Experimental and computational energetic study of two halogenated 2-acetylpyrrole derivatives: 2-Trichloroacetylpyrrole and 2-trifluoroacetylpyrrole

J. Chem. Thermodynamics 42 (2010) 1079–1086

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Experimental and computational energetic study of two halogenated 2-acetylpyrrole derivatives: 2-Trichloroacetylpyrrole and 2-trifluoroacetylpyrrole Ana Filipa L.O.M. Santos, Manuel A.V. Ribeiro da Silva * 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 22 March 2010 Accepted 4 April 2010 Available online 10 April 2010 Keywords: Standard molar enthalpy of formation Standard molar enthalpy of sublimation Combustion calorimetry Knudsen effusion technique Vapour pressures 2-Trichloroacetylpyrrole 2-Trifluoroacetylpyrrole Computational thermochemistry

a b s t r a c t The present work reports the values of the standard (p ¼ 0:1 MPa) molar enthalpies of formation, in the crystalline phase, of 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, which were derived from the standard molar energies of combustion, in oxygen, to yield HCl600H2O(l) and HF10H2O(l), respectively, as well as CO2(g) and N2(g), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The values of the standard molar enthalpies of sublimation, at T = 298.15 K, derived from the Knudsen mass-loss effusion technique are also presented. From the above mentioned experimental quantities, the standard molar enthalpies of formation, in the gaseous phase, were derived: Df Hm (2-trichloroacetylpyrrole, g) = (102.5 ± 1.6) kJ  mol1 and Df Hm (2-trifluoroacetylpyrrole, g) = (704.7 ± 3.0) kJ  mol1. These experimental values are compared with estimates based on high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level, which have also been extended to the calculation of other thermodynamic properties namely N–H bond dissociation enthalpies, gas-phase acidities and basicities, proton affinities, and adiabatic ionization enthalpies. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The pyrrole ring is the basic chemical structure of many pharmacologically active molecules and biologically relevant natural products. Thus, a wide range of synthetic drugs with high therapeutical potential incorporates this heterocyclic moiety [1,2]. Porphyrin is an important cyclic tetrapyrrole that is the core structure of heme and chlorophyll. Moreover, polymers incorporating pyrrole units have interesting chemical, thermal, and electrical properties, and are promising candidates for use as battery materials and catalysts [3]. Both halogenated title compounds are widely used in synthetic organic chemistry. The 2-trichloroacetylpyrrole is used in the synthesis of naturally occurring pyrrole alkaloids, extracted from marine sponges [4–6]; these alkaloids have found applications in the treatment of neurodegenerative disorders, diabetes, cancer, inflammatory pathologies, and ocular diseases. Furthermore, 2-trichloroacetylpyrrole is used as starting material in the preparation of antibacterial agents of potential interest in human chemotherapy [7]. 2-Trifluoroacetylpyrrole is employed in the synthesis of trifluoromethylimines, which are precursors of biological molecules [8]. Thermochemical data, namely enthalpies of formation and bond dissociation enthalpies, are of crucial importance in establishing energetics–structure–reactivity relationships. These ther* 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 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2010.04.001

mochemical parameters are fundamental to several fields ranging from chemistry, medicine, pharmaceutical sciences, biology, environmental, and industrial chemistry. They provide information concerning stability, reactivity, and biodegrability of chemical compounds in the environment, helpful in choosing the most appropriate method for their elimination. Despite the large number of important applications of pyrrole derivatives, information on their energetic properties is still scarce. In order to improve this situation, we are presently involved in a study of thermodynamic and thermochemical properties of pyrrole derivatives, in which the main goal is to analyze and evaluate the enthalpic effects produced by different substituent groups in this family of compounds. In recent work we reported experimental and theoretical thermochemical studies of some pyrrole derivatives, in particular 2- and 3-acetylpyrroles [9], 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid [10], 2- and 3-acetyl1-methylpyrroles [11], 1-phenylpyrrole and 1-(4-methylphenyl)pyrrole [12]. In this work, we have carried out the experimental determination of the standard (p ¼ 0:1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of two halogenated 2-acetylpyrrole derivatives: 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole (figure 1). The standard molar enthalpies of formation, in the crystalline phase, of both compounds, were obtained by rotating bomb combustion calorimetry and their standard molar enthalpies of sublimation were determined using the Knudsen mass-loss effusion technique, by means of the Clausius–Clapeyron equation. Furthermore, we have also

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N H

COCX3

FIGURE 1. Structural formula of 2-trichloroacetylpyrrole (X = Cl) and 2-trifluoroacetylpyrrole (X = F).

