The Intramolecular Hydrogen Bond NâHÂ·Â·Â·S in 2,2 - ACS Publications

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Intramolecular Hydrogen Bond N−H···S in 2,2′-Diaminodiphenyl Disulfide: Experimental and Computational Thermochemistry Fernando Ramos,† Henoc Flores,*,† Julio M. Hernández-Pérez,† Jacinto Sandoval-Lira,†,‡ and E. Adriana Camarillo† †

Facultad de Ciencias Químicas de la Benemérita Universidad Autónoma de Puebla 14 sur y Av. San Claudio, C.P. 72570, Puebla Pue, México S Supporting Information *

ABSTRACT: The intramolecular hydrogen bond of the N− H···S type has been investigated sparingly by thermochemical and computational methods. In order to study this interaction, the standard molar enthalpies of formation in gaseous phase of diphenyl disulfide, 2,2′-diaminodiphenyl disulfide and 4,4′diaminodiphenyl disulfide at T = 298.15 K were determined by experimental thermochemical methods and computational calculations. The experimental enthalpies of formation in gasphase were obtained from enthalpies of formation in crystalline phase and enthalpies of sublimation. Enthalpies of formation in crystalline phase were obtained using rotatory bomb combustion calorimetry. By thermogravimetry, enthalpies of vaporization were obtained, and by combining them with enthalpies of fusion, the enthalpies of sublimation were calculated. The Gaussian-4 procedure and the atomization method were applied to obtain enthalpies of formation in gas-phase of the compounds under study. Theoretical and experimental values are in good agreement. Through natural bond orbital (NBO) analysis and a topological analysis of the electronic density, the intramolecular hydrogen bridge (N−H···S) in the 2,2′-diaminodiphenyl disulfide was confirmed. Finally, an enthalpic difference of 11.8 kJ·mol−1 between the 2,2′-diaminodiphenyl disulfide and 4,4′diaminodiphenyl disulfide was found, which is attributed to the intramolecular N−H···S interaction.

1. INTRODUCTION

Despite the importance of disulfides in the chemical industry, information on their thermo-chemical properties, such as enthalpies of phase changes and formation enthalpies, is very scarce. To the best of our knowledge, only Mackle has reported the enthalpy of formation of the diphenyl disulfide, which was obtained experimentally by combustion calorimetry, and their enthalpies of vaporization and fusion, which were obtained by estimation methods.9 This paper presents a calorimetric study of three aryl disulfides: diphenyl disulfide, 2,2′-diaminodiphenyl disulfide and 4,4′-diaminodiphenyl disulfide, the structures of which are shown in Figure 1. We are interested in analyzing the effect of the amino group, at positions 2 and 4 of the aromatic rings of the diphenyl disulfide, on the thermochemical properties, since the chemical environment is different in both positions. In the 2,2′diaminodiphenyldisulfide, a possible intramolecular interaction between a sulfur of the disulfide and a hydrogen of the amino group at position 2 can be assumed. In the case of 4,4′-diaminodiphenyl disulfide, this interaction is not present. Comparing the enthalpies of formation in the gas phase of both compounds, it is possible to associate an energy value to this interaction. The experimental methodology

The organic compounds that contain sulfur in its molecular structure constitute an important set of chemical compounds useful in the industry of dyes, drugs, and detergents. A particularly important group are the disulfides that have the general structure R−S−S−R′, where R and R′ are alkyl or aryl groups. Disulfides are also important due to the presence of the S−S bond. Although this bond is structurally similar to the O− O bond of peroxides, S−S bonds render compounds that are very different chemically, relative those having O−O bonds. Furthermore, disulfide reactions play a key role in biological systems due to the presence of the disulfide bond. For example, in proteins, S−S bond is one of the main factors that determine their structural properties and their biological function.1 These compounds have applications in the chemical industry, for instance, in the recovery of latex rubber,2,3 in improving the process of devulcanization with organic thiophene,4 and in the process of dissolving sulfur for acid gas production,5 to name a few. Disulfides are also used as flame retardants in the polymer industry because they disrupt or retard the combustion process of several polymers.6 They are used as dynamic cross-linking agents in poly(urea-urethane) systems as well.7 In organic synthesis they are used as precursors of substances that act as inhibitors of the cholesteryl ester transferase protein.8 © XXXX American Chemical Society

Received: September 5, 2017 Revised: December 7, 2017 Published: December 8, 2017 A

DOI: 10.1021/acs.jpca.7b08838 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 1. Structural formulas of the studied disulfides.

includes, in the first place, a thermal analysis by differential scanning calorimetry, by means of which the purity, the heat capacity of the solid and liquid phases, the enthalpy, and the melting temperature were obtained. By rotatory bomb combustion calorimetry, the combustion energy was determined, and from this the standard molar enthalpy of formation in the crystalline phase was derived. By thermogravimetric measurements and using the Langmuir and Clausius− Clapeyron equations, the molar enthalpy of vaporization was derived. In this method, the diffusional effect during the determinations was taken into account. The sublimation enthalpy at 298.15 K was calculated from the enthalpies of melting, vaporization, and heat capacities of the solid, liquid, and gaseous phases of the compounds under study, and appliying Kirchhoff’s equation. The standard molar enthalpy of formation in gas phase at T = 298.15 K was calculated by combining the standard molar enthalpy of formation in the crystalline phase and the enthalpy of sublimation. The computational methodology includes the calculation of the enthalpy of formation of the gas phase by appliying the Gaussian-4 method and using atomization reactions. Both experimental and theoretical values were compared. The intramolecular interaction N−H S was analyzed by performing a topological analysis of the electronic density. With the experimental and theoretical data obtained, the influence of the amino group in positions 2 and 4 of the diphenyl disulfide is analyzed.

