J. Phys. Chem. B 2010, 114, 16253–16262
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Energetics of the Lighter Chalcogen Analogues of Carboxylic Acid Esters† Carol A. Deakyne* and Alicia K. Ludden Department of Chemistry, UniVersity of Missouri, Columbia, Missouri 65211-7600, United States
Maria Victoria Roux and Rafael Notario Instituto de Quimica Fisica “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain
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, United States
Joel F. Liebman Department of Chemistry and Biochemistry, UniVersity of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250-1000, United States ReceiVed: July 31, 2010; ReVised Manuscript ReceiVed: September 30, 2010
In the current paper we present the results of our quantum chemical (G2, G2(MP2), and G3) study of the structure and energetics of carboxylic acids and their chalcogen analogues. In the particular, calculations and accompanying natural bond orbital (NBO) and atoms in molecules (AIM) analyses were performed on all species with the generic formula RC(dX)YR′ (X, Y ) O, S, Se and R ) R′ ) CH3). Energies, enthalpies, and free energies of formation, resonance energies, interchalcogen methyl transfer energies and their energies of activation, and heavy atom bond lengths and angles are all discussed. A comparison of the calculated results with the sparse experimentally available data shows good agreement. Trends are also presented. Introduction Carboxylic acid esters [RC(dO)OR′] are nearly ubiquitous. They are found in foods as definitional components of fats (cf. triglycerides) and in flavorings (e.g., methyl anthranilate and grapes). They are found in plastics as monomeric components, (e.g., methyl methacrylate, vinyl acetate) and as plasticizers (e.g., dibutyl phthalate). They are found in drugs, both legal (e.g., lidocaine) and illegal (e.g., heroin). It is thus not surprising that they have also caught the attention of the thermochemical community: a quick look at diverse data compendiasDomalski,1 Stull et al.,2 Pedley,3 and the WebBook (Afeefy, Liebman, Stein chapter)4sshows data for well over a hundred esters. Carboxylic acid esters containing sulfur are rather much rarer in any of their three incarnations, S-thiocarboxylate esters (thiolesters, [RC(dO)SR′]), O-thiocarboxylate esters (thionesters, [RC(dS)OR′]), and dithioesters [RC(dS)SR′], respectively. Of these, S-thiocarboxylate esters (thiolesters), as perhaps befits their relevance to biochemical energetics and coenzyme A,5 have been studied by calorimetrists, but these efforts have been dominated by thiolacetates.6-9 Exclusive of carbamic acid derivatives,10-15 O-thioesters (thionesters) have seemingly been ignored by this community. Nonetheless, some qualitative inferences may be drawn, such as O-thioesters are less stable than the corresponding isomeric S-thioester as evidenced by the rearrangement for the former into the latter.16-19 This oversight is surprising, indeed disappointing, given the extensive chemical interest in sulfurcontaining species. Selenium-containing esters (with one or two †
Part of the “Robert A. Alberty Festschrift”. * To whom correspondence should be addressed. E-mail: deakynec@ missouri.edu, phone: 573-882-1347, fax: 573-882-2754.
