Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1355 149

added to the basis set. Gonzalez´ and coworkers56 have shown that the correct stability of 2b is well reproduced at the G2(MP2) level of theory, this tautomer being found to be 3.4 kcal mol 1 more stable than 2a.

Another example is provided by the relative stabilities of the different conformers of thioformic acid. It is well established that the s-cis thiol form (4a) of this compound is the global minimum, the s-trans conformer (4b) being 0.16 kcal mol 1 less stable. Ab initio calculations at the Hartree Fock level using different basis sets (see Table 5) also predict the s-cis thiol conformer to be the most stable, but fail to yield the correct energy difference, which at the HF/6-31G(d) level is still about three times greater than the experimental value. This situation does not change appreciably when electron correlation effects are included at second order, and the MP2/6-31 C G(d,p) energy difference is still almost three times too large. Only when higher-order correlation contributions are included in the theoretical treatment by means of G2(MP2) calculations is the correct energy gap between both conformers obtained.

Finally, we should mention that there are also some extreme cases that are not well described even at the G2 level of theory. The most significant examples are the sulfur oxides. Neither the atomization energy of SO nor that of SO2 are correctly reproduced by G2 theory65,71. For the particular case of sulfur monoxide and some of its derivatives, Esseffar and collaborators72 have shown that the dissociation energy of SO and the heat of formation of HSO can only be accurately reproduced by using a very large basis set of the 6-311 C G(5d2f,2p) quality at the quadratic configuration interaction QCISD(T) level, which includes contributions to the energy beyond the fourth order.

2. Thermodynamic aspects

As a consequence of the high reactivity of these compounds, experimentally determined standard enthalpies of formation, fH°m, for thiocarbonyl compounds are quite scarce.

TABLE 5. Relative stability (kcal mol 1) of the s-cis (4a) conformer of thioformic acid with respect to the s-trans (4b)

1366 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

Pilcher73 has recently reviewed the experimental data for a variety of carbonyl and thiocarbonyl compounds. Enthalpies of formation for dithiocarbonic acids, R2N C(DS)SH, thioamides and thioureas were determined by standard calorimetric methods. For an important, older review on the thermochemistry and thermochemical kinetics of sulfurcontaining compounds, see work of Benson74.

fH°m values for thioformaldehyde, H2CS (5) have been obtained by several workers using mass-spectrometric techniques. The most recent results, together with data for formaldehyde (6) and several thiocarbonyl and carbonyl compounds, are presented in Table 6.

Over the last few years, high level ab initio calculations have been performed on 5, 6 and simple cognate species. As indicated earlier, these calculations, either alone or combined with experimental data, have led to the determination of several energetic magnitudes of great relevance. These values were arrived at by combining the appropriate G2 energies (which include the zero-point energy correction) with the formation energies at 0 K for the various atoms. At 0 K, the standard enthalpies of formation,f0H°m, are the same as the standard energies of formation. Representative results are given below.

a. Bond energies and enthalpies. It should be kept in mind that different definitions have been used by different workers and great care should be exercised when comparing their results. Pilcher73 defines bond enthalpies in terms of the enthalpy of atomization of a molecule, aH, the enthalpy for reaction 1:

In order to derive bond enthalpies, Pilcher selects ‘those molecules for which it is reasonable to assume absence of strain energy and stabilization energy’.

Representative results for relevant bonds are given in Table 7.

TABLE 6. Thermochemical data for representative thiocarbonyl and carbonyl compoundsa

aAll values in kcal mol 1. bFrom Reference 75.

c Upper limit.

dFrom Reference 76. eFrom Reference 73. fFrom Reference 77. gFrom Reference 78.

Bond dissociation energies, D0 (A B), defined by equation 3, have been determined experimentally by Berkowitz and coworkers using photoionization mass spectrometry75.

These results are summarized in Table 8 and portray the energy changes for the stepwise atomization of 5. Data for 6 are given for comparison purposes.

The high quality of the G2 results allows one to use bond energies computed at this level for quantitative studies. Representative results are given in Table 9.

