Experimental and computational study of the energetics of 5- and 6-aminoindazole

J. Chem. Thermodynamics 42 (2010) 1240–1247

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Experimental and computational study of the energetics of 5- and 6-aminoindazole Manuel A.V. Ribeiro da Silva *, Joana I.T.A. Cabral, Álvaro Cimas 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 23 April 2010 Accepted 27 April 2010 Available online 5 May 2010 Keywords: Standard molar enthalpy of formation Standard molar enthalpy of sublimation Combustion calorimetry Knudsen effusion technique Vapour pressures 5-Aminoindazole 6-Aminoindazole Computational thermochemistry Electron affinity Ionization enthalpy Bond dissociation enthalpy

a b s t r a c t This work aims to study the influence of the amino group in positions 5 and 6 of the benzene ring of indazole. For that purpose, the standard (p ¼ 0:1 MPa) molar enthalpies of formation of 5- and 6-aminoindazole, in the gaseous phase, at T = 298.15 K were determined. These values were calculated from the standard massic energies of combustion, measured by combustion calorimetry, and from the standard molar enthalpies of sublimation, computed from the variation with the temperature of the vapour pressures of each compound, measured by the Knudsen effusion technique. Compound

Dc u =ðJ  g1 Þ

Dgcr Hm =ðkJ  mol1 Þ

Df Hm ðgÞ=ðkJ  mol1 Þ

5-Aminoindazole (cr) 6-Aminoindazole (cr)

29299.4 ± 3.7 29255.9 ± 3.9

118.34 ± 0.69 121.95 ± 0.63

265.1 ± 1.8 263.0 ± 1.7

The final results for the enthalpies of formation in the gaseous phase are discussed in terms of structural contributions of the amino group. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that the 6-aminoindazole is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, N–H bond dissociation enthalpies. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The interest in indazoles is associated with the crucial importance of this class of compounds. Although indazoles are rare in nature [1], they play an important role in biological and pharmaceutical areas. Due to the relevance of these compounds, the determination of physical and chemical properties of indazoles derivatives has been the object of many studies in order to contribute for the elucidation of reaction mechanisms, for example in the development of new pharmaceuticals [2]. Numerous studies are published regarding the properties of the aminoindazoles. For example, 3-aminoindazole derivatives are reported as having anti-inflammatory, analgesic, and antipyretic properties [3]. Studies with 5-aminoindazole prove that it is potentially active in reproductive systems, having antispermatogenic and antifertility properties [4]. The compound 6-aminoindazole reduces the gastric acid secretion in rats [5]. The study of the influence of different groups in the benzene ring of the indazole molecule has recently been started in our Research * 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.2010.04.026

Group. The results for 5- and 6-nitroindazole were already published and it was concluded that the introduction of the nitro group has a stabilization effect in the indazole molecule and that this effect is similar, whether the nitro group is in position 5 or 6 [6]. In the present work, we intend to study the influence of the amino groups in the indazole molecule. Combustion experiments with a static bomb calorimeter were carried out with 5-aminoindazole and 6-aminoindazole to determine the respective standard massic energies of combustion and, consequently, the standard molar enthalpies of formation in the solid phase. The vapour pressures on the temperature ranges of (358 to 380) K for 5-aminoindazole and, (366 to 388) K for 6-aminoindazole, were measured by the Knudsen effusion technique. From the vapour pressures, the respective standard molar enthalpies of sublimation were computed applying the Clausius–Clapeyron equation. The results obtained by these two techniques enabled us to calculate the standard molar enthalpies of formation, at T = 298.15 K, in the gaseous phase. Additionally, the gas-phase standard molar enthalpies of formation of these compounds were estimated computationally as well as the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies, and N–H bond dissociation enthalpies.