melting method.10,11 This equipment was calibrated for temperature and heat flow using high purity metallic indium as reference material (>0.99999).12 In the analysis of each substance, heating rates of 10 K·min −1 were used from room temperature to 20 K above the melting temperature, and no impurities, crystalline transitions, or some other thermal phenomena were detected in addition to the fusion process. For subsequent experiments the heating range was restricted to ±10 K with respect to the melting temperature and with heating rates of 1 K·min−1, under a nitrogen atmosphere of 50 cm3·min−1. From the DSC thermograms, the melting enthalpy was derived by calculating the area under the melting peak. Compound masses of 1 to 3 mg were used, which were placed in hermetically sealed aluminum cells. 2.2. Heat Capacity of the Solid and Liquid Phases. The heat capacities of the solid and liquid phases of the three compounds were also measured by differential scanning calorimetry, using a PerkinElmer DSC 8000 calorimeter and applying the two step method using synthetic sapphire (αaluminum oxide) as the reference material.12 For the solid phase, the ranges analyzed were (258.15−318.15) K, (258.15− 348.15) K and (258.15−333.15) K for DPDS (cr), 2ADPDS (cr), and 4ADPDS (cr), respectively. For the liquid phase, the ranges analyzed were (338.15−398.15) K, (371.15−423.15) K and (355.15−458.15) K for DPDS (1), 2ADPDS (1) and 4ADPDS (1), respectively. The heating rates used were 10 K· min−1, under a dynamic nitrogen atmosphere of 20 cm3·min−1. In each experiment, masses of 5 to 8 mg of compound were used, which were placed in hermetically sealed aluminum cells. The masses of the samples for the DSC experiments were determined with a Mettler Toledo UMX2 balance (sensitivity of ±10−7 g). Before performing the heat capacity measurements of the compounds, the reliability of the equipment was tested by measuring the heat capacity of synthetic sapphire at different temperatures: 298.15 K, 350.0 K, 400 and 450 K. The values obtained were 78.61 J·mol−1·K−1, 89.01 J·mol−1·K−1, 96.14 J· mol−1·K−1 and 101. J·mol−1·K−1, which are in good agreement with the values reported in the literature: 79.01 J·mol−1·K−1, 88.84 J·mol−1·K−1, 96.08 J·mol−1·K−1, and 101.71 J·mol−1· K−1.12 The densities of the disulfides studied are 1.34 g·cm−3 for diphenyl disulfide,13 1.34 g·cm−3 for 2,2′-diaminodiphenyl disulfide,14 and 1357 g·cm−3 for 4,4′-diaminodiphenyl disulfide.15 For the calculation of the molecular weights of the studied compounds and of all the molar quantities in this work, the atomic weight values recommended by the IUPAC commission in 2013 were used.16 2.3. Rotating-Bomb Combustion Calorimetry. The specific combustion energies of the three disulfides were determined using a rotatory bomb combustion calorimeter,

2. EXPERIMENTAL SECTION 2.1. Materials and Purity Control. The compounds studied were obtained commercially from Sigma-Aldrich Co. with the following mole fractions: diphenyl disulfide (CAS 88233-7) 0.99, 2,2′-diaminodiphenyl disulfide (CAS 1141-88-4) 0.97 and 4,4′-diaminodiphenyl disulfide (CAS 722-27-0) 0.98. A preliminary analysis by differential scanning calorimetry confirmed a molar fraction higher than 0.99 for the diphenyl disulfide, thus it was used without further purification for combustion calorimetry and thermogravimetric experiments. 2,2′-Diaminodiphenyl disulfide and 4,4′-diaminodiphenyl disulfide were purified by double recrystallization from an ethanol-ethyl acetate mixture, after which, the molar fraction was approximately 0.999 for both compounds. To remove the solvent from the crystals formed, they were dried at 313.15 K for 48 h; subsequently the amount of water in the samples was determined by means of a Karl Fischer titration and less than 100 ppm of water was detected therein. Finally, all samples were stored under a nitrogen atmosphere. The molar fraction (x) and the melting temperature (Tfus) were determined by differential scanning calorimetry (DSC) using a TA Instruments Q2000 DSC equipment, and applying the fractional B


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Table 1. Molar Masses, Density, and the Properties Determined by DSC of the Disulfides Studied in Present Work at p° = 0.1 MPa compound DPDS 2ADPDS 4ADPDS benzoic acid cotton

M

a

g·mol−1

218.3378 248.3671 248.3671 122.1210 28.182

ρ

xg

g·cm−3

0.9999 ± 0.0001 0.9989 ± 0.0003 0.9992 ± 0.0004

1.34b 1.34c 1.357d 1.32e 1.50f

Tfus g K

331.9 ± 0.4 364.5 ± 0.4 349.3 ± 0.4

l ° Δcr Hm(Tfus) g

kJ·mol−1

27.99 ± 1.19 31.52 ± 0.54 25.77 ± 1.25

a

Molar masses based on IUPAC 2013 recommendation.16 bValues taken from ref 13. cValues taken from ref 14. dValues taken from ref 15. eValues taken from ref 35. fValues taken from ref 36. gExperimental values, uncertainty corresponds to the expanded one, which includes the contributions of the calibration, was calculated with a coverage factor k = 1.96 and confidence level of 0.95 for two-tailed normal distribution.

Here B = ln(DMS/R) + C, and C is the integration constant from the Clausius−Clapeyron equation. Equation 2 can be applied to isothermal thermogravimetric measurements, and it has recently been tested to determine vaporization and sublimation enthalpies under dynamic conditions.21 From eq 2, the enthalpy of vaporization can be obtained by plotting ln(dm dt·T) versus 1/T; therefore, it is only necessary to measure with sufficient precision the rate of mass loss as a function of temperature. The above can be achieved using a TA Instruments Q500 device. The maximum capacity of the balance is 1 g with a sensitivity of 0.1 μg (accuracy 0.01%). The heating furnace operates in the range of (298.15−1273.15) K with heating rates from 0.01 to 100 K·min−1, and it has a temperature control of ±1 K with a sensitivity in the measurement of 0.1 K. The sample cell has a 10.6 mm diameter and is 1.50 mm deep. Mass calibration was performed using NIST reference masses of 100 and 1000 mg. Curie temperatures of alumel and nickel (425.75 and 631.35 K) were determined in order to calibrate the equipment for temperature. Some preliminary experiments in the thermogravimetric device showed that quantifiable mass losses did not occur before the fusion; hence the phase change analyzed, for the three compounds, was vaporization, as opposed to direct sublimation. Compound masses of 10 to 20 mg were used in each experiment. Heating rates were 10 K·min−1 from room temperature to 570 K under a nitrogen flow of 160 cm3·min −1. In order to verify that the diphenyl disulfides do not exhibit any thermal phenomena during heating, DSC experiments were performed over the same temperature range as those performed by TGA. No process, other than fusion, was observed within the detection limits of the equipment. The nondecomposition of the diphenyl disulfides during vaporization was verified by analyzing a sample (which was condensed at the exit of the heating furnace) by 1H NMR and 13C NMR. Only the compounds’ own signals were observed (the spectra are shown in Figures 1 to 6 of the Supporting Information). 2.5. Computational Details. To understand the stability stemming from structural features and to support the experimental results, we performed several theoretical analyses. All calculations were carried out using Gaussian 09.22 For all geometry optimizations, we used the X-ray crystallography data as starting geometries.13,14,23 In order to ensure the robustness of our electron density analyses, and to trust possible intramolecular interactions stemming from such analyses, the MP2(FULL)/cc-pVDZ level of theory was used. Geometries were fully optimized, and minima were confirmed through vibrational analysis. To evaluate the direction and magnitude of the donor−acceptor