performed high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level, from which the gas-phase standard molar enthalpies of formation were estimated, as well as N–H bond dissociation enthalpies, gas-phase acidities and basicities, proton affinities and adiabatic ionization enthalpies. 2. Experimental 2.1. Materials and purity control The 2-trichloroacetylpyrrole [CAS 35302-72-8] and 2-trifluoroacetylpyrrole [CAS 2557-70-2] were supplied from Sigma–Aldrich Chemical Co., with a minimum mass fraction purity of 0.99. The two crystalline compounds were purified by repeated vacuum sublimations and the final purity of each one was checked by gas–liquid chromatography. No impurities greater than 103 in mass fraction were found in each respective sample. The specific densities for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole were taken as, q = (1.602 and 1.488) g  cm3, respectively, determined from the ratio mass/volume of a pellet of the compound (made in vacuum, with an applied pressure of 105 kg  cm2). 2.2. Combustion calorimetry The standard (p ¼ 0:1 MPa) molar energies of combustion of 2trichloroacetylpyrrole and 2-trifluoroacetylpyrrole were measured in two different isoperibol rotating bomb calorimetric systems: (i) The calorimetric study of 2-trichloroacetylpyrrole was carried out in an isoperibolic rotating bomb combustion calorimeter, previously described in the literature [13,14], equipped with a twin valve bomb lined with tantalum, with an internal volume of 0.329 dm3. This system was calibrated with benzoic acid NIST Thermochemical Standard 39i, with a certified massic energy of combustion, under bomb conditions of (26434 ± 3) J  g1 [15]. The value of the energy equivalent of the calorimeter was found to be e(calor) = (20369.0 ± 2.3) J  K1, for an average mass of water added to the calorimeter of 3969.2 g, where the quoted uncertainty refers to the standard deviation of the mean. The crystalline samples of 2-trichloroacetylpyrrole were ignited in pellet form at a pressure p = 3.04 MPa of oxygen and in the presence of 25.00 cm3 of aqueous solution of As2O3(aq) 0.09073 mol  dm3, which reduces all the free chlorine produced in the combustion to hydrochloric acid. The n-hexadecane (Aldrich, mass fraction >0.999), stored under nitrogen, was used as a combustion auxiliary, for the purpose of avoiding carbon soot residue formation. Its mass-related energy of combustion was found to be Dc u ¼ ð47132:7  2:6Þ J  g1 . After the combustion experiment, the remaining quantity of As2O3(aq) was determined by titration with a standardized iodine solution and the nitric acid formed was analysed by the Devarda’s alloy method [16]. The energy of oxidation of aqueous As2O3 to As2O5, DU(As2O3), was calculated following the procedure described by Hu et al. [17] using the value of the enthalpy of oxidation of As2O3(aq) by Cl2, determined by Sunner and Thorén [18] and the thermal effects of mixing As2O5(aq) with strong acids

[19]. The amount of H2PtCl6(aq) was determined from the loss of mass of the platinum crucible, and the energy correction was based 1 on Df Hm ðH2 PtCl6 ; aqÞ ¼ ð676  0:1Þ kJ  mol [20]. In the cases where a small amount of carbon residue soot was formed in the platinum crucible, the necessary energetic correction for its formation was based on Dc u ¼ 33 kJ  g1 [21]. The standard state corrections, DUR, were calculated following the method given by Hubbard et al. [22] using the solubility constants and energies of solution of CO2 and O2 as given by Hu et al. [17]. (ii) The enthalpy of combustion of 2-trifluoroacetylpyrrole was measured in another calorimeter using an isoperibol rotating bomb calorimeter, developed in Lund, Sweden, by Sunner [23]. This system, already described in the literature [24], is equipped with a twin valve platinum lined bomb, whose internal volume is 0.258 cm3. Benzoic acid NIST Thermochemical Standard 39j [25] was also used for calibration of this bomb. From seven calibration experiments the value of the energy equivalent of the calorimeter was determined as e(calor) = (25157.4 ± 1.1) J  K1, for an average mass of water added to the calorimeter of 5222.5 g; the quoted uncertainty is the standard deviation of the mean. The 2-trifluoroacetylpyrrole was burnt in pellet form enclosed in previously weighed polyethylene bags, {Dc u ¼ ð46282:4 4:8Þ J  g1 , a value determined in our laboratory}, under oxygen at p = 3.04 MPa and with 10.00 cm3 of deionised water placed in the bomb, to yield an acid of uniform and well-defined concentration. In 2-trifluoroacetylpyrrole, the atomic ratio of hydrogen to fluorine is very close to the unity; the combustion reaction can yield a mixture of hydrogen fluoride and carbon tetrafluoride as fluorine combustion products. To prevent and avoid the formation of carbon tetrafluoride, a rich hydrogen containing compound was used to ensure that the hydrogen to fluorine atomic ratio is greater than the unity. Therefore, in the combustion experiments of 2-trifluoroacetylpyrrole, the polyethylene was used with a double purpose: to enclose this fluorine compound due to its high volatility and to increase the atomic ratio of hydrogen to fluorine. The nitric acid formed in the combustion experiments of this fluorine compound was also determined using the Devarda’s alloy method [16]. The corrections to the standard state to calculate the standard massic energy of combustion, Dc u , were made by the procedure given by Good and Scott [26] for fluorine containing compounds. The corrections were based on the method implemented by Hubbard et al. [22] and included the values of the solubility of CO2 in HF solutions, as recommended by Cox et al. [27]. For both halogenated compounds, the calibration experiments were carried out in oxygen, at a pressure of 3.04 MPa, with 1.00 cm3 of deionised water introduced into the bomb, according to the procedure suggested by Coops et al. [21], without bomb rotation. The calorimeter temperature readings were collected at time intervals of 10 s, with a precision of ±(1  104) K, with a quartz crystal thermometer (Hewlett Pachard HP 2804 A), interfaced to a PC programmed to compute the adiabatic temperature change, through the LABTERMO program [28]. In all experiments, the ignition temperature was chosen so that the final temperature was as close as possible to 298.15 K and the rotation of the bomb was started when the temperature rise in the main period reached about 63% of its final value and continued throughout the experiment, according to the procedure recommended by Good et al. [29]. In this way, the frictional work produced by the bomb rotation is included in the correction for heat exchange and work of stirring. The electrical energy for the ignition was determined from the change in potential across a condenser when ca. 40 V were discharged through a platinum wire of diameter 0.05 mm. The empirical formula and the massic energy of combustion of the cotton

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thread used as fuse in all the experiments are, respectively, CH1.686O0.843 and 16240 J  g1 [29]; this value has been previously confirmed in our laboratory. The corrections for the amount of nitric acid formed in all combustion experiments were based on 59.7 kJ  mol1 for the molar energy of formation of 0.1 mol  dm3 HNO3(aq) from N2(g), O2(g), and H2O(l) [20]. The values for the pressure coefficient of massic energy, (ou/op)T, for the title compounds were assumed to be 0.2 J  g1  MPa1, at T = 298.15 K, a typical value for most organic solids [30]. The relative atomic masses used throughout this paper were those recommended by the IUPAC Commission in 2005 [31].

