Experimental and Theoretical Study of the Structures and Enthalpies

6336

J. Phys. Chem. A 2010, 114, 6336–6341

Experimental and Theoretical Study of the Structures and Enthalpies of Formation of 3H-1,3-Benzoxazole-2-thione, 3H-1,3-Benzothiazole-2-thione, and Their Tautomers Maria Victoria Roux,* Manuel Temprado,† Pilar Jime´nez, Concepcio´n Foces-Foces, and Rafael Notario Instituto de Quı´mica Fı´sica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain

Archana R. Parameswar, Alexei V. Demchenko, and James S. Chickos Department of Chemistry and Biochemistry, UniVersity of Missouri-St. Louis, One UniVersity BouleVard, St. Louis, Missouri 63121-4499

Carol A. Deakyne Department of Chemistry, UniVersity of Missouri-Columbia, 601 S. College AVenue, Columbia, Missouri 65211-7600

Joel F. Liebman Department of Chemistry and Biochemistry, UniVersity of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250-1000 ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: April 14, 2010

This paper reports an experimental and theoretical study of the structures and standard (po ) 0.1 MPa) molar enthalpies of formation of 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione. The enthalpies of combustion and sublimation were measured by rotary bomb combustion calorimetry and the Knudsen effusion technique, and gas-phase enthalpies of formation values at T ) 298.15 K of (42.0 ( 2.7) and (205.5 ( 3.8) kJ · mol-1 for 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione, respectively, were determined. G3-calculated enthalpies of formation are in excellent agreement with the experimental values. The present work discusses the question of tautomerism explicitly for both compounds and compares the energetics of all the related species. A comparison of the theoretical results with the structural data is also reported. Introduction As briefly reviewed by Mentado et al.1 in their recent calorimetric study, 2-mercaptobenzazoles have been of considerable interest and importance to the medicinal and industrial communities. Admittedly, there is the possibility that these species exist as the tautomers 1,3-benzoxa/thiazolidine-2-thione but this was ignored in their study. Their monocyclic saturated counterparts, 1,3-oxazolidine-2-thione and 1,3-thiazolidine-2thione, are also of interest and importance as noted in a recent calorimetric study of these species.2,3 In this study, the energetics of the tautomeric azoline-2-thiols were ignored. The current study offers new measurements for the enthalpy of formation of the benzoxa/thiazole derivatives, discusses the question of tautomerism explicitly for both classes of species, and compares the energetics of all of the aforementioned species. A comparison of the theoretical results with the structural data is also reported. Experimental Procedures Materials and Purity Control. 3H-1,3-Benzoxazole-2-thione [CAS: 2382-96-9] (1) and 3H-1,3-benzothiazole-2-thione * To whom correspondence should be addressed. E-mail: victoriaroux@ iqfr.csic.es. † Current address: Departamento de Quı´mica Fı´sica, Universidad de Alcala´, 28871 Alcala´ de Henares (Madrid), Spain.

[CAS: 149-30-4] (2) commercially available from Acros (supplied under the names of the corresponding thiol tautomers: 2-mercaptobenzoxazole and 2-mercaptobenzothiazole) were additionally purified by crystallization using acetone or dichloromethane as the solvents, respectively. The samples were carefully dried under vacuum at T ) 323 K. Determination of purities, assessed by GC (Trace GC Ultra from Thermo Electron Co.) and by DSC (Pyris 1 from Perkin-Elmer) using the fractional fusion technique4 indicated that the mole fraction of impurities in the compounds was less than 0.004 and 0.001 for 1 and 2, respectively. No solid-solid phase transitions were observed over the temperature interval from T ) 268.15 K to the corresponding melting points Tfus(1) ) 470.0 K and Tfus(2) ) 455.9 K.3 Thermochemical Measurements. The enthalpies of formation in the crystalline state were determined by combustion calorimetry using an isoperibol combustion calorimeter equipped with a rotary bomb. Details of the technique and procedure used have been previously described.5,6 The energies of combustion

