ARTICLE pubs.acs.org/JPCB
Diaminobenzenes: An Experimental and Computational Study Ana Filipa L. O. M. Santos and Manuel A. V. Ribeiro da Silva* Centro de Investigac-~ao em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal
bS Supporting Information ABSTRACT: In the present work, the values of the standard (po = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of 1,2-diaminobenzene, 1,3-diaminobenzene, and 1,4-diaminobenzene are reported as 86.6 ( 1.6, 89.6 ( 1.6, and 99.7 ( 1.7 kJ 3 mol1, respectively. These values were derived from experimental thermodynamic parameters, namely the standard (po = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, Δf Hmo(cr), at T = 298.15 K, obtained from the standard molar enthalpies of combustion, Δc Hmo, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at T = 298.15 K, derived from the temperature-vapor pressure dependence, determined by the Knudsen mass loss effusion method. The results were compared with estimates obtained by standard ab initio molecular calculations at the G3(MP2)//B3LYP level. Experimental and calculated data are in very good agreement and show that the 1,2-diaminobenzene is, thermodynamically, the most stable isomer. Finally, proton and electron affinities, basicities and adiabatic ionization enthalpies were also computed at the same level.
1. INTRODUCTION Diaminobenzenes, as aromatic amines, are an important class of molecules with interest in both biological and materials science.1 The 1,2-diaminobenzene is used in the preparation of the agricultural fungicide benomyl,2,3 as corrosion inhibitors,4 in sensors,57 rechargeable batteries,8 and as pharmaceutical intermediate in the synthesis of anti-HIV agents,9 and anticancer drugs.10 The 1,3-diaminobenzene is a low toxic diamine which is applied in the synthesis of thermoplastics,11 polymers,12 in biosensors,1316 and in hair colorant creams.17 Its fire resistance, excellent chemical and temperature stability are properties that make this meta-isomer so versatile.18 In turn, the para-isomer, 1,4-diaminobenzene is a precursor of Kevlar, a p-aramid synthetic fiber with high tensile properties. As a result of its remarkable combination of properties particularly its strength it is used today in a wide variety of applications.19,20 This isomer also appears in dyes, used as colorants,21 and it is the main aromatic amine used in hair dyeing,22,23 which was found to be mutagenic. The 1,4-diaminobenzene derivatives are key components in retardation of rubber degradation (vulcanization accelerators and antioxidants),24,25 as well as agents in color photographic processes,26,27 and their polymers, the poly(1,4-phenylenediamines) are used as biosensors.28 In view of their large number of applications and their important properties, the understanding of the molecular and energetic properties of this kind of compounds as well as their stability and reactivity has great importance, so this is why investigations on their energetics-structure relationships are carried out. The crystalline structure of 1,3-diaminobenzene has been determined by Betz and collaborators from single-crystal X-ray r 2011 American Chemical Society
diffraction.29 This compound has a unit cell with a monoclinic P21/c crystal system, and there are four molecules in the asymmetric unit. The 1,4-diaminobenzene has been subject of several experimental,30 and computational studies focused on its molecular structure.3134 From X-ray diffraction, it was determined that the crystals of 1,4-diaminobenzene are monoclinic, with a P21/c space group, as in the 1,3- isomer.30 The crystal structure also shows that the molecules in 1,4-diaminobenzene are held together by a complex three-dimensional network of NH 3 3 3 N hydrogen bonds. The electron diffraction, in the gasphase, revealed that the molecule has a C2h symmetry.30 Tzeng and Narayanan performed an ab initio molecular orbital study of 1,4-diaminobenzene, having determined its structure and vibrations in the S0 and S1 states.31 Later, Akalin and Aky€uz have reported a theoretical IR spectrum of this isomer.32 Two other theoretical treatments of the para-isomer were obtained: Rauhut and Clark,33 published an AM1 and ab initio calculations to investigate the electron self-exchange reaction between this compound and its radical cation and Palafox et al. presented a study in which they establish relationships between the geometric parameters of the amino group in para-substituted anilines.34 Noto et al. carried out ab initio calculations for full geometry optimization of the diaminobenzene isomers and a vibrational analysis for both gas-phase and in chloroform solution of these compounds.35 Finally, a study of the structural and electronic properties of aniline and substituted anilines, in Received: January 21, 2011 Revised: February 24, 2011 Published: April 08, 2011 4939
dx.doi.org/10.1021/jp200670s | J. Phys. Chem. B 2011, 115, 4939–4948
