Comprehensive Thermochemical Study of Cyclic Five- and Six

Article pubs.acs.org/jced

Comprehensive Thermochemical Study of Cyclic Five- and SixMembered N,N′‑Thioureas Ana L. R. Silva and Maria D. M. C. Ribeiro da Silva* Centro de Investigaçaõ em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal S Supporting Information *

ABSTRACT: An experimental and computational study of the thermochemical and structural properties of ethylenethiourea (ETU) has been carried out. The enthalpies of combustion and sublimation, measured respectively by rotating-bomb combustion calorimetry and Calvet microcalorimetry, yielded the gas-phase enthalpy of formation of ETU at T = 298.15 K. This latter parameter was also derived from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, leading to a value in excellent agreement with the one obtained from experimental data. With the purpose of evaluating the influence of the ring size in the enthalpy of formation of cyclic N,N′thiourea derivatives, the calculation of the enthalpy of formation of N,N′-trimethylenethiourea (MTU) was performed using the G3(MP2)//B3LYP approach. The effects of substituents (carbonyl and thiocarbonyl) on the molecular stability of several N-alkyl (cyclic) ureas/thioureas were also studied.

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inherent metal−biological species interactions.7 As an extension of our previous studies on the structural and energy properties of cyclic ureas, specifically ethyleneurea and N,N′-trimethyleneurea,8 parabanic acid,9 barbital,10 hydantoin, and 2thiohydantoin11 and other heterocycles with a benzene ring fused to a five-membered ring containing two nitrogen heteroatoms, namely, 2-mercaptoimidazole12 and 2-mercaptobenzimidazole,13 this article reports an experimental and computational thermochemical study of ETU. The standard (p° = 0.1 MPa) molar energy of combustion in the condensed phase at T = 298.15 K for the compound studied was measured by rotating bomb combustion calorimetry. The standard molar enthalpy of sublimation at T = 298.15 K was obtained by the vacuum sublimation drop microcalorimetric technique using a high-temperature Calvet microcalorimeter. These experimental results enabled the derivation of the standard molar enthalpy of formation of ETU in the gaseous state at T = 298.15 K. The gas-phase enthalpy of formation for ETU was also calculated from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory. In addition, we have also calculated the gas-phase enthalpy of formation of N,N′trimethylenethiourea (MTU) from the same high-level molecular orbital calculations to develop a possible relation between the ring size and the enthalpy of formation. The main purpose of these studies is to provide reliable data for these key

INTRODUCTION Ethylenebisdithiocarbamate and propylenebisdithiocarbamate have been two of the species most widely used as fungicides in agriculture.1 These fungicides are considered to have low toxicity, but their derivatives ethylenethiourea (imidazolidine-2thione, ETU) and other degradation products are a toxicologically serious problem.2−4 The teratogenicity and molecular structure of compounds related to ETU have been assessed,5 via modifications of the ring size and the substituents, in order to evaluate the structural effects on the characteristics of the compounds. The results indicated a teratogenic activity and molecular structure correlation that was closely linked to the imidazolidinethione structure, and although there is interest in the energetic vs structural effects for the prediction of the chemical behavior of ETU derivatives, such studies are scarce but are important to carry out. Ethylenethiourea (ETU) and N,N′-trimethylenethiourea (MTU), whose structures contain a thioamide group (Figure 1), are well known as ligands for the synthesis of coordination compounds,6 appearing in the literature as extensively studied

Special Issue: Memorial Issue in Honor of Ken Marsh Figure 1. Structural formula of ethylenethiourea (ETU) and N,N′trimethylenethiourea (MTU) studied experimentally and computationally, respectively. © XXXX American Chemical Society

Received: January 25, 2017 Accepted: March 30, 2017

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DOI: 10.1021/acs.jced.7b00083 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Standard (p° = 0.1 MPa) Massic Energy of Combustion, Molar Energy of Combustion, Enthalpy of Combustion, and Enthalpy of Formation for ETU in the Crystalline Phase at T = 298.15 K Δcu°(cr)/(J·g−1)

ΔcUm ° (cr)/(kJ·mol−1)

ΔcHm ° (cr)/(kJ·mol−1)

ΔfHm ° (cr)/(kJ·mol−1)