M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 1240–1247

2. Experimental 2.1. Compounds and purity control In this work, the compounds 5-aminoindazole [CAS 19335-116] and 6-aminoindazole [CAS 6967-12-0] whose structural formulæ are depicted in figure 1 were supplied by Aldrich Chemical Co., with initial mass fraction purity of 0.97 and 0.98, respectively. Both compounds were purified by sublimation. The purity of the compounds was determined recovering the carbon dioxide produced in the combustion experiments and it was confirmed by glc. The average ratios of the mass of carbon dioxide recovered after combustion experiments to that calculated from the mass of sample were: 5-aminoindazole (99.945 ± 0.022) and 6-aminoindazole (100.016 ± 0.021), where the uncertainties are the standard deviations of the means. The densities of each compound were estimated from the mass and the dimensions of pellets made in vacuum, with an applied pressure of 105 kg  cm2, as (1.40 and 1.45) g  cm3 for 5-aminoindazole and 6-aminoindazole, respectively. The relative atomic masses used were those recommended by the IUPAC Commission in 2005 [7]. 2.2. Combustion calorimetry The standard massic energies of combustion of 5- and 6-aminoindazole were determined by static bomb calorimetry, using an isoperibol system with a twin valve combustion bomb, model 1105 (Parr Instrument, Illinois, USA), with an internal volume of 0.340 cm3. The bomb calorimeter, subsidiary apparatus, and working technique have been described previously in the literature [8,9]. The energy equivalent of the calorimeter was determined by combustion of Thermochemical Standard benzoic acid, sample NBS 39j, with Dc u ¼ ð26434  3Þ J  g1 [10] under bomb conditions, adopting the same procedure described by Coops et al. [11]. For 5-aminoindazole, the energy equivalent of the calorimeter used was found to be ecal = (15906.6 ± 1.9) J  K1 and for 6-aminoindazole, ecal = (15917.4 ± 1.4) J  K1, since the calorimeter undergone some small changes. The results from the calibration were corrected to give the energy equivalents, ecorr, corresponding to the average mass of water added to the calorimeter: 3119.6 g; the uncertainties quoted are the standard deviations of the mean. In all combustion experiments, 1.00 cm3 of water was introduced into the bomb, and the bomb was purged twice to remove air, before being charged with 3.04 MPa of oxygen. Both compounds were burnt in pellet form; however, it was necessary to use paraffin oil (Aldrich Gold Label, mass fraction >0.999) as combustion auxiliary with 6-aminoindazole, since during the preliminary combustion experiments of this compound, a significant amount of carbon soot was found. The standard massic energy of combustion of this oil was measured in our laboratory resulting the value of Dc u ¼ ð47193:3  3:3Þ J  g1 . For all experiments, the calorimeter temperatures were measured to ±(1  104) K, at time intervals of 10 s, with a quartz crystal thermometer (Hewlett Packard HP 2804A), interfaced to a PC. The igniH N

H N

H2N

N

N H2N

A

B

FIGURE 1. Structural formula of 5-aminoindazole (A) and 6-aminoindazole (B).

1241

tion of the samples was made at T = (298.150 ± 0.001) K by the discharge of a 1400 lF capacitor through the platinum ignition wire. At least 100 readings were taken for the initial and for the main periods and 200 readings for the after period. For the cotton-thread fuse, with empirical formula CH1.686O0.843, the massic energy of combustion was assigned to Dc u ¼ 16250 J  g1 [11], a value that was previously confirmed in our laboratory. The amount of substance used in each experiment, on which the energy of combustion was based, was determined from the mass of CO2 produced during the experiments, taking into account that formed from the combustion of the cotton-thread fuse and of the paraffin oil, in the experiments with 6-aminoindazole. The amount of HNO3 produced during the experiment was quantified by titration of the aqueous solution resulting from the washing inside of the bomb. Corrections for carbon soot formation were based on the standard massic energy of combustion of carbon, Dc u ¼ 33 kJ  g1 [11]. 2.3. Knudsen effusion technique For the crystalline 5-aminoindazole and 6-aminoindazole, the vapour pressures were measured using a Knudsen mass-loss effusion apparatus, enabling the simultaneous operation of nine effusion cells at three different temperatures. The experimental temperatures were chosen in order to obtain pressures between (0.1 and 1.0) Pa. The temperature ranges used in the experiments with 5-aminoindazole was (358 to 380) K and for 6-aminoindazole (366 to 388) K. The detailed description of the apparatus, procedure, and the technique was previously done [12]. In each effusion experiment, the loss of the mass of the sample, Dm, during the effusion period t, is determined by weighing the effusion cells before and after the effusion time, in a system evacuated to a pressure near 1  104 Pa. At the temperature T of the experiment, the vapour pressure p is calculated by equation (1),

p¼



Dm Ao wo t

rffiffiffiffiffiffiffiffiffiffiffiffi 2pRT ; M

ð1Þ

where M is the molar mass of the effusing vapour, R is the gas constant, Ao is the area of the effusion orifice, and wo is the respective Clausing factor calculated by equation (2), where l is the thickness of effusion orifice and r its radius:

