Computational determination of the enthalpy of formation of alkylthial

S-oxides and alkylthione S-oxides: a study of (Z)-propanethial-S-oxide, the lachrymatory factor of the onion (Allium cepa)

Martina Kieninger and Oscar N. Ventura*

CCPG, Dequifim, Facultad de Quımica, CC 1157, 11800, Montevideo, Uruguay.E-mail: oscar@bilbo.edu.uy; Fax: 5982 924 1906; Tel: 5982 924 8396

Received 14th May 2002, Accepted 25th July 2002First published as an Advance Article on the web 20th August 2002

A general computation procedure for the determination of unknown enthalpies of formation of alkylthial oralkylthione S-oxides, based on the recent accurate determination of the enthalpy of formation of methanethialS-oxide, is proposed. The method relies on the use of isodesmic reactions involving the sulfoxide and sulfide ofthe species of interest, with the individual energies calculated at the CBS-QB3 level. A discussion is given aboutthe convenience of using density functional theory (DFT) instead of the CBS-QB3 model chemistry,especially for large compounds. To exemplify the method, the enthalpies of formation of ethanethial-S-oxide(�74.7� 8.4 kJ mol�1), propanethione-S-oxide (�112.1� 8.4 kJ mol�1) and propanethial-S-oxide(�94.1� 8.4 kJ mol�1) are calculated, as well as those of the necessary sulfides propanethial (50.4� 8.4 kJmol�1) and propanethione (28.0� 8.4 kJ mol�1). These values were derived on the basis of an independenttheoretical estimation of the enthalpy of formation of thioformaldehyde (113.2� 4.2 kJ mol�1, in agreementwith the experimental value, 118� 8.4 kJ mol�1). The use of the same method for evaluating the enthalpy offormation of ethanethial however, gave 68.1� 4.2 kJ mol�1, higher than the experimental value, 50� 8 kJmol�1. Convincing evidence is given that the theoretical estimation should be preferred to the experimental onein this case. Moreover, it was determined that the most stable isomer of propanethial S-oxide is in fact theZ-conformer, where the terminal methyl group is about 120� out of the plane of the C=S=O group. However,the C=S and=CH–CH2– bond lengths are very different from the recent experimental determination.A comparison with the experimental and theoretical results for the lower members of the series suggests that themodel assumed for deriving the bond lengths from the microwave spectrum may be at fault. The whole processis validated by comparison of the enthalpies of formation of thioaldehydes and thioketones to the knownenthalpies of formation of aldehydes and ketones.

Introduction

Organosulfur compounds are essential components in the tis-sues of Allium species (onion, garlic) which are responsiblefor some of the sensory responses to these species1 and mayexhibit therapeutic activities.2 One of the best known examplesof the behavior of these substances is the lachrymatory actionproduced by sliced or crushed onions. S-alk(en)yl-L-cysteinesulfoxides are transformed by the alliinase enzymes to transi-ent alk(en)yl sulfenic acids from which the lachrymatory fac-tor, propanethial-S-oxide (CH3CH2CH=S=O, 1), is produced.3

Condensation to odoriferous thiosulfinates occurs at the sametime, leading to the widespread (but incorrect) associationbetween the smell of the sliced onion and the lachrymatoryaction.Sulfine chemistry in general is not a well explored field. For

instance, the enthalpy of formation of the lowest member ofthe series, methanethial S-oxide (sulfine, H2C=S=O, 4), wasfor some time a source of controversy,4 settled only recently.5

Very few thermochemical properties are known for the highermembers in the series3 and even the geometrical structure ofpropanethial S-oxide (1) was determined experimentally onlyin recent times.6 Very recently, Shen and Parkin7 developeda system that allows the generation of pure thiosulfinatesand propanethial S-oxide, 1, thus ensuring that this family ofcompounds will receive much more attention in the next few

years. Our purpose in this connection will be the determinationof thermochemical properties which may help in the follow upof the complicated chemical reactions observed in freshly pre-pared Allium extracts.Work performed in the last decade has shown conclusively

that computational procedures are rapidly becoming themethod of choice for the accurate determination of enthalpiesof formation and reaction (as well as other thermochemicalproperties) of chemical compounds.8 Several papers haveshown that different model chemistries9,10 can be used for thispurpose. Also density functional methods11 may work well,12

provided that isodesmic reactions13 are employed in the deter-mination. In this paper we want to demonstrate the power ofthis approach in connection with organosulfur chemistry,developing a general procedure for computing the enthalpyof formation of alkylthials and alkylthiones, as well as thoseof their S-oxide derivatives.