which has a Parr 1004 C combustion bomb with an internal platinum coating and an internal volume of 0.348 dm3. This equipment was previously tested and calibrated in our laboratory.17 Recently, the equipment was calibrated again using benzoic acid (NIST Standard Reference material 39j, with a specific combustion energy of (−26434 ± 3) J·g−1), obtaining a value ε (calor) = (14362.7 ± 2.2) J·K−1,18 where the associated uncertainty corresponds to the standard deviation of the mean. Details of the experimental procedure are described in the Supporting Information. Pawelec et al.6 have reported that aryl type disulfides, such as those studied in this work, can act as flame retardants; therefore, to promote the complete combustion of the samples studied here, it was necessary to use benzoic acid (NIST Standard Reference Material 39j) as the auxiliary material. For each combustion experiment, the sample of the diphenyl disulfides in the pellet form was surrounded by benzoic acid and then compressed in such a way that only a pellet of about 1 g was made. A scheme of the distribution of diphenyl disulfides and benzoic acid in the pellet is shown in the Supporting Information. The calculation of the specific combustion energy and the reduction to the standard state was performed for each experiment as described by Hubbard et al.19 with a Computer program developed in our laboratory, which has been used previously.18 The apparent mass correction for the effect of air buoyancy was applied. 2.4. Termogravimetry. The vaporization enthalpies of the compounds studied in the present work were determined by thermogravimetry, TGA, and using the Langmuir and Clausius−Clapeyron equations. Since the Langmuir equation is strictly valid only under vacuum conditions, Pieterse and Focke20 suggested that the diffusional effect of the gaseous phase must be taken into account, and to this end they propose eq 1.

⎛ MS ⎞ dm ⎟D = p⎜ ⎝ RT ⎠ dt

(1)

where dm/dt is the rate of mass loss at temperature T; A is the exposed area of the sample, in the sample cup, during the sublimation or vaporization process; p is the vapor pressure of the substance; M is the molar mass of compound; R is the universal gas constant; D is the diffusion coefficient; and S = A/ z. Here z is the depth of the gas-filled part of the sample cup. Combining eq 1 and the integrated Clausius−Clapeyron equation gives eq 2. Δg H 1 ⎛ dm ⎞ ln⎜ ·T ⎟ = B − l m ⎝ dt ⎠ R T

(2) C


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Table 2. Heat Capacity Equations of the Liquid, Gas, and Solid Phases as a Function of Temperature, Determined by DSCa compound DPDS

2ADPDS

4ADPDS

phase

temperature range/K

equation

cr l g cr l g cr l g

258.15−318.15 338.15−398.15 298.15−460.15 258.15−348.15 371.15−423.15 298.15−460.15 258.15−333.15 355.15−458.15 298.15−460.15

Cp(DPDS, cr)/J·mol−1·K−1=243.416−1.117 (T/K) + 3.735 × 10−3 (T/K)2 Cp(DPDS, l)/ J·mol−1·K−1=690.827−2.079 (T/K) + 3.100 × 10−3 (T/K)2 Cp(DFDS, g)/ J·mol−1·K−1=-30.088 + 0.936 (T/K) - 0.484 × 10−3 (T/K)2 Cp(2ADPDS, cr)/ J·mol−1·K−1=426.360−2.080(T/K) + 4.702 × 10−3 (T/K)2 Cp(2ADPDS, l)/ J·mol−1·K−1= 521.478−0.840 (T/K) + 1.599 × 10−3 (T/K)2 Cp(2ADFDS, g)/ J·mol−1·K−1= −19.112 + 1.102 (T/K) - 0.616 × 10−3 (T/K)2 Cp(4ADPDS, cr)/ J·mol−1·K−1= −96.703 + 1.649 (T/K) - 1.123 × 10−3 (T/K)2 Cp(4ADPDS, l)/ J·mol−1·K−1= 244.015 + 0.507 (T/K) − 0.385 × 10−4 (T/K)2 Cp(4ADFDS, g)/ J·mol−1·K−1= −12.212 + 1.081 (T/K) - 0.599 × 10−3 (T/K)2

correlation coefficient r2 r2 r2 r2 r2 r2 r2 r2 r2

= = = = = = = = =

0.9992 0.9908 1.0000 0.9995 0.9985 1.0000 0.9993 0.9992 1.0000

a

For compounds in solid and liquid phase, the uncertainty corresponds to the expanded uncertainty which include the contribution of the calibration and was calculated with a coverage factor k = 2.36 and confidence level of 0.95 for two tailed normal distribution: U(Cp, DPDS, cr) = 0.4 J·mol−1· K−1, U(Cp, DPDS, l) = 0.9 J·mol−1·K−1, U(Cp, 2ADPDS, cr) = 0.4 J·mol−1·K−1, U(Cp, 2ADPDS, l) = 0.9 J·mol−1·K−1, U(Cp, 4ADPDS, cr) = 0.5 J· mol−1·K−1, and U(Cp, 4ADPDS, l) = 1.6 J·mol−1·K−1.

interactions, the natural bond orbital (NBO)24−27 analysis for all structures was performed using the NBO 3.0 program.28 The topological analysis of the electron density29 was carried out using DensToolKit program.30 The enthalpies of formation were estimated through the atomization approach, and we calculated the electronic energies using the G4 composite method.31 Atomic enthalpies of formation in gas-phase, at T = 0 K, as well as their thermal corrections, at T = 298.15 K, were taken from the literature.32

Table 3. Individual Values of the Massic Energy of Combustion of Disulfides at p° = 0.1 MPa and T = 298.15 Ka DPDS 34.2542 34.2402 34.2436 34.2560 34.2576 34.2629 34.2600 34.2430