seleniums, and if but one, with the other element oxygen or sulfur) likewise remain unstudied by thermochemical researchers. This omission is less surprising given the paucity of such studies with selenium compounds in general as compared to those containing its lighter chalcogen congeners, as in the brief review in the “Patai series” on organoselenium and organotellurium thermochemistry,20 as opposed to that on general sulfur compounds,21 and the earlier, but more specialized one, on sulfonic acids and derivatives.22 As befits our earlier studies of the energetics of chalcogencontaining 16 valence electron triatomics and pentaatomics23-25 and of a pair of thiocarbamate and dithiocarbamate heterocycles,15,16 we now report our computational study of S- and O-thiocarboxylate esters (thiol and thiocarboxylates), dithiocarboxylate esters, and their selenium counterparts. In particular, we have investigated the structures and energetics of the nine compounds, H3CC(dX)YCH3 (X, Y ) O, S, Se), at the G2, G2(MP2), and G3 levels of theory. The focus of this work is on the following properties of these compounds: resonance energies, enthalpies of formation, barrier heights for intramolecular 1,3-methyl migration, and syn (Z) vs anti (E) relative stabilities. We are interested in how these properties differ down the group and between the C(dX)Y and the C(dY)X arrangements. Relatively few previous quantum mechanical studies of sulfurcontaining esters have appeared in the literature. Moreover, although the relative energies of HC(dSe)OH and HC(dO)SeH and the barrier for OfSe hydrogen transfer have been examined computationally,26 as have the gas-phase acidities of the series of selenocarboxylic acids RCSeOH and RCOSeH, R ) H, F, Cl, NH2, CH3,27 we have found no computational studies performed on selenoesters. The theoretical work with the most
10.1021/jp107208q 2010 American Chemical Society Published on Web 10/28/2010
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CHART 1
overlap with this work is a MNDO investigation of the ketene-thioketene formation and 1,3-methyl transfer reactions of methyl acetate H3CC(dO)OCH3, S-methyl thioacetate H3CC(dO)SCH3, O-methyl thioacetate H3CC(dS)OCH3, and methyl dithioacetate H3CC(dS)SCH3.28 The 1,3-methyl transfer reaction was found to be preferred for all compounds but S-methyl thioacetate. The majority of the remaining calculations discussed below involve the sulfur-containing compounds as models for thioesters. The energy difference between the Z and the E rotamers (Chart 1) and the barrier to internal rotation for the model S-thiocarboxylate ester H3CC(dO)SCH3 have been evaluated by several research groups.29-32 The destabilization of the E conformer compared with the Z has generally been rationalized in terms of its less effective nSfπ*CdO electron delocalization, less favorable dipole-dipole interactions, and larger steric strain. S-methyl thioacetate and methyl dithioacetate H3CC(dS)SCH3 have also been used as model compounds to assay CsS versus CsO bond polarities, relevant to the higher reaction rates of thiol esters with respect to dithio esters in the deacylation of the cysteine protease papain.33 The reactivities of oxoesters and thioesters in nucleophilic acyl transfer reactions have been probed by a comparison of the model reactions of methyl acetate and S-methyl thioacetate with hydroxide, ammonia, and methylcyanoacetate carbanion.34 The relative reactivities correlate with the net loss of delocalization energy accompanying the formation of the transition state. Substituent effects on the gasphase basicities of the O-thiocarboxylate H3CC(dS)OCH3 and H3CC(dS)OC2H5 have been examined as part of a combined mass spectrometric and computational study of the intrinsic reactivities of carbonyl versus thiocarbonyl compounds.35 Again, differences in electron delocalization effects, in this case from the substituent lone pairs to the CdO or CdS π-orbitals, help to elucidate the greater sensitivity of the carbonyl compounds to substitution. The greater basicity of the S counterparts was attributed to the lower electronegativity and greater polarizability of S than O. (Similar effects were seen when the carbonyl compounds were compared with the selenocarbonyl derivatives.26) The 33S NMR properties of H3CC(dO)SCH3, H3CC(dS)OCH3, and H3CC(dS)SCH3 have also been calculated.36 G337 and G238 energies have been evaluated for methyl acetate to determine the proton affinity of [CH2COOCH3]- and the enthalpy of formation of methyl peroxy acetate (via an isodesmic reaction), respectively. A G3 energy has also been evaluated for S-methyl thioacetate39 to determine the homolytic bond dissociation energy of the CsS bond. However, no systematic study of the entire series of H3CC(dX)YCH3 (X, Y ) O, S, Se) molecules has been carried out at any level of calculation. In addition, Z and E relative stabilities were not reported in the earlier investigations. Calculational Details The calculations on the nine chalcogen-containing compounds H3CC(dX)YCH3 (X, Y ) O, S, Se) were performed with Gaussian 0340 and Gaussian 0941 using the G342 and G243
