Substituent Effects on the Thermochemistry of Thiophenes. A

Article pubs.acs.org/JPCA

Substituent Effects on the Thermochemistry of Thiophenes. A Theoretical (G3(MP2)//B3LYP and G3) Study Rafael Notario,*,† Manuel Temprado,‡ María Victoria Roux,† and Joel F. Liebman§ †

Instituto de Química Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain Departamento de Química Física, Universidad de Alcalá, 28871 Alcalá de Henares, Madrid, Spain § Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250-1000, United States ‡

S Supporting Information *

ABSTRACT: Very good linear correlations between experimental and calculated enthalpies of formation in the gas phase (G3(MP2)//B3LYP and G3) for 48 thiophene derivatives have been obtained. These correlations permit a correction of the calculated enthalpies of formation in order to estimate more reliable “experimental” values for the enthalpies of formation of substituted thiophenes, check the reliability of experimental measurements, and also predict the enthalpies of formation of new thiophenes that are not available in the literature. Moreover, the difference between the enthalpies of formation of isomeric thiophenes with the same substituent in positions 2 and 3 of the ring has been analyzed. Likewise, a comparison of the substituent effect in the thiophene and benzene rings has been established.

1. INTRODUCTION Thiophene is a chemically stable compound and is easy to process, and its applications have been a continuing matter of investigation. Several books and reviews are available on the chemistry of thiophenes.1−5 It is the simplest representation of an aromatic structure bearing sulfur. Thiophene obeys the 4n + 2π electron rule, and it is generally considered to be aromatic.6 Its structure can be assumed to be derived from benzene by the replacement of two annular CH groups by sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet, and thiophene is thus considered to be an electron-rich heterocycle. Thiophene-based compounds have been investigated extensively and have also found widespread use in modern drug design,7,8 biodiagnostics,9 electronic and optoelectronic devices,10−12 and conductive polymers.13−18 Moreover, condensed thiophenes comprise a significant portion of the organosulfur compounds in petroleum and in other products from fossil fuels, being obtainable as a byproduct of petroleum distillation.19 The knowledge of the energetic properties of thiophenes was very sparse until recent years. The enthalpy of formation in the condensed phase of the parent thiophene was determined by combustion calorimetry experiments for the first time in 1940 by Moore et al.20 New measurements were made by Hubbard et al.21 in 1955, who also measured the vaporization enthalpy, and Sunner22 in 1963. The enthalpies of formation in the gas phase of only two substituted thiophenes were also determined © 2012 American Chemical Society

several years ago: 3-methylthiophene was measured in 1953 by McCullough et al.,23 2-methylthiophene was measured in 1956 by Pennington et al.,24 and the enthalpy of formation in the condensed phase of 2-isopropylthiophene was measured by Good25 in 1972. There were also experimental enthalpies of formation in the gas phase for two compounds with one or two benzene rings fused to a thiophene ring, benzo[b]thiophene25,26 and dibenzothiophene.25,27,28 This situation has significantly improved during the past few years. Since 2002 our group29−34 in Spain and since 2007 the group of Ribeiro da Silva32−46 in Portugal have published thermochemical studies on thiophenes with a variety of substituents in different positions of the ring. Ribeiro da Silva’s group has also published other studies on thiophenes including complicated substituents,47,48 unrelated to others of interest, and some metal-containing substituents.49,50 Now we have a great body of data on these species, and it is possible to establish correlations between experimental and theoretical data that permit to check the reliability of experimental measurements and also to predict the enthalpies of formation of new thiophenes that are not available in the literature. In this paper, the gas-phase enthalpies of formation of 77 substituted thiophenes and other five-membered heterocycles containing sulfur derived from thiophene have been calculated Received: January 26, 2012 Revised: March 16, 2012 Published: April 6, 2012 4363

dx.doi.org/10.1021/jp300857v | J. Phys. Chem. A 2012, 116, 4363−4370

The Journal of Physical Chemistry A

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at the G3(MP2)//B3LYP and G3 levels using the atomization reaction method.51,52