3. RESULTS AND DISCUSSION 3.1. Fusion and Heat Capacity Data. Table 1 shows the molar fractions, melting temperatures, and melting enthalpies obtained by DSC of the diphenyl disulfides studied in this work. In addition, the molar mass and density of each compound, obtained from the literature, are shown. The heat capacity equations of the solid, liquid and gaseous phases are shown in Table 2. The heat capacity data for each phase were adjusted to quadratic equations, obtaining high correlation coefficients. The uncertainty associated with each of the properties determined by DSC corresponds to the expanded uncertainty, with a coverage factor k = 1.96 and confidence level of 0.95 for two tailed normal distribution,33,34 which includes the contributions of the equipment calibration. The melting experiments and the heat capacity results of the solid and liquid phases, obtained by DSC, are shown in detail in the Supporting Information. The heat capacity of the gas phase in the temperature range analyzed is also shown. 3.2. Energies and Enthalpies of Combustion. The results of the combustion experiments are shown in Table 3, which are associated with the idealized combustion reactions: eq 3 for DPDS, and eq 4 for 2ADPDS and 4ADPDS. The uncertainty associated with each specific combustion energy corresponds to the standard deviation of the mean. The detail of each experiment presented in Table 3 is shown in the Supporting Information.

34.2522 ± 0.0031

31.1112 31.0861 31.1150 31.1218 31.0994 31.1039 31.1206

⟨Δcu° (298.15 K)/kJ·g−1⟩ 31.1293 ± 0.0045

31.1083 ± 0.0048

The uncertainty associated with each average value of massic energy of combustion is the standard uncertainty for eight experiments for DPDS and seven experiments for 2ADPDS and 4ADPDS.

Table 4. Standard Molar Energies and Enthalpies of Combustion and Formation of the Studied Compounds at T = 298.15 K and P° = 0.1 MPaa compound

° (cr) −ΔcUm

° (cr) −ΔcHm

kJ·mol−1

DPDS 2ADPDS 4ADPDS

7478.6 ± 1.8 7731.5 ± 2.1 7726.3 ± 2.1

kJ·mol−1

7492.2 ± 1.8 7743.9 ± 2.1 7738.7 ± 2.1

° (cr) Δ f Hm kJ·mol−1

137.0 ± 2.4 102.8 ± 2.6 97.6 ± 2.6

a

The uncertainty associated with the values corresponds to the expanded uncertainty which includes the uncertainties of the calibration, the combustion energy of the benzoic acid, the auxiliary materials and the compounds studied and was calculated with a coverage factor of k = 1.96 and with a confidence level of 0.95.

compound at T = 298.15 K. The uncertainty associated with the values corresponds to the expanded uncertainty, which includes the calibration uncertainties, as well as the uncertainties from combustion energies of the benzoic acid, the auxiliary materials, and the compounds studied. These expanded uncertainties were calculated with a coverage factor of k = 1.96 and a confidence level of 0.95.33,34 The enthalpies of formation of the compounds were calculated using the formation enthalpy values of CO2 (g), H2O (l), and H2SO4· 115H2O (aq) as (−393.51 ± 0.13) kJ·mol−1, (−285.830 ± 0.042) kJ·mol−1 and (−887.811 ± 0.044) kJ·mol−1, respectively.37,38

(3)

C12H12N2S2 (cr) + 18O2 (g) + 226H 2O(l) → 12CO2 (g) + 2(115H 2O· H 2SO4 ) + N2(g)

4ADPDS

a

C12H10S2 (cr) + 17.5O2 (g) + 227H 2O(l) → 12CO2 (g) + 2(115H 2O· H 2SO4 )

2ADPDS (Δcu°) (compound)/kJ·g1 31.1352 31.1369 31.1133 31.1237 31.1432 31.1153 31.1376

(4)

Table 4 presents the standard molar energy of combustion, ΔcU°, the standard molar enthalpy of combustion, ΔcH°, and the standard molar enthalpy, ΔfH°, of the solid phase for each D


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The Journal of Physical Chemistry A Mackle et al.9 reported a calorimetric study of DPDS. The values of specific combustion energy and enthalpy of formation of the crystalline phase that the authors obtained are (−34.3082 ± 0.0027) kJ·g−1 and (149.33 ± 2.72) kJ·mol−1. These values differ from ours by 56 J·g−1 and 12 kJ·mol−1, respectively. The authors report that it was necessary to burn the compound in the presence of 32 atm of oxygen and 3 atm of nitrogen. In our laboratory, we tried to reproduce some experiments under these conditions; however, the complete combustion of the samples was not obtained. 3.3. Enthalpies of Vaporization by Thermogravimetry. The vaporization enthalpies were obtained from four independent experimental measurements over the ranges of (360.0−380.0) K, (400.0−420.0) K and (450.0−470.0) K, for DPDS, 2ADPDS and 4ADPDS respectively. Thermogravimetric curves of the studied compounds are shown in Figure 2. The

Table 5. Thermogravimetric Data of a Series of DPDS and Calculation of the Vaporization Enthalpy of the Compounds Studied Herea T K

m mg

(dm / dt )·109

(1 / T )·103

kg·s−1

K −1

ln(dm/dt·T)