The Journal of Physical Chemistry B particular, the three diaminobenzene isomers, obtained through DFT-based methods, have been reported by Vaschetto and coworkers.36 Because of the importance of these diaminobenzene isomers and derivatives, some thermodynamic properties were determined previously. The standard molar enthalpy of formation, in the crystalline phase, of 1,2-diaminobenzene was first determined in 1973, by Kunyavskaya et al. as 0.0 kJ 3 mol1, without indication of associated uncertainty.37 Later, Contineanu et al. obtained the value 39.1 ( 5.7 kJ 3 mol1.38 Kunyavskaya and collaborators also determined, for 1,3- and 1,4-diaminobenzene, values of ΔfHom(cr), as 7.9 kJ 3 mol1 and þ6.3 kJ 3 mol1, respectively.37 For the para-isomer, two values of ΔcHom are reported: 3509.3 ( 0.63 kJ 3 mol1,39 yielding a value of ΔfHom(cr) = 4.9 ( 1.0 kJ 3 mol1 and 3507.4 ( 0.63 kJ 3 mol1 (value reanalyzed by Cox and Pilcher,40 original value 3506.9 ( 0.63 kJ 3 mol1 41), which leads to a value of Δf Hom(cr) = 3.0 ( 1.0 kJ 3 mol1. In the Pedley compendium, the enthalpies of formation of these three isomers are only presented in the crystalline phase: 0.3 ( 4.1 kJ 3 mol1 for 1,2-diaminobenzene, 7.8 ( 4.1 kJ 3 mol1 for 1,3-diaminobenzene and 3.0 ( 0.7 kJ 3 mol1 for 1,4-diaminobenzene.42 More recently, Sabbah and Perez performed a thermodynamic study of the three benzenediamine isomers.43 From combustion calorimetry, they reached to the following results for ΔfHom(cr): 6.5 ( 1.3 kJ 3 mol1 for 1,2-diaminobenzene, 6.4 ( 1.4 kJ 3 mol1 for 1,3-diaminobenzene and 3.5 ( 1.4 kJ 3 mol1 for 1,4-diaminobenzene. The enthalpies of sublimation obtained for the 1,2-, 1,3- and 1,4- isomers, measured by Tian-Calvet microcalorimetry were, respectively, 85.51 ( 0.29, 90.36 ( 0.36, and 92.22 ( 0.24 kJ 3 mol1. There is also available in the literature gas-phase ionic data for the three isomers. For 1,2-diaminobenzene, there are several values for the ionization enthalpies (IE), ranging from 694.7 to 771.9 kJ 3 mol1.44 From charge transfer spectra, values of 718.8,45 710.1,46 and 714.047 kJ 3 mol1 were obtained for this isomer. Tsuji et al.,48 obtained a value of 694.7 kJ 3 mol1, after photoelectron spectroscopy experiments, and from electron impact method, due to Crable and Kearns,49 the IE obtained was 771.9 kJ 3 mol1, values somewhat different from the ones obtained from charge transfer spectra. The reported proton affinity (PA) and gas-phase basicity (ΔGbasicity) for the ortho- isomer are 896.5 kJ 3 mol1 and 865.8 kJ 3 mol1, which are review values due to Hunter and Lias.50 A quantum chemical calculations, performed by Bagno and Terrier,51 show that the PA calculated for the most favorable site, the N atom, is 905.0 kJ 3 mol1, 8.5 kJ 3 mol1 higher than the reviewed value.50 For the m- and p-isomers, the values of (PA) and (ΔGbasicity) are also from the evaluation carried out by Hunter and Lias.50 The values of PA are, respectively, 929.9 kJ 3 mol1 and 905.9 kJ 3 mol1; ΔGbasicity = 899.2 kJ 3 mol1 for 1,3-diaminobenzene and 874.0 kJ 3 mol1 for 1,4-diaminobenzene. The gas-phase basicities of the three diaminobenzene isomers were determined by Lau et al., measuring proton transfer equilibria with a pulsed electron beam high-pressure mass spectrometer.52 These researchers reported that the 1,3-diaminobenzene is ring protonated and also that the ΔGbasicity of the three isomers follows the order 1,3- > 1,4- > 1,2-. The ionization enthalpy of 1,3-diaminobenzene was first determined by Crable and Kearns, through the electron impact technique, as 768.0 kJ 3 mol1.49 In 1966, Farrell and Newton, obtained an IE value of 723.6 kJ 3 mol1.47 About a decade later,
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Figure 1. Structural formula of (1,2-, 1,3- or 1,4-) diaminobenzene.
Tsuji et al.48 and Pykhtina and collaborators53 reported the values of 717.8 kJ 3 mol1 (photoelectron spectroscopy) and 688.9 kJ 3 mol1 (photoionization mass spectrometry), respectively. The selected value for IE of 1,4-diaminobenzene, in the NIST webbook, resulting form an evaluation performed by Lias, is 662.8 ( 4.8 kJ 3 mol1.54 There are several values for the IE of this isomer, going from 659.9 kJ 3 mol1 to 731.3 kJ 3 mol1. Briegleb and Czekalla,45 as well as Farrell and Newton,47 reported the values of 689.9 kJ 3 mol1 and 675.4 kJ 3 mol1, respectively, obtained from charge transfer spectra. From the electron impact method, values of 731.3 kJ 3 mol1 49 and 690.8 kJ 3 mol1 55 were obtained for this compound. In the 1970s, two other values of IE, due to Potapov et al.56 and Tsuji et al.,48 664.8 kJ 3 mol1 and 659.9 kJ 3 mol1, respectively, were reported. Because of the importance of the diaminobenzene isomers and due to the wide dispersion of the thermochemical and thermodynamic data reported in the literature, it prompted us to carry out the present study to get accurate values and to obtain additional thermochemical knowledge about the 1,2-, 1,3-, and 1,4-diaminobenzene. In this work, static bomb combustion calorimetry and the Knudsen mass-loss effusion experiments were performed aiming, respectively, the determination of the enthalpies of formation, in the crystalline phase, and the standard molar enthalpies of sublimation of the three diaminobenzene derivatives, whose general structural formula is depicted in Figure 1. Additionally, high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level were performed and the estimated gas-phase standard molar enthalpies of formation were compared with the experimental data. Other thermodynamic properties were also computed at the same level.