wo ¼

  1 3l 1þ : 8r

ð2Þ

The thickness of the effusion holes was 0.0125 mm and their areas and Clausius factors of effusion orifices are present in the supporting information, table S1. 2.4. Computational methods Standard ab initio molecular orbital calculations were performed with the Gaussian 03 series of programs [13]. The G3MP2B3 composite method was used throughout this work [14]. This is a variation of the G3MP2 theory [15] which uses the B3LYP density functional method [16,17] for geometries and zero-point energies. The B3LYP functional uses a combination of the hybrid three-parameter Becke’s functional, first proposed by Becke [16], together with the Lee–Yang–Parr non-local correlation functional [18]. The computations carried out with the G3MP2B3 composite approach use the B3LYP method and the 6-31G(d) basis set for both the optimization of geometry and calculation of frequencies. Introduction of high-order corrections to the B3LYP/6-31G(d) enthalpy is done in a manner that follows the Gaussian-3 philosophy, albeit

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The energies computed at T = 0 K were thermally corrected for T = 298.15 K by introducing the vibrational, translational, rotational, and the pV terms. The vibrational term is based on the vibrational frequencies calculated at the B3LYP/6-31G(d) level. The same computational approach was used to calculate also the ionization enthalpies, proton and electron affinities, gas-phase acidities, and N–H bond dissociation enthalpies. For that purpose, the G3MP2B3 computations were also extended to cationic, anionic, and radicalar species of both the 5-amino and 6-aminoindazole.

using a second-order Moller–Plesset perturbation instead of MP4, as in the original G3 method [19]. The enthalpy of formation of 5-aminoindazole was estimated after the consideration of the following gas-phase working reactions: H N

7C + 3N + 7H

N

ðIÞ

H2N

5-Aminoindazole H N

3. Experimental and theoretical results

H N +

N

+

N

H2N

OH

OH

5-Aminoindazole

3.1. Condensed phase

ðIIÞ

H 2N

Phenol

1H-Indazole

The results of the combustion experiments carried out with 5- and 6-aminoindazole are presented, respectively, in tables 1 and 2 , where Dm(H2O) is the deviation of the mass of water added to the calorimeter from 3116.9 g, with the isothermal bomb process, DU(IBP), calculated according the equation: DU(IBP) = [ecal + Dm(H2O)cp(H2O(l)) + ef] + DU(ign), where DU(ign) corresponds to the energy supplied for the sample ignition. DU(fuse) is the energy of combustion of the cotton-thread fuse and DU(paraffin oil) is the energy of combustion of the oil used as combustion auxiliary of the 6-aminoindazole. The corrections for nitric acid formation, DU(HNO3), were based on 59.7 kJ  mol1, for the molar energy of formation of a solution of HNO3(aq) 0.1 mol  dm3 from N2(g), O2(g), and H2O(l) [20]. For each compound, an estimated pressure coefficient of massic energy, (ou/op)T = 0.2 J  g1  MPa1, a typical value for most organic compounds [21] was used. The correction to the standard state, DUR, and the massic energy of combustion, Dc u , were calculated by the procedure of Hubbard et al. [22]. Table 3 lists, for each compound, the derived standard molar values for the energy, Dc U m , and enthalpy, Dc Hm , of combustion, according to equation (3), and the standard molar enthalpy of formation, Df Hm in the condensed phase. To derive Df Hm from Dc Hm , the standard molar enthalpies of formation of water in liquid 1 phase, Df Hm ðH2 O; lÞ ¼ ð285:830  0:042Þ kJ  mol [23] and of  carbon dioxide in the gaseous phase, Df Hm ðCO2 ; gÞ ¼ ð393:51 1 0:13Þ kJ  mol [23], were used

2-Aminophenol

CH 3

CH 3

H N

H N N

+

+

N

H2N

ðIIIÞ H 2N

5-Aminoindazole

Toluene

1H-Indazole

3-Methylbenzenamine

The gas-phase reactions shown below were used to estimate the enthalpy of formation of 6-aminoindazole: H N

H2N

7C+3N+7H

N

ðIVÞ

6-Aminoindazole H2N

H N

H N +

N

H 2N +

N

ðVÞ OH

6-Aminoindazole

OH

Phenol

1H-Indazole

3-Aminophenol

CH 3 H N

H2N

H N N

+

H 2N N

ðVIÞ

+ CH3

6-Aminoindazole

Toluene

1H-Indazole

4-Methylbenzenamine

These reactions have been chosen on the basis of the available experimental thermochemical data for the compounds used. TABLE 1 Results of combustion experiments at T = 298.15 K for 5-aminoindazole. Experiment

