Calorimetric and computational study of (1H-Indol-n-yl)methanol and 2-(1H-Indol-n-yl)ethanol (n=2, 3)

Thermochimica Acta 673 (2019) 169–176

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Calorimetric and computational study of (1H-Indol-n-yl)methanol and 2-(1H-Indol-n-yl)ethanol (n=2, 3)

T

Tânia M.T. Carvalhoa,1, Luísa M.P.F. Amarala,2, , Victor M.F. Moraisa,b, ⁎ Maria D.M.C. Ribeiro da Silvaa, ⁎

a

Centro de Investigação em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal b Instituto de Ciências Biomédicas Abel Salazar, ICBAS, University of Porto, Rua de Jorge Viterbo Ferreira, 228, P-4050-313 Porto, Portugal

ARTICLE INFO

ABSTRACT

Keywords: Enthalpy of formation Combustion calorimetry Sublimation enthalpy G3 calculations Indole methanol Indole ethanol

In the present work, the gas-phase standard molar enthalpy of formation of 2-(1H-indol-3-yl)ethanol was derived, at T = 298.15 K, from the enthalpy of combustion for the crystalline compound, measured by static-bomb calorimetry, and its enthalpy of sublimation obtained from Calvet microcalorimetry measurements. The standard molar enthalpies of formation of this compound and for (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol and 2-(1H-indol-2-yl)ethanol were calculated using the composite G3 method. The experimental value of the gasphase enthalpy of formation of 2-(1H-indol-3-yl)ethanol is ‒(48.5 ± 3.3) kJ mol−1, being in excellent agreement with the G3 value, thus giving confidence to the estimates. The results were analysed in terms of the enthalpic methylene increments and compared with other related systems.

1. Introduction Indole derivatives constitute an important class of therapeutic agents in medicinal chemistry, presenting diverse biological properties [1,2] and ample industrial applications [3]. Due to their high potential, there has been the emphasis on the novel and improved process for the synthesis of indole derivatives to overcome problems faced by existing therapeutic agents. As so, the knowledge of their thermodynamic properties is highly valuable in clarifying the chemical behavior inherent to those species. The present work is part of a systematic thermochemical study of indole derivatives aiming at investigating the energetic effect of different substituents on the indole ring as well as to get information on their relative stabilities [4–6]. In this work, we report the standard (po = 0.1 MPa) molar enthalpy of formation in the crystalline phase and the standard molar enthalpy of sublimation for 2-(1H-indol-3-yl) ethanol, at T = 298.15 K, determined by static-bomb combustion calorimetry and Calvet microcalorimetry techniques, respectively. The obtained experimental values led to the enthalpies of formation of 2-(1H-indol-3-yl)ethanol in the gaseous phase. Additionally, high level ab initio molecular orbital calculations have been performed at the G3 level for (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol, 2-(1H-

indol-2-yl)ethanol and 2-(1H-indol-3-yl)ethanol, in order to estimate the gas-phase standard molar enthalpies of formation of these systems and, complementarily, to evaluate the energetic effect of the −CH2OH and −CH2CH2OH groups on the indole structure. 2. Experimental and computational methods 2.1. Compound and purity control 2-(1H-indol-3-yl)ethanol was obtained commercially from TCI Europe, with certified mole fraction purity of 0.997, and was purified by sublimation under reduced pressure (1 Pa). The purity was checked by gas-liquid chromatography performed on an Agilent 4890D Gas Chromatography equipped with an HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpolysiloxane (15 x 0.530 mm i.d x 1.5 μm film thickness), using nitrogen as the carrier gas, (≥ 0.9998 mass fraction). The purity of the compound was also checked by the amount of carbon dioxide recovered after the combustion experiments. The average ratio of the mass of carbon dioxide recovered to that calculated from the mass of sample was (1.0002 ± 0.0002), where the uncertainty is the standard deviation of the mean. Details of the origin and purification of the sample are presented in Table 1.

