [4] M.A.V. Ribeiro da Silva, L.M. Spencer S. Lima, A.R.G. Moreno, A.I.M.C. Lobo Ferreira, J.R.B. Gomes, J. Chem. Thermodyn. 40 (2008) 155–165. [5] M.A.V. Ribeiro da Silva, A.I.M.C. Lobo Ferreira, L.M. Spencer S. Lima, S.M.M. Sousa, J. Chem. Thermodyn. 40 (2007) 137–145. [6] M.A.V. Ribeiro da Silva, A.I.M.C.L. Ferreira, J.R.B. Gomes, Chem. Phys. Lett. 422 (2006) 565–570. [7] W. Martin Owtonz, J. Chem. Soc., Perkin Trans. 1 (1994) 2131–2135. [8] J.B. Blair, D. Kurrasch-Orbaugh, D. Marona-Lewicka, M.G. Cumbay, V.J. Watts, E.L. Barker, D.E. Nichols, J. Med. Chem. 43 (2000) 4701–4710. [9] N.J. Lawrence, L.A. Hepworth, D. Rennison, A.T. McGown, J.A. Hadfield, J. Fluorine Chem. 123 (2003) 101–108. [10] M.A. Huffman, J.D. Rosen, R.N. Farr, J.E. Lynch, Tetrahedron 63 (2007) 4459– 4463. [11] B.C. Hamper, K.L. Leschinsky, S.S. Massey, C.L. Bell, L.H. Brannigan, S.D. Prosch, J. Agric. Food Chem. 43 (1995) 219–228. [12] S. Bergaoui, A. Haj Saîd, S. Roudesli, F. Matoussi, Electrochim. Acta 51 (2006) 4309–4315. [13] H.M. Seip, R. Seip, Acta Chem. Scand. 27 (1973) 4024–4027. [14] M. Onda, A. Toda, S. Mori, I. Yamaguchi, J. Mol. Struct. 144 (1986) 47–51. [15] O. Desyatnyk, L. Pszczółkowski, S. Thorwirth, T.M. Krygowski, Z. Kisiel, Phys. Chem. Chem. Phys. 7 (2005) 1708–1715. [16] C.G. Eisenhardt, G. Pietraperzia, M. Becucci, Phys. Chem. Chem. Phys. 3 (2001) 1407–1410. [17] R. Matsumoto, K. Sakeda, Y. Matsushita, T. Suzuki, T. Ichimura, J. Mol. Struct. 735–736 (2005) 153–167. [18] S. Tsuzuki, H. Houjou, Y. Nagawa, K. Hiratani, J. Phys. Chem. A 104 (2000) 1332–1336.

[19] M. Bossa, S. Morpurgo, S. Stranges, J. Mol. Struct. (Theochem) 618 (2002) 155– 164. [20] V.P. Novikov, L.V. Vilkov, H. Oberhammer, J. Phys. Chem. A 107 (2003) 908– 913. [21] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, K. Tsuji, K. Shibuya, Chem. Phys. Lett. 409 (2005) 93–97. [22] D.G. Lister, N.L. Owen, J. Chem. Soc., Faraday Trans. 2 (1973) 1304–1309. [23] D.G. Lister, J. Mol. Struct. 68 (1980) 33–40. [24] R. Cervellati, D.G. Lister, J. Mol. Struct. 82 (1982) 307–309. [25] N.I. Giricheva, G.V. Girichev, J.S. Levina, H. Oberhammer, J. Mol. Struct. 703 (2004) 55–62. [26] O.V. Dorofeeva, Y.V. Vishnevskiy, A.N. Rykov, N.M. Karasev, N.F. Moiseeva, L.V. Vilkov, H. Oberhammer, J. Mol. Struct. 789 (2006) 100–111. [27] B. Bogdanov, D. van Duijn, S. Ingemann, S. Hammerum, Phys. Chem. Chem. Phys. 4 (2002) 2904–2910. [28] J.D. Cox, A Method for Estimating the Enthalpies of Formation of Benzene Derivatives in the Gas State. NPL Report CHEM 83, June 1978. [29] Handbook of Fine Chemicals and Laboratory Equipment. Aldrich – España/ Portugal, 2007–2008. [30] M.E. Wieser, Pure Appl. Chem. 78 (2006) 2051–2066. [31] Certificate of Analysis, Standard Reference Material 39j, Benzoic Acid Calorimetric Standard. NIST, Gaithersburg, 1995. [32] S. Sunner, in: S. Sunner, M. Månsson (Eds.), Experimental Chemical Thermodynamics, vol. 1, Pergamon Press, Oxford, 1979 (Chapter 2). [33] M.L.C. Vale, Graduation Thesis, Faculty of Sciencs, University of Porto, 1989. [34] A.M.R.O.A. Silva, Master Thesis, Faculty of Sciencs, University of Porto, 1993. [35] M.A.V. Ribeiro da Silva, L.M. Spencer S. Lima, L.M.P.F. Amaral, A.I.M.C.L. Ferreira, J.R.B. Gomes, J. Chem. Thermodyn. 