1

2

m(CO2, total)/g m(cpd)/g m(paraffin oil)/g m(fuse)/g DTad/K ef/(J  K1) Dm(H2O)/g DU(IBP)a/J DU(fuse)/J DU(HNO3)/J DU(ign)/J DU(carbon)/J DUR/J Dc u =ðJ  g1 Þ % (CO2)

1.67235 0.72063

1.68290 0.72538

0.00313 1.33497 15.75 0.0 21254.99 50.83 67.10 0.87

0.00285 1.34329 15.71 0.0 21387.36 46.28 72.24 0.92

16.02 29309.13 100.094

16.08 29298.80 99.951

3

4

5

6

0.84389

1.82768 0.78795

1.47808 0.63700

0.82105

0.00287 1.45822 15.82 0.0 23217.48 46.61 77.61 0.91

0.00265 1.17980 15.55 0.0 18784.00 43.04 58.74 0.95

0.00249 1.51920 15.98 0.0 24188.91 40.44 69.85 0.67

17.56 29285.74 99.959

14.10 29306.31 99.919

18. 54 29304.04 (99.981)

0.00282 1.56174 15.88 0.0 24865.80 45.80 86.57 0.97 4.95 18.81 29292.41 (99.981)

Dc u ¼ ð29299:4  3:7Þ J  g1 m(CO2) is the mass of CO2 recovered in the combustion; m(cpd.) is the mass of compound burnt in each experiment; m(paraffin oil) is the mass of paraffin oil used as combustion auxiliary, m(fuse) is the mass of fuse (cotton) used in each experiment; DTad is the corrected temperature rise; ef is the energy equivalent of the contents in the final state; Dm(H2O) is the deviation of mass of water added to the calorimeter from 3119.6 g; DU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; DU(paraffin oil) is the energy of combustion of the paraffin oil; DU(fuse) is the energy of combustion of the fuse (cotton); DU(HNO3) is the energy correction for the nitric acid formation; DU(ign) is the electrical energy for ignition; DU(carbon) is the massic energy correction for the carbon residue soot; DUR is the standard state correction; Dc u is the standard massic energy of combustion. a DU(IBP) already includes the DU(ign).

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M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 1240–1247 TABLE 2 Results of combustion experiments at T = 298.15 K for 6-aminoindazole. Experiment

1

2

3

4

5

6

m(CO2, total)/g m(cpd)/g m(paraffin oil)/g m(fuse)/g DTad/K ef/(J  K1) Dm(H2O)/g DU(IBP)a/J DU(n-hexadecane)/J DU(fuse)/J DU(HNO3)/J DU(ign)/J DU(carbon)/J DUR/J Dc u =ðJ  g1 Þ % (CO2)

1.46658 0.47427 0.11742 0.00257 1.22441 15.88 0.1 19508.65 5541.22 41.74 47.52 0.75

1.59582 0.52215 0.12320 0.00300 1.33092 15.98 0.0 21205.37 5814.34 48.72 52.89 0.68 1.98 13.09 29260.38 99.927

1.31337 0.43817 0.09494 0.00275 1.09249 15.65 0.1 17405.47 4479.47 44.66 48.36 0.77

0.59127 0.13133 0.00284 1.48238 16.14 0.0 23618.83 6198.02 46.12 59.82 0.73

1.59603 0.54882 0.10340 0.00291 1.32130 15.97 0.0 21051.67 4879.70 47.26 55.40 1.09

1.64004 0.53543 0.12783 0.00229 1.36866 16.00 0.0 21806.65 6032.64 37.19 56.60 0.76

10.68 29263.30 100.065

14.84 29259.10 (100.028)

13.42 29255.29 100.084

13.41 29260.24 100.091

11.88 29237.12 99.981

Dc u ¼ ð29255:9  3:9Þ J  g1 a

U(IBP) already includes U(ign).