DPDS Series 1 360.0 19.6336 0.6300 2.778 −15.299 362.0 19.6257 0.7120 2.762 −15.171 364.0 19.6165 0.8085 2.747 −15.039 366.0 19.6062 0.9264 2.732 −14.897 368.0 19.5941 1.0667 2.717 −14.751 370.0 19.5805 1.1889 2.703 −14.637 372.0 19.5654 1.3484 2.688 −14.505 374.0 19.5482 1.5172 2.674 −14.382 376.0 19.5289 1.6964 2.660 −14.265 378.0 19.5074 1.9082 2.646 −14.142 380.0 19.4831 2.1307 2.632 −14.027 Series 1 ln(dm/dt T) = 9.0−8755.7/T; r2 = 0.9996; σa = 0.2; σb = 56.5; Δgl Hm(370.0 K)/kJ·mol−1=72.8 ± 0.5 Series 2 ln(dm/dt T ) = 9.4−8897.7/T; r2 = 0.9987; σa = 0.3; σb = 108.4; Δgl Hm(370.0 K)/kJ·mol−1 = 74.0 ± 0.9 Series 3 ln(dm/dt T) = 9.1−8832.0/T; r2 = 0.9996; σa = 0.2; σb = 55.9; Δgl Hm(370.0 K)/kJ·mol−1 = 73.4 ± 0.5 Series 4 ln(dm/dt T) = 9.1−8816.2/T; r2 = 0.9998; σa = 0.1; σb = 43.1; Δgl Hm(370.0 K)/kJ·mol−1 = 73.3 ± 0.4 weighted average ⟨Δgl Hm(DPDS, 370.0 K)⟩/kJ·mol−1 = 73.3 ± 0.5 2ADPDS Series 1 ln(dm/dt T) = 9.4−10437.5/T; r2 = 0.9985; σa = 0.3; σb = 134.4; Δgl Hm(410.0 K)/kJ·mol−1 = 86.8 ± 1.1 Series 2 ln(dm/dt T) = 9.5−10371.6/T; r2 = 0.9910; σa = 0.8; σb = 329.7; Δgl Hm(410.0 K)/kJ·mol−1 = 86.2 ± 2.7 Series 3 ln(dm/dt T) = 9.4−10386.1/T; r2 = 0.9974; σa = 0.4; σb = 176.5; Δgl Hm(410.0 K)/kJ·mol−1 = 86.4 ± 1.5 Series 4 ln(dm/dt T) = 10.4−10887.6/T; r2 = 0.9987; σa = 0.3; σb = 129.3; Δgl Hm(410.0 K)/kJ·mol−1 = 90.5 ± 1.1 weighted average ⟨Δgl Hm(2ADPDS, 410.0 K)⟩/kJ·mol−1 = 88.1 ± 1.3 4ADPDS Series 1 ln(dm/dt T) = 10.7−12184.0/T; r2 = 0.9961; σa = 0.6; σb = 253.1; Δgl Hm(460.0 K)/kJ·mol−1 = 101.3 ± 2.1 Series 2 ln(dm/dt T) = 11.0−12304.9/T; r2 = 0.9983; σa = 0.4; σb = 171.0; Δgl Hm(460.0 K)/kJ·mol−1 = 102.3 ± 1.4 Series 3 ln(dm/dt T) = 11.23−12399.7/T; r2 = 0.9989; σa = 0.3; σb = 139.9; Δgl Hm(460.0 K)/kJ·mol−1 = 103.1 ± 1.2 Series 4 ln(dm/dt T)=10.3−11979.6/T; r2=0.9939; σa = 0.7; σb = 312.9; Δgl Hm(460.0 K)/kJ·mol−1=99.6 ± 2.6 /kJ·mol−1= 102.3 ± 1.6

Figure 2. Thermogravimetric plot of diphenyl disulphyde (); 2aminodipheyl disulphyde (− − − ); 4-aminodiphenyl disulphyde (― -).

data of an individual thermogravimetric experiment, for the DPDS, are shown in Table 5; this includes the temperature T, the mass m, the rate of mass loss dm/dt, and the terms ln(dm/ dt·T) and 1/T. From the graph of these two last terms, a straight line is generated, and from its slope, the enthalpy of phase change was obtained, which is associated with the average temperature (Tav) of the analyzed interval. From the linear adjustment of the data, we obtained the correlation coefficient r2 and the uncertainty associated with the intersection σa and the slope σb. The average of the four experimental series is the weighted average,39 and its uncertainty corresponds to the combined uncertainty, which includes slope, temperature, and dm/dt uncertainty.33,34 In the Supporting Information, all the data of the thermogravimetric and the detail of the uncertainty calculation are shown. Considering that the enthalpy is a state function and that its magnitude depends only on the initial and final states, it can be calculated at 298.15 K using a path such as that proposed in Figure 3. According to Figure 3, the trajectory begins with the compound being in crystalline phase at 298.15 K, followed by a heating to its melting temperature; the enthalpy associated with this heating is ΔH1, which is calculated using the heat capacity of the phase crystalline form from 298.15 K to the melting temperature of the compound (Tfus). The next step is the fusion of the compound, and the enthalpy of the process

Standard uncertainties u are u(T) = 0.1 K, u(m) = 0.1 μg, and the combined expanded uncertainty Uc is Uc(dm/dt) = 0.066 × 109 kg·s−1, Uc(1/T) = 0.001 × 103 K−1, Uc(ln(dm/dt·T) = 0.020; Uc(ln(dm/dt·T) = 0.020 (with a coverage factor k = 2.45 and confidence level of 0.95 for two tailed normal distribution). a

corresponds to the enthalpy of fusion. After the liquid compound is heated until average vaporization temperature (Tav), the enthalpy corresponding to this heating is ΔH2 and is calculated using the heat capacity of the liquid phase from Tfus to Tav. At the average vaporization temperature occurs the vaporization of the compound, which has an associated enthalpy of vaporization. Once the compound is in the gaseous phase at Tav, it is cooled down to the Tfus, and the enthalpy associated with this cooling is ΔH3 and can be obtained using the heat capacity of the gas phase from Tav to Tfus. Finally, the gas phase compound is cooled down from Tfus to 298.15 K, where the enthalpy is calculated by ΔH4. The sum of all stages of the cycle corresponds to the sublimation enthalpy at 298.15 K. E


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Figure 3. Pathway to calculate the enthalpies of sublimation from the enthalpies of vaporization, fusion, and heat capacity.

Table 6. Experimental Values of Fusion and Vaporization Enthalpies, and Temperature Corrections Required to Calculate the Sublimation Enthalpies at T = 298.15 K by Using Figure 3 Tfus K

2ADPDS 4ADPDS

331.9 364.5 349.3

l ° Δcr Hm(Tfus) a −1

kJ·mol

27.99 ± 1.19 31.52 ± 0.54 25.77 ± 1.25

Tav K

370.0 410.0 460.0

T

fus C p(cr)dT c ∫298.15K

g ° (Tav ) b Δl Hm

−1

−1

kJ·mol

kJ·mol

73.3 ± 1.2 88.1 ± 3.2 102.3 ± 3.9

8.9 ± 0.4 16.9 ± 0.4 16.3 ± 0.5

T

∫T vap C p(l)dT c fus

−1

kJ·mol

13.1 ± 0.9 19.8 ± 0.9 49.0 ± 1.6

T

∫T fus C p(g)dT vap

−1

kJ·mol

−9.2 −14.5 −36.4

298.15K

∫T

fus

C p(g)dT

kJ·mol−1

−7.4 −18.6 −14.1

g ° Δcr Hm(298.15K) d

kJ·mol−1

106.7 ± 2.0 123.2 ± 3.4 142.9 ± 4.4

a

The uncertainty associated with values corresponds to the expanded uncertainty, which includes the contributions of the calibration with a coverage factor k = 1.96 and confidence level of 0.95 for two tailed normal distribution. bThe uncertainty associated with values corresponds to the expanded uncertainty, which includes the contributions of the calibration with a coverage factor k = 2.45 and confidence level of 0.95 for two tailed normal distribution. cThe uncertainty associated with values corresponds to the expanded uncertainty, which includes the contributions of the uncertainty of the heat capacity of the solid or liquid phase with a coverage factor k = 2.36 and confidence level of 0.95. dUncertainty was calculated by applying the root method of the sum of the squares of each term of pathway of Figure 3.