Corresponding authors. E-mail addresses: [email protected] (L.M.P.F. Amaral), [email protected] (M.D.M.C. Ribeiro da Silva). 1 LEPABE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, 4200-465 Porto, Portugal. 2 REQUIMTE, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal. ⁎

https://doi.org/10.1016/j.tca.2019.01.021 Received 9 December 2018; Received in revised form 22 January 2019; Accepted 23 January 2019 Available online 28 January 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Purification details of the 2-(1H-indol-3-yl)ethanol. Chemical name

CAS

Provenance

Initial mass fraction purity*

Purification method

Final mass fraction purity

Analysis method

2-(1H-indol-3-yl)ethanol

526-55-6

TCI

0.997

Sublimation

1.0002

CO2 recovery

* Commercial information.

2.2. Combustion calorimetry

mass, to within ± 10 μg, into each of the twin calorimetric cells [19]. The calibration constant, kcal, of the calorimeter was determined as the average of six independent experiments with naphthalene, using its standard molar enthalpy of sublimation (72.6 ± 0.6) kJ mol−1 [20], kcal (376.0 K) = (1.0377 ± 0.0077); the uncertainty presented is the standard deviation of the mean.

The standard massic energy of combustion for the compound was measured using a static-bomb combustion isoperibol calorimeter, equipped with a twin valve bomb (internal volume: 0.290 dm3), whose detailed description and procedure has been reported elsewhere [7–9]. The calorimetric system was calibrated using NBS benzoic acid Reference Material 39 j, having a massic energy of combustion of (−26,434 ± 3) J g−1 [10], under certificate conditions. Calibration experiments were carried out in oxygen at a pressure of 3.04 MPa, with 1.00 cm3 of deionized water added to the bomb. The calibration results were corrected to give the energy equivalent of the calorimeter, εcal, corresponding to the average mass of 2900.0 g of water added to the calorimeter. From six calibration experiments an average value, εcal = (15,551.2 ± 1.6) J K−1, was obtained, where the uncertainty quoted is the standard deviation of the mean. The samples (≈ 0.5 – 0.7 g) of the compound, in the pellet form, were ignited at T = 298.15 K, in an oxgen atmosphere (p = 3.04 MPa), with 1.00 cm3 of deionised water added to the bomb. The ignition of the samples was made at T = (298.150 ± 0.001) K and the electrical energy for ignition was determined from the change in potential difference across a 1400 μF condenser when discharged through the platinum ignition wire. For the cotton thread fuse used, empirical formula CH1.686O0.843, Δcuo = −16,240 J g−1 [11]. The corrections for nitric acid formation were based on −59.7 kJ mol−1 [12], for the molar energy of formation of 0.1 mol dm−3 HNO3(aq) from N2, O2, and H2O(l). In the experiments with carbon residue soot formation during the combustion, the necessary energetic correction for its formation was based on Δcuo = −33 J g−1 [11]. A typical value of the pressure coefficient of specific energy, (∂u/∂p)T, for organic solids was assumed to be −0.2 J g−1 MPa−1 [13]. The amounts of the compound used in each experiment were determined from the total mass of carbon dioxide produced during the experiments, taking into account that formed from the combustion of the cotton-thread fuse and that lost due to eventual carbon formation. For each compound, Δcuo was calculated by the procedure given by Hubbard et al. [14]. The literature value of the specific density used was ρ = 1.0183 g cm−3 [15] for 2-(1H-indol-3-yl)ethanol, the specific densities used for the cotton thread fuse and for the n-hexadecane were, respectively, ρ = 1.50 g cm3 and ρ = 0.773 g cm-3 [16]. The relative atomic masses used for the elements were those recommended by the IUPAC Commission in 2013 [17].