−25 437.0 ± 6.0

−2598.6 ± 1.4

−2603.6 ± 1.4

−36.4 ± 1.5

sublimation drop method described by Skinner.18 The details of the apparatus and the technique were previously reported.19 Samples of ∼5 mg were dropped simultaneously with the corresponding blank tube at room temperature (T ≈ 298.15 K) into the reaction vessel of the microcalorimeter. After the tubes reached thermal equilibrium, the sample was removed from the hot zone by sublimation under reduced pressure. The studies were done at T = 447.57 K. Six analogous experiments with anthracene led to a calorimeter constant of k = (1.0243 ± 0.0056). Computational Details. The G3(MP2)//B3LYP20 composite method was used throughout this work. This is a variation of the G3(MP2) theory21 that uses the B3LYP density functional method22 for geometry optimization and zero-point energies. The B3LYP functional uses a combination of the hybrid three-parameter Becke’s functional, first proposed by Becke,22 and the Lee−Yang−Parr nonlocal correlation functional.23 The 6-31G(d) basis set is adopted for both the optimization of geometry and the calculation of the harmonic vibrational frequencies. The introduction of a series of highorder corrections to the initial QCISD(T)/6-31G(d) singlepoint calculation is accomplished in a manner that closely follows the Gaussian-3 philosophy but using second-order Møller−Plesset perturbation corrections instead of the much more expensive MP4 calculation used in the original G3 method.24

compounds in order to be used both for the estimation of the thermochemical data of related compounds not studied experimentally and also to understand the effects of the size of the carbon chain in the ring as well as the thiocarbonyl and carbonyl substituents on the stability of several N-alkyl (cyclic) ureas/thioureas.

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MATERIALS AND METHODS Materials. Ethylenethiourea, ETU [96-45-7], supplied by TCI Europe (>98.0%), was purified by sublimation under reduced pressure (T ≈ 430 K). The purity of the sublimed sample (0.9999, mass fraction) was checked by gas chromatography analysis. This analysis was carried out on an Agilent 4890D gas chromatography−flame ionization detector (GC−FID) apparatus equipped with an HP-5 column (crosslinked 5% diphenyl and 95% dimethylpolysiloxane; 15 m × 0.530 mm i.d. with a 1.5 μm film thickness); nitrogen was used as the carrier gas. The combustion calorimetric system was calibrated with benzoic acid, a NIST Standard Reference Material (SRM 39j), while the Calvet microcalorimeter was calibrated with anthracene (Aldrich Chemical Co., zone-refined mass fraction >0.99). Rotating-Bomb Combustion Calorimetry. The standard (p° = 0.1 MPa) massic energy of combustion of ETU was measured in an isoperibol rotating-bomb calorimeter, originally developed at Lund University14 and installed in our department, where some improvements have been made. The apparatus and procedure were first described,15 with only the most relevant aspects being reported herein. The calorimetric system was equipped with a platinum -lined stainless steel twin valve bomb of 0.258 dm3 internal volume. Calorimeter temperatures were measured to ±0.1 mK at time intervals of 10 s using a quartz thermometer (HP-2804A). The final temperature was T ≈ 298.15 K, and the bomb rotation was started when the temperature rise in the main period reached about 0.63 of its total value.16 The energy equivalent of the calorimeter was determined using benzoic acid with a certified massic energy of combustion, under bomb conditions, of −(26 434 ± 3) J·g−1. Calibration experiments were made in oxygen at 3.04 MPa, with 1.00 cm3 of water added to the bomb without bomb rotation. The energy equivalent of the calorimeter was εcal = (25 161.3 ± 1.5) J·K−1, where the assigned uncertainty is the standard error of the mean of six independent experiments. The εcal value refers to the average mass of water added to the calorimeter of 5222.5 g. The solid samples of ETU were burnt in pellet form and enclosed in Melinex bags under an oxygen atmosphere at 3.04 MPa in the presence of 15.00 cm3 of deionized water to ensure the formation of H2SO4·115H2O(l). The standard massic energy of combustion, Δcu°, of ETU was calculated by a procedure similar to that developed by Good et al.17 Further details about the experimental procedure are provided in the Supporting Information. Calvet-Drop Microcalorimetry. The enthalpy of sublimation of ETU was measured on a high-temperature Calvet microcalorimeter (Setaram HT 1000), using the vacuum