The integrals corresponding to ΔH1 and ΔH2 were calculated using the heat capacity equations of the crystalline and liquid phases respectively (Table 2), which were determined by DSC. The integrals corresponding to ΔH3 and ΔH4 were calculated using the partition function, Q, from which, eq 1 was obtained. Details of the derivation of the eq 5 are in the Supporting Information. ΔH = 4R(T2 − T1) + ⎛ hν ⎞⎤ − coth⎜ i ⎟⎥ ⎝ 2kT1 ⎠⎥⎦

R 2

∑ i=1

studied is shown in Table 6. The uncertainty associated with each sublimation enthalpy value corresponds to the expanded uncertainty calculated with a coverage factor k = 1.96 and confidence level of 0.95.33,34 According to the sublimation enthalpies at 298.15 K shown in Table 6, 4ADPDS has the most intense interactions in the solid phase, followed by the 2ADPDS and finally the DPDS. This trend is presumably due to more intense intermolecular hydrogen bond interactions between two adjacent amino groups in the crystalline phase. The same trend is observed in the enthalpy of vaporization. Mackle9 estimated the fusion and vaporization enthalpies of DPDS, using the Walden’s rule and the Laidler methods, respectively. The values obtained were 16.3 ± 2.1 and 78.66 ± 2.92 kJ·mol−1, which differ from those obtained in the present work (Table 6). The difference can be attributed to the inherent error of the estimation methods used to determine these properties. 3.4. Gas-Phase Molecular Structures. Crystallographic studies of the molecular structures for the three isomers studied here have been reported in the literature. We used these structures as starting points for optimizing geometries. The optimized molecular structures of DPDS, 2ADPDS, and 4ADPDS are shown in Figure 4. The dihedral angles (C1−

⎛ hν ⎞ hνi ⎡ ⎢coth⎜ i ⎟ k ⎢⎣ ⎝ 2kT2 ⎠

(5)

The integrals related to ΔH3 and ΔH4 were calculated using the eq 5. In order to be consistent with the enthalpy calculation, we used the vibrational frequencies obtained by the G4 method. In other words, we used vibrational frequencies calculated at the B3LYP/6-31G(2df,p) level, scaled by a factor of 0.9854.31 The enthalpy of fusion was determined by DSC, and the enthalpy of vaporization was obtained from thermogravimetric experiments. A summary of the values required to calculate the sublimation enthalpy at 298.15 K for each of the disulfides F


Article

The Journal of Physical Chemistry A S1−S2−C7) are −86.4°, −36.9° and −61.9° for DPDS, 2ADPDS, and 4ADPDS, respectively.

From the enthalpies of formation in gas-phase, the enthalpic contribution of the amino group at the 2 and 4 position of diphenylsulfide was derived, as shown in Figure 5. Figure 5 shows that the enthalpic contribution of the amino group at position 2 is exothermic by − (17.7 ± 5.3) kJ·mol−1 at position 4, the contribution is also exothermic by (−3.2 ± 6.0) kJ·mol−1. The enthalpic change due to the change ortho-para of the amino substituent is endothermic by 14.5 ± 6.7 kJ·mol−1, which implies some enthalpic stability in the 2ADPDS. This is most likely attributed to a hydrogen bonding interaction between sulfur of the disulfide bond with a hydrogen of the amino group of the 2-position. In order to provide further evidence of the existence of these intramolecular interactions, we performed a more detailed analysis using natural bond orbital (NBO) analysis, and an electron density analysis from the Quantum Theory of Atoms in Molecules (QTAIM). Briefly, in the NBO method, filled orbitals correspond to localized Lewis structures, while unoccupied orbitals are related to antibonding or Rydberg orbitals. The interaction between filled (donor) and unoccupied (acceptor) NBO orbitals is characterized by an energy E(2) (obtained through a second order perturbation approach), which serves as a measure of delocalization. The higher the energy E(2) of the interaction, the higher the degree of its electron delocalization. Table 8 lists the results of NBO analysis: the atomic charges of H [q(H)] and S (q(Y)) atoms, the occupancy in the lone pair orbital [δ(nY)] and the antibonding orbital [δ(σ*N−H)], as well as the second-order perturbative interactions in the DPDS, 2ADPDS, and 4ADPDS. A charge increase in the H10(12) atoms (0.40 au) and a charge decrease in the S1(2) atoms (0.10 au) of the 2ADPDS are both observed with respect to their counterparts, i.e., H10(12) atoms (0.38 and 0.38 au, respectively) in 4ADPDS and S1(2) atoms (0.14789 au) of the DPDS. These values are reflected in the orbital occupation of σ*N1(2)‑H10(12) antibonding orbital (0.012 au) for the 2ADPDS in comparison with the population in σ*N1(2)‑H10(12) antibonding orbital (0.006 au) the 4ADPDS. This shows that 2ADPDS has nS1(2) → σ*N1(2)‑H10(12) intramolecular hydrogen bond interactions. The first interaction occurs between a lone pair of the sulfur atom (S1) and the σ* N1−H10 antibonding orbital of the amino group (nS1 → σ*N1−H10 with E(2) energy of 12.6 kJ/mol), and the second interaction is between a lone pair of the sulfur atom (S2) and the σ* N2−H12 antibonding orbital of the amino group (nS2 → σ*N2−H12 with E(2) energy of 12.6 kJ/mol); see Figure 6. All of these numbers support the existence of two intramolecular interactions of N−H···S type. The Quantum Theory of Atoms In Molecules (QTAIM) is based on the topological analysis of electron density.29 The study of electron density critical points (which are points at which the gradient of electron density is zero) provides information about the molecular structure and atomic interactions. In this work, the QTAIM was used to obtain a greater insight into the N−H···S type hydrogen bonds. AIM calculations were done using wave functions computed at the MP2(FULL)/cc-pVDZ level of theory for the 2ADPDS. The 2ADPDS molecular graph is depicted in Figure 7. It shows the bond critical points (BCPs) along the lines joining the N−H and S atoms, which establishes the existence of the N−H···S hydrogen bond between N1(2)−H10(12) and S1(2). The AIM criteria proposed by Popelier40 that establishes what can be considered a classical hydrogen bond were applied to the N− H···S bond. The topological parameters for 2ADPDS are listed

Figure 4. Optimized structures of DPDS, 2ADPDS, and 4ADPDS at the MP2(FULL)/cc-pVDZ level of theory.