Scheme 1. Different Possible Conformations for the Compounds Studied

2. COMPUTATIONAL DETAILS Standard ab initio molecular orbital calculations53 were performed with the Gaussian03 series of programs.54 Energies were obtained using the Gaussian-3 theory, at the G3(MP2)// B3LYP level.55 This is a variation of G3(MP2) theory56 that uses the B3LYP density functional method57 for geometries and zero-point energies. The B3LYP density functional used is a linear combination of Hartree−Fock exchange, Becke exchange,58 and Lee, Yang, and Parr (LYP) correlation.59 Two modifications have been made to derive G3(MP2)// B3LYP. First, the geometries are obtained at the B3LYP/631G(d) level instead of MP2(full)/6-31G(d), and further reoptimization is not carried out. Second, the zero-point energies and thermal corrections are obtained at the B3LYP/631G(d) level and scaled by 0.960 instead of HF/6-31G(d) scaled by 0.893. All of the other steps remain the same with the exception of the values of the higher-level correction parameters.55 The energy of the compounds studied was also calculated using Gaussian-n theory, at the G3 level.60 G3 corresponds effectively to calculations at the QCISD(T)/G3large level, G3large being a modification of the 6-311+G(3df,2p) basis set, including more polarization functions for the second row (3d2f), less on the first row (2df), and other changes to improve uniformity. In addition, some core polarization functions are added.60 Single-point energy calculations are carried out on MP2(full)/6-31G(d) optimized geometries, incorporating scaled HF/6-31G(d) zero-point vibrational energies, a so-called higher-level correction to accommodate remaining deficiencies, and spin−orbit correction for atomic species only.60

Using eq 2 we have obtained the compositions in the gas phase at T = 298 K for the compounds studied. The results are collected in Tables S5 and S6 of the Supporting Information. The G3(MP2)//B3LYP and G3-calculated enthalpies of formation in the gas phase, taking into account the conformational compositions of each species, are shown in Table S7 of the Supporting Information.

3. RESULTS AND DISCUSSION G3(MP2)//B3LYP and G3-calculated energies at 0 K, and enthalpies at 298 K, for the 77 compounds studied in this work are given in Tables S1 and S2 of the Supporting Information, respectively. Since different rotamers can exist for numerous compounds (see Scheme 1), we have carried out a conformational analysis for all compounds optimizing the molecular structures of the lowest-energy conformers. All of the structures are minima on the potential energy surfaces. In Tables S1 and S2 are considered those conformers that contribute significantly to the populated states. The standard procedure to obtain enthalpies of formation in Gaussian-n theories is through atomization reactions.51,52 The G3(MP2)//B3LYP and G3-calculated enthalpies of formation of the studied compounds, for all the optimized conformers, using atomization reactions are shown in Tables S3 and S4 of the Supporting Information, respectively. To obtain the conformational composition of the studied compounds in the gas phase, at T = 298 K, we need the ΔfG0m values. They can be calculated through eq 1 Δf Gm0(i) = Δf Hm0(i) − T[S 0(i) −

∑ S 0(el)]

xi =

e −[ n

0 (i) Δf Gm RT ]

∑i = 1 e−[

0 (i) Δf Gm RT ]

(2)

As it can be seen in Table S7, the G3(MP2)//B3LYPcalculated values are usually far from the experimental ones; they are lower than the experimental values. The mean absolute deviation is very high, 11.0 kJ·mol−1, for the set of 48 thiophenes (compounds number 1−48 in Table S7) with known experimental enthalpy of formation values. Recently, Anantharaman and Melius62 have developed a bond additivity correction (BAC) procedure for the G3(MP2)//B3LYP method, applicable to compounds containing atoms from the first three rows of the Periodic Table, and so including organosulfur species. The BAC procedure applies atomic, molecular, and pairwise bond corrections to theoretical heats of formation of molecules. The procedure requires parameters for each atom type but not for each bond type. The authors have applied the method to an extended test set involving 273 compounds, neutral and ionic, and the average error was only 4.4 kJ·mol−1. We have carried out the BAC correction following the steps indicated in ref 62, and the values obtained are collected in Table S7 of the Supporting Information. The mean absolute deviation has dramatically decreased to 6.4 kJ·mol−1 but is higher than that obtained using G3-calculated values, 4.4 kJ·mol−1. Very good linear correlations between experimental and calculated enthalpies of formation have been obtained:

(1)

where ΣS0(el) is the sum of the entropy of the elements in their standard state. Using for the elements the entropy values, at 298 K, taken from ref 61, ΔfG0m values have been obtained for all the conformers. 4364