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RESULTS AND DISCUSSION The standard atomic weight recommended by the IUPAC Commission in 2011 was used in the calculation of all molar thermodynamic quantities.25 Crystalline-Phase Enthalpy of Formation of ETU. Results of the combustion experiments with ETU and the individual values of the massic energy of combustion, Δcu°, with the mean value and the corresponding standard deviation of the mean for ETU are given in the Supporting Information (Table S1). The internal energy for the isothermal bomb process, ΔU(IBP), was calculated according to eq 1. ΔU (IBP) = {− εcal + Cp(H 2O, l) Δm(H 2O)}ΔTad + (Ti − 298.15K)εi + (298.15 − Ti − ΔTad)εf + ΔU (ign) (1)

The standard massic energy of combustion, Δcu°, for ETU refers to combustion reaction 2. Table 1 lists the massic energy of combustion, Δcu° (cr), the derived standard molar energy, ΔcU°m (cr), and the standard molar enthalpies of combustion, ΔcHm ° (cr), and formation in the crystalline phase, ΔfHm ° (cr), for ETU at T = 298.15 K. The uncertainty associated with the standard molar enthalpy of combustion is twice the overall standard deviation of the mean and includes the uncertainties from calibration and from the combustion energies of the auxiliary materials.26,27 To derive ΔfHm ° (cr) from ΔcHm ° (cr), the standard molar enthalpies of formation, at T = 298.15 K, were used for H2O(l),28 −(285.830 ± 0.040) kJ·mol−1, CO2(g),28 −(393.51 ± 0.13) kJ·mol−1, and H2SO4 in 115 H2O(l),29 −(887.81 ± 0.42) kJ·mol−1. B



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Figure 2. Front (a) and side (b) views of the most stable conformation found for ETU (1) and MTU (2) obtained at B3LYP/6-31G(d). Selected bond distances are in nanometers, and angles are in degrees.

Molecular Structure of ETU and MTU. Figure 2 represents the front and side views of the B3LYP/6-31G(d)optimized most stable structures of ETU and MTU, where the relevant bond lengths and the bond angles are also included. The most stable molecular structure for ETU, obtained at the B3LYP/6-31G(d) level, is nonplanar, as could be expected from systems having mainly sp3-hybridized atoms. Dihedral angles N1−C2−N3−C4 and N1−C5−C4−N3 are 10.3 and 28.1°, respectively. However, the atoms belonging to the thioamide group were found to be nearly in the same plane. This fact allows the occurrence of electronic delocalization between the nitrogen atoms of the ring and the thiocarbonyl group, providing energy stabilization. In the case of the MTUoptimized structure, the thioamide group is planar but the C5 atom lies out of the plane so that the six-membered ring approximates the envelope form, with a dihedral angle C2−N3− C4−C5 of −30.7°. The crystal structure of ETU was reported by Wheatley30 by X-ray diffraction. There is no conclusive evidence for the establishment of hydrogen bonds, through the hydrogen atoms linked to N1 or N3, or to the sulfur of the neighboring molecules, despite the closeness of these atoms. This molecule is expected to be planar in isolation but is distorted slightly out of plane by the crystal forces. We could not find experimental

C3H6N2S(cr) + 6O2 (g) + 113H 2O(l) → 3CO2 (g) + N2(g) + H 2SO4 ·115H 2O(l)

(2)

Enthalpy of Sublimation of ETU. The enthalpy of sublimation measurements on ETU by Calvet microcalorimetry led to Δg,T ° = 123.6 ± 0.5 kJ·mol−1, where T = 447.57 cr,298.15KHm K. The indicated uncertainty is twice the overall standard error of the mean of six determinations and includes the uncertainty from the calibration. The correction of Δg,T ° to T = cr,298.15KHm 298.15 K gave ΔsubHm ° = 107.4 ± 1.4 kJ·mol−1. The correction, T Δ298.15k Hm° = 16.2 kJ·mol−1, was performed through the equation ΔT298.15K Hm° (g) =

T

∫298.15K C°p,m(g) dT

(3)

where Cp,m ° (g) is the molar heat capacity of gaseous ETU derived from statistical thermodynamics using the vibrational frequencies calculated at the B3LYP/6-31G(d) level and scaled by a factor of 0.9614. The theoretically computed C°p,m(g) values in the range of 250−650 K (Table S2 in the Supporting Information) are represented by eq 4. C °p,m(g)/(J·K−1·mol−1) = − 2.233 × 10−7(T /K)3 + 1.629 × 10−4(T /K )2 + 2.175 × 10−1(T /K ) + 1.630 × 101