10.1021/jp102126j  2010 American Chemical Society Published on Web 05/07/2010

Enthalpy of Formation of Mercaptobenzazole Derivatives

J. Phys. Chem. A, Vol. 114, No. 21, 2010 6337

of 1 and 2 were determined by burning the solid samples in pellet form. The pelleted compounds were enclosed in polyethylene bags. Vaseline was used as auxiliary material to limit the amount of sulfur in the samples to only 8 mmol following the recommendation of Sunner and Lundin.7 This insures complete oxidation of the atoms of sulfur to hexavalent sulfur and also avoids high concentrations of sulfuric acid in the final state. The bomb was filled with oxygen to a pressure of p ) 3.04 MPa. Following combustion, the calorimeter was disassembled and the gases slowly released. The absence of CO and SO2 was confirmed using Dra¨ger tubes (sensitivity levels were approximately 1 × 10-6 and 1 × 10-7 mass fractions, respectively). Analysis of the liquid phase in the bomb was made as described in ref 5. The absence of SO32- and NO2- was confirmed by calibrated ionic chromatography (Perkin-Elmer Optima 4300 DV). The quantity of nitric acid was taken as the difference between the total acid and the theoretical quantity of sulfuric acid calculated from the mass of sample. The energy of the combustion experiments was always referenced to the final temperature of Tth ) 298.15 K. From the combustion energies, the enthalpies of formation in the condensed state were calculated. Additional details are provided in the Supporting Information. The enthalpies of sublimation were evaluated from the temperature dependence of the vapor pressures (ClausiusClapeyron equation). Vapor pressures of 1 and 2 evaluated over a 15 K temperature interval were measured by a mass-loss Knudsen-effusion method8 using the technique described in ref 9. The apparatus consists, essentially, of a stainless steel sublimation chamber immersed in a thermo-regulated water jacket and connected to a high-vacuum system (p ) 1 × 10-4 Pa). The Knudsen cell was weighed with a Mettler AT-21 microbalance, reproducible to within (0.000 005 g, before and after each effusion time period, t, to determine the mass, m, of sublimed material. The vapor pressure, p, for each temperature, T, was calculated from eq 1.

p ) (∆m/Waat)(2πRT/M)1/2

(1)

where R is the gas constant, a is the area of the effusion orifice, Wa is the corresponding Claussing coefficient,10 and M is the molar mass of the compounds studied. The atomic weights of the elements used are those recommended by IUPAC in 2005.11 X-ray Diffraction Studies. A search of the Cambridge Crystallographic Database12 reveals that several crystal structure determinations of the 3H-1,3-benzoxazole/benzothiazole-2thione compounds have been reported (CSD refcodes: BZOXZT,13 BZOXZT01,14 and SBTHAZ,15 SBTHAZ02,16 SBTHAZ11,17 SBTHAZ12,18 SBTHAZ13, 19 respectively). For compound 1, analysis of the unit cell dimension, space group, and atomic coordinates indicate that no phase transition has occurred between room temperature (BZOXZT) and T ) 165 K (BZOXZT01). For compound 2, all five determinations were carried out at room temperature, and all of them correspond to the same polymorph after transforming the P21/c space group into the P21/n equivalent one. Furthermore, the simulated X-ray powder diffraction patterns are included in the Supporting Information. Computational Details. Standard ab initio molecular orbital calculations20 were performed with the Gaussian 03 series of programs.21 The energy of each of the compounds studied was calculated using Gaussian-n theory, at the G3 level.22

TABLE 1: Results of Typical Combustion Experiments of 1 and 2a compound

1

2

b

0.65297 0.09424 0.20664 0.00225 1.0825 -31.2437 -0.0541 0.0005 0.0574 0.0007 0.0305 4.3702 9.5236 0.0392 -26.4571

0.64321 0.08262 0.21253 0.00273 1.1073 -31.9916 -0.0553 0.0005 0.0582 -0.0035 0.0237 3.8314 9.7949 0.0475 -28.4423

m′(compound) (g) m′′(vaseline)b (g) m′′′(polyethylene)b (g) m′′′′(fuse)b (g) ∆Tc (K) ε(calor)c(-∆Tc) (kJ) ε(cont.)d(-∆Tc) (kJ) ∆Uigne (kJ) ∆Udec(HNO3)f (kJ) ∆Udiln(H2SO4)g (kJ) ∆U(corr. to std. states)h (kJ) -m′′∆cu0(vaseline) (kJ) -m′′′∆cu0(polyethylene) (kJ) -m′′′′∆cu0(fuse) (kJ) ∆cu0(compound) (kJ g-1)

Tth ) 298.15 K; Vbomb ) 0.260 dm3; pigas ) 3.04 MPa; miwater ) 10.00 g. b Masses obtained from apparent mass. c ε(calor), energy equivalent of the whole system but the content of the bomb. d ε(cont.), energy equivalent of the contents of the bomb ε(cont.)(-∆Tc) ) εi(cont.)(Ti - 298.15 K) + εi(cont.)(298.15 K Tf + ∆Tcorr.). e Experimental energy of ignition. f Experimental energy of decomposition of nitric acid. g Experimental energy of dilution of sulfuric acid. h ∆U(corr. to std. states) is the standard state correction. a