tion interaction calculation is performed using the frozen core approximation for the correlation calculation, QCISD(T)/6-31G(d). In a third step, a second order Moller–Plesset perturbation theory computation (MP2) is performed with the basis set GTMP2Large. Finally, the energies computed at T = 0 K are thermally corrected to T = 298.15 K by introducing the vibrational, translational, rotational, and the pV terms. All the computations were performed with the Gaussian 03 series of programs [35]. This composite method was used to compute the enthalpies of the gas-phase reactions described by the following equation, in which X = Cl, for 2-trichloroacetylpyrrole and X = F for 2-trifluoroacetylpyrrole:

2.3. Knudsen effusion technique The mass-loss Knudsen effusion technique was used to measure the vapour pressures of the crystals at several temperatures. For 2trifluoroacetylpyrrole, due to its low melting point, an apparatus that enables work at temperatures below room temperature was used, that employed the simultaneous operation of three Knudsen cells, with three different effusion holes. This apparatus will be referred as Knudsen-1; a detailed description of the apparatus, procedure, and technique have been reported previously [32]. The vapour pressures were measured in the range (0.4 to 1.1) Pa. For 2-trichloroacetylpyrrole, the vapour pressures were also measured at several temperatures using a Knudsen effusion apparatus which enables the simultaneous operation of nine aluminium effusion cells [33]. Hereafter, this apparatus will be referred as Knudsen-2. The nine effusion cells are contained in cylindrical holes inside three aluminium blocks, three cells per block. Each block is kept at a constant temperature, different from the other two blocks. For this compound, the measurements were extended through a selected temperature interval of ca. 20 K, chosen to correspond to measured vapour pressures in the range (0.1 to 1.0) Pa. The vapour pressure, p, of each compound, in an effusion experiment, is calculated through equation (1), knowing the mass loss of the crystalline sample, Dm (determined by weighing the effusion cells to ±0.01 mg, before and after each effusion experiment), during a convenient effusion time period, t, at the temperature T of the experiment, in a system evacuated to a pressure near 1  104 Pa:

p ¼ ðDm=Ao wo tÞð2pRT=MÞ1=2 ;

ð1Þ

where Ao represents the area of the effusion orifice, wo is the respective Clausing factor, R is the gas constant and M is the molar mass of the effusing vapour. For 2-trifluoroacetylpyrrole, studied with the Knudsen-1 apparatus, the thickness of the effusion holes was 0.0125 mm and their areas and Clausing factors were: hole 1, Ao/mm2 = 0.5053, wo = 0.989; hole 2, Ao/mm2 = 0.7765, wo = 0.991; hole 3, Ao/ mm2 = 1.1370, wo = 0.992. For the Knudsen-2 apparatus, the areas and Clausing factors of the effusion orifices, made of platinum foil of 0.0125 mm thickness, are presented in the Supporting Information, table S1. 3. Computational details The enthalpies of all species considered were obtained using the G3(MP2)//B3LYP method, which is based on standard ab initio molecular calculations and empirically based corrections. Full details and the theoretical basis of the method can be found in Baboul et al. [34]. Here, only a brief description is provided. In a first step of the method, the geometries of the molecules are optimized at the B3LYP/6-31G(d) level and the vibrational frequencies are determined. The zero-point energies (ZPE) obtained from this level are scaled by a factor of 0.96. Next, a single point quadratic configura-

COCX3

N H

+

CH4 N H

CH3

+ C2HOX3

ð2Þ Using the calculated enthalpies of the above reactions and the experimental gas-phase standard molar enthalpies of formation, Df Hm ðgÞ, for methane, (74.4 ± 0.4) kJ  mol1 [36], 2-methylpyrrole, 74.91 kJ  mol1 [37], trichloroacetaldehyde, (193.5 ± 1.4) kJ  mol1 {calculated as the sum of the respective standard molar enthalpies of formation, in the liquid phase Df Hm ðlÞ ¼ ð234:5 1 1 [36] and of vaporization, Dgl Hm ¼ 41:0 kJ  mol 1:4Þ kJ  mol [38]}, and trifluoroacetaldehyde, 776.4 kJ  mol1 [37] the enthalpies of the two titled compounds were estimated. Their enthalpies of formation have also been estimated from the computed enthalpy of atomization reaction, at T = 298.15 K, obtained by the computational approach using the experimental gas-phase enthalpies of formation for carbon, 716.67 kJ  mol1, hydrogen, 218.00 kJ  mol1, oxygen, 249.17 kJ  mol1, nitrogen, 472.68 kJ  mol1, chlorine, 121.30 kJ  mol1, and fluorine, 79.39 kJ  mol1 [39]. At the G3(MP2)//B3LYP level, N–H bond dissociation enthalpies, gas-phase acidities and basicities, proton affinities, and adiabatic ionization enthalpies were also calculated for 2-trichloroacetylpyrrole and for 2-trifluoroacetylpyrrole. By convention, gas-phase basicity (DGbasicity) and proton affinity (PA) were calculated as:

A þ Hþ ! AHþ þ

A þ H ! AH

þ

DGbasicity ¼ DGr ;

ð3Þ

PA ¼ DHr ;

ð4Þ

where A = 2-trichloroacetylpyrrole or 2-trifluoroacetylpyrrole. 4. Results 4.1. Condensed phase and phase transition The detailed results of one combustion experiment of each compound are given in table 1. The values of the energy associated to the isothermal bomb process, DU(IBP), were calculated from equation (5):

DUðIBPÞ ¼ feðcalorÞ þ cp ðH2 O; lÞDmðH2 OÞgDT ad þ ðT i  298:15Þei þ ð298:15  T i  DT ad Þef þ DUðignÞ