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formation in the gas phase derived using the correlation eqs 3 and 4, respectively, are shown in Table 1. As can be observed in the Table, the mean absolute deviations have decreased to 3.7 and 3.4 kJ·mol−1, for the G3(MP2)//B3LYP- and G3-calculated values, respectively, clearly inside the “chemical accuracy” (±1 kcal·mol−1 ≈ 4.2 kJ·mol−1). The higher discrepancies between the experimental and the calculated enthalpy of formation values are those found for the substituents 2-CO2Et 21, 2-CH2CO2Me 22, 2-CHO-5-NO2 32, 2-COMe-5-NO2 38, and also 2,2′−bithiophene 47, with deviations closer or slightly higher than 10 kJ·mol−1. In order to further assess the reliability of the experimental measurements for these compounds, the enthalpy of formation in the gas phase was also calculated through isodesmic reactions 5−9.

Δf Hm 0(exp) = (0.9973 ± 0.0035)Δf Hm 0 (G3(MP2)//B3LYP) − (5.98 ± 0.73) (in kJ ·mol−1) n = 48; R = 0.999 71; sd = 4.9 kJ·mol−1

(3)

Δf Hm 0(exp) = (0.9879 ± 0.0034)Δf Hm 0(G3) + (1.89 ± 0.72)(in kJ ·mol−1)

n = 48; −1

R = 0.999 72; sd = 4.8 kJ ·mol

(4)

The G3(MP2)//B3LYP calculated values corrected with the BAC procedure were used in the correlation equations. The graphic representations of the correlations are shown in Figure 1. The correlated enthalpies of formation span about 880

It is to be expected that the errors of the computational method would cancel by the use of isodesmic reactions 5−9, and in fact this behavior is observed for compounds 22, 32, 38, and 47 with a decrease in the difference between the theoretical and experimental values from 10.1 to 6.2 kJ·mol−1 (22), from −10.8 to 1.3 kJ·mol−1 (32), from −10.6 to −4.7 kJ·mol−1 (38), and from −10.1 to −4.6 kJ·mol−1 (47). For 21, the difference between the computed and experimental values calculated using the isodesmic reaction 5 of −10.0 kJ·mol−1 is similar to the one obtained by the atomization method (−9.0 kJ·mol−1) suggesting that the experimental value for this compound is somewhat less negative than it should be. Recently, Zauer64 has published an article related to the calculation of the enthalpy of formation of 21 carbonyl compounds of the thiophene series by low-level semiempirical methods. Moreover, Dorofeeva et al.65 in an assessment of Gaussian-4 theory66 for the computation of enthalpies of formation of large organic molecules found that the largest deviations between experiment and G4 values were found for nitro compounds and oxygen and sulfur heterocycles, almost all G4 values being 7−17 kJ·mol−1 less than experimental values, similar to the trend observed in our current study. In the test set used in that work, compounds 16, 17, 43, 46, 47, and 48 (see Table 1) were included. Differences between computed and experimental values of −13.6, −7.2, −8.5, −16.6, −15.2, and −11.8 kJ·mol−1, respectively, were calculated for these compounds at the G4 level. Similar values have been obtained in the current study for the three first compounds (see Table

Figure 1. Plots of the experimental vs calculated (at G3(MP2)// B3LYP and G3) enthalpies of formation for the set of 48 thiophene derivatives.

kJ·mol−1, from compound 11 (3-cyanothiophene, ΔfH0m(exp) = (248.6 ± 2.4) kJ·mol−1) to compound 42 (thiophene-2,5dicarboxylic acid, ΔfH0m(exp) = −(632.6 ± 2.2) kJ·mol−1). Both statistical correlations are of essentially the same level of quality with slopes close to one and intercepts close to zero. They permit a correction of the calculated enthalpies of formation in order to estimate more reliable “experimental” values for the enthalpies of formation of substituted thiophenes. The G3(MP2)//B3LYP- and G3-calculated enthalpies of 4365



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Table 1. Experimental Enthalpies of Formation, ΔfH0m (in kJ·mol−1), of Substituted Thiophenes, and Deviations from G3(MP2)//B3LYP- and G3-Calculated Values Using Correlation Eqs 3 and 4

a

no.