We have also reoptimized the geometries at the MP2(full)/ 6-31G(3df,2p) level to obtain more reliable molecular structures for the compounds studied. The charge distribution has been analyzed using a population partition technique, the natural bond orbital (NBO) analysis of Reed and Weinhold,23-25 using the NBO program26 implemented in the Gaussian 03 package.21 Results Experimental Determination of the Enthalpies of Formation in the Gas Phase. The enthalpies of formation in the gas 0 state, ∆fHm (g), were determined by combining the standard 0 (cr), enthalpies of formation of the crystalline 1 and 2, ∆fHm g 0 Hm, both with their standard enthalpies of sublimation, ∆cr referenced to T ) 298.15 K. Results of a typical combustion experiment on each compound are given in Table 1. The values of the standard massic energy of combustion, ∆cuo, refer to the combustion reactions 2 and 3 for 1 and 2, respectively.

C7H5ONS(cr) + 9.25 O2(g) + 113.5 H2O(l) ) 7 CO2(g) + [H2SO4 · 115 H2O](l) + 0.5 N2(g) (2) C7H5NS2(cr) + 11.25 O2(g) + 229.5 H2O(l) ) 7 CO2(g) + 2[H2SO4 · 115 H2O](l) + 0.5 N2(g) (3) The individual results for the standard massic energy of combustion, ∆cu0, obtained in all the combustion experiments, together with the mean value and its standard deviation, are given in Table 2. g 0 Hm, for both compounds The enthalpies of sublimation, ∆cr were obtained from the temperature dependence of the vapor pressures from the Clausius-Clapeyron equation (eq 4).

ln(p/Pa) ) -B(T (K))-1 + A

(4)

6338

J. Phys. Chem. A, Vol. 114, No. 21, 2010

Roux et al.

TABLE 2: Individual and Mean Values of the Standard Massic Energy of Combustion, ∆cu0, at 298.15 K

TABLE 4: Parameters A and B as well as Enthalpy and Entropy of Sublimation

∆cu0 (kJ g-1) 3H-1,3-Benzoxazole-2-thione

compound

Tm (K)

A

B

g 0 ∆cr Hm[Tm] (kJ · mol-1)

g 0 ∆cr Sm[Tm] (J · mol-1 K-1)

1 2

363.40 382.98

32.8 ( 0.6 33.1 ( 0.8

12682 ( 216 13072 ( 288

105.4 ( 1.8 108.7 ( 2.4

290.0 ( 5.0 283.8 ( 6.3

3H-1,3-Benzothiazole-2-thione

-26.4571 -26.4631 -26.4644 -26.4568 -26.4466

-28.4423 -28.4656 -28.4567 -28.4762 -28.4374 -28.4280 -28.4271 -28.4429 ( 0.0063

-26.4576 ( 0.0031

TABLE 5: Experimentally Determined Standard Molar Enthalpy of Combustion, Sublimation, and Formation in the Crystalline and Gaseous States at T ) 298.15 K for 3H-1,3-Benzoxazole-2-thione and 3H-1,3-Benzothiazole-2-thione experimental valuesa

where A is a constant and B ) ∆gcrH0m[Tm]/R. The standard molar enthalpy of sublimation at the mean temperature [Tm] of the experimental range was derived. The results of the Knudsen-effusion experiments for 1 and 2 calculated by means of eq 1 are summarized in Table 3. The largest error for the vapor pressure, p, is 5 × 10-3p, computed as the sum of the estimated errors of all quantities in eq 1. The mean experimental temperature, Tm, parameters A and B, and enthalpy and entropy of sublimation at the mean experimental temperature for 1 and 2 are given in Table 4. g 0 Hm[Tm] are The uncertainties assigned to the values of ∆cr based on the standard deviations of B values. The standard molar enthalpies of sublimation at T ) 298.15 K were calculated from eq 5.