ð5Þ

in which Dm(H2O) is the deviation of the mass of water added to the calorimeter from the mass assigned for e(calor): 3969.2 g for 2-trichloroacetylpyrrole and 5222.5 g for 2-trifluoroacetylpyrrole, DTad is the calorimeter temperature change corrected for the heat exchange, work of stirring and the frictional work of bomb rotation, DUR is the correction to the standard state and the remaining terms are as previously defined [22,30]. The detailed results for all the combustion experiments of each compound are presented in the Supporting Information (tables S2 and S3). The individual values of the standard massic energy of combustion, Dc u , for all the combustion experiments of each compound, together with the mean values, hDc u i, and their standard deviations of the mean are given in table 2. The values of Dc u for 2-trichloroacetylpyrrole and for 2-trifluoroacetylpyrrole are related to the idealized combustion reactions, as described by equations (6) and (7), respectively:

C6 H4 Cl3 NOðcrÞ þ 5:75O2 ðgÞ þ 1799:5H2 Oð1Þ ! 6CO2 ðgÞ þ 0:5N2 ðgÞ þ 3ðHCl  600H2 OÞð1Þ;

ð6Þ

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TABLE 1 Typical combustion results, at T = 298.15 K, (p ¼ 0:1 MPa), for the compounds studied.

m(cpd)/g m0 (fuse)/g m00 (n-hex)/g m000 ðpolÞ=g Ti/K Tf/K DTad/K ei/(J  K1) ef/(J  K1) ecorr/(J  K1) Dm(H2O)/g DU(IBP)a/J DU(fuse)/J DU(n-hex)/J DU(pol)/J DU(HNO3)/J DU(As2O3) / J DU(ign)/J DU(H2PtCl6)/J DU(carb)/J DUP/J Dc u =ðJ  g1 Þ

2-Trichloroacetylpyrrole

2-Trifluoroacetylpyrrole

0.58218 0.00269 0.10568

0.61873 0.00314 0.27375 297.2200 298.1514 0.91265 51.60 53.59 25153.63 0.9 23002.38 50.99

297.4904 298.1656 0.63624 116.86 114.01 20375.69 1.6 13037.15 43.69 4980.93

12669.85 30.03

23.40 265.27 1.10 0.70 5.94 23.61 13235.58

1.14

28.26 16522.96

m(cpd) is the mass of compound burnt in each experiment; m0 (fuse) is the mass of the fuse (cotton) used in each experiment; m00 (n-hex) is the mass of n-hexadecane used as auxiliary of combustion; m000 ðpolÞ is the mass of polyethylene 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 contents in the initial state; ef is the energy equivalent of contents in the final state; ecorr is the energy equivalent of the calorimeter corrected for the deviation of mass of water added to the calorimeter; Dm(H2O) is the deviation of mass of water added to the calorimeter from 3969.2 g for 2-trichloroacetylpyrrole and from 5222.5 g for 2-trifluoroacetylpyrrole; DU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions and includes DU(ignition); DU(fuse) is the energy of combustion of the fuse (cotton); DU(n-hex) is the energy of combustion of nhexadecane used as auxiliary of combustion; DU(pol) is the energy of combustion of polyethylene; DU(HNO3) is the energy correction for the nitric acid formation; DU(As2O3) is the energy correction for the oxidation of the aqueous solution of As2O3; DU(ign) is the electric energy for the ignition; DU(H2PtCl6) is the energy correction for the formation of the platinum complex; DU(carb) is the correction energy for carbon soot formation; DUP is the standard state correction; Dc u is the standard massic energy of combustion. a DU(IBP) includes DU(ignition).

TABLE 2 Individual values of standard (p ¼ 0:1 MPa) massic energies of combustion, Dc u , of the compounds, at T = 298.15 K. 2-Trichloroacetylpyrrole

2-Trifluoroacetylpyrrole 

Dc u =ðJ  g

1

Þ

13245.79 13241.84 13232.92 13235.58 13241.67 13232.35

Table 3 lists the derived values of the standard molar energies, and enthalpies of combustion, Dc Hm ðcrÞ, as well as the standard molar enthalpies of for mation, Df Hm ðcrÞ, for the two title compounds in the crystalline phase, at T = 298.15 K. These last values were derived form the values of Dc Hm ðcrÞ and from the standard molar enthalpies of formation, at T = 298.15 K, of H2O(l), (285.830 ± 0.042) kJ  mol1 [40], CO2(g), (393.51 ± 0.13) kJ  mol1 [40], HCl600H2O(l), (166.540 ± 0.005) kJ  mol1 [20,40] for 2-trichloroacetylpyrrole, and HF10H2O(l), (322.034 ± 0.650) kJ  mol1 [41] for 2-trifluoroacetylpyrrole. The uncertainties assigned to the standard molar energies of combustion correspond to twice the overall standard deviation of the mean and include the uncertainties in calibration with benzoic acid and in the values of the energies of combustion of polyethylene or n-hexadecane used as combustion auxiliaries [42,43]. The standard molar enthalpies of sublimation, at the mean temperature of the experimental temperature range, hTi, of the two compounds studied, obtained from the mass-loss Knudsen effusion method, were derived from the integrated form of the Clausius–Clapeyron equation, ln(p) = a  b  (T/K)1, where a is a constant and b ¼ Dgcr Hm ðhTiÞ=R. The experimental results obtained from each effusion cell, together with the residuals of the Clausius–Clapeyron equation {102  D lnðpÞ}, derived from least squares adjustments are summarized in tables 4 and 5 for 2-trichloroacetylpyrrole and for 2-trifluoroacetylpyrrole, respectively. Table 6 collects for each group of holes used and for the global treatment of all the (p, T) points obtained for each compound studied, the detailed parameters of the Clausius–Clapeyron equation, together with the calculated standard deviations and the standard molar enthalpies of sublimation at the mean temperature of the experiments T ¼ hTi. The equilibrium pressures at this temperature pðhTiÞ and the entropies of sublimation, at equilibrium conditions, relatively to the global treatment are also presented. The enthalpies of sublimation, at T = 298.15 K, Dgcr Hm , were derived from the same thermodynamic parameter, at the mean temperature hTi of the experiment, by equation (8):