TABLE 3 Condensed phase standard (p ¼ 0:1 MPa) molar thermochemical functions at T = 298.15 K. Dc U m =

Compound

ðkJ  mol 5-Aminoindazole 6-Aminoindazole

C7 H7 N3 ðcrÞ þ

1

Dc Hm = 1

ðkJ  mol

Þ

3901.2 ± 1.4 3895.4 ± 1.3

Df Hm ðcrÞ= ðkJ  mol

Þ

3901.8 ± 1.4 3896.0 ± 1.3

1

Þ

146.8 ± 1.7 141.0 ± 1.6

35 7 3 O2 ðgÞ ! 7CO2 ðgÞ þ H2 OðlÞ þ N2 ðgÞ: 2 2 2

ð3Þ

3.2. Phase transition The integrated form of the Clausius–Clapeyron equation,

lnðpÞ ¼ a  b  ðT=KÞ1 ;

ð4Þ Dgcr Hm ðhTiÞ was used to derive the standard R

where a is a constant and b ¼ molar enthalpies of sublimation, at the mean temperature of the experimental temperature range. The experimental results obtained from each effusion cell, together with the residuals of the Clausius– Clapeyron equation, derived from the least squares adjustment, are presented, in tables 4 and 5, for 5- and 6-aminoindazole, respectively. For each compound, the calculated enthalpies of sublimation obtained from each individual orifice are in agreement within experimental error. The entropy of sublimation, at equilibrium conditions, were calculated as

Dgcr Sm fhTi; pðT ¼ hTiÞg ¼

Dgcr Hm ðhTiÞ : hTi

ð5Þ

Table 6 presents for each orifice used and for the global treatment of all the (p, T) points obtained for each studied compound, the detailed parameters of the Clausius–Clapeyron equation together with the calculated standard deviation and the standard molar enthalpies of sublimation at the mean temperature of the experiments T = hTi. The equilibrium pressure at this temperature p(T = hTi) and the entropies of sublimation, at equilibrium conditions, Dgcr Sm fhTi; pðT ¼ hTiÞg ¼ Dgcr Hm ðhTiÞ=hTi, are also presented. Table 7 lists the (p, T) values calculated from the (p, T) equations for the crystalline compounds, within the experimental pressure range (0.1 to 1) Pa. Sublimation enthalpies, at T = 298.15 K, were derived from the sublimation enthalpies calculated at the mean temperature hTi of the experiments by equation (6)

Dgcr Hm ðT ¼ 298:15 KÞ ¼ Dgcr Hm ðhTiÞ þ Dgcr cp;m ð298:15 K  hTiÞ;

ð6Þ

where Dgcr cp;m was estimated as being 50 J  K1  mol1 [24], a value that we have already used for other organic compounds [25–50]. Table 8 presents, for each compound, the values of the standard molar enthalpies, entropies, and Gibbs free energies of sublimation, at T = 298.15 K.

TABLE 4 Knudsen effusion results for 5-aminoindazole. T/K

t/s

Orifices

102  Dln(p)

p/Pa pA

pB

pC

pA

pB

pC

5-Aminoindazole 358.123 360.177 362.163

23824

A1–C7 A2–B5–C8 A3–B6–C9

0.0843 0.1010 0.1305

0.1064 0.1265

0.0830 0.1030 0.1259

1.4 2.5 2.1

2.7 1.0

0.2 0.5 1.5

364.114 366.166 368.167

18011

A1–B4–C7 A2–B5–C8 A3–B6–C9

0.1589 0.1991 0.2397

0.1556 0.1966 0.2332

0.1563 0.1879 0.2247

1.3 2.7 0.7

0.8 1.4 2.0

0.3 3.1 5.7

370.135 372.179 374.155

14631

A1–C7 A2–B5–C8 A3–B6–C9

0.2948 0.3483 0.4435

0.3696 0.4334

0.2986 0.3499 0.4198

1.5 2.3 2.2

3.6 0.1

2.7 1.9 3.3

376.133 378.172 380.162

10806

A1–B4–C7 A2–B5–C8 A3–B6–C9

0.5419 0.6618 0.7639

0.5340 0.6680 0.7696

0.5297 0.6232 0.7519

2.9 3.1 1.7

1.4 4.0 1.0

0.6 2.9 3.3

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M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 1240–1247