0 ∆ cHm 0 ∆ fH m (cr) g 0 ∆cr Hm 0 ∆ fH m (g) a

m

2 -4767.6 ( 2.8 94.5 ( 2.9 111.0 ( 2.4 205.5 ( 3.8

All values in kJ mol-1.

standard molar enthalpies of formation of H2O(l) and CO2(g), at T ) 298.15 K, -(285.830 ( 0.042) kJ mol-1 and -(393.51 ( 0.13) kJ mol-1, respectively, were used.28 The value of the enthalpy of formation of [H2SO4 · 115H2O], -(887.811 ( 0.042) kJ mol-1, was taken from ref 29. X-ray Crystallography. The 3H-1,3-benzoxazole/benzothiazole-2-thione compounds exist in the solid state in the tautomeric thione form (Table 6). The CdS distances are in good agreement with values observed in the 3H-oxazolidine/thiazolidine-2-thione compounds2 and with the average values retrieved from the CSD for the X-CS-NH (X ) O, S) fragments. It is interesting to note that the thione form has also been observed in the 3H1,3-benzothiazol-2-one/1 molecular complex (QELYAI)30 and in the 5-amino-3H-1,3-benzothiazole-2-thione compound (VATKEH).31 The supramolecular aggregation (Figure 1) in 1 consists of hydrogen-bonded catemers wherein the adjacent molecules are bonded to one another through bifurcated N-H · · · (O, SdC) contacts. In 2, the molecules are associated into dimers through N-H · · · SdC bonds (Figure 1) as happens in the molecular complex 3H-1,3-benzothiazole-2-one/1 (QELYAI)30 between both components. Similar hydrogen-bonding motifs have been displayed by the corresponding ketone analogues: 3H-1,3-benzoxazole-2-one (BZOXZO)13 and 3H-1,3-benzothiazole-2-one (WUHPUK).32 For each network, the relevant differences between the thione and ketone derivatives concern the donor · · · acceptor distances and the disposition of the hydrogen to the acceptor as measured by the CdO/S · · · H angle, which are longer and lower in the thione analogues, resulting in weaker hydrogen bonds.33 In

0 0 ∆gcrHm [T ) 298.15 K] ) ∆gcrHm [Tm] +

∫T298.15K [Cp,0 m(g) - Cp,0 m(cr)]dT

1 -4005.6 ( 1.8 -65.5 ( 2.0 107.5 ( 1.8 42.0 ( 2.7

(5)

0 (g) ) -13.414 + 0.5028T - 0.0002T 2 for 1, and where Cp,m 0 Cp,m(g) ) -9.5063 + 0.5278T - 0.0002T 2 for 2 have been calculated from their respective vibrational contributions cal0 (cr) were culated at the HF/6-31G(d) level, and values of Cp,m experimental values determined by DSC taken from ref 3. Values for the standard molar sublimation enthalpy at T ) 298.15 K, (107.5 ( 1.8) and (111.0 ( 2.4) kJ · mol-1 for 1 and 2, respectively, were deduced. Table 5 collects the values determined for the standard molar 0 g 0 , sublimation, ∆cr Hm, and enthalpies of combustion, ∆cHm 0 formation in the crystalline, ∆fHm(cr), and gaseous state, 0 (g), of 1 and 2. ∆fHm In accordance with normal thermochemical practice, the uncertainties assigned are, in each case, twice the overall standard deviation of the mean and include the uncertainties in calibration and in the values of the auxiliary quantities.27 To 0 0 (cr) from ∆cHm (cr), the CODATA28 values of the derive ∆fHm

TABLE 3: Vapor Pressures, p, of 1 and 2a T (K)

t (s)

∆m (mg)

p (Pa)

102(δp/p)

355.41 358.70 361.81 364.83

23 100 21 300 28 200 19 800

2.59 3.24 5.53 5.34

0.0596 0.0811 0.105 0.146

-2.79 0.869 -3.79 -0.195

375.61 378.98 380.33

21 660 24 720 19 500

7.30 11.82 10.38

0.175 0.249 0.278

-1.46 2.80 1.58

T (K)

t (s)

∆m (mg)

p (Pa)

102(δp/p)

366.62 368.31 369.88 371.39

25 560 26 700 15 600 20 220

8.02 9.98 6.85 10.29

0.169 0.202 0.238 0.276

-2.19 -0.219 1.53 2.38

383.68 386.95 390.34

18 000 22 500 15 600

12.44 20.69 19.84

0.362 0.484 0.672

-2.07 -1.84 1.61

1

2

a

p represents the vapor pressure; ∆m is the mass loss during the time t, at temperature T; and δp/p is the fractional deviation of the experimental vapor pressures from those computed using eq 4.