35 (2003) 1343–1359. [36] L.M.N.B.F. Santos, Ph.D. Thesis, University of Porto, 1995. [37] W.D. Good, D.W. Scott, G. Waddington, J. Phys. Chem. 60 (1960) 1080–1089. [38] Certificate of Analysis Standard Reference Material 39j, Benzoic Acid Calorimetric Standard. NBS, Washington, 1968. [39] J. Coops, R.S. Jessup, K. Van Nes, in: F.D. Rossini (Ed.), Experimental Thermochemistry, vol. 1, Interscience, New York, 1956 (Chapter 3). [40] R. Sabbah, A. Xu-wu, J.S. Chickos, M.L. Planas Leitão, M.V. Roux, L.A. Torres, Thermochim. Acta 331 (1999) 93–204. [41] A. Snelson, H.A. Skinner, Trans. Faraday Soc. 56 (1960) 1776–1783. [42] A.I. Vogel, Quantitative Inorganic Analysis, Longmans, London, 1978. [43] The NBS Tables of Chemical Thermodynamic Properties, J. Phys. Chem. Ref. Data 11 (Suppl. 2) (1982). [44] E.N. Washburn, J. Res. Natl. Bur. Stand. (US) 10 (1933) 525–558. [45] W.D. Good, D.W. Scott, in: H.A. Skinner (Ed.), Experimental Thermochemistry, vol. 2, Interscience, New York, 1962 (Chapter 2). [46] W.N. Hubbard, D.W. Scott, G. Waddington, in: F.D. Rossini (Ed.), Experimental Thermochemistry, vol. 1, Interscience, New York, 1956 (Chapter 5). [47] J.D. Cox, H.A. Gundry, A.J. Head, Trans. Faraday Soc. 60 (1964) 653–665. [48] F.A. Adedeji, D.L.S. Brown, J.A. Connor, M.L. Leung, M.I. Paz-Andrade, H.A. Skinner, J. Organomet. Chem. 97 (1975) 221–228. [49] M.A.V. Ribeiro da Silva, M.A.R. Matos, L.M.P.F. Amaral, J Chem. Thermodyn. 27 (1995) 565–574. [50] L.M.N.B.F. Santos, B. Schröder, O.O.P. Fernandes, M.A.V. Ribeiro da Silva, Thermochim. Acta 415 (2004) 15–20. [51] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [52] A.D. Becke, Phys. Rev. A 38 (1988) 3098–3100. [53] M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265–3269. [54] J.P. Merrick, D. Moran, L. Radom, J. Phys. Chem. A 111 (2007) 11683–11700. [55] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian03, Revision C.02, Gaussian Inc., Pittsburgh, PA, 2004. [56] J.D. Cox, D.D. Wagman, V.A. Medvedev (Eds.), CODATA Key Values for Thermodynamics, Hemisphere, New York, 1989. [57] G.K. Johnson, P.N. Smith, W.N. Hubbard, J. Chem. Thermodyn. 5 (1973) 793– 809. [58] F.D. Rossini, in: F.D. Rossini (Ed.), Experimental Thermochemistry, vol. 1, Interscience, New York, 1956 (Chapter 14). [59] G. Olofsson, in: S. Sunner, M. Månsson (Eds.), Combustion Calorimetry, Pergamon, Oxford, 1979 (Chapter 6). [60] J.B. Pedley, Thermochemical Data and Structures of Organic Compounds, Thermodynamics Research Center, College Station, Texas, 1994.

JCT 08-332