Methods

Isodesmic reactions13 are particularly suited for minimizationof the error related to the calculation of enthalpies of forma-tion. A reaction of the type

Aþ B ! CþD; DrH�298 ð1Þ

4328 Phys. Chem. Chem. Phys., 2002, 4, 4328–4333 DOI: 10.1039/b204643a

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is isodesmic if all the bond types and lone pairs present on thereactant side are also present (eventually not in the same mole-cule) on the product side. The common practice implies theknowledge of accurate experimental enthalpies of formationfor all but one of the species in reaction (1), which we will callgenerically S, the calculation of the enthalpy of reaction withthe theoretical method at hand, and the obtention of theunknown enthalpy of formation of the remaining species X as

DfH�298ðXÞ ¼ arDrH

�298 � SS6¼X aSDfH

�298ðS; expÞ ð2Þ

where ar and aS are either 1 or �1 depending on which side ofthe equation the X target species is situated. The main problemin using reaction (2) is that it is not always simple to find anisodesmic reaction for the target compound, in which all theother required enthalpies of formation are known experimen-tally. This is exactly the case for propanethial S-oxide, 1, forwhich the following isodesmic reactions can be written

Et CHSOð1Þ þH2CSð2Þ ! Et CHSð3Þ þH2CSOð4Þ ð3ÞEt CHSOð1Þ þMe CHSð5Þ ! Et CHSð3Þ þMe CHSOð6Þ

ð4ÞEt CHSOð1Þ þ SO2ð7Þ ! Et CHSð3Þ þ SO3ð8Þ ð5Þ

Et CHSOð1Þ þH2CSð2Þ ! Me CHSð5Þ þMe CHSOð6Þð6Þ

Et CHSOð1Þ þMe CHSð5Þ ! ðMeÞ2CSð9Þ þMe CHSOð6Þð7Þ

None of these reactions is directly useful however, becausethe necessary experimental data for propanethial (3), etha-nethial S-oxide (6), or propanethione (9) are not available.Therefore, we need a general scheme to calculate the missingdata, at the same time that we get an estimation of the errorinvolved. Our approach will be to construct a ladder of isodes-mic reaction leading to the ones we need to calculate theenthalpy of formation of propanethial S-oxide, 1. In the firstplace, we will use experimental data and our own previouscomputational estimation of the enthalpy of formation of sul-fine (H2CSO, 4)5 in the isodesmic reaction

Me CHSO ð6Þ þH2CS ð2Þ ! CH2SO ð4Þ þMe CHS ð5Þð8Þ

to calculate the enthalpy of reaction of ethanethial S-oxide (6)which is not known experimentally. This isodesmic reactioncan be formally looked at as if a methyl group has been‘‘ transfered ’’ between the species. The possible error can begauged by comparing the results with those obtained usinganother isodesmic reaction

Me CHSO ð6Þ þ SO2 ð7Þ ! Me CHS ð5Þ þ SO3 ð8Þ ð9Þ

where the formal ‘‘ transfer ’’ is now true with respect to an oxogroup. Notice that in the first case there is no change in thevalency of any of the sulfur atoms, while in the second casetwo S(IV) atoms turn into a S(II) and a S(VI) atom. This factcan cause some slight problem that will be discussed whenappropriate.Once the enthalpy of formation of ethanethial S-oxide, 6, is

available through reactions (8) and (9), it can be used to deter-mine the enthalpy of formation of other alkylthial andalkylthiones whose values are not available experimentally.As an example, we will determine the enthalpy of formationof propanethione, (Me)2CS, 9, using the isodesmic reaction

Me CHSOð6Þ þMe CHSð5Þ ! H2CSOð4Þ þ ðMeÞ2CSð9Þð10Þ

With the available enthalpies of formation of 6 and 9, it is nowpossible to determine the enthalpies of formation of higher

members of this series. As an example, we chose to calculatepropanethione-S-oxide, 10, for which we can write the reac-tions