Deakyne et al. composite methods. Ethane and the three corresponding H3CC(dX)CH3 and H3CYCH3 compounds (eq 1 below) have been studied previously at these levels of calculation.26,38,39,42-45 All possible arrangements of the hydrogens within the Z and E C(dX)YC configurations of H3CC(dX)YCH3 (Chart 1) were examined for each X, Y pair. Structures with both CS and C1 symmetry were considered. (The C(dX)YC gauche conformations tested relaxed to syn or anti.) Transition structures were located for the YfX intramolecular 1,3-methyl shifts for all six relevant combinations of X and Y. The designation TS-YX has been used to more easily differentiate among the various interconversions. For the transition structures, normal-mode vibrational frequencies were reevaluated at the MP2/6-31G(d) calculational level to verify that the structures had one imaginary frequency at that level. Intrinsic reaction coordinate calculations (IRC)46 were carried out for the transition structures to confirm that they connected the desired reactants and products. The Cartesian coordinates for the nine Z and nine E equilibrium structures and for the six transition structures can be found in Table S1 of the Supporting Information (SI). The G2, G2(MP2), and G3 energies, enthalpies, and free energies of these molecules and the reference molecules are listed in Table S2 of the SI. As our measure of the resonance energies associated with these compounds, for each X, Y pair, where X, Y ) O, S, Se, we have computed the enthalpy change for the following isodesmic group separation reaction (eq 1).
H3CsC(dX)sYsCH3 + H3CsCH3 f H3CsC(dX)sCH3 + H3CsYsCH3 (1) This reaction has also been used to compare the resonance energy and rotational barriers of esters and amides (Y ) NCH3).47 Together with the experimental enthalpies of formation4,48 of C2H6, H3CsC(dX)sCH3 and H3CsYsCH3, the above reaction enthalpies allow us to obtain enthalpies of formation for the H3CC(dX)YCH3 (X, Y ) O, S, Se) species. When experimental enthalpies of formation are not available for the products, we have estimated some enthalpies of formation for H3CC(dX)YCH3 from the following rearrangement reaction (eq 2).
H3CsC(dY)sXsCH3 f H3CsC(dX)sYsCH3
(2) Natural bond orbital (NBO)49,50 and atoms in molecules (AIM)51 analyses were performed for the nine H3CC(dX)YCH3 systems to elucidate the influence of electron delocalization effects on the relative conformational and group separation energies of these systems. The AIM analysis provides bond critical point densities (Fb), which we use as a measure of bond strength.26,51-55 The NBO analysis examines deviations of the molecule from the strictly localized Lewis structure. For these molecules, the Lewis NBOs describe about 99% of the total electron density. We utilized the NBO analysis of the Hartree-Fock orbitals to examine the role of the nXfσ*(CsY), nYfσ*(CdX), and nYfπ*(CdX) hyperconjugations in governing the above relative energies. Each of these orbital interactions involves a lone pair on atom X(Y) delocalizing into an unfilled CsY(CdX) NBO. The energies of the individual orbital interactions ∆E(2)(donorfacceptor) were estimated with the second-order NBO perturbation approach.50
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TABLE 1: Relative Enthalpies and Free Energies, Dipole Moments, and %Ys Character molecule
∆H298a
∆G298a
µb (Z)
µb (E)
%Ysc (C(X)sY)
%Ysc (YsCH3)
H3CC(dO)OCH3 H3CC(dS)OCH3 H3CC(dSe)OCH3 H3CC(dO)SCH3 H3CC(dS)SCH3 H3CC(dSe)SCH3 H3CC(dO)SeCH3 H3CC(dS)SeCH3 H3CC(dSe)SeCH3
30.6 [31.2] 32.5 [33.4] 32.0 [33.1] 17.7 [17.7] 17.9 [18.1] 17.1 [17.4] 13.4 [14.0] 13.3 [13.9] 12.6 [13.2]
30.7 [31.2] 32.1 [33.0] 31.6 [33.0] 25.0 [24.5] 20.1 [20.3] 18.1 [19.0] 15.1 [17.3] 14.4 [17.2] 13.9 [21.2]
1.95 2.52 2.57 1.51 2.07 2.13 1.46 2.00 2.04
5.07 5.43 5.47 4.70 4.87 4.83 4.48 4.63 4.59
32.6 34.8 35.3 17.5 19.4 20.0 13.9 15.4 16.0
29.3 29.6 29.5 18.0 18.2 18.2 14.7 15.0 15.0
a G2 and G3 (square brackets) thermochemical data (kJ/mol) for the E local minimum relative to the Z global minimum. b Calculated dipole moments in Debye. c % contribution from the s-orbitals of atom Y to the C(X)sY or YsCH3 σ-bond of the Z rotamer.