2.4. Computational details In the present work the absolute enthalpies, at T = 298.15 K, of (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol, 2-(1H-indol-2-yl) ethanol, 2-(1H-indol-3-yl)ethanol and all the auxiliary species considered were calculated using the composite G3 approach [21]. This protocol involves an initial geometry optimization at the Hartree-Fock (HF) level with the 6–31 G(d) basis set [22] and the subsequent determination of the harmonic frequencies, which are then scaled by a factor of 0.8929 to take account of known deficiencies at this level [23,24]. These frequencies are used to evaluate the zero-point vibrational energy (E(ZPE)) and thermal effects. The equilibrium geometry is further refined at the MP2(Full)/6–31 G(d) level, using all electrons for the calculation of correlation energy. Finally, a series of single point energy calculations, including QCISD(T) (quadratic configuration interaction including single and double excitations with triple excitations being taken into account perturbatively) using the 6–31 G(d) basis set, are then performed and the results properly corrected and combined into a final energy which is effectively at the QCISD(T,FC)/ 6–311+G(3df,2p) level. The values of the molar heat capacity in the gaseous phase at difo (g) , have been derived from statistical therferent temperatures, Cp,m modynamics [25] using the vibrational frequencies calculated at the RHF/6–31 G(d) level of theory. All standard G3 level calculations were performed with the Gaussian 03 series of programs [26] while additional more accurate ab initio calculations were conducted with Molpro 2012.1 [27] ab initio program package. 3. Results 3.1. Combustion calorimetry The detailed results for the combustion experiments of 2-(1H-indol3-yl)ethanol are given in Table 2, together with the mean value of the standard massic energy of combustion ⟨Δcu°⟩, and its standard deviation of the mean. Δm(H2O) is the deviation of the mass of water added to the calorimeter from 2900.0 g (the mass assigned for εcal), ΔUƩ is the correction to the standard state, and the remaining terms are as previously described [13,28]. For the static-bomb measurements, as samples were ignited at T = (298.150 ± 0.001) K, the energy associated to the isothermal bomb process ΔU(IBP), was calculated through:

2.3. High temperature Calvet microcalorimetry The standard molar enthalpy of sublimation of 2-(1H-indol-3-yl) ethanol was measured in a high temperature Calvet microcalorimeter (Setaram, model HT 1000), using the technique described by Skinner et al. [18]. The measuring procedure and a detailed description of the technique are reported in the literature [19]. For each experiment, a sample of about 4–5 mg of the compound was introduced in a thin glass capillary tube sealed at one end. Two thin glass capillary tubes, one containing the crystalline sample and the other being the reference tube were simultaneously dropped into the hot reaction vessel, held at a suitable predefined temperature, T, and then removed from the hot zone by vacuum sublimation. The thermal corrections for the glass capillary tubes were determined in separate experiments, and were minimized by dropping tubes of nearly equal

ΔU(IBP) = –{εcalor + cp (H2O, l). Δm(H2O) + εf}ΔTad + ΔU(ign),

(1)

were cp(H2O, l) is the specific heat capacity, at constant pressure, for liquid water, εf is the energy of the bomb contents after ignition, ΔU (ign) is the ignition energy and ΔTad is the adiabatic temperature rise. The values of Δcuo, together with the respective mean value, ⟨Δcuo⟩, and the standard deviation, presented in Table 2 refer to the general idealized combustion reaction represented by equation (2): 170

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Table 2 Results of combustion experiments of 2-(1H-indol-3-yl)ethanol, at T = 298.15 K. 1 m(CO2)/g 2.04086 m’(cpd)/g 0.74603 m’’(fuse)/g 0.00255 ΔTad/K 1.59172 Ti/K 25.0015 26.6513 Tf/K εf /(J·K−1) 14.82 Δm(H2O)/g 0.3 a 24778.18 −ΔU(IBP) /J −ΔU(fuse)/J 41.41 −ΔU(HNO3)/J 34.87 ΔU(ign)/J 0.56 ΔU(Carbon)/J 0.00 ΔUƩ/J 15.02 o −1 − Δcu / J g 33091.00 −⟨Δcuo〉 = 33,092.6 ± 4.2 J g−1

2

3

4

5

6

1.37195 0.50074 0.00301 1.06915 25.0009 26.1364 14.21 0.9 16645.20 48.88 23.15 0.58 0.00 9.65 33078.08

1.37601 0.50265 0.00230 1.07274 25.0016 26.154 14.20 0.8 16700.62 37.35 23.82 0.60 0.00 9.68 33084.19

1.39157 0.50822 0.00253 1.08539 25.0013 26.1585 14.20 1.0 16898.51 41.09 26.15 0.56 0.00 9.80 33098.8

1.37765 0.50318 0.00243 1.07511 25.0006 26.1532 14.18 0.0 16733.89 39.46 26.59 0.61 0.00 9.69 33105.75

1.38594 0.50634 0.00301 1.08135 25.0013 26.1464 14.21 −0.2 16830.20 48.88 24.37 0.55 11.55 9.77 33097.78

m(CO2) is the mass of CO2 recovered in the combustion; m’(cpd) is the mass of compound burnt in each experiment; m’’(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 2900.0 g; ΔU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; 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(Carbon) is the energy correction for the carbon residue soot formation; ΔU∑ is the standard state correction; Δcuo is the standard massic energy of combustion. a ΔU(IBP) already includes ΔU(ign).