TABLE 5 Knudsen effusion results for 6-aminoindazole. T/K

t/s

Orifices

102  Dln(p)

p/Pa pA

pB

pC

pA

pB

pC

0.1025 0.1221 0.1468

1.9 1.8 1.0

1.1 0.3 2.0

1.3 2.5 5.0

366.142 368.169 370.179

25532

A1–B4–C7 A2–B5–C8 A3–B6–C9

0.1031 0.1275 0.1528

6-Aminoindazole 0.1023 0.1257 0.1513

372.138 374.159 376.174

19204

A1–B4–C7 A2–B5–C8 A3–B6

0.1923 0.2356 0.2875

0.1919 0.2340 0.2739

0.1878 0.2228

1.8 1.5 1.1

1.6 0.8 3.8

0.6 4.1

378.149 380.164 382.172

16248

A1–B4–C7 A2–B5–C8 A3–B6–C9

0.3523 0.4332 0.5244

0.3592 0.4280 0.5044

0.3463 0.4138 0.5114

1.7 2.5 2.0

3.6 1.2 1.9

0.0 2.1 0.6

384.151 386.158 388.167

11774

A1–B4–C7 A2–B5–C8 A3–B6–C9

0.6281 0.7813 0.9109

0.6132 0.7708 0.8984

0.6304 0.7396 0.8878

0.9 3.5 0.2

1.5 2.1 1.6

1.2 2.0 2.8

TABLE 6 1 Experimental results for the compounds, where a and b are from Clausius–Clapeyron equation ln(p) = a  b  (K/T), and b ¼ Dgcr Hm ðhTiÞ=R; R ¼ 8:314472 J  mol  K1 . Orifice number

a

b

hTi/K

1 2 3

36.178 36.263 35.768

13837.2 13876.7 13702.7

369.143 369.143 369.143

Global

36.065

13806.5

369.143

1 2 3

36.552 36.501 36.532

14217.6 14147.1 14221.4

377.154 377.154 377.154

Global

36.476

14192.2

377.154

p(hTi)/Pa 5-Aminoindazole 0.2652 0.2642 0.2585 0.2628 6-Aminoindazole 0.3183 0.3137 0.3089 0.3138

Dgcr Hm ðhTiÞ=ðkJ  mol1 Þ

Dgcr Sm ðhTi; phTiÞ=ðJ  mol1  K1 Þ

115.05 ± 0.99 155.4 ± 1.2 113.9 ± 1.1

311.7 ± 2.7 312.6 ± 3.2 308.6 ± 2.9

114.79 ± 0.26

311.0 ± 1.9

118.21 ± 0.60 117.6 ± 1.1 118.2 ± 1.1

313.4 ± 1.6 311.9 ± 2.9 313.5 ± 2.8

118.02 ± 0.63

312.9 ± 1.7

TABLE 7 (p, T) values from the vapour pressure equation. p/Pa

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

T/K 5-Aminoindazole 6-Aminoindazole

359.8 366.0

366.5 372.6

370.4 376.7

373.3 379.5

375.6 381.8

377.5 383.7

379.1 385.3

380.5 386.7

381.7 388.0

382.8 389.1

TABLE 8 Derived 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. Compound

Dgcr Hm = ðkJ  mol

5-Aminoindazole 6-Aminoindazole

Dgcr Sm = 1

Þ

118.34 ± 0.69 121.95 ± 0.63

Dgcr Gm = 1

ðkJ  mol

Þ

214.8 ± 1.9 219.3 ± 1.7

ðkJ  mol

1

Þ

54.30 ± 0.89 56.56 ± 0.80

nitrogen atom to adopt a pyramidal distribution. The C–NH2 distance is shorter for 6-aminoindazole due to its major conjugation. In this isomer, the lone-pair is conjugated in the whole structure while for 5-aminoindazole it is only conjugated with the benzene ring. We cannot find other theoretical or experimental data in the available literature. At the G3(MP2)//B3LYP level, the most stable isomer is 6-aminoindazole with 5-aminoindazole lying 5.78 kJ  mol1 above.

3.3. Gas-phase Combining the experimental results obtained for the standard molar enthalpies of formation in the crystalline phase with the standard molar enthalpies of sublimation, the standard molar enthalpies of formation of 5- and 6-aminoindazole, in the gasphase at T = 298.15 K have been obtained. The results are summarized in table 9. The optimized geometries of both 5- and 6-aminoindazole calculated at the G3(MP2)//B3LYP level of theory are reported in figure 2. Within this approach, the optimization of the geometry is carried out at the B3LYP/6-31G(d) level of theory. Bond distances and angles are included. Both isomers are quasi-planar with the NH2 group lying out of the plane. This is due to the nitrogen’s lone-pair which forces the