Dgcr Hm ðT ¼ 298:15 KÞ ¼ Dgcr Hm ðhTiÞ þ Dgcr C p;m ð298:15  hTiÞ:

ð8Þ Dgcr C p;m

According to estimations made by other authors [44], the value of considered was 50 J  K1  mol1, a value that has already been used in previous works devoted to other organic compounds [9,10,12,45–53]. Table 7 presents, for each compound, the values, at T = 298.15 K, of the standard molar enthalpies, entropies, and Gibbs energies of sublimation. The combination of the derived standard molar enthalpies of formation, in the crystalline phase, with the standard molar enthalpies of sublimation, yields the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, for 2-trichloroacetylpyrrole and for 2-trifluoroacetylpyrrole, registered in table 8.

4.2. Gas-phase – molecular structures The most stable conformations obtained for 2-trichloroacetylpyrrole and for 2trifluoroacetylpyrrole, taking into account the geometry optimization performed at the B3LYP/6-31G(d) level of theory (G3(MP2)//B3LYP calculations) are those represented in figure 2. Selected bond lengths and bond angles are also included. No structural data for both compounds have been found in the literature for comparison with our results. The geometrical parameters obtained for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole are similar and agree very well with the corresponding ones found for 2-acetylpyrrole, previously reported by us [9].

For both compounds studied, the calculated molecular structures are almost planar, pertaining to the symmetry point group C1, as the non-halogenated analogue 2-acetylpyrrole [9]. At this level of theory, the N–H,O-syn is the most stable conformation adopted by both molecules; the N–H,O-anti conformation is (8.3 and 10.8) kJ  mol1 less stable than the NH,O-syn, for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, respectively.

hDc u i=ðJ  g1 Þ a

ð7Þ Dc U m ðcrÞ,

5. Discussion

16514.30 16513.54 16534.77 16522.96 16533.25 16505.47

(13238.4 ± 2.2)a

C6 H4 F3 NOðcrÞ þ 5:75O2 ðgÞ þ 29:5H2 Oð1Þ ! 6CO2 ðgÞ þ 0:5N2 ðgÞ þ 3ðHF  10H2 OÞð1Þ:

(16520.7 ± 4.8)a

Mean value and standard deviation of the mean.

TABLE 3 Derived standard (p ¼ 0:1 MPa) molar energies of combustion, Dc U m , standard molar enthalpies of combustion, Dc Hm , and standard molar enthalpies of formation, Df Hm , for the crystalline compounds, at T = 298.15 K. Compound

Dc U m ðcrÞ=ðkJ  mol

2-Trichloroacetylpyrrole 2-Trifluoroacetylpyrrole

2812.6 ± 1.3 2694.5 ± 1.8

1

Þ

Dc Hm ðcrÞ=ðkJ  mol 2810.7 ± 1.3 2692.6 ± 1.8

1

Þ

Df Hm ðcrÞ=ðkJ  mol 192.9 ± 1.6 777.5 ± 2.8

1

Þ

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Ana Filipa L.O.M. Santos, M.A.V. Ribeiro da Silva / J. Chem. Thermodynamics 42 (2010) 1079–1086 TABLE 4 Knudsen effusion results for the 2-trichloroacetylpyrrole. T/K

t/s

Orifices

102  D lnðpÞ

p/Pa Small

295.12 297.17 299.22 301.14 303.23 305.20 307.10 309.13 311.21 313.11 315.16 317.20

22795 22795 22795 20341 20341 20341 12840 12840 12840 10226 10226 10226

A1–B4–C7 A2–B5–C8 A3–B6–C9 A1–B4–C7 A2–B5–C8 A3–B6–C9 A1–B4–C7 A2–B5–C8 A3–B6–C9 A1–B4–C7 A2–B5–C8 A3–B6–C9

Medium 2-Trichloroacetylpyrrole 0.0992 0.129 0.165 0.213 0.272 0.336 0.430 0.531 0.661 0.843 1.043 1.289

0.103 0.132 0.169 0.218 0.274 0.344 0.434 0.539 0.674 0.861 1.060 1.327

TABLE 5 Knudsen effusion results for the 2-trifluoroacetylpyrrole. T/K

t/s

Hole 1 263.19 264.14 265.11 266.12 267.12 268.12 269.11 270.11 271.13

12185 11558 10989 10823 10444 11471 10504 10761 10165

0.433 0.480 0.533 0.640 0.716 0.809 0.900 1.147

Hole 2

Hole 3

Hole 1

2-Trifluoroacetylpyrrole 0.414 0.406 3.1 0.469 0.480 1.1 0.531 0.9 0.623 0.597 4.6 0.690 0.670 3.2 0.796 0.771 3.0 0.894 0.866 1.4 1.022 0.993 1.130 1.101 0.9

Small

Medium

Large

0.101 0.129 0.162 0.213 0.268 0.335 0.424 0.527 0.652 0.837 1.028 1.270

2.0 1.2 0.6 3.1 1.5 1.2 2.3 0.8 0.1 3.2 1.6 1.9

2.1 0.8 1.6 0.9 0.8 1.4 1.4 0.6 2.0 1.1 0.0 1.0

0.1 1.4 3.2 1.1 0.9 1.6 0.1 1.3 3.5 0.4 1.5 2.4

TABLE 8 Comparison between the experimental and computed G3(MP2)//B3LYP gas-phase enthalpies of formation of 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, at T = 298.15 K.