Results and Analysis of Results The thermochemical data reported in this work was evaluated at the G2, G2(MP2), and G3 levels of calculation. Because the G2(MP2) and G2 values vary by no more than 3 kJ/mol and usually by less than 1 kJ/mol, the G2(MP2) values have not been tabulated. The G2 and G3 ∆E0 and ∆H298 values deviate by 4 kJ/mol or less, but the ∆G298 values deviate by as much as 10 kJ/mol. Another general trend is that all ∆E0 and ∆H298 thermochemical quantities are essentially equal. Thus, we can concentrate on solely the stereoelectronic aspects of the chemical energetics. Z versus E Conformers. The conformational preference of the XdCsYsZ backbone in RC(dX)YR′ compounds (Chart 1) has been of interest for some time. For example, the relative stability of the two possible conformations has been important in interpreting spectral data31,56-59 and photodecomposition data.30 The previous experimental30,31,59-63 and computational30,31,38,59,63-65 studies on a variety of RC(dX)YR′ species indicate that the Z conformation is preferred over the E conformation (Chart 1), although the difference in stability varies with the R and R′ substituents. The IR and (resonance) Raman investigations of methyl acetate H3CC(dO)OCH3,59 S-methyl thioacetate H3CC(dO)SCH3,30,58 O-methylthioacetate H3CC(dS)OCH3,57 and methyl dithioacetate H3CC(dS)SCH356,60,62 also suggest that all of the heavy atoms lie in the same plane. In agreement with these results and those from earlier computational work on H3CC(dO)OCH3,59,64 H3CC(dO)SCH3,29-32 and H3CC(dS)OCH3,35 the G2 and G3 results for these compounds show that the Z conformer, with CS symmetry, is the more stable of the two conformers (Table 1). We found the same preferred symmetry and conformation for Se-methyl selenoacetate H3CC(dO)SeCH3, O-methyl selenoacetate H3CC(dSe)OCH3, Se-methyl thioacetate H3CC(dS)SeCH3, and methyl diselenoacetate H3CC(dSe)SeCH3. However, the Z global minima of methyl dithioacetate H3CC(dS)SCH3 and S-methyl selenoacetate H3CC(dSe)SCH3 have C1 symmetry at these levels of calculation. The 1-2° deviation from planarity of the X ) CsSsC backbone and twisting of the H3CC hydrogens observed for these C1 structures (Table 2) were confirmed with MP2(full)/6-31G(d) vibrational frequency calculations for both molecules and with higher level MP2/6311+G(d,p), MP2/cc-pVDZ, and MP2/aug-cc-pVDZ optimizations and frequencies for H3CC(dS)SCH3. For all nine compounds, the E local minima have CS symmetry. The primary determining factor in the energy difference between the Z and the E conformations (Chart 1) appears to be the nature of the Y atom in the XdCsYsC framework. The thermochemical data summarized in Table 1 show that, for each X, as Y proceeds from O to S to Se the E form is increasingly favored with respect to the Z. The calculated Z/E enthalpy difference (31 kJ/mol) for H3CC(dO)OCH3 is in good agree-