(4) C



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Table 2. Working Reactions for ETU and Corresponding Values for the Enthalpies of Formation in the Gaseous Phase at T = 298.15 K

(Table S3). In this table, we also present the corresponding experimental standard molar enthalpies of formation in the gaseous phase at T = 298.15 K. The results obtained were also analyzed in terms of enthalpic increments. The presence of a carbonyl group at position 4 of the ethyleneurea and ethylenethiourea is illustrated in the scheme of Figure 3. From that scheme, one can clearly see that the introduction of a −CO group in the C4 of the two compounds referred to above causes the same stabilizing effects (∼156 kJ·mol−1) within the experimental uncertainties, showing that the presence of a carbonyl/thiocarbonyl group in the C2 of the ring does not induce additional enthalpic effects. On the other hand, the difference in the enthalpies of formation of a five-membered-ring compound (ethyleneurea or ethylenethiourea) and the corresponding six-membered-ring compound, presenting an additional CH2 group, is thus about 26 kJ·mol−1 (Figure 3). This observation is in accordance with that previously reported in the literature by Roux et al.,31 where the enthalpies of formation of the five-membered rings, 1,3oxazolidine-2-thione and 1,3-thiazolidine-2-thione, decrease on changing to the corresponding six-membered rings of current interest, tetrahydro-2H-1,3-oxazine-2-thione and tetrahydro2H-1,3-thiazine-2-thione, by (29.5 ± 7.2) kJ·mol−1 and 24.7 kJ·mol−1, respectively. Also, the transformation associated with the presence of a sulfur instead of an oxygen atom in the fivemembered-ring species presented in Figure 3 is considerably endothermic (ca. 246 kJ·mol−1). Continuing our interest in the establishment of energy− structural correlations for heterocyclic compounds with one or

data in the available literature about the crystal structure of MTU. Gas-Phase Enthalpy of Formation of ETU and MTU. From the ΔfHm° (ETU,cr) and ΔsubHm° values previously reported, it is possible to derive ΔfHm ° (ETU, g) = 71.0 ± 2.1 kJ·mol−1. The enthalpy of formation of ETU and MTU was estimated from the calculated G3(MP2)//B3LYP absolute gas-phase enthalpies by using the gas-phase working reactions (5−18) shown in Tables 2 and 3. The computationally calculated enthalpies of reaction were combined with the experimental standard molar enthalpies of formation of all of the intervening atoms/molecules in order to obtain estimates of the gas-phase standard molar enthalpy of formation at T = 298.15 K for ETU and MTU. The computational results for the standard gasphase enthalpy of formation of ETU, reported in Table 2, are between 65.2 and 78.3 kJ·mol−1, where the mean value of the estimated gas-phase enthalpy of formation, 71.4 kJ·mol−1, is in excellent agreement with the experimental ΔfHm ° (ETU, g) = (71.0 ± 2.1) kJ·mol−1 obtained in this work. The calculated values for MTU span the interval from 38.5 to 47.1 kJ·mol−1, resulting in an average gas-phase enthalpy of formation of 42.4 kJ·mol−1. The calculated data obtained for ETU are supported by its experimental result, giving us confidence in the preference of using the G3(MP2)//B3LYP approach to estimate the ΔfH°m(g) for this class of compounds. The calculated G3(MP2)//B3LYP absolute enthalpies at T = 298.15 K for ETU, MTU, and all the auxiliary molecules used in this study are collected in the Supporting Information D



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Table 3. Working Reactions for MTU and Corresponding Values for the Enthalpies of Formation in the Gaseous Phase at T = 298.15 K

Figure 3. Enthalpic increments associated with the presence of carbonyl, thiocarbonyl, and methylene groups on a series of cyclic ureas/thioureas (values in kJ·mol−1). aReference 8. bReference 11.cValue estimated at the G3(MP2)//B3LYP level of theory (Table 3). dThis work.