102  D lnðpÞ

p/Pa

Large

Hole 2

Hole 3

1.6 1.4 1.2 1.9 0.5 1.3 0.7 1.9 0.5

3.6 1.0

Compound

Experimental

G3(MP2)//B3LYP Equation (2)

Atomization reaction 1

2.4 3.3 1.9 2.4 1.0 3.2

The C5NH angle in 2-trichloroacetylpyrrole is larger that the C2NH (127.9° vs. 122.0°, respectively), as well as in 2-trifluoroacetylpyrrole (127.8° vs. 122.3°, respectively), indicating that the pyrrole N–H hydrogen atom in both molecules is slightly moved in the direction of carbonyl oxygen atom, which was also found for 2acetylpyrrole [9], and for 2-pyrrolecarboxylic acid [10]. This suggests the existence of an interaction of type N–H  O, as in the case of these two last compounds. Thus, a topological analysis with the

2-Trichloroacetylpyrrole 2-Trifluoroacetylpyrrole

102.5 ± 1.6 704.7 ± 3.0

Df Hm =ðkJ  mol Þ 103.4 (0.9) 111.1 (8.6) 697.4 (7.3) 698.5 (6.2)

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

Topmod program [54] was performed aiming at the location of critical points in the electronic charge density distribution in the region between the H and O atoms. This analysis revealed a bond critical point in each compound studied in the referred region: the values of electron density in the critical point are g = 0.034 (electron localization function), q = 0.0110 (electron density), r2q = 0.0416 (Laplacian of the electron density) for 2-trichloroacetylpyrrole and, g = 0.027, q = 0.0091 and r2q = 0.0344, for 2-trifluoroacetylpyrrole, calculated at the B3LYP/6-311+G(2d,2p) level of

TABLE 6 Experimental results for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, where a and b are from Clausius–Clapeyron equation, ln(p) = a  b  (K/T) and b ¼ Dgcr Hm ðhTiÞ=R; R = 8.314472 J  K1  mol1. Orifices

a

b

A1–A2–A3 B4–B5–B6 C7–C8–C9

34.43 ± 0.13 34.51 ± 0.16 34.25 ± 0.18

10832 ± 40 10862 ± 50 10784 ± 56

Global results

34.39 ± 0.13

10826 ± 39

Hole 1 Hole 2 Hole 3

32.98 ± 0.71 33.87 ± 0.43 32.84 ± 0.58

8902 ± 190 9146 ± 115 8876 ± 154

Global results

33.09 ± 0.48

8937 ± 128

pðhTiÞ=Pa

hTi=K

Dgcr Hm ðhTiÞ=ðkJ  mol1 Þ

Dgcr Sm ðhTi; pðhTiÞÞ=ðJ  K1  mol1 Þ

2-Trichloroacetylpyrrole 90.1 ± 0.3 90.3 ± 0.4 89.7 ± 0.5 306.16

0.379

90.0 ± 0.3

294.0 ± 1.0

2-Trifluoroacetylpyrrole 74.0 ± 1.6 76.0 ± 1.0 73.8 ± 1.3 267.16

0.696

74.3 ± 1.1

278.1 ± 4.1

TABLE 7 Values of the standard (p ¼ 0:1 MPa) molar enthalpies, Dgcr Hm , entropies, Dgcr Sm , and Gibbs energies Dgcr Gm , of sublimation, at T = 298.15 K, for the compounds studied. Compound

Dgcr Hm =ðkJ  mol1 Þ

Dgcr Sm =ðJ  K1  mol1 Þ

Dgcr Gm =ðkJ  mol1 Þ

2-Trichloroacetylpyrrole 2-Trifluoroacetylpyrrole

90.4 ± 0.3 72.8 ± 1.1

191.5 ± 1.0 173.9 ± 4.1

33.3 ± 0.4 21.0 ± 1.6

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Ana Filipa L.O.M. Santos, M.A.V. Ribeiro da Silva / J. Chem. Thermodynamics 42 (2010) 1079–1086

theory. These values suggest a weak bond, with the features of a hydrogen bond, according to the data obtained by Koch and Popelier [55]. Experimentally, through NMR studies, Ushakov and collaborators confirmed the formation of an intramolecular hydrogen bond, N–H  O, in both 2-acetylpyrrole and 2-trifluoroacetylpyrrole [56]. In 2-trichloroacetylpyrrole the distance between the H atom from the N–H group in the pyrrole ring and the oxygen atom from the acetyl group is 0.2425 nm, which is close to the N–H  O distance found for 2-acetylpyrrole [9], and slightly lower than the corresponding distance in 2-trifluoroacetylpyrrole (0.2536 nm). 5.1. Gas-phase computational enthalpies of formation The gas-phase enthalpies of formation of both compounds studied were estimated by combining the respective enthalpies of reaction described by equation (2) and of atomization reaction, with

the enthalpies of formation, in the gaseous phase, of the atoms and molecules there included. The values estimated with the G3(MP2)//B3LYP approach are reported in table 8. For 2-trichloroacetylpyrrole, when reaction (2) is used, the estimated value almost matches the experimental one, with an enthalpic difference of only 0.9 kJ  mol1, falling within the experimental uncertainty. However, a poorer estimate was obtained when the atomization reaction was considered; the calculated value differs by 8.6 kJ  mol1 from the experimental one. Similar differences have also been reported by Ribeiro da Silva et al. [57,58] for other organic chlorine compounds, namely, monochloro- and dichloroanilines, when the atomization reaction was considered for predicting their gas-phase enthalpies of formation. For 2-trifluoroacetylpyrrole, the computational results are in satisfactory agreement with the experimental one. The atomization reaction provides a better estimate than the obtained through the reaction described by equation (2), with a deviation from the experimental result of ca. 6 kJ  mol1. When equation (2) is considered, a larger deviation was obtained (7.3 kJ  mol1), which can be justified by the use of values of gas-phase enthalpies of formation of 2-methylpyrrole and trifluoroacetaldehyde estimated computationally, since no experimental values have been found in the literature. The computed G3(MP2)//B3LYP enthalpies for the compounds studied, auxiliary molecules, and atoms used in the working reactions are listed in table S4 in the Supporting Information. 5.2. Other gas-phase thermodynamic properties

FIGURE 2. Optimized most stable configurations for the 2-trichloroacetylpyrrole (syn), (a), and for 2-trifluoroacetylpyrrole (syn), (b). Distances are in nm and angles in degrees.