ment with the experimental measurement of about 36 ( 4 kJ/ mol59 and with the MP2/6-311+G(d,p) ∆E value of 35 kJ/mol.65 The ca. 25 kJ/mol greater stability of the Z conformer of H3CC(dO)SCH3 is comparable to the value of about 22 kJ/ mol obtained at the MP2/6-31+G(d)30 and MP2/6-31G(d)31 levels of calculation. S-Methyl thioacetate and methyl diselenoacetate are the only compounds for which the effect of the X atom on any thermochemical value is larger than 1-2 kJ/mol. Again with the exceptions of H3CC(dO)SCH3 and H3CC(dSe)SeCH3 the ∆H298 and ∆G298 values are within 3 kJ/mol of each other (Table 1), indicating that, generally, entropy does not play a significant role in the relative stabilities of the two conformations. The larger 7 kJ/mol contribution of the -T∆S term to ∆G298 for H3CC(dO)SCH3 was also obtained with the B3LYP/6-311+G(d,p) calculations of Nagy et al. and can be explained by the difference in the vibrational frequencies of the methyl torsions for the two forms.65 Selected MP2(full)/6-31G(d) geometrical parameters are collected in Table 2 for the Z forms of the nine H3CC(dX)YCH3 molecules and for the transition structures for the 1,3-methyl migrations among them. Because the overall structures of the minima are so similar for the nine systems, only the optimized structures of the Z and E rotamers of H3CC(dSe)SCH3 are displayed in Figure 1. The optimized structures of the transition structures for the three symmetric and three asymmetric interconversions are also depicted in the figure. Experimental geometries are available for H3CC(dO)OCH359 and H3CC(dO)SCH3,31 and as has been reported previously for methyl thioacetate, there is good agreement between the theoretical and the experimental results. However, the gas electron diffraction data for H3CC(dO)SCH3 were refined by fixing two of the tilt angles and a dihedral angle to their MP2/6-31G(d) calculated values.31 Also, only the mean SsC distance and the difference in the two SsC distances were determined from the experimental data. Nevertheless, the resulting values of 1.793 Å and 0.024 Å, respectively, are in excellent agreement with the MP2/ 6-31G(d) values.31 Several patterns emerge from the geometrical parameters for the Z conformers. Whether X ) O, S, or Se, the CdX bond length is the same within 0.006 Å for all three compounds containing that bond. Similarly small differences are found for the YsCH3 bonds. There is more diversity among the C(X)sY bond distances, with a range of 0.017 Å for Y ) O, 0.050 Å for Y ) S, and 0.057 Å for Y ) Se. These changes in CdX and C(X)sY bond lengths are reminiscent of the behavior of amide bonds upon rotation about the CsN bond.64 For formamide, for example, Wiberg and Laidig have shown that the rotation elongates the CsN bond by 0.08 Å but shortens the CdO bond by only 0.01 Å.64