C10 H11NO (cr) + 12.25 O2 (g)

10 CO2 (g) + 5.5 H2 O(l) + 0.5 N2 (g)

(2)

scaled by a factor of 0.8929 [23,24]. The Cpo, m (g) = f(T) obtained for 2(1H-indol-3-yl)ethanol is represented by:

Table 3 lists the derived standard molar energy, c Umo (cr), and enthalpy, f Hmo (cr), of combustion and the standard molar enthalpy of formation in the crystalline state, f Hmo (cr), of 2-(1H-indol-3-yl) ethanol, at T = 298.15 K. To obtain f Hmo (cr), from f Hmo (cr), the standard molar enthalpies of formation of CO2(g) and H2O(l), at T = 298.15 K, –(393.51 ± 0.13) kJ mol−1 [29], and –(285.830 ± 0.040) kJ mol−1 [29], respectively, were used. The uncertainties assigned to the standard molar energy of combustion and standard molar enthalpy of combustion are, in each case, twice the overall standard deviation of the mean and include the uncertainties in calibration and in the values of auxiliary quantities used [30,31].

Cpo, m (g) kJ mol 1K

Cpo, m (g)dT ,

298.15 K

(3)

Table 3 Derived standard (po = 0.1 MPa) molar energy of combustion, c Umo , standard molar enthalpy of combustion, c Hmo , and standard molar enthalpy of formation, f Hmo , for the crystalline compound at T = 298.15 K.a Values in kJ mol−1.

2-(1H-indol-3-yl)ethanol

o c Um (cr)

5334.6 ± 1.8

o c Hm (cr)

5338.9 ± 1.8

1.561

05(T /K)2 + 6.445E (4)

The calculated molecular structures of the title compounds, optimized at the MP2(Full)/6–31 G(d) level of theory (G3 calculations), are shown in Fig. 1. For 2-(1H-indol-2-yl)ethanol and (1H-indol-2-yl)methanol the oxygen atom of the hydroxyl group orients in such a way that it approaches the NeH fragment of the indole ring, thus enabling a stabilizing interaction to occur. The distance between the oxygen atom and the hydrogen atom of the NeH fragment is found to be 0.2141 nm (n = ethanol) or 0.2434 nm (n = methanol), while the N‒H⋯O bond angle is 121.9° (n = ethanol) and 95.5° (n = methanol). The distance N⋯O, 0.2812 nm (n = ethanol) or 0.2724 nm (n = methanol), is less than the sum of the van der Waals radii of nitrogen and oxygen (0.155 + 0.152 = 0.307 nm). Note also that the O⋯H distance satisfies the van der Waals cut-off criterion, for both 2-(1H-indol-2-yl)ethanol and (1H-indol-2-yl)methanol, being the sum of the van der Waals radii of H and of O: 0.120 + 0.152 = 0.272 nm. For (1H-indol-3-yl)methanol and 2-(1H-indol-3-yl)ethanol the interactions involving the oxygen atom and the NeH fragment of the indole ring are completely precluded. In the case of (1H-indol-3-yl) methanol the C–O bond assumes a near coplanar conformation with respect to the pyrrole ring (dihedral angle 26.3°), while for 2-(1H-indol3-yl)ethanol the C‒C(O) bond of the hydroxyl group is nearly perpendicular with respect to the plane of the indole ring (dihedral angle 95.2° with the OeH fragment point in the direction of the pyrrole ring. We have considered several working reactions to obtain estimates

where T is the temperature of the hot reaction cells and Cpo, m (g) is the gas-phase molar heat capacity derived from statistical thermodynamics using the vibrational frequencies obtained at the HF/6–31 G(d) level,

Compound

07(T /K)3 + 1.115E

3.3. Computational calculations

T g,T o cr,298.15K Hm

2.839E

The gas-phase molar heat capacity values, as a function of the temperature, between 200 and 700 K, are given in Supporting Information (Table S1). The standard molar enthalpy of formation in the gaseous state, at T = 298.15 K, was obtained for the 2-(1H-indol-3-yl)ethanol by the addition of the derived standard molar enthalpy of formation in the crystalline state and the standard molar enthalpy of sublimation. The results are summarized in Table 5.