It has been already shown, in previous work that focused on the study of pyrrole derivatives that the G3(MP2)//B3LYP method seems to work well with this family of compounds, not only for estimating gas-phase enthalpies of formation, but also for calculating some other thermodynamic properties [9–12]. Therefore, this computational approach was also used, in this work, to compute N–H bond dissociation enthalpies (BDE), gas-phase acidities (DGacidity) and basicities (DGbasicity), proton affinities (PA), and adiabatic ionization enthalpies (IE) for the title compounds, properties that may be difficult to measure experimentally. The whole set of results obtained for the two halogenated 2-acetylpyrrole derivatives studied are presented in table 9. Thermodynamic data for these two isomers are scarce in the literature; only the IE, a property that measures the capacity of a compound to act as electron-donor species, of 2-trifluoroacetylpyrrole, measured by Linda et al. in the early 1970s using an electron impact technique, was found. The experimental value obtained by Linda et al. [59], (885.7 ± 4.8) kJ  mol1, differs by 16.8 kJ  mol1 from the result computed in this work {(868.9 kJ  mol1)}. Similar enthalpic differences between the experimental and calculated data obtained with the same G3(MP2)//B3LYP approach were found for 2- and 3- fluoroanilines [60], as well as for 2-acetylpyrrole [9]. The energy required to remove an electron increases in the order: 2-acetylpyrrole (818.7 kJ  mol1) [9], 2-trichloroacetylpyrrole (838.8 kJ  mol1), and 2-fluoroacetylpyrrole (868.9 kJ  mol1). As already found for 2-acetylpyrrole [9], 2-pyrrolecarboxylic acid [10], and 2-nitropyrrole [61], protonation at the carbonyl oxygen atom is also the most favorable site in 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole and the N atom is the least favorable

TABLE 9 G3(MP2)//B3LYP computed N–H bond dissociation enthalpies (BDE), gas-phase acidities, DGacidity, and basicities, DGbasicity, proton affinities, PA, and adiabatic ionization enthalpies, IE, at T = 298.15 K, for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole. All values are in kJ  mol1. Compound

N–H BDE

DGacidity

DGbasicity

PA

IE

2-Trichloroacetylpyrrole 2-Trifluoroacetylpyrrole

419.9 424.0

1380.2 1382.4

826.0 808.1

857.8 (O) 839.9 (O)

838.8 868.9

Ana Filipa L.O.M. Santos, M.A.V. Ribeiro da Silva / J. Chem. Thermodynamics 42 (2010) 1079–1086

protonation site. The computed values also show that O-protonation is easier for 2-trichloroacetylpyrrole than for 2-trifluoroacetylpyrrole by ca. 18 kJ  mol1. All the computed proton affinity values of each protonation site of the two halogenated 2-acetylpyrrole derivatives are reported in table S5, in the Supporting Information. The computed N–H BDE for 2-trichloroacetylpyrrole and for 2trifluoroacetylpyrrole are, respectively (21.8 and 25.9) kJ  mol1 larger than for pyrrole [9], which means that a higher enthalpy is required for N–H bond scission. This observation supports the existence of an interaction of type N–H  O, already referred above, and if one considers only the N–H BDE values, this interaction will be more effective in 2-trifluoroacetylpyrrole than in 2-trichloroacetylpyrrole. The knowledge of the energy required for the N–H bonds cleavage is important to evaluate the antioxidant strength of compounds with this type of bonds. The lower the N–H BDE is, the higher the antioxidant activity. Thus, focusing just on the N–H BDE values reported in table 9, the two compounds studied have similar antioxidant properties, although, the 2-trichloroacetylpyrrole will be a slightly more effective antioxidant than the trifluoro- derivative. The calculated DGacidity values presented in table 9 indicate that both compounds studied have similar acidic character. The values obtained in this work for 2-trichloroacetylpyrrole (1380.2 kJ  mol1) and for 2-trifluoroacetylpyrrole (1382.4 kJ  mol1) when compared with the corresponding ones obtained in a previous work for 2-acetylpyrrole (1419.7 kJ  mol1) [9], show that the deprotonation in 2-acetylpyrrole is 38 kJ  mol1 more unfavourable than in its two halogenated derivatives.

6. Conclusions The standard molar gas-phase enthalpies of formation, at T = 298.15 K, of 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole have been obtained through experimental and computational methods. Experimentally, rotating-bomb combustion calorimetry and Knudsen effusion mass-loss technique have been used. Computational calculations at the G3(MP2)//B3LYP level have been carried out. The estimated gas-phase enthalpy of formation of 2-trichloroacetylpyrrole compares well with the experimental result whereas for 2-trifluoroacetylpyrrole a satisfactory agreement was found. The molecular structure of both compounds has been established and the structural parameters have been determined. Furthermore, a topological analysis of both compounds revealed a bond critical point in the region between the H (from N–H) and O atoms, suggesting the existence of an interaction of type N–H  O in both compounds, similarly to those previously reported for 2-acetylpyrrole and 2-pyrrolecarboxylic acid. Other thermodynamic properties of 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, namely N–H bond dissociation enthalpies, gas-phase acidities and basicities, proton affinities, and adiabatic ionization enthalpies have also been obtained by means of G3(MP2)//B3LYP calculations.