2. EXPERIMENTAL SECTION 2.1. Materials and Purity Control. The compounds 1,2diaminobenzene [CAS 95545], 1,3-diaminobenzene [CAS 108452], and 1,4-diaminobenzene [CAS 106503] were purchased from Sigma-Aldrich Chemical Co., with a minimum massic fraction purity of 0.99. The purification of these compounds, crystals at room temperature, was performed by successive vacuum sublimations under reduced pressure. Control of the purity was made by gasliquid chromatography and also by the percentage of carbon dioxide recovered during the combustion experiments. The purity obtained by gasliquid chromatography for each isomer was 99.93%, for the 1,2-diaminobenzene, 100.00% for the 1,3diaminobenzene and 99.94% for the 1,4-diaminobenzene. The average ratios of the mass of carbon dioxide recovered to those calculated from the mass of samples used in each experiment, together with the uncertainties (twice the standard deviation of the mean) were: 1,2-diaminobenzene (1.0000 ( 0.0012); 1,3-diaminobenzene (0.9999 ( 0.0017); 1,4-diaminobenzene (0.9995 ( 0.0014). 4940
dx.doi.org/10.1021/jp200670s |J. Phys. Chem. B 2011, 115, 4939–4948
The Journal of Physical Chemistry B The values of densities used were F = 1.030 g 3 cm3 for 1,2diaminobenzene,57 F = 1.231 g 3 cm3 for 1,3-diaminobenzene,29 and F = 1.246 g 3 cm3 for 1,4-diaminobenzene.30 The relative atomic masses used for the elements were the ones recommended by the IUPAC Commission in 2007.58 2.2. Combustion Calorimetry. The massic energies of combustion of the three isomers were measured using an isoperibol static bomb calorimeter, equipped with a Parr 1108 model twin valve bomb, made of stainless steel and with an internal volume of 0.342 dm3.59,60 The calibration of the calorimeter was made by combustion of benzoic acid, NIST Thermochemical Standard 39j, with a certified massic energy of combustion, when burnt under the bomb conditions, of 26434 ( 3 J 3 g1.61 Six calibration experiments, that were performed under oxygen, at p = 3.04 MPa, with 1.00 cm3 of deionized water added to the bomb, leading to the value of the energy equivalent of the calorimeter ε(calor) = 15995.3 ( 2.0 J 3 K1 (the uncertainty quoted is the standard deviation of the mean), for an average mass of water added to the calorimeter of 3119.6 g. The procedure suggested by Coops et al.,62 was followed. Samples of compounds, in pellet form, were ignited in oxygen, at T = 298.150 ( 0.001 K, under a pressure of 3.04 MPa, with 1.00 cm3 of deionized water introduced into the bomb. Temperature measurements of the calorimeter were automatically collected every 10 s, with a precision of (1 104 K, using a Hewlett-Packard (HP 2804 A) quartz crystal thermometer interfaced to a PC programmed to data acquisition and to compute the adiabatic temperature change, through the program LABTERMO.63 The electrical energy for ignition was measured from the change in potential difference across a capacitor when discharged through a platinum ignition wire (diameter φ = 0.05 mm). For the cotton thread fuse, whose empirical formula is CH1.686O0.843, Δcuo = 16240 J 3 g1,64 a value which has been previously confirmed in our Laboratory. The nitric acid formed was determined by titration and the respective correction was based on 59.7 kJ 3 mol1 for the molar energy of formation of 0.1 mol 3 dm3 HNO3 (aq) from N2(g), O2(g), and H2O(l).65 The amount of compound, m(cpd), burnt in each experiment and on which the energy of combustion was based, was determined from the mass of CO2 produced taking into account that formed from the combustion of the cotton thread fuse. The value of the pressure coefficient of specific energy, (∂u/∂p)T, was assumed to be 0.2 J 3 g1 3 MPa1, at T = 298.15 K, a typical value for most organic compounds.66 The standard massic energies of combustion of the compounds, Δcuo, were calculated following the procedure given by Hubbard et al.67 2.3. Knudsen Effusion Technique. The mass-loss Knudsen effusion technique was applied to measure the vapor pressure of the purified crystalline samples of each of the three diaminobenzene isomers, at several temperatures. The apparatus used allows the simultaneous operation of nine aluminum effusion cells, contained in cylindrical holes inside three temperaturecontrolled aluminum blocks. A detailed description of the system, procedure, technique as well as the results obtained with five test compounds, namely, benzoic acid, phenanthrene, anthracene, benzanthrone, and 1,3,5-triphenylbenzene has been reported before.68 During an effusion experiment, each block is kept at a constant temperature, different from the other two blocks, with three effusion cells inside and with different effusion
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areas. The areas and Clausing factors of the nine effusion orifices, made of platinum foil of 0.0125 mm thickness, are presented in the Supporting Information, Table S1. The vapor pressure, p, of the crystalline compounds in the effusion experiments is calculated by eq 1, knowing the mass of sublimed compound, Δm, during a convenient time period, t, at the temperature, T, of the experiment, in a system evacuated to a pressure near 1 104 Pa. The uncertainty of the temperature measurements is estimated to be less than ((1 102) K, and the uncertainty of the calculated vapor pressures is estimated to be less than 0.01 Pa. p ¼ ðΔm=Ao wo tÞð2πRT=MÞ1=2
ð1Þ
where A0 represents the area of the effusion orifice, w0 is the respective Clausing factor, R is the gas constant, and M is the molar mass of the effusing vapor. The amount of compound sublimed is determined by weighing the effusion cells to (0.01 mg, before and after each effusion experiment.
3. COMPUTATIONAL DETAILS Ab initio molecular calculations,69 have been performed with the G3(MP2)//B3LYP composite method,70 aiming the estimation of the gas-phase standard molar enthalpies of formation of the three diaminobenzene isomers. In this method, the B3LYP/ 6-31G(d) approach yield the optimized geometry and the thermal corrections for T = 298.15 K. The geometries obtained have been characterized as true minima after the computation of the vibrational frequencies at the same level of theory. Then, for the previously optimized structures, two additional calculations are carried out to correct the energy calculated with the DFT method, the QCISD(T)/6-31G(d) and MP2/GTMP2Large approaches. The absolute enthalpies, at T = 298.15 K, were obtained by adding the energies computed at T = 0 K with the vibrational, translational, rotational and the pV terms. All the calculations have been performed by means of the Gaussian 03 computer code.71 These enthalpies, at T = 298.15 K, were then used to estimate the enthalpies of formation of the compounds under study, by combining the enthalpy of the gas-phase working reactions 2 and 3 and the experimental enthalpies of formation of the atoms and molecules there involved:
Proton and electron affinities, basicities and adiabatic ionization enthalpies of the diaminobenzene isomers were also calculated through the G3(MP2)//B3LYP approach. Following the convention, gas-phase basicity (ΔGbasicity), proton affinity (PA), and electron affinity (EA) were calculated as: A þ Hþ f AHþ 4941
ΔGbasicity ¼ ΔGr
ð4Þ
dx.doi.org/10.1021/jp200670s |J. Phys. Chem. B 2011, 115, 4939–4948
The Journal of Physical Chemistry B
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Table 1. Typical Combustion Results, at T = 298.15 K, (p = 0.1 MPa), for the Studied Compoundsa 1,2-diaminobenzene
1,3-diaminobenzene
1,4-diaminobenzene
m(CO2, total)/g
1.472 71
1.472 13
1.474 75
m(cpd)/g
0.600 79
0.600 91
0.601 74
m0 (fuse)/g
0.003 54
0.003 00
0.003 36
ΔTad/K
1.221 89
1.220 80
1.225 49
εf/J 3 K1
15.69
15.66
15.74
Δm(H2O)/g
0
0
0
ΔU(IBP)b/J
19562.88
19545.15
19620.27
ΔU(fuse)/J ΔU(HNO3)/J
57.49 55.03
48.72 58.31
54.57 51.43 1.10
ΔU(ign)/J
0.79
1.03
ΔU∑/J
11.83
11.81
11.93
Δcuo/J 3 g1
32354.95
32328.15
32409.91
a m(CO2, total) is the mass of CO2 recovered in each combustion; m(cpd) is the mass of compound burnt in each experiment; m0 (fuse) is the mass of the fuse (cotton) used in each experiment; ΔTad is the corrected temperature rise; εf is the energy equivalent of the contents in the final state; Δm(H2O) is the deviation of mass of water added to the calorimeter from 3119.6 g; ΔU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions and includes ΔU(ignition); ΔU(fuse) is the energy of combustion of the fuse (cotton); ΔU(HNO3) is the energy correction for the nitric acid formation; ΔU(ign) is the electric energy for the ignition; ΔU∑ is the standard state correction; Δcuo is the standard massic energy of combustion. b ΔU(IBP) includes ΔU(ignition).