Measurements of the standard molar enthalpy of sublimation of 2(1H-indol-3-yl)ethanol by Calvet microcalorimetry, as well as the associated uncertainty given by its standard deviation of the mean, are given in Table 4. The uncertainty associated to the standard molar enthalpy of sublimation, at T = 298.15 K, is twice the standard deviation of the mean and include the uncertainties associated with the calibration process. The observed enthalpy of sublimation was corrected to T = 298.15 K using the equation,

=

=

01(T /K)

3.2. High temperature Calvet microcalorimetry

g o cr Hm (298.15K)

1

o f Hm (cr)

168.3 ± 2.2

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

171

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Table 4 Standard (po = 0.1 MPa) molar enthalpy of sublimation for 2-(1H-indol-3-yl)ethanol, at T = 298. Values in kJ mol−1. Compound

Number of experiments

Ta K

2-(1H-indol-3-yl)ethanol

6

375.8

a b c

g, T ob cr,298K Hm

135.8 ± 1.0

16.0

g o c cr Hm (298.15K)

119.8 ± 2.5

u(T) = ± 0.1 K. Mean value and standard deviation of the mean. The uncertainty is twice the overall standard deviation of the mean, and include the contributions from the calibration.

decided to use our G3 results, as well as those obtained from the explicitly-correlated version of the very accurate W1 computational composite method [32,33] to check the accuracy of their standard gasphase enthalpy of formation. To this end we used standard atomization reactions and the absolute enthalpies obtained from the above mentioned calculations. The geometries used in the W1-F12 calculations were optimized at the B3LYP/cc-pVTZ+d- level and the frequencies to obtain the thermal corrections were scaled by the factor 0.985. For atomic species we used the spin-orbit corrections, ΔE(SO), obtained from experiment [34]. The crystalline enthalpy of formation of 3-methyl-1H-indole and its enthalpy of sublimation were determined by our group in a previous study [35] and is the only experimental value found in the literature (see Table 6). Combining these two experimental data, the corresponding enthalpy of the formation in gaseous phase results as f Hmo (g) = 137.8 ± 3.0 kJ mol−1. The G3(MP2)//B3LYP composite approach was used to estimate the enthalpy of formation of 3-methyl-1H-indole using the atomization reaction, f Hmo (g) = 125.9 kJ mol−1 and f Hmo (g) = 133.8 kJ mol-1 are obtained when using an isodesmic reaction [35]. Both approaches suggest a lower enthalpy of formation than the experimental value. Dorofeeva [36] performed G4 calculations, using atomization and several isodesmic reactions, having estimated the value 126 kJ mol-1 (atomization procedure) and 131.2 kJ mol-1 (isodesmic reactions). The results obtained from our calculations are given in Table 6, together with the literature ones. As the f Hmo (g) values of 3-methyl-

Table 5 Standard (po = 0.1 MPa) molar enthalpy of formation for 2-(1H-indol-3-yl) ethanol, in both crystalline and gaseous phases, and standard molar enthalpy of sublimation, at T = 298.15 K.a Values in kJ mol−1. Compound 2-(1H-indol-3-yl)ethanol a

T o 298.15K Hm

o f Hm (cr)

−168.3 ± 2.2

g o cr Hm

119.8 ± 2.5

o f Hm (g)

−48.5 ± 3.3

Uncertainties calculated through the RSS (Root-Sum-Square) method.

for the enthalpy of formation of the studied compounds. These reactions must be chosen in order to fulfil the following two criteria: on one hand, they must involve maximum bonding pattern similarity between the products and the reactants; thus, with the exception of the atomization reactions they constitute a set of isodesmic or even homodesmotic reactions whose thermal behaviour is likely to be well reproduced by our accurate calculations. On the other hand, reliable experimental enthalpies of formation must be available for all the auxiliary molecular species involved in such reactions. The last criterion is often difficult to be fulfilled. Indeed, we found that the quality of the available experimental data for the species, 3-methyl-1H-indole, furan-2-ylmethanol, and cyclohexanol revealed to be not very reliable, delivering estimates of the enthalpy of formation of 2-(1H-indol-3-yl) ethanol evidencing systematic unacceptably large disagreement with our experimental measurements. Since all the three molecular systems above are not exaggeratedly large and possess molecular symmetry, we