Enthalpy of Formation of Mercaptobenzazole Derivatives

J. Phys. Chem. A, Vol. 114, No. 21, 2010 6339

TABLE 6: Experimental and Calculated Selected Geometrical Parameters (Å, °) for 1 and 2

Figure 2. MP2(Full)/6-31G(3df,2p)-optimized structures of 1 and 2.

1

2

compound

CSDa

ab initiob

CSDa

ab initiob

C1-S2 C1-N1 N1-C2 C2-C7 C7-O1/S1 C1-O1/S1 N1-C1-O1/S1 C1-N1-C2 N1-C2-C7 C2-C7-O1/S1 C7-O1/S1-C1 S2-C1-N1 S2-C1-O1/S1

1.645(2) 1.351(4) 1.398(18) 1.392(1) 1.398(8) 1.365(4) 107.6(2) 110.4(3) 105.2(2) 108.1(10) 108.6(1) 129.7(2) 122.7(4)

1.617 1.362 1.377 1.389 1.363 1.369 106.5 111.1 104.6 109.4 108.5 129.0 124.5

1.658(3) 1.344(3) 1.384(3) 1.388(4) 1.743(3) 1.739(6) 109.2(3) 116.8(3) 112.1(1) 109.7(2) 92.4(1) 127.7(3) 123.2(1)

1.629 1.360 1.374 1.399 1.728 1.744 108.0 117.5 111.4 110.3 92.8 126.5 125.5

a Average values together with the standard deviation of the sample. b Values calculated for the molecular structure at the MP2(Full)/6-31G(3df,2p) level of theory, and therefore necessarily refer to gas phase species.

Figure 1. Hydrogen-bonding patterns: catemeric N-H · · · (O, SdC) association in 1 and dimeric N-H · · · SdC association in 2.

addition, the molecules of the ketone analogue in the catemeric motif are oriented in such a way that the O of the five-membered ring does not participate in bifurcated N-H · · · (O, OdC) interactions as in the thione derivative. The pairs of analogues ketone-thione are not isomorphous. Computational Results Theoretical Molecular and Electronic Structures. Molecular structures of 1 and 2 were optimized at the MP2(Full)/631G(3df,2p) level of theory. The equilibrium structures of 1 and 2 are shown in Figure 2, and calculated bond distances and angles are collected in Table 6. As it can be seen, the optimized structures agree very well with the experimental crystal structures. In both compounds, the thione tautomers are significantly more stable than the thiol tautomers. At the G3 level of theory,

Figure 3. MP2(Full)/6-31G(3df,2p)-calculated NBO atomic charges with hydrogens summed into heavy atoms in 1 and 2.

TABLE 7: G3-calculated Energies, at 0 K, and Enthalpies, at 298 K, for 1 and 2 in their Thione and Thiol Tautomeric Formsa species

E0

H298

1 1,3-benzoxazole-2-thiol (1 thiol tautomer) 2 1,3-benzothiazole-2-thiol (2 thiol tautomer)

-797.521 001 -797.513 577

-797.512 626 -797.504 902

-1120.399 791 -1120.392 525

-1120.390 849 -1120.383 187

a

All Values are in Hartrees.

the thione forms are 20.3 and 20.1 kJ mol-1 more stable than the thiol forms, for 1 and 2, respectively. A population analysis using the natural bond orbital (NBO) analysis has also been carried out,23-25 to obtain the natural atomic charges (the nuclear minus summed populations of the natural atomic orbitals on the atoms) that characterize the ground electronic state of the compounds studied. The calculated charges with the hydrogen atoms summed into the adjoining heavy atoms for both compounds are reported in Figure 3. As expected, differences in size, electronegativity, and bond polarities associated with oxygen and sulfur result in large differences in the electronic structures of the two compounds. In the case of 2, the sulfur atom of the ring and the carbon nearer to the N atom have positive charges associated with them in relation to the other heavy atoms in the five-membered ring, which are associated with negative charges. In 1, the oxygen atom and also the other two heteroatoms have a negative charge, whereas positive charge in the five-membered ring is located at the three carbon atoms. The NBO analysis also describes the bonding in terms of the natural hybrid orbitals. In 2, the hybridization of the ring sulfur atom is sp4, with more than 80% p character to form C-S bonds, whereas the hybridization of the ring oxygen atom in 1 is sp2 (ca. 68% p character), to form the C-O bonds. Moreover, sulfur and oxygen atoms have sp and p lone pairs that may delocalize into the vicinal antibonding orbitals. Theoretical Enthalpies of Formation. G3-calculated energies at T ) 0 K and enthalpies at T ) 298 K for the two compounds studied, both in thione and thiol tautomeric forms, are given in Table 7. The standard procedure in obtaining enthalpies of formation in Gaussian-n theories is through atomization reactions.34,35 Raghavachari et al.36 have proposed to use a standard set of isodesmic reactions, the “bond separation reactions”,20 where all formal bonds between non-hydrogen atoms are separated

6340

J. Phys. Chem. A, Vol. 114, No. 21, 2010

Roux et al.