A þ Hþ f AHþ A þ e f A
PA ¼ ΔHr EA ¼ ΔHr
ð5Þ ð6Þ
Table 2. Individual Values of Standard (po = 0.1 MPa) Massic Energies of Combustion, Δcu, of the Compounds, at T = 298.15 K 1,2-diaminobenzene
1,3-diaminobenzene
1,4-diaminobenzene
where A = diaminobenzene isomer. Δcuo/J 3 g1
4. RESULTS AND DISCUSSION
32359.07
32332.27
32392.85
4.1. Condensed Phase and Phase Transition. Table 1
32354.95
32320.25
32418.99
collects the combustion results for a typical experiment of each compound studied, in which Δm(H2O) represents the deviation of the mass of water added to the calorimeter from 3119.6 g, the mass assigned to ε(calor), ΔTad is the calorimeter temperature change corrected for the heat exchange and the work of stirring, ΔUΣ is the correction to the standard state and the remaining terms are as previously defined.67,72 The detailed results for all the combustion experiments of each compound are presented in the Supporting Information, Tables S2, S3, and S4. The energy associated with the isothermal bomb process, ΔU(IBP), was calculated using the following equation:
32353.40
32332.18
32398.05
32343.49
32328.15
32420.71
32346.47
32327.25
32414.47
32363.17
32330.41
ΔUðIBPÞ ¼ fεðcalorÞ þ cp ðH2 O, lÞ 3 ΔmðH2 OÞ þ εf gΔTad þ ΔUðignÞ
ð7Þ
The results of all the combustion experiments of each compound, together with the mean values, ÆΔc uoæ, and their standard deviations of the mean are presented in Table 2. These values are referred to the combustion reaction described by the following equation: C6 H8 N2 ðcrÞ þ 8O2 ðgÞ f 6CO2 ðgÞ þ 4H2 OðlÞ þ N2 ðgÞ ð8Þ The derived standard molar energies, ΔcUom(cr), and enthalpies, ΔcHom(cr), of combustion, and the standard molar enthalpies of formation, ΔfHom(cr), in the crystalline phase, at T = 298.15 K, for each compound are given in Table 3.
(32353.4 ( 3.0)a a
32409.91 1
ÆΔcu æ/J 3 g (32328.4 ( 1.8)a o
(32409.2 ( 4.6)a
Mean value and standard deviation of the mean.
In accordance to Rossini,73 and Olofsson,74 the uncertainties assigned for the standard the molar energies and enthalpies of combustion are twice the overall standard deviation of the mean and include the uncertainties in calibration and in the values of the auxiliary quantities used. The CODATA values of the ΔfHom, at T = 298.15 K, of H2O(l), 285.830 ( 0.042 kJ 3 mol1 and CO2(g), 393.51 ( 0.13 kJ 3 mol1, combined with the ΔcHom(cr), were used to derive the values of ΔfHom(cr), at T = 298.15 K.75 Values of vapor pressures at different temperatures, obtained for each diaminobenzene isomer and for each effusion orifice, together with the residuals of the ClausiusClapeyron equation {102 Δ[ln(p/Pa)]}, derived from least-squares adjustments, are summarized in Table 4. The standard molar enthalpies of sublimation at the mean temperature of the experimental temperature range, ÆTæ, were derived from the integrated form of the ClausiusClapeyron equation, ln(p/Pa) = a b 3 (T/K)1, where a is a constant and b = ΔgcrHom(ÆTæ)/R. For each isomer studied, the equations obtained, jointly with the calculated standard deviations were the following ones: 4942
dx.doi.org/10.1021/jp200670s |J. Phys. Chem. B 2011, 115, 4939–4948
The Journal of Physical Chemistry B
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Table 3. Derived Standard (po = 0.1 MPa) Molar Energies of Combustion, ΔcUom, Standard Molar Enthalpies of Combustion, ΔcHom, and Standard Molar Enthalpies of Formation, ΔfHom, for the Crystalline Compounds, at T = 298.15 K compound