The structure of 2ADPDS shows some closeness between two pairs of sulfur and hydrogen atoms: The first between sulfur atom (S1) and hydrogen atom (H10) of the amino group (N1) with a distance of 2.57 Å, and second sulfur atom (S2) and hydrogen atom (H12) of the amino group (N2) with a distance of 2.57 Å. Both bond angles ∠N1(2)H10(12)··· S1(2) and are 110.5°. These geometrical features in the 2ADPDS suggest the presence of two intramolecular interactions of N−H···S type. 3.5. Formation Enthalpies in Gas-Phase. From the enthalpy of sublimation and the molar standard enthalpy of formation in crystalline phase, the enthalpy of formation of all compounds in gas phase was obtained at T = 298.15 K. The experimental results and the comparison with the theoretical values are shown in Table 7. Table 7. Summary of Final Results of Disulfides at T = 298.15 K compound

° (cr) a Δ f Hm

DPDS 2ADPDS 4ADPDS

137.0 ± 2.4 102.8 ± 2.6 97.6 ± 2.6

kJ·mol−1

g ° Δcr Hm b

kJ·mol−1

106.7 ± 2.0 123.2 ± 3.4 142.9 ± 4.4

° (g)(Exp) c Δ f Hm kJ·mol−1

243.7 ± 3.1 226.0 ± 4.3 240.5 ± 5.1

° (g)(Theor) d Δ f Hm kJ·mol−1

238.8 (4.9) 227.1 (−1.1) 242.6 (−2.1)

a

The uncertainty associated with each average value corresponds to the expanded uncertainty, which includes the uncertainties of the calibration, the combustion energy of the benzoic acid, the auxiliary materials and the compounds studied with a coverage factor k = 1.96 and confidence level of 0.95 for two tailed normal distribution. bThe uncertainty associated with values corresponds to the expanded uncertainty which includes the contributions of the uncertainty of the heat capacity of the solid or liquid phase with a confidence level of 0.95 for two tailed normal distribution. cUncertainty calculated by applying the root of the sum of squares method. dThe number enclosed in parentheses is the difference between the experimental and theoretical values in kJ·mol−1. G


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Figure 5. Enthalpic contribution of the amine group at positions 2 and 4 of DPDS, and isomerization enthalpy of the compounds 2ADPDS and 4ADPDS.

Table 8. NBO Analysis with E(2) in kJ/mol (All Other Values in au) DPDS

2ADPDS

4ADPDS

NBO parameters

S1

S2

N1−H10···S1

N2−H12···S2

N1−H10

N2−H12

q(H) q(S) δ(nS) δ(σ*N−H) E(2)

0.15 1.988 -

0.15 1.988 -

0.40 0.10 1.952 0.012 12.6

0.40 0.010 1.952 0.012 12.6

0.38 0.11 1.98 0.006 -

0.38 0.11 1.98 0.006 -

in Table 9. The charge densities and their Laplacians at the BCPs of the N−H···S hydrogen bonds are 0.0159 and 0.0522 Table 9. Values of Electron Density and Laplacian of Electron Density in Bond Critical Point in au

Figure 6. NBO analysis shows the overlap of the n(S) orbitals of the sulfur atoms and σ*N−H antibonding orbital the amino groups in 2ADPDS.

2ADPDS

ρ (BCP)

∇2ρ(r) (BCP)

N1−H10···S1 N2−H12···S2

0.0159 0.0159

0.0522 0.0522

au, respectively. These values of the electron density and its Laplacian are well within the range specified for the existence of the hydrogen bond in terms of electron density (0.002−0.040 au) and its Laplacian (0.024−0.139) au.

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CONCLUSIONS

According to the thermochemical data determined by combustion calorimetry, DSC and thermogravimetry, it was found that the compound 2ADPDS presents greater enthalpic stability compared to 4ADPDS due to the formation of an intramolecular hydrogen bond between the hydrogen of the amino group and a sulfur of the Disulfide bond. This is supported by a topological analysis of the electronic density and an NBO study. The substitution of the amino group at the position 2 of the diphenyl disulfide is exothermic and has an energy difference of (−17.7 ± 5.3) kJ·mol−1. The substitution of the same group at the position 4 has an associated energy difference of (−3.2 ± 6.0) kJ·mol−1. The isomerization process has an enthalpic increase of (14.5 ± 6.7) kJ·mol−1. The differences between the experimental and calculated formation enthalpies in gas-phase fall under the uncertainty of the experimental results.

Figure 7. Molecular graph of 2ADPDS. Bond critical points (BCPs) are denoted by blue dots; yellow dots denote the ring critical points (RCPs). The black boxes enclose the bond paths associated with hydrogen bonds.