TABLE 8: G3-calculated Enthalpies of Formation for the Compounds Studied, Using Atomization and Isodesmic Reactionsa compound

atomization

isodesmic (6) or (7)

isodesmic (8) or (9)

exp

1 2

33.0 197.1

34.5 195.9

42.8 202.1

42.0 ( 2.7 205.5 ( 3.8

a

All values in kJ mol-1.

into the simplest parent molecules containing these same kinds of linkages, to derive the theoretical enthalpies of formation. In this work it is not convenient to use the isodesmic bond separation reactions because the experimental enthalpy of formation of one of the reference compounds, thioformaldehyde H2CdS, has not been accurately determined. There are two values reported in the literature,37,38 very different one from another, and with very large uncertainties. Consequently, two similar isodesmic reactions, eqs 6 and 7, using thiourea ((H2N)2CdS) as a reference in both, have been used, for 1, C7H5NOS, and 2, C7H5NS2, respectively:

C7H5NOS + 8CH4 + NH3 + H2O f 3C2H6 + 3C2H4 + 2CH3OH + (H2N)2CdS (6) C7H5NS2 + 8CH4 + NH3 + H2S f 3C2H6 + 3C2H4 + 2CH3SH + (H2N)2CdS (7) The enthalpies of formation of 1 and 2 have been calculated using the G3-calculated enthalpies of reaction and the experimental enthalpies of formation of the species involved39-43 in these reactions. The results are shown in Table 8. There is a reasonable agreement between experimental and theoretical results, with the theoretical values predicting the compounds to be more stable than measured experimentally. Very recently, we have carried out2 an experimental and theoretical study of the thermochemistry of two compounds related with those studied in this work, 1,3-oxazolidine-2-thione and 1,3-thiazolidine-2-thione, and we can use them in isodesmic reactions 8 and 9 in order to obtain more reliable values for the enthalpy of formation of the compounds studied in this work.

values, particularly for 1 are very different from ours. How do we determine the relative plausibility of these two sets of numbers? We start with Mentado’s assumption that they are both thiols, certainly a possibility in the gas phase. If the thiol group is assumed to interact weakly or similarly with the benzoxazole and benzothiazole backbone, then a difference between the two of (44.8 ( 0.5) - (135.9 ( 0.3) ) -91.1 ( 0.6 kJ mol-1 is calculated using the enthalpies of formation of gaseous benzoxazole and benzothiazole taken from Pedley.41 The difference using our gas phase numbers is (42.0 ( 2.7) (205.5 ( 3.8) ) -163.3 ( 4.6 kJ mol-1. Although our results are slightly closer to the difference of the backbone heterocycles than Mentado’s, both results remain significantly disparate compared to the parents, consistent with the theoretical calculations and X-ray studies that thiones tautomers are the more stable form as in both the gas and condensed phases. Now consider the two species as their thione (thiocarbamate and dithiocarbamate) tautomers, that is, with -NHC()S)Oand -NHC()S)S- substructures as known in the solid state from crystallographic determinations. Let us assume that both species have the same degree of aromaticity and similar geometric requirements so their contribution to molecular strain energies are essentially the same whether found in species 1 and 2, and in any pair of compounds with the same 5-membered ring backbones. Making use of the model of aromaticity recently suggested by Matos and Liebman in ref 44, we predict that the difference of the enthalpies of formation of gaseous 1 and 2 would be very nearly equal to those of the saturated heterocyclic derivatives, 1,3-oxazolidine-2-thione and 1,3-thiazolidine-2thione. As determined previously,2 the difference for the latter pair of species is (-74.4 ( 4.6) - (97.1 ( 4.0) ) -171.5 ( 6.1 kJ mol-1, in good agreement with the results of -163.3 ( 4.6 kJ mol-1 as calculated above. This corroborates the accuracy of our results compared to those of Mentado et al.,1 as well as our preferred assignment of the thione tautomers in the gas phase. Because of the unexplained disagreement with the results of Mentado et al.1 for the enthalpies of formation of 1 and 2, we are hesitant to accept the value of the enthalpy of formation of 3H-benzimidazole-2-thione from these authors. As such, the comparison of the enthalpy of formation of 1 and the corresponding -2-one, and of 3H-1,3-benzimidazole-2-thione and the corresponding -2-one will not be attempted despite uncontested values for the enthalpy of formation of the latter two carbonyl compounds.45 Acknowledgment. The support of the Spanish Ministerio de Ciencia e Innovacio´n under Project CTQ2007-60895/BQU and CTQ2006-10178/BQU is gratefully acknowledged. M.T. would like to thank MEC/SEUI, FPU AP2002-0603, Spain for financial support. A.V.D. thanks the National Science Foundation (CHE0547566) and American Heart Association (0855743G) for financial support.