ΔcUom(cr)/kJ 3 mol1
ΔcHom(cr)/kJ 3 mol1
ΔfHom(cr)/kJ 3 mol1
1,2-diaminobenzene
3498.7 ( 1.4
3501.2 ( 1.4
3.2 ( 1.6
1,3-diaminobenzene 1,4-diaminobenzene
3496.0 ( 1.2 3504.8 ( 1.6
3498.5 ( 1.2 3507.3 ( 1.6
5.9 ( 1.5 2.9 ( 1.7
Table 4. Knudsen Effusion Results for the Three Diaminobenzene Isomers 102 Δ[ln(p/Pa)]a
p/Pa T/K
t/s
orifices
small medium large small medium large 1,2-diaminobenzene
292.10 20 197 A1B4C7 0.0963 0.0985 0.0974 - 0.6 294.16 20 197 A2B5C8 0.127
0.125
0.124
1.7
0.5
1.1
- 0.5
- 1.4 - 2.1
296.21 20 197 A3B6C9 0.170
0.159
0.158
4.6
- 1.6
298.10 18 754 A1B4C7 0.207
0.205
0.205
1.3
0.5
0.7
300.16 18 754 A2B5C8 0.269
0.265
0.258
3.0
1.3
- 1.3
302.20 18 754 A3B6C9 0.343
0.327
0.324
2.9
- 1.9
- 2.6
304.10 12 620 A1B4C7 0.409 306.16 12 620 A2B5C8 0.531
0.411 0.519
0.407 0.515
- 1.8 0.4
- 1.3 - 1.7
- 2.3 - 2.6
308.20 12 620 A3B6C9 0.680
0.652
0.634
1.9
- 2.3
- 5.1
310.08 10 806 A1B4C7 0.837
0.835
0.823
1.5
1.2
- 0.2
312.13 10 806 A2B5C8 1.069
1.054
1.006
3.1
1.6
- 3.0
314.20 10 806 A3B6C9 1.388
1.297
1.285
6.4
- 0.4
- 1.3
1,3-diaminobenzene 301.10 20 138 A1B4C7 0.101
0.102
0.104
- 2.0
- 0.4
0.9
303.18 20 138 A2B5C8 0.133 305.20 20 138 A3B6C9 0.176
0.133 0.168
0.132 0.167
- 0.4 2.9
0.1 - 1.4
- 0.7 - 2.6
307.10 18 008 A1B4C7 0.219
0.218
0.218
1.6
1.3
1.1
309.17 18 008 A2B5C8 0.282
0.277
0.273
2.2
0.3
- 1.4 - 3.1
311.19 18 008 A3B6C9 0.361
0.344
0.341
2.7
- 2.1
313.09 15 409 A1B4C7 0.443
0.445
0.442
0.9
1.3
0.6
315.15 15 409 A2B5C8 0.562
0.562
0.549
0.8
0.9
- 1.6
317.18 15 409 A3B6C9 0.722
0.687
0.681
2.6
- 2.2
- 3.1
319.11 10 423 A1B4C7 0.888 321.18 10 423 A2B5C8 1.112
0.885 1.125
0.868 1.064
1.7 1.1
1.4 2.2
- 0.5 - 3.4
323.19 10 423 A3B6C9 1.421
1.345
1.326
3.5
- 2.0
- 3.4
1,4-diaminobenzene
a
310.10 20 150 A1B4C7 0.0936 0.0940 0.0921
0.5
0.9
- 1.1
312.17 20 150 A2B5C8 0.120
0.8
1.8
- 0.7
Figure 2. Plots of ln(p/Pa) against 1/T for the three diaminobenzene isomers: O, small holes; Δ, medium holes; 0, large holes.
1; 2-diaminobenzene : ln p ¼ ð34:60 ( 0:17Þ 10788 ( 53 ðR 2 ¼ 0:9992Þ T ð9Þ 1; 3-diaminobenzene : ln p ¼ ð35:66 ( 0:15Þ 11442 ( 47 ðR 2 ¼ 0:9994Þ T ð10Þ 1; 4-diaminobenzene : ln p ¼ ð34:96 ( 0:16Þ 11577 ( 52 ðR 2 ¼ 0:9993Þ T ð11Þ
318.14 16 701 A2B5C8 0.246
0.241
0.235
2.9
0.5
- 1.9
In Figure 2 are depicted the plots of ln(p/Pa) = f(1/T) for the global results obtained for the three studied compounds. The enthalpies of sublimation, at T = 298.15 K, ΔgcrHom, were derived from the same parameter at the mean temperature ÆTæ of the experiment, ΔgcrHom(ÆTæ) by
320.19 16 701 A3B6C9 0.311 322.08 13 508 A1B4C7 0.377
0.293 0.370
0.292 0.374
3.0 1.0
- 3.0 0.9
- 3.5 0.1
Δgcr Hmo ðT ¼ 298:15KÞ ¼ Δgcr Hmo ðÆTæÞ þ Δgcr Cop, m ð298:15 ÆTæÞ
324.13 13 508 A2B5C8 0.470
0.467
0.459
0.3
0.4
- 2.0
0.122
0.118
314.20 20 150 A3B6C9 0.157
0.150
0.147
3.6
- 1.1
- 3.2
316.08 16 701 A1B4C7 0.189
0.191
0.186
0.3
1.2
- 1.5
326.19 13 508 A3B6C9 0.606
0.579
0.568
3.2
1.4
- 3.3
328.10 10 367 A1B4C7 0.732
0.730
0.725
1.4
1.1
0.5
330.16 10 367 A2B5C8 0.918
0.901
0.877
2.0
0.2
- 2.6
332.18 10 367 A3B6C9 1.163
1.111
1.083
4.3
- 0.2
- 2.8
The deviations of the experimental results from those given by the ClausiusClapeyron equations are denoted by Δln(p/Pa).