X

rotamera

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

H 2-Me 3-Me 2-Et 2-nPr 2-nBu 3-nBu 2-CHO 3-CHO 2-CN 3-CN 2-CH2CN 3-CH2CN 2-CO2H 3-CO2H 2-CH2CO2H 3-CH2CO2H 2-COMe 3-COMe 2-CO2Me 2-CO2Et 2-CH2CO2Me 3-CH2CO2Me 2-CH2CO2Et 3-CH2CO2Et 2-CONH2 2-CONHNH2 2-CH2CONH2 2-CHO-3-Me 2-CHO-5-Me 2-CHO-5-Et 2-CHO-5-NO2 2-CN-3-Me 2-COMe-3-Me 2-COMe-4-Me 2-COMe-5-Me 2-CO2H-3-Me 2-COMe-5-NO2 2-CO2H-5-Me 2-CO2H-5-COMe 2,5-di-Me 2,5-di-CO2H 2,5-di-Me-3-COMe 2-Me-3-COMe-5-Ph benzo[b]thiophene dibenzothiophene 2,2′-bithiophene 3,3′-bithiophene |dev|e

− a b c c c c e d − − a b d d c c e d d d c c c c dc dd c d, a e, a e, c e, h −, b d, a e, b e, a d, a e, h d, a d, e a, a d, d b, e, a b, e, − − − − −

G3 − exptlb 1.4 −0.3 0.6 7.8 5.3 8.8 2.4 0.8 3.3 3.5 −1.3 5.1 0.5 −1.4 −0.4 −6.8 −0.8 2.7 0.5 −1.6 −9.0 10.1 6.9 6.7 0.7 −0.8 0.7 −0.8 0.9 −5.6 4.9 −10.8 −0.3 0.7 1.8 3.8 −1.5 −10.6 −2.7 −4.4 −0.8 0.9 0.1 −4.4 −0.9 −0.5 −10.1 −5.6 3.4

[−2.2] [−3.0] [−2.1] [5.5] [4.1] [8.1] [1.7] [−0.4] [2.0] [3.8] [−1.4] [4.2] [−0.4] [−3.0] [−2.1] [−9.0] [−1.3] [2.2] [0.0] [−1.7] [−7.3] [11.7] [7.6] [10.9] [4.6] [−2.3] [−2.2] [−1.4] [0.7] [−5.7] [5.5] [−10.8] [1.0] [0.9] [2.1] [4.4] [−2.2] [−9.5] [−3.2] [−3.5] [−2.8] [0.0] [0.9] [0.7] [0.0] [4.8] [−7.2] [−3.1] [3.7]

exptl 115.0 83.5 82.5 53.3 32.6 8.0 15.4 −7.1 −7.4 248.0 248.6 228.1 227.8 −259.2 −261.8 −265.7 −275.5 −59.2 −54.5 −243.6 −270.6 −268.5 −267.6 −300.0 −295.7 −64.0 49.4 −88.0 −40.6 −37.3 −70.1 4.4 212.0 −90.9 −92.4 −96.0 −295.6 −48.8 −293.8 −424.3 50.6 −632.6 −123.2 4.6 166.3 205.1 247.5 244.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

ref 1.0 0.9 0.9 2.1 2.4 2.6 2.6 1.9 1.9 1.9 2.4 2.2 2.2 1.9 1.7 2.2 2.4 2.1 1.8 2.1 2.4 2.8 2.4 2.6 2.6 1.3 1.4 1.9 2.1 2.1 2.4 1.3 2.1 3.1 3.3 3.3 1.7 1.6 1.6 1.9 1.9 2.2 2.7 3.1 0.9 1.5 2.7 2.6

63 63 63 35 35 35 35 36 36 37 37 37 37 29 29 31 31 32 32 43 43 33 33 43 43 38 42 38 36 36 36 44 37 39 39 39 41 44 41 41 40 30 40 46 63 63 34 34

Lowest energy conformation at the G3 level according to Scheme 1. bG3MP2//B3LYP values between brackets. cDihedral SCCO angle = −161.9°. Dihedral SCCO angle = −158.9°. eMean absolute deviation.

d

S7); however, values of −9.1, −4.6, and −0.1 kJ·mol−1 have been derived at the G3 level for compounds 46, 47, and 48 respectively, suggesting a better performance of G3 compared to the G4 method for these compounds. Furthermore, the deviations are reduced significantly with the use of correlation eqs 3 and 4 yielding differences between the calculated and the

experimental values of −6.8, −0.8, and 0.9 kJ·mol−1 for compounds 16, 17, and 43, respectively. To test the robustness of our correlation equations, we have calculated the enthalpies of formation of eight compounds derived from thiophene (dihydro- and tetrahydrothiophenes) with measured enthalpies of formation available in the literature (compounds 49−56 in Table 2). 4366