4. Gas-phase theoretical enthalpies of formation The gas-phase enthalpies of formation of the two isomers studied were estimated taking into account the computed enthalpies of reactions described by equations (I)–(VI) and the experimental enthalpies of formation in the gaseous phase of the other atoms and molecules involved. The values of Df Hm ðgÞ used were as follows: carbon, 716.7 kJ  mol1 [51]; hydrogen, 218.0 kJ  mol1 [51]; nitrogen, 472.7 kJ  mol1 [51]; benzene, 82.6 kJ  mol1 [52]; ethene, 52.5 kJ  mol1 [52]; aniline, 87.1 kJ  mol1 [52]; 1H-pyrazole, 179.4 kJ  mol1 [52]; 1H-indazole, 243.0 kJ  mol1 [52]; phenol, 96.4 kJ  mol1 [52]; 2-aminophenol, 87.2 kJ  mol1, methylbenzene, 50.5 kJ  mol1 [52]; and 3-aminophenol, 89.4 kJ  mol1,

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3-methylbenzenamine, 61.1 kJ  mol1, and 4-methylbenzenamine, 62.2 kJ  mol1 [52].

TABLE 9 Derived standard (p ¼ 0:1 MPa) molar enthalpies of formation and of phase transition at T = 298.15 K. Compound

Df Hm ðcrÞ= ðkJ  mol

5-Aminoindazole 6-Aminoindazole

1

Df Hm ðgÞ=

Dgcr Hm = Þ

146.8 ± 1.7 141.0 ± 1.6

ðkJ  mol

1

Þ

118.34 ± 0.69 121.95 ± 0.63

ðkJ  mol

1

Þ

265.1 ± 1.8 263.0 ± 1.7

The calculated enthalpies of formation along with the experimental values are registered in table 10. As can be seen from the table, the agreement between the experimental and G3(MP2)// B3LYP calculated values is fairly good. The maximum deviations from the experimental results are (5.3 and 4.6) kJ  mol1 for 5aminoindazole, and (10.5 and 9.5) kJ  mol1 for 6-aminoindazole. These deviations are of the same order of magnitude defined by the uncertainty associated to the experimental and calculated values [14]. The computed G3(MP2)//B3LYP enthalpies for the compounds studied, auxiliary molecules, and atoms used in the atomization and working reactions are listed in table S2 in the supporting information.

FIGURE 2. Front and side views of the B3LYP/6-31G(d) optimized geometries of both 5- and 6-aminoindazole. Distances are given in nm and angles in degrees. The atom numbering is the same for both isomers.

TABLE 10 Comparison between the experimental and computed G3(MP2)//B3LYP gas-phase enthalpies of formation of both 5-aminoindazole and 6-aminoindazole at T = 298.15 K. Enthalpic differences between the experimental and computed values are given in parentheses. Compound

G3(MP2)//B3LYP Atomization reaction (equation (I))

Equation (II)

Df Hm ðgÞ=ðkJ 5-Aminoindazole

260.9 (4.2) Atomization reaction (equation (IV))

6-Aminoindazole

255.1 (7.9)

Equation (III)

Experimental value

260.5 (4.6) Equation (VI)

265.1 ± 1.8

253.5 (9.5)

263.0 ± 1.7

1

 mol Þ 259.8 (5.3) Equation (V) 252.5 (10.5)

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Table 11 G3(MP2)//B3LYP computed gas-Phase acidities, DHacidity, gas-phase basicities, DGbasicity, proton, PA, and electron affinities, EA, ionization enthalpies, IE, and N–H bond dissociation enthalpies, DHN–H, at T = 298.15 K, for the two isomers of aminoindazole. All values are given in kJ  mol1. Compound

DHacidity

DGbasicity

PA

EA

IE

DHN–H

5-Aminoindazole

1452.7 (NH) 1523.5 (NH2)

917.4 (N) 846.5 (NH) 886.6 (NH2)

917.3 (N) 841.5 (NH) 884.3 (NH2)

45.2

720.1

404.8 (NH) 385.6 (NH2)

6-Aminoindazole

1456.0 (NH) 1500.6 (NH2)

928.2 (N) 843.5 (NH) 876.4 (NH2)

926.4 (N) 841.7 (NH) 872.8 (NH2)

55.3

743.9

408.4 (NH) 391.2 (NH2)

H N N

22.1 ± 2.2

H N

H2N N

243.0 ± 1.3

20.0 ± 2.1

265.1 ± 1.8

Δ isomH = − 2.1 ± 2.9 H N

H2N

N

263.0 ± 1.7 SCHEME 1. Enthalpic effect due to the introduction of an amino group in position 5 and 6 of the indazole. All values in kJ  mol1.