Fig. 1. MP2(Full)/6–31 G(d) optimized most stable configurations for the (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol, 2-(1H-indol-2-yl)ethanol and 2-(1Hindol-3-yl)ethanol. 172

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Table 6 Comparison of experimental enthalpies of formation. at T = 298.15 K for 3-methyl-1H-indole, furan-2-ylmethanol and cyclohexanol with values calculated from atomization and isodesmic reactions. (Values in kJ mol−1). All values taken from NIST [37], except as indicated. Compound

o f Hm (cr,l)

g o cr,l Hm

o f Hm (g)

Experimental

G3(MP2)//B3LYP [35]

G4 [36]

G3this

(1)a

(2)b

(1)a

(2)b

(1)a

(2)b

(1)a

125.9

133.8

126

131.2R2

133.4

128.4 R3 average −221.6 R4

129.1 129.3 R3 129.6 ± 1.1 −217.6 −219.5 R4

average −295.3 −295.1

−218.6 ± 1.2 −292.0 −291.3 −291.1

R1

work

3-methyl-1H-indole

47.4 ± 2.3 [35]

90.4 ± 1.9 [35]

137.8 ± 3.0 [35]

furan-2-ylmethanol

−277.0 −276.4 −276.7c −352.0 ± 0.67 −350 ± 2 −347.4 ± 2.2 −349.2 ± 0.2 −359.2

64.4 53.6

−211.8 ± 2.1 −212.3d

−215.8

60.1 49.8 61.2 ± 0.6 55 59.9 62.7 49.3 58.4 60.4 52.6 54.8 45.44 62 ± 0.9 [40]

−286 ± 2 [39] −290 ± 8

−290.9

Cyclohexanol

W1-F12

average

R5 R6

this work

(2)b

R5 R6

−292.6 ± 0.8

R1

3-methyl-1H-indole + benzene → indole + toluene; R2Average value calculated from several isodesmic reactions, see table S4 of supplementary data [36; R33methyl-1H-indole + CH4 → indole + C2H6; R4furan-2-ylmethanol + methane → furan + ethanol; R5 Cyclohexanol + methane → cyclohexane + methanol; R6 Cyclohexanol + ethane → cyclohexano + Ethanol. a Atomization reaction. b Isodesmic reaction. c Average. d Calculated from the experimental values.

1H-indole estimated using different methodologies, are in perfect agreement, one may suggest that the average calculated value (129.6 ± 1.1 kJ mol−1) is more reliable estimation of the enthalpy of formation than the experimental one. Two values of enthalpy of formation for the liquid furan-2-ylmethanol were found in the literature; f Hmo (l) = ‒277.0 kJ mol−1 and −1 o , measured in 1936 and 1950, respectively f Hm (l) = ‒276.4 kJ mol [37]. For the vaporization enthalpy, the reported values for, gl Hmo are 64.4 kJ mol−1, measured in 1929 and a more recent value, 53.6 kJ mol−1, measured in 1987 [37]. The value f Hmo (g) = ‒211.8 ± 2.1 kJ mol−1 was derived by Cox and Pilcher [38]. The average of the four values obtained from G3 and W1-F12 calculations is ‒218.6 ± 2.1 kJ mol−1, 6.8 kJ mol−1 lower than the value suggested in the Cox and Pilcher compilation. With respect to cyclohexanol, there are five values of the enthalpy of formation found in the literature for the liquid compound, and thirteen values for the enthalpy of vaporization, ranging from 45 to 63 kJ mol−1 (see Table 6). The large disagreement in the available experimental data can lead to quite different predictions of the gas-phase enthalpy of formation for the compound. The suggested value for the gaseous enthalpy of formation of cyclohexanol in Pedley compilation [39] is o o = ‒286 ± 2 kJ mol−1, obtained from = f Hm (l) f Hm (g) g o −1 −1 ‒348 ± 2 kJ mol and l Hm = 62 kJ mol . On the other hand in NIST [37], the suggested value is f Hmo (g) = ‒290 ± 8 kJ mol−1. The average obtained from our theoretical calculations, f Hmo (g) = ‒292.6 ± 0.8 kJ mol−1, seems to be a good estimate. The calculated enthalpy variations for the working reactions were further combined with the experimental gas-phase standard molar enthalpies of formation of the auxiliary molecules involved, thereby providing estimates of the enthalpies of formation, f Hmo , in the gasphase, at T = 298.15 K. The calculated reaction enthalpies and the standard gas-phase molar enthalpies of formation, at T = 298.15 K, are collected in Tables S2, S3, S4 and S5, of the Supporting Information, respectively, for (1H-indol-2-