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Table 2. G3(MP2)//B3LYP- and G3-Calculated Enthalpies of Formation, ΔfH0m (in kJ·mol−1), of Several FiveMembered Heterocycles Containing Sulfur, Using Correlation Eqs 3 and 4 no.

compound

49 50 51 51

tetrahydrothiophene 2,3-dihydrothiophene 2,5-dihydrothiophene tetrahydro-2methylthiophene tetrahydro-3methylthiophene dihydro-2(3H)thiophenone dihydro-3(2H)thiophenone 2(5H)-thiophenone

53 54 55 56 a

G3 − exptla

Table 3. G3(MP2)//B3LYP- and G3-Calculated Enthalpies of Formation, ΔfH0m (in kJ·mol−1), of Substituted Thiophenes and Five-Membered Heterocycles Containing Sulfur, Using Correlation Eqs 3 and 4

ref

no.

X

rotamera

G3(MP2)//B3LYP

G3

1.3 1.5 1.5 0.8

63 63 63 63

0.4 [−2.4]

−60.5 ± 0.8

63

26.9 [26.3]

−196.2 ± 1.9

63

2.9 [2.0]

−135.3 ± 1.9

63

−66.3 ± 2.0

45

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

3-Et 3-CO2Me 3-CONH2 2-F 3-F 2-Cl 3-Cl 2-OH 3-OH 2-OMe 3-OMe 2-NH2 3-NH2 2-NO2 3-NO2 2-CF3 3-CF3 2-COCl 3-COCl 2(3H)-thiophenone 3(2H)-thiophenone

c d eb − − − − g f g f j i h h a b d d − −

59.8 −246.3 −66.0 −50.6 −66.5 97.9 89.1 −36.0 −47.6 −20.5 −32.5 138.8 128.8 106.5 97.4 −557.7 −565.8 −74.4 −75.5 −54.0 −46.0

62.1 −246.0 −64.4 −48.1 −63.9 99.7 90.7 −32.6 −44.1 −19.4 −30.6 141.2 132.7 107.7 98.8 −553.7 −561.5 −75.9 −76.7 −52.4 −45.0

3.7 −6.6 0.0 2.2

[0.3] [−10.1] [−4.1] [−0.4]

0.7 [−0.2]

exptl −33.5 90.7 86.9 −64.2

± ± ± ±

G3MP2//B3LYP values between brackets.

The agreement between experimental and calculated enthalpy of formation values is very good at both level of calculations with one exception, dihydro-2(3H)-thiophenone 54, with a deviation of ca. 27 kJ·mol−1. This very large deviation suggests problems with the experimental value. Another problem with this species is seen by comparing the enthalpy of the hypothetical ring-closing reaction 10 with that of the likewise hypothetical reaction 11. a

CH3CH 2CH 2SC(O)CH3 → dihydro‐2(3H )‐thiophenone + CH4

b

Lowest energy conformation at G3 level according to Scheme 1 . Dihedral CCCO angle = −18.4°

(10)

counterparts, species 54 and 55, a keto group adjacent to the sulfur in the isomeric thiophenones is stabilizing. We hesitate to say that the group −S−CH2−C(O)− is destabilizing although it is found that 3-oxo-tetrahydrothiapyran with a 6-membered sulfur containing heterocyclic ring has been reported68 to be ca. 20 kJ·mol−1 less stable than its 4-isomer. In Table 4 we compare the difference between the enthalpies of formation of thiophenes with the same substituent in positions 2 and 3 of the ring evaluating the energetics of reaction 12.