4.1. Other gas-phase thermodynamic properties The G3(MP2)//B3LYP approach was used to compute other thermodynamic properties for the two isomers of aminoindazole. The calculated values of gas-phase acidity (DHacidity), gas-phase basicity (DGbasicity), proton (PA) and electron affinities (EA), adiabatic ionization enthalpies (IE), and N–H bond dissociation enthalpies (DHN–H) are registered in table 11. The calculated gas-phase acidities show that the NH group for both isomers has a similar acid behaviour whereas the NH2 group is more acidic for the 6-aminoindazole. This fact can be explained based on the stability of the corresponding anion. In the case of 6aminoindazole, the negative charge in the resulting anion is more stabilized by resonance effects than in the corresponding 5-aminoindazole anion. With respect to the gas-phase basicity for both isomers, the more basic centre is the nitrogen atom, followed by the NH2 group and, finally, the NH group. The proton affinity follows the same pattern. The lowest basicity for the NH group is due to the high delocalization of its lone-pair along the whole structure. Between the N and NH2 groups, the nitrogen atom greater basicity can be explained taking into account the corresponding cation. In the latter case, the positive charge is more stabilized by conjugation effects than in the NH2 corresponding cation. As can be seen from table 11, the addition of an electron to 5- or 6-aminoindazole is not favourable as shown by its negative value of the electron affinity for both isomers. This is due to the fact that

NO2

NH2

− 14.2 ± 1.3

68.53 ± 0.67

4.4 ± 1.3

82.7 ± 1.1

87.1 ± 1.1

SCHEME 2. Enthalpic effects due to the introduction of a nitro and an amino group in benzene. Values from reference [52] and in kJ  mol1.

an anti-bonding orbital is occupied and destabilizes the whole molecule. Regarding the ionization enthalpies, it is possible to conclude that both isomers studied have almost the same electron donor capacity, being the 5-aminoindazole the species which looses the electron easier. Finally, the N–H bond dissociation enthalpies show that the NH group in both isomers has the strongest N–H bond. The NH2 group presents similar bond dissociation enthalpies for both isomers, the bigger value corresponding to the 6-aminoindazole. No experimental or computational data have been found in the literature for comparison with our results on any of these properties. 5. Discussion Considering the values for the standard molar enthalpies of formation in the gaseous phase for indazole, Df Hm ðg; indazoleÞ ¼ 1 ð243:0  1:3Þ kJ  mol [52], and those calculated for the compounds under study, the effects due to the introduction of a – NH2 group in positions 5 and 6 of the indazole ring were derived. As can be seen in scheme 1, the introduction of the –NH2 group produces the same effect in both positions, since within the experimental uncertainties, the enthalpic increment is the same. This fact is also reflected in a low isomerisation enthalpy: Dis1 (see scheme 1). omH = (2.1 ± 2.5) kJ  mol As can be seen, the positive value of the increment shows that the amino group, a donor group, causes a destabilization in the indazole molecule. The opposite effect was verified for the introduction of the nitro group, which results in a negative increment [6]. This could allow us to expect that donor groups destabilize the indazole molecule, possibly due to the reduction of the electronic delocalization, while the withdrawing groups provide an increase of the delocalization and, consequently, the stabilization of the molecule. Although the magnitude of the respective increments are different, the same tendency is verified for the benzene, where the nitro group has a stabilizing effect whereas in the introduction of the

M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 1240–1247

amino group results a destabilization of the benzene molecule (scheme 2). Acknowledgements 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. J.I.T.A.C. thanks FCT and the European Social Fund (ESF) under Community Support Framework (CSF) for the award of a Post-Doc research grant (BPD/27140/2006). Appendix A. Supplementary material Detailed data of the effusion orifices (diameter and areas of the effusion orifices and Clausing factors) of the Knudsen effusion apparatus; G3(MP2)//B3LYP computed enthalpies (energies plus thermal corrections for T = 298.15 K) for 5- and 6-aminoindazoles, for the auxiliary molecules and for the atoms used in the gas-phase working reactions, are presented. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2010.04.026. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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JCT 10-136