H


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Healing Elastomers Based on Aromatic Disulfide Metathesis. Mater. Horiz. 2014, 1, 237−240. (8) Shinkai, H.; Maeda, K.; Yamasaki, T.; Okamoto, H.; Uchida, I. Bis(2-(Acylamino)Phenyl) Disulfides, 2-(Acylamino)benzenethiols, and S-(2-(Acylamino)phenyl) Alkanethioates as Novel Inhibitors of Cholesteryl Ester Transfer Protein. J. Med. Chem. 2000, 43, 3566− 3572. (9) Mackle, H.; Mayrick, R. G. Studies in the Thermochemistry of Organic Sulphides. Trans. Faraday Soc. 1962, 58, 238−243. (10) Plato, C.; Glasgow, S. R. Differential Scanning Calorimetry as a General Method for Determining the Purity and Heat of Fusion of High-Purity Organic Chemicals. Application to 95 Compounds. Anal. Chem. 1969, 41, 330−336. (11) Brown, M. E. Determination of Purity by Differential Scanning Calorimetry (DSC). J. Chem. Educ. 1979, 56, 310−313. (12) Sabbah, R.; An, X.-w.; Chickos, J. S.; Planas Leitão, M. L.; Roux, M. V.; Torres, L. A. Reference Materials for Calorimetry and Differential Thermal Analysis. Thermochim. Acta 1999, 331, 93−204. (13) Lee, J. D.; Bryant, W. R. The Crystal Structure of Diphenyl Disulfide. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 2094−2101. (14) Gomes de Mesquita, A. H. The Crystal Structure of 2, 2′Diaminodiphenyl Disulfide. Acta Crystallogr. 1967, 23, 671−672. (15) Vittal, J. J.; Anjali, K. S. Sinusoidal Ribbon Waves in the Crystal Structure of Bis (4-aminophenyl) Disulfide. Cryst. Eng. 1998, 1, 147− 152. (16) Meija, J.; Coplen, T. B.; Berglund, M.; Brand, W. A.; De Biévre, P.; Gröning, M.; Holden, N. E.; Irrgeher, J.; Loss, R. D.; Walczyk, T.; Prohaska, T. Atomic Weights of the Elements 2013 (IUPAC Technical Report). Pure Appl. Chem. 2016, 88, 265−291. (17) Flores, H.; Mentado, J.; Amador, P.; Torres, L. A.; Campos, M.; Rojas, A. Redesigning the Rotating-Bomb Combustion Calorimeter. J. Chem. Thermodyn. 2006, 38, 756−759. (18) Ramos, F.; Flores, H.; Rojas, A.; Hernández-Pérez, J. M.; Camarillo, E. A.; Amador, M. P. Experimental and Computational Thermochemical Study of Benzofuran, Benzothiophene and Indole Derivatives. J. Chem. Thermodyn. 2016, 97, 297−306. (19) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry; Rossini, F. D. Ed.; Interscience: New York, 1956; Vol. 1, Chapter 5. (20) Pieterse, N.; Focke, W. W. Diffusion-Controlled Evaporation Through a Stagnant Gas: Estimating Low Vapour Pressures from Thermogravimetric Data. Thermochim. Acta 2003, 406, 191−198. (21) Ramos, F.; Ledo, J. M.; Flores, H.; Camarillo, E. A.; Carvente, J.; Amador, M. P. Evaluation of Sublimation Enthalpy by Thermogravimetry: Analysis of the Diffusion Effects in the Case of Methyl and Phenyl Substituted Hydantoins. Thermochim. Acta 2017, 655, 181− 193. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2009. (23) Lee, J. D.; Bryant, M. W. R. An Accurate Crystal Structure Determination of 2, 2′-Diaminodiphenyl Disulfide. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1970, 26, 1729−1735. (24) Foster, P.; Weinhold, F. Natural Hybrid Orbitals. J. Am. Chem. Soc. 1980, 102, 7211−7218. (25) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural-Population Analysis. J. Chem. Phys. 1985, 83, 735−746. (26) Reed, A. E.; Weinhold, F. Natural Localized Molecular Orbitals. J. Chem. Phys. 1985, 83, 1736−1740. (27) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor−Acceptor Viewpoint. Chem. Rev. 1988, 88, 899−926. (28) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1; University of Wisconsin: Madison, WI, 2001 (29) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Clarendon Press: Oxford, U.K., 1990.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b08838. Data associated with differential scanning calorimetry experiments (Tables S1 and S2), heat capacity values obtained by computational calculations (Table S2), details of the combustion experiments (Tables S3−S5) and the thermogravimetric data (Tables S6−S8), and 1H NMR and 13C NMR spectra (Figure S2−S7) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: henoc.fl[email protected]; Phone: (+52) 222 2295500, Ext. 7532 or 7533. ORCID

Henoc Flores: 0000-0003-3661-9684 E. Adriana Camarillo: 0000-0001-8922-2344 Present Address ‡

Centro de Investigación en Materiales Avanzados, S. C. Alianza Norte 202, PIIT, Carretera Monterrey-Aeropuerto Km. 10, C.P. 66628 Apodaca, Nuevo León, México.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank VIEP-BUAP for financial support through project FLSH-NAT17-G and CONACYT for financial aid to purchase equipment (INFRA 2015, 0254854). F.R. thanks CONACYT for his grants (Registration Numbers 268314). J.S.-L. thanks CONACyT and SNER for a postdoctoral fellowship. H.F. and J.M.H.P. acknowledge the computer resources, technical expertise and support provided by the Laboratorio Nacional de Supercómputo del Sureste de México (Grant Number O-2016/026). The authors thank J. M. SolanoAltamirano for his revision and useful suggestions to the manuscript.

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REFERENCES

(1) Khoo, K. K.; Norton, R. S. Role of Disulfide Bonds in Peptide and Protein Conformation, in Amino Acids, Peptides and Proteins in Organic Chemistry: Analysis and Function of Amino Acids and Peptides; Hughes, A. B., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; Vol. 5. (2) Rajan, V. V.; Dierkes, W. K.; Joseph, R.; Noordermeer, J. W. M. Science and Technology of Rubber Reclamation with Special Attention to NR-based Waste Latex Products. Prog. Polym. Sci. 2006, 31, 811−834. (3) Le Beau, D. S. Science and Technology of Reclaimed Rubber. Rubber Chem. Technol. 1967, 40, 217−37. (4) Tawfic, M. L.; Younan, A. F. Feasible and Economic Process for Using Organic Disulfide and Organic Mercaptane as a Reclaiming Agent for Ground Tire Powder (GTP). J. Elastomers Plast. 2014, 46, 19−32. (5) Clark, P. D.; Oriakhi, C. O. Studies on the Stability of Organic Disulfide-Polysulfide Sulfur Solvents Used in Sour Gas Production. Energy Fuels 1992, 6, 474−477. (6) Pawelec, W.; Holappa, A.; Tirri, T.; Aubert, M.; Hoppe, H.; Pfaendner, R.; Wilen, C. E. Disulfides e Effective radical Generators for Flame Retardancy of Polypropylene. Polym. Degrad. Stab. 2014, 110, 447−456. (7) Rekondo, A.; Martin, R.; Ruiz de Luzuriaga, A.; Cabañero, G.; Grande, H. J.; Odriozola, I. Catalyst-Free Room-Temperature SelfI


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