CH3CH 2CH 2SCH 2CH3 → tetrahydrothiophene + CH4 (11) 63

Using experimentally measured enthalpies of formation of all of the species above, we find that reaction 10 is exothermic by 20.2 kJ·mol−1. However, using the average of our two nearly identical calculated values for dihydro-2(3H)-thiophenone, this reaction is found to be endothermic by 6.4 kJ·mol−1. Using only experimentally measured enthalpies of formation of all of the species above,63 reaction 11 is seen to be exothermic by 3.1 kJ·mol−1. The difference of the two reaction enthalpies is reduced to 9.5 kJ·mol−1, a very much smaller difference than the earlier one. We would have liked also to compare CH3SCH2C(O)CH3 with dihydro-2(3H)-thiophenone, but since this data is absent, as it is for all substances with a −S− CH2−C(O)− backbone save our just enunciated cyclic species and its thiapyrano counterpart, we did not proceed with this analysis. Regardless, it is clear that dihydro-2(3H)-thiophenone is more stable than dihydro-3(2H)-thiophenone. This is consistent with the former being a thiolester, and that they, like esters, amides, and other acyl derivatives attached to electronegative groups, enjoy significant stabilization relative to alkyl derivatives with the same groups.67 In view of the excellent results obtained with correlation eqs 3 and 4, we have also carried out calculations on thiophenes with new substituents in order to predict their enthalpies of formation in the gas phase, and the results are collected in Table 3. As can be seen in Table 3, 2(3H)-thiophenone (compound 76) is almost 20 kJ·mol−1 more stable than the corresponding enol tautomer (compound 64). However, in the case of the 3tautomers (compounds 65 and 77), the enthalpy of formation in the gas phase is essentially the same. As with their saturated

Classically, the substituent effect on aromatic rings has been explained as a function of Hammett, Taft, or other linear free energy relationships.69 As can be seen in Table 4, the 3-isomers of thiophene are stabilized compared to their 2-counterparts by the addition of strong π-donor substituents (σR < 0) or by strong σ-attracting groups (large σI). Otherwise, the energetics of the substitution in positions 2 and 3 are nearly identical. This may be understood in terms of resonance structure reasoning (Scheme 2) and the observation that sulfur stabilizes both carbocations and carbanions, a phenomena seen best in its pinnacle in 1,3-dithiolanes and dithianes.70 For the 2-isomer with a suitably strong electron-donating substituent, there is the nonpolar resonance structure (I in Scheme 2) and also two for which the substituent is positively charged (II and III, Scheme 2). In structure II the negative charge is found on ring carbon3, while in structure III it is found on carbon-5. In structure II, the sulfur offers no stabilization because of no carbocation− heteroatom interaction. In structure III the dipolar structure 4367



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charges. For the 3-isomer, there are the nonpolar resonance structure IV and the dipolar structure V with the negative charge on carbon-2 where it is adjacent to sulfur and thus there is significant stabilization. In structure III, the negative charge is also adjacent to sulfur and a similar stabilization should be expected. In addition there is a shorter distance between opposite charges in structure V as compared to structure III. The same argument can be applied for the derivatives with a suitably strong electron-withdrawing substituent as show in structures VI−X in Scheme 2. The stabilization of the 3isomers as compared to the 2-counterparts is more pronounced when the substituent is a strong π-donor group compared to a π-accepting group as can be seen in Table 4. The sulfur in thiophene is positively charged; thus, the stabilization of structure V should be expected to be more important than in structure X. Furthermore, the computed values for the enthalpy of reaction 13 (Table 5) refer to the stabilization or destabilization due to a substituent change from thiophene to benzene.71

Table 4. Calculated Isomerization Enthalpies (Reaction 12), in kJ·mol−1, along with σI and σR Values for the Substituentsa

a

substituent

σI

σR

G3(MP2)//B3LYP

G3

Me Et nBu CHO COMe CO2H CO2Me CONH2 COCl CH2CO2H CH2CO2Me CH2CO2Et CH2CN CN CF3 F Cl NO2 NH2 OH OMe

0.01 0.00 −0.01 0.33 0.33 0.34 0.34 0.26 0.46 na 0.19 0.15 0.17 0.51 0.38 0.45 0.42 0.65 0.08 0.33 0.29

−0.18 −0.15 −0.15 0.09 0.17 0.11 0.11 0.10 0.15 na na na 0.01 0.15 0.16 −0.39 −0.19 0.13 −0.74 −0.70 −0.56

0.1 −1.0 −1.0 −2.1 −2.5 1.7 1.0 −0.3 1.1 2.1 3.2 2.0 4.9 4.6 8.1 15.9 8.8 9.1 10.0 11.6 12.0

0.1 −1.0 −1.0 −2.2 −2.5 1.6 0.8 −0.4 0.8 3.8 2.3 1.7 4.9 4.2 7.8 15.8 9.0 8.9 8.5 11.5 11.2