The results are shown in Table 8. The agreement between experimental and computational results is excellent. Comparison with Earlier Thermochemical Results. Recently, Mentado et al.1 have reported the values of the enthalpy of combustion and formation in the crystalline state for 1 (∆cH0m ) -3958.4 ( 0.5 kJ mol-1, ∆fH0m(cr) ) -112.7 ( 1.4 kJ mol-1) 0 0 ) -4758.4 ( 0.6 kJ mol-1, ∆fHm (cr) ) 85.3 ( and 2 (∆cHm -1 1.5 kJ mol . Using our enthalpy of sublimation with their enthalpy of formation measurements for the solids, we derive for the enthalpy of formation of these species in the gas phase [(-112.7 ( 1.4) + (107.5 ( 1.8)] ) -5.2 ( 2.1 kJ mol-1 and [(85.3 ( 1.5) + (111.0 ( 2.4)] ) 195.3 ( 2.8 kJ mol-1. These

Supporting Information Available: Procedure for thermochemical measurements; simulated X-ray powder diffraction patterns and Cartesian coordinates and frequencies, of the compounds studied. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mentado, J.; Flores, H.; Amador, P. J. Chem. Thermodyn. 2008, 40, 1106–1109. (2) Roux, M. V.; Temprado, M.; Jime´nez, P.; Foces-Foces, C.; Notario, R.; Parameswar, A. R.; Demchenko, A. V.; Chickos, J. S.; Deakyne, C. A.; Ludden, A. K.; Liebman, J. F. J. Phys. Chem. A 2009, 113, 10772–10778.

Enthalpy of Formation of Mercaptobenzazole Derivatives (3) Temprado, M.; Roux, M. V.; Parameswar, A. R.; Demchenko, A. V.; Chickos, J. S.; Liebman, J. F. J. Therm. Anal. Calorim. 2008, 91, 471–475. (4) Marti, E. E. Thermochim. Acta 1973, 5, 173–220. (5) Roux, M. V.; Da´valos, J. Z.; Jime´nez, P.; Flores, H.; Saiz, J. L.; Abboud, J.-L. M.; Juaristi, E. J. Chem. Thermodyn. 1999, 31, 635–646. (6) Roux, M. V.; Temprado, M. Thermochemistry in Handbook of Thermal Analysis and Calorimetry; Brown, M. , Ed., Elsevier: Amsterdam, 2008; Vol. 5, Ch 14. (7) Sunner, S.; Lundin, B. Acta Chem. Scand. 1953, 7, 1112–1118. (8) Knudsen, M. Ann. Phys. 1909, 28, 999–1016. (9) Jime´nez, P.; Roux, M. V.; Da´valos, J.; Martı´n-Luengo, M. A.; Abboud, J.-L. M. J. Chem. Thermodyn. 1997, 29, 1281–1288, and references therein. . (10) Freeman, R. D.; Searcy, A. W. J. Chem. Phys. 1954, 22, 762–763. (11) Wieser, M. E. Pure Appl. Chem. 2006, 78, 2051–2066. (12) Allen, F. H. Acta Crystallogr. 2002, B58, 380–388. (13) Groth, P. Acta Chem. Scand. 1973, 27, 945–969. (14) Kamat, M. N.; Rath, N. P.; Demchenko, A. V. J. Org. Chem. 2007, 72, 6938–6946. (15) Tashpulatov, Y.; Zvonkova, Z. V.; Zhdanov, G. S. Kristallografiya (Russ.) (Crystallogr. Rep.) 1957, 2, 33–41. (16) Radha, A. Z. Kristallogr. 1985, 171, 225–228. (17) Chesick, J. P.; Donohue J. Acta Crystallog. 1971, B27, 1441–1444. (18) Zhang, R.-F.; Qiu, L.-L.; Li, F.; Ma, C.-L. Chin. J. Chem. 2004, 22, 768–773. (19) Jian, F.-F.; Zhao, P.-S.; Bai, Z.-S.; Zhang, L. Jiegou Huaxue (Chin. J. Struct. Chem.) 2006, 25, 271–278. (20) Hehre, W. J.; Radom. L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. ; Gaussian 03, ReVision E.01; Gaussian, Inc., Wallingford CT, 2004. (22) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1998, 109, 7764–7776. (23) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066–4073. (24) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899–926. (25) Weinhold, F. Natural Bond Orbital (NBO) Analysis in Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Ed.; Wiley: New York, 1998; Vol. 3.