two benzene rings fused to a five- or six-membered-ring containing oxygen or sulfur heteroatoms,32 in this work we focused our attention on a set of six pairs of urea/thiourea derivatives (Ur−X, X = O, S) with five- or six-membered rings containing two nitrogen heteroatoms at positions (1, 3). Thus, to understand and predict the thermochemical properties of

cyclic urea/thiourea derivatives, their corresponding gas-phase enthalpies of formation were compiled in Table 4. A plot correlating the experimental enthalpies of formation for this set of heterocyclic compounds listed in Table 4 is presented in Figure 4, showing an excellent linear correlation among the gaseous standard molar enthalpies of formation (expressed in E



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Table 4. Experimental Enthalpy of Formation (in kJ·mol−1) in the Gaseous State at T = 298.15 K for Cyclic N,N′-Disubstituted Urea/Thiourea Derivatives

Δf Hm◦ (Ur−X, X = S, g) = (0.955 ± 0.009)·Δf Hm◦ (Ur−X, X = O, g) + (237 ± 3) (19)

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CONCLUSIONS The gas-phase enthalpy of formation of ETU, ΔfHm ° (g) = (71.0 ± 2.1) kJ·mol−1, at T = 298.15 K has been derived from experimental data of the enthalpies of formation of the crystalline compound and of sublimation, as determined by static bomb combustion calorimetry and Calvet microcalorimetry, respectively. The result obtained is in excellent agreement with that derived from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, 71.2 kJ·mol−1. This fact gives us confidence to use an identical methodology to estimate the values of the corresponding property for other related compounds. The study also involves the computational determination of the gas-phase enthalpy of formation of N,N′trimethylenethiourea (MTU), ΔfHm ° (g) = 42.1 kJ·mol−1. The results obtained are discussed by comparison with others reported for cyclic ureas, namely, hydantoin, 2-thiohydantoin, ethyleneurea, and N,N′-trimethyleneurea. In particular, the effects of the size of the carbon chain in the ring as well as that of the thiocarbonyl and the carbonyl substituents on the stability of several N-alkyl (cyclic) ureas/thioureas were evaluated, and a new energy−structural correlation for cyclic urea/thiourea derivatives (Ur−X, X = O, S) is presented.

Figure 4. Plot of the experimental enthalpies of formation for the set of urea/thiourea derivatives listed in Table 4 (values in kJ·mol−1). (a) XO, S for ethyleneurea or ethylenethiourea. (b) XO, S for hydantoin or 2-thiohydantoin. (c) XO, S for barbituric acid or 2thiobarbituric acid. (d) XO, S for 1,3-diethylbarbituric acid or 1,3diethyl-2-thiobarbituric acid. (e) XO, S for uracil or 2-thiouracil. (f) XO, S for 2-benzimidazolinone or 2-benzimidazolinethione.

kJ·mol−1) of urea/thiourea derivatives, yielding eq 19, with r2 = 0.9997. Moreover, we used this relationship (eq 19) to estimate the ΔfHm ° (g) of MTU, considering the same data for the corresponding cyclic urea,8 ΔfHm ° (N,N′-trimethyleneurea, g) = (−201.1 ± 1.3) kJ·mol−1, yielding the value of 44.9 kJ·mol−1. This value is in agreement with the one calculated (G3(MP2)//B3LYP method) in the context of this work, ΔfHm ° (MTU, g) = 42.4 kJ·mol−1. Our experimental results and the calculations performed at the G3(MP2)//B3LYP level of theory emphasize the validation of this predictive scheme in order to obtain reliable values for the enthalpies of formation of other cyclic urea/thiourea derivatives (Ur−X, X = O, S).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00083. Data for all of the combustion calorimetry experiments for ETU as well as additional information about the experimental procedure, values of the standard molar F



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heat capacity in the gaseous phase for ETU and MTU, and calculated energies at the G3(MP2)//B3LYP level of theory (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +351 220 402 659. Phone: + 351 220 402 538. ORCID

Maria D. M. C. Ribeiro da Silva: 0000-0003-0482-0308 Funding

This work was supported by the Foundation for Science and Technology (FCT) of Portugal, project UID/QUI/UI0081/ 2013, and FEDER, projects POCI-01-0145-FEDER-006980 and NORTE-01-0145-FEDER-000028. A.L.R.S. thanks FCT for the award of a Ph.D. grant (SFRH/BD/69606/2010). Notes

The authors declare no competing financial interest.

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