Some important conclusions derived from the results given in Tables 6 to 9 are as follows: (1) Computed and experimental D0 values are in excellent agreement. (2) By all criteria, CO bonds in H2CO and CO are stronger than CS bonds in H2CS and CS. The differences between the two homologous couples are, however, quite important,

TABLE 8. Experimental bond energies, D0a, for 5, 6 and species derived therefrom

1368 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

TABLE 9. Standard enthalpies, ofH°m

(g) and dissociation energies at O K, D0, computed at the G2 level for 5 and 6a

aAll values in kcal mol 1. bFrom Reference 66.

cFrom Reference 80.

namely 46.6 and 85.6 kcal mol 1, respectively (see Table 9). (3) The CH bonds in H2CO and HCOÐ are significantly weaker than those in H2CS and HCSÐ. (4) The CS bond in H2CS is substantially weaker than the CO bond in H2CO. Its stability, however, is appreciably higher than that of many ‘strong’ single bonds (see Table 7) and barely 12 kcal mol 1 lower than that of the CC bond in C2 H4 (computed using Pilcher’s scheme). Thus, the apparent instability of thiocarbonyl compounds is not a consequence of the intrinsic weakness of the thiocarbonyl group but rather follows from the fact that other atomic arrangements are energetically more favorable. In this respect, if we consider the thiol enethiol equilibrium embodied in reaction 4, use of the data given in Table 7 leads to an upper limit (i.e. neglecting resonance stabilization of the enethiol) for H 5 of17 kcal mol 1, a substantial value.

CH3CSH ! H2CDCHSH H 4 4

From a chemical standpoint, the decomposition of the energies of the double bonds into the corresponding and components, E and E , is important. Unfortunately, this is a conceptual division and these quantities are not quantum-mechanical observables. Thus, approximate methods have been devised to estimate these contributions.

In the case of 5 and 6, Schleyer and Kost81 used reactions 5 and 6 to compare the energies of the CDX double bonds with those of the single bonds C X:

E 7 provides D0(C X), taken as a measure of E , the contribution of the bond to the overall stability of the CDX bond; E 5 , when substracted from 2D0(C X), is intended to yield an estimate of the total energy of the bond, E C . The bond contribution, E , is given by E C E . Representative results, based on experimental data, are given in Table 10.

erence 78.

Schmidt’s group82, developed a scheme based on the definition of E as the enthalpy change at 0 K for reaction 7:

This magnitude can be determined by combining the enthalpy change at 0 K for the hydrogenation reaction 8 with the appropriate bond dissociation energies:

Selected results are also given in Table 10.

Although the two sets of data are not identical, they nevertheless define the same pattern:

E (CS)/E CC ³ E CS/E CC ³ 0.80 and E CS/E CO

³ E CS/E CO ³ 0.66

An important contributor to the enhanced stability of the CO bond relative to that of the ethylenic double bond originates in the sizable contribution of the limiting structure 6b.

This follows from the difference between the electronegativities of oxygen and carbon1. As indicated by Wiberg and coworkers83 85, the Coulombian attraction between C and O contributes significantly to the overall stability of the bond. It is remarkable that, even in the presence of electron-donor substituents, structures such as 6b are very important in the case of carbonyls and much more so than in the case of thiocarbonyls83. The importance of structure 6b brings about a reduction in the bond order of the carbonyl group: 1.371 vs 2.026 for C2H4. At variance with this, the bond order in the thiocarbonyl group of 5 reaches 1.71881. Other factors are to be considered when comparing the stability of the thiocarbonyl and carbonyl bonds. They were very ably examined by Kutzelnigg86.

b. Conjugative and electronegativity effects. In substituted carbonyl and thiocarbonyl groups, conjugation between the systems of these groups and the electronic systems of

1370 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

appropriate symmetry of the substituents can take place. These interactions may confer importance to limiting structures such as 5d, 5f, 6d and 6f.