yl)methanol, (1H-indol-3-yl)methanol, 2-(1H-indol-2-yl)ethanol, and 2(1H-indol-3-yl)ethanol, together with deviations, Δ, from the experimental values, for 2-(1H-indol-3-yl)ethanol. The G3 absolute enthalpies and the experimental gas-phase enthalpies of formation, at T = 298.15 K, for the compounds under study, as well as for all the relevant auxiliary molecules considered in this work are provided as Supporting Information (Table S6). Thus, using appropriate reactions, we were able to estimate the enthalpy of formation for the compounds studied with deviations, relative to the experimental value, that are not larger than about 5.9 kJ mol−1. A mean value was calculated from the computational estimates:−(21.6 ± 0.9) kJ mol−1 for (1H-indol-2-yl) methanol,−(15.5 ± 0.9) kJ mol−1 for (1H-indol-3-yl)methanol, −(57.4 ± 0.9) kJ· mol−1 for 2-(1H-indol-2-yl)ethanol and −(49.8 ± 0.9) kJ mol−1 for 2-(1H-indol-3-yl)ethanol. 4. Discussion For 2-(1H-indol-3-yl)ethanol, the gas-phase standard molar enthalpy of formation determined experimentally and that obtained from computational calculations are in excellent agreement, which confers confidence to the estimates of this thermodynamic parameter for the compounds which have not been studied experimentally. In the Fig. 2 we show a scheme which allows analysing the effect of the addition of a −CH2‒ group in the linear chain of (1H-indol-2-yl) methanol and (1H-indol-3-yl)methanol, to give 2-(1H-indol-2-yl) ethanol and 2-(1H-indol-3-yl)ethanol, respectively. We note that the −CH2‒ increments are equal, within the associated uncertainty, in (1Hindol-2-yl)methanol and (1H-indol-3-yl)methanol. The average value of this increment is −(34.4 ± 3.6) kJ mol−1, a value that is similar to the value corresponding to the addition of the same group in methyl indole2-carboxylate to give ethyl indole-2-carboxylate, −(32.2 ± 1.3) kJ mol−1, and methyl indole-3-carboxylate → ethyl indole-3-carboxylate, −(31.3 ± 1.4) kJ mol−1 [16]. For the thiophene derivatives [41–43], where the enthalpic 173

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Fig. 2. Enthalpic increments relative to the addition of the −CH2– group to (1H-indol-2-yl)methanol and (1H-indol-3-yl)methanol. Enthalpy of isomerization 2 → 3 for (1H-indol-yl)methanol and for the (1H-indol-3-yl)methanol.

Fig. 3. Enthalpic increments relative to the introduction of the −CH2– group on methyl thiophene-2-carboxylate and alkyl 2- and 3-thiophenoacetate (alkyl = methyl or ethyl).

variations are shown in Fig. 3., the average value of the enthalpic increment relative to the addition of a methylene group in the linear chain of the thiophene derivatives, −(31.2 ± 6.3) kJ mol−1 is identical to the value corresponding to the same substitution in indole derivatives. As regards the pyrrole derivatives [44], with respect to the addition of the −CH2– group in the linear chain the methyl pyrrole-2-carboxylate, the value of the respective enthalpic increment −(34.2 ± 3.3) kJ mol−1 is identical to the value corresponding to the same addition in indoles and thiophenes. By analyzing all of the above systems it can be concluded that the values of the enthalpic increments relative to the increase of the carbon chain of the substituent on the heterocyclic ring (N or S as heteroatoms) are very close, ca. −33 kJ mol−1. Comparing these values with the value of the enthalpic increment associated with the addition of the methylene group in methanol and methylacetate to give, respectively, ethanol and ethylacetate [39]

Fig. 4. Enthalpic increment corresponding to the introduction of the −CH2– group in methanol and methylacetate.