ð12Þ For the three isomers, the values of ΔgcrCop,m were calculated from the respective molar heat capacities, in the crystalline phase, Cop,m(cr), available in the literature,37,76,77 and the values of the gas phase molar heat capacities, Cop,m(g), at T = 298.15 K, obtained in this work from statistical thermodynamics using the vibrational frequencies from DFT calculations, B3LYP/6-31G(d) approach (scaled by 4943
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Table 5. Values of the Standard (po = 0.1 MPa) Molar Enthalpies, ΔgcrHom, Entropies, Δgcr Som, and Gibbs Energies, Δgcr Gom, of Sublimation, at the Mean Temperature of the Experiments T = ÆTæ and T = 298.15 K, for the Compounds Studied T = 298.15 K ΔgcrSm(ÆTæ, 1
ΔgcrSom/
p(ÆTæ))/
compound
ÆTæ/K
ΔgcrHom(ÆTæ)/kJ 3 mol1
1,2-diaminobenzene
303.15
89.7 ( 0.4
295.9 ( 1.3
89.8 ( 0.4
192.8 ( 1.3
32.3 ( 0.6
1,3-diaminobenzene 1,4-diaminobenzene
312.15 321.14
95.1 ( 0.4 96.3 ( 0.4
304.7 ( 1.3 299.9 ( 1.2
95.5 ( 0.4 96.8 ( 0.4
203.0 ( 1.3 198.8 ( 1.2
35.0 ( 0.6 37.5 ( 0.5
J 3 K
0.9614).78 For 1,2-diaminobenzene, Cop,m(cr) = 150.82 J 3 K1 3 mol1,76 and Cop,m(g) = 130.3 J 3 K1 3 mol1, yielding a value of Δgcr Cop,m= 20.5 J 3 K1 3 mol1. A value of Δgcr Cop,m = 25.4 J 3 K1 3 mol1 was obtained for 1,3-diaminobenzene, derived from Cop,m(cr) = 159.6 J 3 K1 3 mol1,77 and Cop,m(g) = 134.2 J 3 K1 3 mol1. Finally, for 1,4-diaminobenzene, Δgcr Cop,m = 21.4 J 3 K1 3 mol1, calculated from Cop,m(g) = 134.2 J 3 K1 3 mol1 and Cop,m(cr) = 155.64 J 3 K1 3 mol1,37 (it was assumed that Cop,m(cr) at T = 298.15 K is equal to Cop,m(cr) at T = 300 K, the only value available in the literature). The values of the standard (po = 0.1 MPa) molar enthalpies,Δgcr Hom, entropies, Δgcr Som, and Gibbs energies Δgcr Gom, of sublimation, at the mean temperature of the experiments T = ÆTæ and at T = 298.15 K, for the studied isomers are given in Table 5. 4.2. Gas-Phase—Molecular Structures. In Figure 3 are represented the front and side views of the B3LYP/6-31G(d) optimized most stable structures of the diaminobenzene isomers. The carbon scaffold of the three molecules is almost planar and the nitrogen atoms are out of the ring plane (φ) for about 2 - 3, which is in agreement with the values found by Vaschetto et al., at the B3LYP/6-31G(d) level,36 and with Noto et al., with the 6-31G(d,p) basis set at the HF and MP2 methods.35 However, the degree of tilting of the amino groups toward the phenylene plane calculated in this work for 1,2-diaminobenzene is slightly lower that the corresponding obtained by Vaschetto et al (φ = 1.7 vs φ = 3.8). The values of the structural parameters calculated by Noto et al. (MP2/6-31G(d,p)) and by Vaschetto and collaborators for the three diaminobenzene isomers are in great agreement with the ones obtained in this work. The most stable conformation adopted by the three isomers is that in which the -NH2 groups are in trans orientation related to the aromatic ring plane (the hydrogen atoms of both amino groups are two above and two below with respect to the aromatic plane). No cis conformer was found for 1,2-diaminobenzene. Trans and cis conformations were obtained for the 1,3-diaminobenzene and 1,4-diaminobenzene, for which the trans is 0.34 kJ 3 mol1 and 0.24 kJ 3 mol1 more favorable than cis conformation, respectively. The optimized structure of 1,2-diaminobenzene is somewhat different when compared with those of both 1,3- and 1,4diaminobenzene; one of the hydrogen atoms of each -NH2 group is above or below ca. 46 of the molecular plane (C2C1N1H2 = C1C2N2H3), while the other H atoms are only about 12 (C6C1N1H1 = C3C2 N2H4); also the C1C2 bond length is larger that the other ring distances (these are the atoms in which the aminic groups are connected), which is in accordance with Noto and collaborators.35 For the meta- and para-isomers, the interatomic ring distances are equivalent, as it is shown in Figure 3, and all the
1
3 mol
ΔgcrHom/kJ 3 mol1
J 3 K1 3 mol1
ΔgcrGom/kJ 3 mol1
dihedral angles referred above are similar, ca. 27 for 1,3diaminobenzene and 29 for 1,4-diaminobenzene. A computational study due to Palafox and Melendez,34 shows that the tilt angle, φ, for 1,4-diaminobenzene ranges from 2.3 to 3.6, depending on the approach used. The parameters obtained with B3LYP/6-31G(d,p) are in good agreement with the corresponding ones calculated in this work, at the B3LYP/6-31G(d) level. The calculated molecular geometry parameters for the para-isomer, from other two computational studies,31,32 agree reasonably well with the values reported in Figure 3. 4.3. Gas-Phase Experimental and Theoretical Enthalpies of Formation. The gas-phase standard (p = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of the three diaminobenzene isomers, derived from the respective standard molar enthalpies of formation in the crystalline phase, ΔfHom(cr), and the standard molar enthalpies of sublimation, are presented in Table 6. For 1,2-diaminobenzene, the value of ΔfHom(cr) derived in this work (3.2 ( 1.6 kJ 3 mol1) is in satisfactory agreement with those reported by Kunyavskaya et al. (0.0 kJ 3 mol1),37 and Pedley (0.3 ( 4.1 kJ 3 mol1),42 but not with those reported by Contineanu et al.,38 and by Sabbah and Perez,43 in which the differences are ca. 42 kJ 3 mol1 (!!!) and 10 kJ 3 mol1, respectively. Regarding the ΔfHom(cr) for 1,3-diaminobenzene, the literature reports the values of Kunyavskaya et al.,37 Pedley,42 and Sabbah et al.,43 which differ from our value (Table 3) by 2.0, 1.9, and 0.5 kJ 3 mol1, respectively. For 1,4-diaminobenzene, there are also in the literature several values for the ΔfHom(cr), where the maximum deviation from our value is from the one due to Sabbah and Perez (6.4 kJ 3 mol1). Differences of 3.4, 2.0, 0.1, and 0.1 kJ 3 mol1 were found from the values reported by Kunyavskaya et al.,37 Sullivan et al.,39 Kliber and Hunt,41 and Pedley,42 respectively. The only values of enthalpies of sublimation of the benzenediamine isomers found in the literature are those reported by Sabbah and Perez;43 their results are, systematically, ca. 5 kJ 3 mol1 lower than the values obtained in this work. By comparing the values of ΔfHom(g) derived by Sabbah and Perez,43 for o-, m- and p-diaminobenzene (92.01 ( 1.3, 83.96 ( 1.4, and 88.72 ( 1.4 kJ 3 mol1) and the corresponding obtained in this work (Table 6), it is possible to notice that there is a great disagreement between them (differences of 5.4, 5.6, and 11.0 kJ 3 mol1). According to the values of Sabbah and Perez, the three isomers follow the order of stability meta- > para- > ortho-. This trend is reversed, regarding the values obtained in this work, ortho- > meta- > para-. For comparison purposes and in order to clarify some doubts concerning the experimental values of thermochemical data of diaminobenzene isomers, caused by the wide range of values found in the literature, we have carried out a computational study aiming the calculation of the gas-phase standard molar enthalpies 4944
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Figure 3. Front and side views of the B3LYP/6-31G(d) optimized structures of the three isomers of diaminobenzene. Selected bond lengths (nm) and bond angles (deg) are included.