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TABLE 2: Selected Geometrical Parameters of the Z Rotamers and the Transition Structuresa molecule H3CC(dO)OCH3, CS
H3CC(dS)OCH3, CS
H3CC(dSe)OCH3, CS
H3CC(dO)SCH3, CS
H3CC(dS)SCH3, C1
H3CC(dSe)SCH3, C1
H3CC(dO)SeCH3, CS
H3CC(dS)SeCH3, CS
H3CC(dSe)SeCH3, CS
TS-OO, CS TS-OS, C1
TS-OSe, C1
TS-SS, CS TS-SSe, C1
TS-SeSe, CS
bond lengths
bond angles b
CdO: 1.218 [1.206] C(O)sO: 1.356 [1.357] OsCH3: 1.439 [1.438] O · · · CH3: 2.632 CdS: 1.631 C(S)sO: 1.345 OsCH3: 1.437 S · · · CH3: 2.960 CdSe: 1.768 C(Se)dO: 1.339 OsCH3: 1.438 Se · · · CH3: 3.040 CdO: 1.222 [1.214]c C(O)sS: 1.781 [1.781] SsCH3: 1.806 [1.805] O · · · CH3: 2.790 CdS: 1.633 C(S)sS: 1.744 SsCH3: 1.797 S · · · CH3: 3.110 CdSe: 1.771 C(Se)sS: 1.731 SsCH3: 1.796 Se · · · CH3: 3.167 CdO: 1.218 C(O)sSe: 1.935 SesCH3: 1.944 O · · · CH3: 2.886 CdS: 1.627 C(S)sSe: 1.893 SesCH3: 1.936 S · · · CH3: 3.202 CdSe: 1.765 C(Se)sSe: 1.878 SesCH3: 1.935 Se · · · CH3: 3.248 CsO: 1.265 O · · · CH3: 2.001 CsS: 1.668 C(S)sO: 1.267 O · · · CH3: 2.006 S · · · CH3: 2.482 CsSe: 1.806 C(Se)sO: 1.263 O · · · CH3: 2.022 Se · · · CH3: 2.611 CsS: 1.662 S · · · CH3: 2.470 CsSe: 1.800 C(Se)sS: 1.656 S · · · CH3: 2.482 Se · · · CH3: 2.596 CsSe: 1.793 Se · · · CH3: 2.604
dihedral angles
OdCsO: 123.4 [123.0] CsOsCH3: 114.1 [116.4] O · · · CH3sO: 59.4 SdCsO: 125.4 CsOsCH3: 117.8 S · · · CH3sO: 63.4 SedCsO: 125.4 CsOsCH3: 117.8 Se · · · CH3sO: 65.3 OdCsS: 122.1 [122.8] CsSsCH3: 98.2 [99.2] O · · · CH3sS: 66.2 SdCsS: 125.3 CsSsCH3: 102.7 S · · · CH3sS: 69.6
SdCsSsC: 1.4
SedCsS: 125.4 CsSsCH3: 102.7 Se · · · CH3sS: 71.7
SedCsSsC: 0.9
OdCsSe: 121.7 CsSesCH3: 94.9 O · · · CH3sSe: 66.9 SdCsSe: 124.9 CsSesCH3: 99.7 S · · · CH3sSe: 70.0 SedCsSe: 124.8 CsSesCH3: 99.8 Se · · · CH3sSe: 72.1 OsCsO: 117.7 CsO · · · CH3: 88.4 O · · · CH3 · · · O: 65.5 SsCsO: 118.2 CsO · · · CH3: 100.7 S · · · CH3 · · · O: 67.6
OsCsO · · · CH3: 1.1 OsCsS · · · CH3: 1.6
SesCsO: 117.9 CsO · · · CH3: 103.6 Se · · · CH3 · · · O: 68.2
OsCsSe · · · CH3: 1.6
SsCsS: 119.4 CsS · · · CH3: 84.7 S · · · CH3 · · · S: 71.0 SedCsS: 119.4 CsS · · · CH3: 89.0 Se · · · CH3 · · · S: 72.0
SsCsS · · · CH3: 2.7
SesCsSe: 119.1 CsSe · · · CH3: 84.0 Se · · · CH3 · · · S: 72.8
SesCsSe · · · CH3: 3.0
SsCsSe · · · CH3: 1.8
a MP2(full)/6-31G(d) distances in Å and angles in degrees. b Experimental data in brackets from ref 59. c Experimental data (in brackets) and calculated data from ref 31.
Comparing the C(X)sY and YsCH3 distances within the same species (Table 2), the variations decrease in the order Y ) O (e0.099 Å) > Y ) S (e0.065 Å) > Y ) Se (e0.057 Å). For a particular Y, ∆r((Y-CH3)-(C(X)-Y)) increases as X changes from O to S to Se. With respect to the bond angles,