J. Phys. Chem. A, Vol. 114, No. 21, 2010 6341 (26) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1; Madison, WI, 1988. (27) Olofsson, G. Assignment of uncertainties. In Combustion Calorimetry. Sunner, S., Månsson, M. Eds.; Pergamon Press: Oxford, 1979; Ch 6. (28) CODATA. Recommended key values for thermodynamics, 1975. J. Chem. Thermodyn. 1976, 8, 603. (29) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11 Supplement 2, 58. (30) Wang, G.-X.; Zhang, Q.-W. Acta Crystallogr. 2006, E62, o3168– o3169. (31) Zhong, H.-P.; Long, L.-S.; Huang, R.-B.; Ng, S. W. Acta Crystallogr. 2003, E59, o1599–1600. (32) Flakus, H. T.; Miros, A.; Jones, P. G. J. Mol. Struct. 2002, 604, 29–44. (33) Wood, P. A.; Pidcock, E.; Allen, F. H. Acta Crystallogr. 2008, B64, 491–496. (34) Notario, R.; Castan˜o, O.; Abboud, J.-L. M.; Gomperts, R.; Frutos, L. M.; Palmeiro, R. J. Org. Chem. 1999, 64, 9011–9014. (35) Notario, R.; Castan˜o, O.; Gomperts, R.; Frutos, L. M.; Palmeiro, R. J. Org. Chem. 2000, 65, 4298–4302. (36) Raghavachari, K.; Stefanov, B. B.; Curtiss, L. A. J. Chem. Phys. 1997, 106, 6764–6767. (37) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1993, 98, 2568–2579. (38) Roy, M.; McMahon, T. B. Org. Mass Spectrom. 1982, 8, 392– 395. (39) Experimental ∆fH°m values for the reference compounds used in isodesmic reactions 6-9: methane, ethane, and ethylene,-74.6,-84.0, and 52.4 kJ mol-1, respectively, have been taken from ref 40; methanol, methanethiol, and thiourea, -201.5, -22.9 and 22.9 kJ mol-1, respectively, have been taken from ref 41; hydrogen sulfide, ammonia, and water, -20.5, -45.9, and -241.8 kJ mol-1, respectively, have been taken from ref 42; benzene, 82.9 kJ mol-1, has been taken from ref 43; and 1,3-oxazolidine-2-thione and 1,3-thiazolidine-2-thione, -74.4, and 97.1 kJ mol-1, respectively, have been taken from ref 2. (40) Manion, J. A. J. Phys. Chem. Ref. Data 2002, 31, 123–172. (41) Pedley, J. B., Thermochemical Data and Structures of Organic Compounds; TRC Data Series, TRC: TX, 1994; Vol. 1. (42) NIST-JANAF Thermochemical Tables, Fourth Edition, Chase, M. W.Jr. J. Phys. Chem. Ref. Data 1998, Monograph 9. (43) Roux, M. V.; Temprado, M.; Chickos, J. S.; Nagano, Y. J. Phys. Chem. Ref. Data 2008, 37, 1855–1996. (44) Matos, M. A. R.; Liebman, J. F. Recent Advances in the Experimental Thermochemistry of Heterocycles and Their Aromaticity: A Study of Nitrogen, Oxygen and Sulfur Derivatives of Indane and Indene. In Topics in Heterocyclic Chemistry: Aromaticity in Heterocyclic Chemistry, 19 ed.; Gupta, R. R., Vol. Ed.; Krygowski, T. M., Cyranski, M. K.; Springer: Heidelberg, 2009; pp 1-26. (45) Morais, V. M. F.; Miranda, M. S.; Matos, M. A. R.; Liebman, J. F. Mol. Phys. 2006, 104, 325–334.

JP102126J