(Fig. 4), it is found that they are similar. Considering the results obtained by our computational calculations, we can observe that the enthalpy of isomerization (1H-indol-2-yl)methanol → (1H-indol-3-yl)methanol is (6.1 ± 1.3) kJ mol−1 and the 174

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isomerization 2-(1H-indol-2-yl)ethanol → 2-(1H-indol-3-yl)ethanol is (8.9 ± 3.4) kJ mol−1, representing a small enthalpic stabilization of the 2-isomers compared to the 3-isomers. These enthalpic increments are compatible with our earlier observations concerning the geometrical structure of the substituted indoles which allow stabilizing interactions involving the oxygen atom of the substituent and the NeH fragment of the pyrrole ring in the case of 2-substitution but not in the case of 3-substitution. In addition, the relative magnitude of both increments is also quantitatively compatible with the geometrical details of those interactions in that the N‒H⋯O interaction is both shorter and more linear for 2-(1H-indol-2-yl)ethanol (H⋯O = 0.2141 nm, N⋯O = 0.2812 nm, 121.9°) than for (1H-indol-2-yl)methanol (H⋯O = 0.2434 nm, N⋯O = 0.2724 nm, 95.5°). We have indeed conducted analyses of the B3LYP-6-311++G(2df,2p)//MP2(Full)/ 6–31 G(d) electronic density, for both systems, in the context of the quantum theory of atoms in molecules (AIM) [45] from which a bond critical point could be detected for 2-(1H-indol-2-yl)ethanol but not for (1H-indol-2-yl)methanol.

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5. Conclusions The standard molar gas-phase enthalpy of formation, at T = 298.15 K, of 2-(1H-indol-3-yl)ethanol has been obtained both by experimental and computational techniques. The enthalpy of formation was obtained from static bomb combustion calorimetry and Calvet microcalorimetry experiments. These values have been also estimated by G3 calculations and by considering a pertinent set of working reactions. The computed value is in excellent agreement with the experimental data herewith reported, therefore, the calculations were also extended to the (1H-indol-2-yl)methanol, (1H-indol-3-yl)methanol and 2-(1H-indol-2-yl)ethanol compounds. The comparative analysis of the isomers studied has shown that the 2-isomers are more stable than the 3-isomers because of a favorable interaction between the oxygen atom of the hydroxyl group and the NeH fragment of the pyrrole ring. The enthalpic increment for the addition of the −CH2– group in the linear chain of the hydroxyl group in the indole derivatives is identical to the value corresponding to the same addition in pyrroles and thiophenes. Further, the insertion of a −CH2– group in methanol and methyl acetate is similar to the value found for the above systems. Acknowledgments Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, for the financial support to Project UID/QUI/0081/ 2013 and to FEDER through Program NORTE2020 for the financial support to Project POCI‐01‐0145‐FEDER‐006,980 and to Project "Sustained Advanced Materials", ref. NORTE-01-0145-FEDER-000028 (FCUP). LMPFA thanks to Programa Ciência 2008. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tca.2019.01.021. References [1] N.K. Kaushik, N. Kaushik, P. Attri, N. Kumar, C.H. Kim, A.K. Verma, E.H. Choi, Biomedical importance of índoles, Molecules 18 (2013) 6620–6662. [2] S. Biswal, U. Sahoo, S. Sethy, H.K.S. Kumar, M. Banerjee, Indole: the molecule of diverse biological activities, Asian J Pharm Clin Res 5 (2012) 1–6. [3] T.C. Barden, Indoles: industrial, agricultural and over-the-counter uses, in: G. Gribble (Ed.), Heterocyclic Scaffolds II:. Topics in Heterocyclic Chemistry, vol 26, Springer, Berlin, Heidelberg, 2010. [4] L.M.P.F. Amaral, T.M.T. Carvalho, J.I.T.A. Cabral, M.D.M.C. Ribeiro da Silva, M.A.V. Ribeiro da Silva, Experimental study on the energetics of the indole derivatives: standard molar enthalpies of formation of indole-2-carboxylic acid and indole-3-carboxaldehyde, J. Therm. Anal. Calorim. 115 (2014) 803–810.

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