of formation of the compounds studied. These values were estimated by computing the enthalpy of each reaction, described by eqs 2 and 3, for each isomer, and from reliable experimental values of ΔfHom(g) of the auxiliary species used: carbon, 716.67 kJ 3 mol1,79 hydrogen, 218.00 kJ 3 mol1,79 nitrogen, 472.68 kJ 3 mol1,79 benzene, 82.6 ( 0.7 kJ 3 mol1,42 and aniline, 87.1 ( 1.1 kJ 3 mol1.42 The values estimated with the G3(MP2)//B3LYP approach are reported in Table 6 and an excellent agreement has been obtained with our experimental values, with a maximum enthalpic differences of 2.9, 2.0, and 1.0 kJ 3 mol1, respectively, for 1,2-, 1,3-, and
1,4-diaminobenzene. The best estimates are achieved when the reaction described by eq 3 is considered, due to a good compensation of different bonds in the reactants and in the products of reaction. It is important to note that, for 1,4diaminobenzene, the deviations obtained are smaller than the uncertainties associated with the experimental value. The computed G3(MP2)//B3LYP enthalpies for the compounds studied, auxiliary molecules and atoms used in the working reactions 2 and 3 are presented in Table S5 in the Supporting Information. Regarding the values of the gas-phase enthalpies of formation of the three isomers studied, presented in Table 6, they suggest 4945
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Table 6. Comparison Between the Experimental and Computed G3(MP2)//B3LYP Gas-Phase Enthalpies of Formation of the Three Diaminobenzene Isomers at T = 298.15 K ΔfHom(g)/kJ 3 mol1
a b
compound
experimental
G3(MP2)//B3LYPa
Δb
1,2-diaminobenzene
86.6 ( 1.6
89.5 (2)
2.9
89.0 (3)
2.4
1,3-diaminobenzene
89.6 ( 1.6
91.6 (2)
2.0
91.1 (3)
1.5
1,4-diaminobenzene
99.7 ( 1.7
100.7 (2) 100.2 (3)
1.0 0.5
The respective working reactions are indicated in parentheses. Difference between the experimental and computed values.
that the ortho- isomer is thermodynamically more stable than the other two isomers. In a rough analysis, we could be led to think that this compound would be the less stable due to the proximity of both NH2 groups, and consequently, a higher steric hindrance between them. Indeed, the 1,2-diaminobenzene is the most stable isomer, since it is stabilized by intramolecular NH 3 3 3 N hydrogen bonds. Experimentally, Krueger,80 through partial deuteration and examination of the cistrans isomerism of the NDH group, confirmed the formation of double intramolecular NH 3 3 3 N hydrogen bonds, and each NH2 group acts as a proton donor and a proton acceptor. Sabbah and Perez, in their thermodynamic study of the three benzenediamine isomers, also considered the existence of an intramolecular hydrogen bond in the ortho- isomer.43 Indeed, the value of the enthalpy of sublimation of the ortho-isomer, determined in this work, is 5.7 and 7.0 kJ 3 mol1 lower than the values of the meta- and para-isomers, respectively, which means that the intermolecular hydrogen bonds in the 1,2-diaminobenzene are smaller and the NH2 groups are less available to establish this type of interaction, due to the existence of intramolecular NH 3 3 3 N hydrogen bonds. More recently, Estacio and collaborators, by means of computational chemistry, evaluated the energetics of intramolecular hydrogen bonding in disubstituted benzenes, namely, the 1,2-diaminobenzene.81 Through the orthopara method (enthalpic difference between the para- and the ortho-isomers), they reached to several values of intramolecular hydrogen bond enthalpies, depending on the computational method used; according to these researchers, their best estimate is 10.8 kJ 3 mol1, determined by a modified complete basis set extrapolation method (CBS-QMPW1). Following the ortho-para method proposed by Estacio et al. the experimental value of the intramolecular hydrogen bond enthalpy obtained in this work is 13.1 ( 2.3 kJ 3 mol1, which fits very well with the computational value. 4.4. Other Gas-Phase Thermodynamic Properties. Values of adiabatic ionization enthalpies (IE), electron (EA) and proton affinities (PA), and gas-phase basicities, ΔGbasicity, at T = 298.15 K, for the three diaminobenzene isomers were also calculated by means of the G3(MP2)//B3LYP composite method; the whole set of results obtained are presented in Table 7. By comparing the IE values obtained for the three isomers, it is possible to notice that the energy required to remove an electron is almost the same for 1,2- and 1,3-diaminobenzene, but ca. 49 kJ 3 mol1 higher than the energy needed for the 1,4- isomer. As stated in the introduction, there are several IE values in the
Table 7. G3(MP2)//B3LYP Computed Proton, PA, and Electron Affinities, EA, Gas-Phase Basicities, ΔGbasicity, and Adiabatic Ionization Enthalpies, IE, at T = 298.15 K, for the Three Diaminobenzene Isomersa
a
ΔGbasicity
EA
IE
905.3 (N1 = N2)
873.3
192.1
711.8
940.0 (C4 = C6)
909.4
176.4
709.2
909.7 (N1 = N2)
884.5
100.2
661.8
compound
PA
1,2-diaminobenzene 1,3-diaminobenzene 1,4-diaminobenzene
All values are in kJ 3 mol1.