This, in turn, will affect the energetics of the molecules, their rotational barriers and their reactivity. The relative importance of these structures will be determined to a large extent by the difference in electronegativities between oxygen and sulfur as well as by the ability of X and Y C to either donate or receive electrons. The electronegativities of X and Y C also play a key role, as they strongly affect the degree of polarization of the carbonyl and thiocarbonyl groups (including both and bonds. Several schemes have been set forth for the purpose of quantifying the influence of substituents on the stability of both families of compounds.

(a) Abboud and coworkers39 have used the families of reactions 9:

Wiberg and coworkers83 85 have used reactions 10:

Most of the energetic data used in these studies were obtained by computational methods [6-31 G(d)39 and G283]. We present in Table 11 a set of data taken from these studies as well as from Notario’s work87.

(b) A direct way of comparing substituent effects on both families is by means of the isodesmic reactions 11 and 12, related to reactions 9 and 10, respectively.

Values of E O,s and H°0 O,s are also given in Table 11. This table lists data for potential electron-donor substituents. Relative to hydrogen, most of them exert a stabilizing effect.

Some conclusions derived from these data are as follows: (1) Substituent effects on carbonyl groups are systematically larger (in absolute value) than those on thiocarbonyl.

(2) Electron-donor groups always exert a stabilizing effect. (3) Substituent effects are not additive.

E H,X,S , E H,X,O and E O,S can be analyzed in terms of the Taft Topsom formalism88 wherein these effects are decomposed into electronegativity, polarizability, field and resonance contributions, respectively measured by the descriptors , ˛, F andR. In view of the size of the database, only two parameters are used. Linear combinations of and R lead to a reasonably good description of the E values indicated above. The main semiquantitative conclusions of such an analysis are as follows:

(1) Electron-donation effects (structures 6d and 5d) are quite important and stabilizing in both cases. According to Wiberg83, this effect is relatively more important in the case of thiocarbonyls. Structures 6f and 5f do not seem to exert a sufficiently strong stabilizing effect, the overall effect in the case of cyano and nitro derivatives being moderately

1372 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

destabilizing. The mutual repulsion of the dipoles also comes into play. This fact notwithstanding, the recent determination of the PES of thioformylcyanide by Pfister-Guillouzo and collaborators89 provided direct proof of the relevance of structure 6f. These authors analyzed their experimental results in terms of a correlation diagram showing that the overlap between the CS MO of H2CS and one of the degenerate CN orbitals of HCN leads to a strongly stabilized in-phase combination essentially isoenergetic with the original MO of H2CS. Elecron-donation from the thiocarbonyl group to CN and C CH groups is also confirmed by analysis of the stretching frequencies and bond lengths in thiopropynal and formyl cyanide40. Similar behavior is also found in ˛-thiocarbonyl cations. As shown by Creary’s group90, the thiocarbonyl cation (7a) displays features suggestive of substantial thiocarbonyl conjugation. For instance, comparison of bond lengths of 7a with those of the corresponding carbonyl analog 7b, in particular the significant shortening of the C C bond (from 1.475 A˚ in 7b to 1.403 A˚ in 7a), indicates that thiocarbonyl conjugative stabilization of a cationic center is more important than carbonyl conjugation. It must be mentioned that, despite the stabilization of 7a due to thiocarbonyl conjugation, the most stable cation corresponds to the cyclic 7c form, which lies 28.1 kcal mol 1 below the open 7a form90.

A strong resonance stabilization is also found for 4-sulfomethylenecyclopropene (8a). Bachrach and Liu91a have estimated that the delocalization energy for 8a is about 4.4 kcal mol 1 higher than that of the carbonyl analog 8b. This finding was explained in terms of the lower electronegativity and larger polarizability of sulfur91a. These and other cyclopropenes have been studied by Burk and coworkers91b.