Acknowledgements Thanks are due to Fundação para a Ciência e a Tecnologia, F.C.T., Lisbon, Portugal, and to FEDER for financial support to Centro de Investigação em Química, University of Porto. A.F.L.O.M.S. thanks F.C.T. and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the post-doctoral fellowship (SFRH/BPD/41601/2007).

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Appendix A. Supplementary material Detailed data of the effusion orifices (diameter and Clausing factors) of the Knudsen-2 effusion apparatus; the data and the details of all the combustion calorimetry experiments for the 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole; G3(MP2)//B3LYP computed enthalpies (energies plus thermal corrections for T = 298.15 K) for the 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, for the auxiliary molecules and for the atoms used in the gas-phase working reactions; G3(MP2)//B3LYP computed proton affinities, PA, at T = 298.15 K for 2-trichloroacetylpyrrole and 2-trifluoroacetylpyrrole, are presented. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jct.2010.04.001. References [1] http://www.lipitor.com (29.07.08). [2] S. Kirner, P.E. Hammer, D.S. Hill, A. Altmann, I. Fischer, L.J. Weislo, M. Lanahan, K.-H. Pée, J.M. Lingon, J. Bacteriol. 180 (1998) 1939–1943. [3] G.B. Street, in: T.A. Skotheim (Ed.), Handbook of Conducting Polymers, vol. 1, Marcel Dekker, New York, 1986 (Chapter 8). [4] G. Papeo, H. Posteri, D. Borghi, M. Varasi, Org. Lett. 7 (2005) 5641–5644. [5] S. Picon, E.T.H. Dau, M.-T. Martin, P. Retailleau, A. Zaparucha, A. Al-Mourabit, Org. Lett. 11 (2009) 2523–2526. [6] C. Behrens, M.W. Christoffersen, L. Gram, P.H. Nielsen, Bioorg. Med. Chem. Lett. 7 (1997) 321–326. [7] D.M. Bailey, R.E. Johnson, J. Med. Chem. 16 (1973) 1300–1302. [8] M. Abid, M. Savolainen, S. Landge, J. Hu, G.K.S. Prakash, G.A. Olah, B. Török, J. Fluorine Chem. 128 (2007) 587–594. [9] A.F.L.O.M. Santos, J.R.B. Gomes, M.A.V. Ribeiro da Silva, J. Phys. Chem. A 113 (2009) 3630–3638. [10] A.F.L.O.M. Santos, M.A.V. Ribeiro da Silva, J. Phys. Chem. A 113 (2009) 9741– 9750. [11] M.A.V. Ribeiro da Silva, A.F.L.O.M. Santos, J. Phys. Chem. B 114 (2010) 2846– 2851. [12] A.F.L.O.M. Santos, M.A.V. Ribeiro da Silva, J. Chem. Thermodyn. 42 (2010) 734– 741. [13] M.A.V. Ribeiro da Silva, J.M. Gonçalves, G. Pilcher, J. Chem. Thermodyn. 29 (1997) 253–260. [14] M.A.V. Ribeiro da Silva, A.F.L.O.M. Santos, J. Therm. Anal. Cal. 88 (2007) 7–17. [15] Certificate of Analysis Standard Reference Material 39i Benzoic Acid Calorimetric Standard, NBS, Washington, 1968. [16] A.I. Vogel, Quantitative Inorganic Analysis, Longman, London, 1978. [17] A.T. Hu, G.C. Sinke, M. Månsson, B. Ringnér, J. Chem. Thermodyn. 4 (1972) 283– 299. [18] S. Sunner, S. Thorén, Acta Chem. Scand. 18 (1964) 1528–1532. [19] P. Sellers, S. Sunner, I. Wadsö, Acta Chem. Scand. 18 (1964) 202–206. [20] The NBS Tables of Chemical Thermodynamic Properties, J. Phys. Chem. Ref. Data 11 (Suppl. 2) (1982). [21] J. Coops, R.S. Jessup, K.G. van Nes, in: F.D. Rossini (Ed.), Experimental Thermochemistry, vol. 1, Interscience, New York, 1956 (Chapter 3). [22] W.N. Hubbard, D.W. Scott, G. Waddington, in: F.D. Rossini (Ed.), Experimental Thermochemistry, vol. 1, Interscience, New York, 1956 (Chapter 5). [23] S. Sunner, in: S. Sunner, M. Månsson (Eds.), Combustion Calorimetry, Pergamon Press, Oxford, 1979 (Chapter 2). [24] M.A.V. Ribeiro da Silva, M.L.C.C.H. Ferrão, F. Jiye, J. Chem. Thermodyn. 26 (1994) 839–846. [25] Certificate of Analysis Standard Reference Material 39j Benzoic Acid Calorimetric Standard, NBS, Washington, 1995. [26] W.D. Good, D.W. Scott, in: H.A. Skinner (Ed.), Experimental Thermochemistry, vol. 2, Interscience, New York, 1962 (Chapter 2). [27] J.D. Cox, H.A. Gundry, A.J. Head, Trans. Faraday Soc. 60 (1964) 653–665. [28] L.M.N.B.F. Santos, Ph. D. Thesis, University of Porto, 1995. [29] W.D. Good, D.W. Scott, G. Waddington, J. Phys. Chem. 60 (1956) 1080–1089. [30] E.W. Washburn, J. Res. Natl. Bur. Stand. (US) 10 (1933) 525–558. [31] M.E. Wieser, Pure Appl. Chem. 78 (2006) 2051–2066. [32] M.A.V. Ribeiro da Silva, M.J.S. Monte, Thermochim. Acta 171 (1990) 169–183. [33] M.A.V. Ribeiro da Silva, M.J.S. Monte, L.M.N.B.F. Santos, J. Chem. Thermodyn. 38 (2006) 778–787. [34] A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghavachari, J. Chem. Phys. 110 (1999) 7650–7657. [35] 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.

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JCT 10-101