literature for the diaminobenzene isomers. The values calculated in this work lie in the range of data determined by other authors. For 1,2-diaminobenzene, the value obtained (711.8 kJ 3 mol1) is in agreement with those determined by charge transfer spectra (718.8,45 710.1,46 and 714.0,47 kJ 3 mol1), but differs by ∼17 kJ 3 mol1 and ∼60 kJ 3 mol1 from the values determined by Tsuji et al.,48 and Crable and Kearns,49 respectively. For the meta- isomer the IE calculated value (709.2 kJ 3 mol1) is near from the value reported by Pykhtina et al. (717.8 kJ 3 mol1),53 and for the para- isomer, the selected value, in the NIST webbook (662.8 ( 4.8 kJ 3 mol1),54 almost matches our calculated value. The computed electron affinities show that the addition of one electron to the 1,4-diaminobenzene is more favorable than in the other two isomers. The entrance of the amino group in the metaposition of aniline favors the electron affinity by ca. 16 kJ 3 mol1 with respect to the ortho-position. The calculated ΔGbasicity presented in Table 7 indicate that the basic character of the compounds studied follow the trend 1,3diaminobenzene >1,4-diaminobenzene >1,2-diaminobenzene, in agreement with Lau et al.,.52 The values obtained in this work are about 10 kJ 3 mol1 higher than the corresponding ones reported in the literature.50 Both 1,2- and 1,4-diaminobenzene protonate, preferentially, on the nitrogen atoms of the aminic groups and the carbon atoms in which the NH2 groups are attached appear to be the least favorable sites for protonation. These two compounds are nitrogen bases. The values of PA obtained for these two isomers differs only by 4.4 kJ 3 mol1, with the N-protonation occurring more easily in the para- isomer. Our calculated value of proton affinity for 1,4-diaminobenzene is in accordance with the review value due to Hunter and Lias,50 (PA = 909.7 kJ 3 mol1 and 905.9 kJ 3 mol1, respectively). For the 1,2-diaminobenzene, our value differs ∼9 kJ 3 mol1 from the value selected by Hunter and Lias. On the other hand, Bagno and Terrier found, at G3(MP2) level, that in the ortho- isomer, the nitrogen atom is the most basic site and they obtain a PA = 905.0 kJ 3 mol1, which is in excellent agreement with our value.51 In opposite, the meta-isomer is ring protonated, i.e. undergoes protonation on the C4 (=C6) atom, as pointed out earlier by Lau et al.52 and by Lee and collaborators,82 for several meta- anilines, because π-electron donating substituents, like NH2, in the meta-position, stabilize the benzenium ions. The proton attachment at the nitrogen atoms was found to be 42.7 kJ 3 mol1 less favorable than in the C4 (=C6) atom. The selected NIST value for the PA of the 1,3-diaminobenzene (929.9 kJ 3 mol1),50 differs about 10 kJ 3 mol1 than the obtained in this work. All the computed proton affinity values of each protonation site of the three diaminobenzene isomers are collected in Table S6, in the Supporting Information. 4946
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5. CONCLUSIONS The experimental thermochemical study carried out allows the determination of new values for the standard molar enthalpies of formation, in the crystalline phase, and standard molar enthalpies of sublimation of the three diaminobenzene isomers, since there are several discrepant values available in the literature. Consequently, new values for the gas-phase were derived and they fit very well with data calculated by the G3(MP2)//B3LYP approach, as referred in section 4.3. Computationally, the molecular structures of the three isomers was established and the structural parameters were determined at the B3LYP/631G(d) level of theory. It was found that the most stable conformation adopted by the three isomers is that in which the NH2 groups are in trans orientation related to the aromatic ring plane. Moreover, proton and electron affinities, basicities and adiabatic ionization enthalpies were also computed at the G3(MP2)//B3LYP approach. In the gas-phase, the meta-isomer is ring protonated, while the nitrogen atom is the most basic site for the ortho- and para-isomers. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed data of the effusion orifices (diameter and Clausing factors) of the Knudsen apparatus, the data and the details of all the combustion calorimetry experiments for the 1,2-diaminobenzene, 1,3-diaminobenzene, and 1,4-diaminobenzene, G3(MP2)//B3LYP enthalpies (energies plus thermal corrections for T = 298.15 K) for the three diaminobenzene isomers and for the auxiliary molecules and atoms used in the several working reactions and G3(MP2)//B3LYP computed proton affinities, and PA, at T = 298.15 K, for the three diaminobenzene isomers.This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ351-22-0402 521. Fax: þ351-22-0402 522. E-mail:
[email protected].
’ ACKNOWLEDGMENT Thanks are due to Fundac-~ao para a Ci^encia e a Tecnologia, F. C.T., Lisbon, Portugal, and to FEDER for financial support to Centro de Investigac-~ao em Química, University of Porto. A.F.L. OMS thanks FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the postdoctoral fellowship (SFRH/BPD/41601/2007). ’ REFERENCES (1) Rappoport, Z. The Chemistry of Anilines, Part 1, Interscience: Jerusalem, 2007. (2) Zimmerman, W. T. J. Labelled Compd. Radiopharm. 2000, 43, 767–772. (3) Unger, T. A. Pesticide Sysnthesis Handbook; Noyes Publications: United States, 1996. (4) Martínez-Palou, R.; Zepeda, L. G.; H€opfl, H.; Montoya, A.; Guzman-Lucero, D. J.; Gusman, J. Mol. Diversity 2005, 9, 361–369. (5) Lukachova, L. V.; Kotel’nikova, E. A.; D’Ottavi, D.; Shkerin, E. A.; Karyakina, E. E.; Moscone, D.; Palleschi, G.; Curulli, A.; Karyakin, A. A. Bioelectrochem. 2002, 55, 145–148.
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