S O

(8a) (8b)

The lower electronegativity of sulfur with respect to oxygen explains also the shortening of the C C bond in thioketones with respect to the corresponding ketones92. In the most stable conformer of thioaldehydes93, thioketones92,94,95 and thioamides57,96, the methyl groups show a systematic preference to eclipse the CDS bond. This is also the most common situation as far as carbonyl derivatives are concerned. However, when this comparison is carried out for further terms in the series of aldehydes, some differences appear with respect to the analogous thioaldehydes. Ab initio calculations95 show that, similarly to what has been found for thiols and thioethers97 98, thiopropionaldehyde preferentially adopt the gauche conformation (9a), in contrast to the analogous oxygen compounds which generally adopt the trans (9b) form97.

(2) The role of electronegativity determines an important difference between the two families. For thiocarbonyl compounds, the contribution is small and barely at the limit of statistical significance. In the case of the carbonyls, the contribution is large and stabilizing.

1374 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

strong contributions from the fluorine orbitals and from the CDO subunit orbitals, respectively, while for amino and hydroxy substituents, these weights are more evenly balanced. From the energetic point of view, this implies that the out-of-phase combination will be much higher in energy when the substituents are NH2 or OH than when the substituent is fluorine. A quantitative calculation shows that in the first two cases this orbital becomes the HOMO of the system (see Scheme 1). Thus, while as in the parent compound (basically an oxygen lone-pair), in formaldehyde and formic acid this orbital lies below the aforementioned -MO. Scheme 1 also shows that these -interactions are slightly more favorable for OH than for NH2 substituents, since for the former both interacting MOs are almost degenerate. There is a second interaction which affects the lower energy orbitals of CDO. These interactions become more effective as the electronegativity of the substituent increases. In constrast to -orbitals, the -MOs of the parent compound contain a contribution from the hydrogen atom orbitals. When hydrogen is replaced by a more electonegative system, the contribution of the substituent orbitals to the corresponding -MO increases significantly. This implies that the larger the electronegativity of the substituent, the lower the participation of the carbon orbitals to the corresponding MO should be. This perturbation should be reflected in a greater CC O polarity of the carboxylic bond and hence in a stabilization. For the thiocarbonyl series the situation is qualitatively similar but quantitatively different from carbonyl systems. The MOs of the parent compound (5) are analogous to those in 6 and follow the same ranking of energies. However, -interactions are now less favorable because the -MO of the CDS subunit lies higher in energy than that in CDO. As a consequence, the energy gap with respect to the substituent orbitals increases (see Scheme 1). This is reflected in the fact that, while formamide presents the two HOMOs in a reversed order with respect to formaldehyde, in thioformamide the order is the same as in thioformaldehyde. Scheme 1 also shows that the interaction is now more favorable with the orbitals of the NH2 group than with those of the OH. This is in good agreement with the results of Table 11, which show that while the carbonyl system is more stabilized by hydroxy than amino substitution, the opposite holds for thiocarbonyl compounds.

Hopkinson and coworker99 have just published ab initio MO calculations at MP2(FULL)/6-311G(d,p) or MP2(FULL)/6-31G(d,p) levels on carbocations RR0 CCHOC , RR0 CCHSC , RR0 CCONH2C and RR0 CCSNH2C where R and R0 D H, CH3, c-C3H5 and C6H5. Primary (R D R0 D H), secondary (R D H, R0 D alkyl or phenyl) and tertiary (R D R0 D CH3) prefer the cyclic oxiranyl or thiiranyl structure 10, with open structures such as 11 being a transition structure for ring opening. Tertiary carbocations with R D R0 D phenyl or cyclopropyl and the 9-thioformamidyl-9-fluorenyl cation (12) have an open structure. These authors provide strong experimental evidence suggesting that 12 has indeed this open structure. Isodesmic reactions show CONH2 to be weakly stabilizing in the methyl cation, and CSNH2 has a larger stabilizing effect, roughly equivalent to that of a methyl group. An ˛-thioamide substituent is less stabilizing in the ethyl cation and even less stabilizing in the isopropyl cation. In C6H5CHCSNH2C the CSNH2 group is slightly destabilizing and, by extrapolation, is more destabilizing in Ar2CCSNH2C .

(Y = O, S)