Table 5. Enthalpy of Reaction 13 at the G3 and G3(MP2)// B3LYP Levels (in kJ·mol−1)

Values taken from ref 69.

provides little stabilization since it corresponds to a vinylogous interaction, i.e., a comparatively long distance, and so therefore there is less stabilizing interaction between the opposite

2-isomer

Scheme 2. Resonance Structures for 2- and 3-Thiophene Derivatives Containing a π-Donor (X) or a π-Withdrawing (Y) Substituent

3-isomer

X

G3(MP2)//B3LYP

G3

G3(MP2)//B3LYP

G3

Me Et nBu CHO COMe CO2H CO2Me CONH2 COCl CN CF3 F Cl NO2 NH2 OH OMe

2.1 3.3 4.2 2.9 2.4 0.3 1.3 −2.5 3.3 −4.0 −9.3 −33.2 −13.8 −6.9 −19.6 −28.3 −15.3

−0.6 1.0 3.5 1.7 1.9 −1.3 1.2 −4.0 4.8 −3.7 −13.3 −35.7 −15.6 −8.1 −22.0 −31.7 −16.4

2.2 2.3 3.2 0.8 −0.1 2.0 2.3 −2.8 4.4 0.6 −1.2 −17.3 −5.0 2.2 −9.6 −16.7 −3.3

−0.5 0.0 2.5 −0.5 −0.6 0.3 2.0 −4.4 5.6 0.5 −5.5 −19.9 −6.6 0.8 −13.5 −20.2 −5.2

Data in Table 5 confirm that the substitution on a benzene ring is energetically preferred to the same on a thiophene ring for substituents having larger π-donor and σ-attracting character, this trend being more marked for the 2-isomers. It has been proposed that the aromaticity in benzene hardly changes upon substitution,74,75 but significant reduction of the aromaticity has been observed for other heterocycles such as pyrazole and imidazole76 and so substituent effects are plausibly larger.

4. CONCLUSIONS The enthalpies of formation in the gas phase of 77 substituted thiophenes and other heterocycles derived from thiophene have been calculated at the G3(MP2)//B3LYP and G3 levels. Very 4368



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good correlations between the calculated and experimental data have been obtained for the 48 compounds for which the experimental enthalpy of formation is available in the literature. These correlations have allowed us to check the reliability of the experimental data for this important class of sulfur heterocycles suggesting that the experimental values of 2ethylthiophenecarboxylate and dihydro-2(3H)-thiophenone are about 10 kJ·mol−1 higher and 25 kJ·mol−1 lower, respectively, than they should be. With the use of the calculated values, the substituent effect has been analyzed as a function of its position in the thiophene ring, 2-isomers vs the 3-analogues, indicating that the 3-isomers are stabilized by the addition of groups having strong π-donor or strong σ-attracting character. Likewise, the substitution on a benzene ring is more favorable energetically than on a thiophene ring by the addition of strong π-donor or strong σ-attracting substituents.

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ASSOCIATED CONTENT

S Supporting Information *

G3(MP2)//B3LYP- and G3-calculated energies at 0 K, and enthalpies at 298 K, for thiophenes and a series of fivemembered heterocycles containing sulfur (Tables S1 and S2, respectively); G3(MP2)//B3LYP- and G3-calculated atomization enthalpies, ΔHa, and enthalpies of formation, ΔfH0m, at 0 and 298 K, in kJ·mol−1, for thiophenes and a series of fivemembered heterocycles containing sulfur (Tables S3 and S4, respectively); G3(MP2)//B3LYP- and G3-calculated enthalpies, ΔfH0m, and Gibbs energies of formation, ΔfG0m, at 298 K, in kJ·mol−1, for thiophenes and a series of five-membered heterocycles containing sulfur (Tables S5 and S6, respectively); G3(MP2)//B3LYP- and G3-calculated enthalpies of formation, ΔfH0m, for all the studied compounds using atomization reactions (Table S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the Spanish Ministerio de Ciencia e Innovación under Projects CTQ2007-60895/BQU and CTQ2010-16402 is gratefully acknowledged. M.T. acknowledges the Spanish Ministerio de Ciencia e Innovación for a “Juan de la Cierva” contract.

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REFERENCES

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Substituent Effects on the Thermochemistry of Thiophenes. A

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