Me CHSOð6Þþ ðMeÞ2CSð9Þ! ðMeÞ2CSOð10ÞþMe HCSð5Þð11Þ

Me CHSOð6Þ þMe CHSð5Þ ! ðMeÞ2CSOð10Þ þH2CSð2Þð12Þ

ðMeÞ2CS ð9Þ þH2CSO ð4Þ ! ðMeÞ2CSOð10Þ þH2CSð2Þð13Þ

ðMeÞ2CSO ð10Þ þ SO2 ð7Þ ! ðMeÞ2CSð9Þ þ SO3ð8Þ ð14Þ

Thus, we have now all the necessary ingredients for calculatingthe enthalpies of formation of propanethial S-oxide, 1, usingreactions (6) and (7), and propanethial, 3, using reactions (3)to (5).Since the whole procedure rests on the correctness of the

enthalpy of formation of 6 determined through eqn. (8) and(9), it is important to have some alternative procedure to assessthe precision and accuracy of these calculations. With this pur-pose in mind, we constructed the following isodesmic reactions

H2CS ð2Þ þ CO ð11Þ ! CS ð12Þ þH2CO ð13Þ ð15ÞMe CHS ð5Þ þ CO ð11Þ ! CS ð12Þ þMe CHO ð14Þ ð16ÞMe CHS ð5Þ þH2CO ð13Þ ! H2CS ð2Þ þMe CHO ð14Þ

ð17Þ

The enthalpies of formation of all the species in reactions (15)to (17) have been determined experimentally. Thus, these reac-tions may serve as a quality control for the results obtainedpreviously.Moreover, we can also employ these experimentally well-

known enthalpies of formation to write eqn. (18) and (19)

ðMeÞ2CS ð9Þ þ CO ð11Þ ! CS ð12Þ þ ðMeÞ2CO ð15Þ ð18ÞðMeÞ2CS ð9Þ þH2CO ð13Þ ! H2CS ð2Þ þ ðMeÞ2CO ð15Þ

ð19Þ

These equations afford an alternative path for calculating theenthalpy of formation of propanethione, 9, one of the inter-mediate links, through reaction (7), for the calculation of theenthalpy of formation of 1.The available experimental enthalpies of formation were

obtained from the NIST Standard Reference DatabaseN�69.14 The enthalpies of reaction for the different moleculeswere obtained using quantum chemistry methods. Two differ-ent models were employed. On the one hand, density func-tional (DFT) methods11 were used to calculate the electronicenergy. Becke’s three parameter methods,15 including Becke’sexchange functional16 and either the Lee–Yang–Parr17 or thePerdew–Wang18 correlation functionals were employed. Theseare the B3LYP and B3PW91 models respectively, and theywere used with the 6–311++G(3df,2pd) Pople basis set.19 Geo-metry optimization of all the molecules intervening in the cal-culations was performed until the bond lengths were convergedto 10�4 A and the angles to 10�2 degrees. Analytical secondderivatives were calculated and it was verified that no negativeeigenvalues were present. Thermochemical functions wereobtained in the standard way (within the harmonic oscilla-tor/rigid rotator model) at standard temperature and pressureconditions.Alternatively, the complete basis set CBS-QB3 model chem-

istry9 was used. Model chemistry methods are characterized bythe fact that geometries and frequencies obtained at some levelof theory are combined with higher level calculations tailoredto include appropriate basis sets and correlation energy correc-tions. In the specific case of the CBS-QB3 method employedhere, the geometries and frequencies are calculated at the

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B3LYP/6-311G(2d,d,p) level (2 d polarization funcions in sec-ond-row atoms, 1 d polarization function in first-row atomsand 1 p polarization function on H). Higher order correctionsare calculated at the UMP2, MP4(SDQ) and CCSD(T) levels,and combined into an extrapolation formula which gives theenergy at the complete basis set limit (hence CBS). Moredetails about the method can be found in the original refer-ence.9 The maximum error of this method is below 12 kJmol�1, while the accuracy (mean error) is about 4 kJ mol�1.A final note with respect to the CBS-QB3 method is that one

can calculate enthalpies of formation using simply the atomi-zation reactions, since spin–orbit coupling and bond-additivitycorrections are built into the method. While the numbersobtained at the DFT level would not be significant, thoseobtained at the CBS-QB3 level may prove valuable. They aregiven here together with those obtained from the isodesmicreactions, and used in the averaging process (see later).All the calculations reported were performed using the

Gaussian 98 suite of computer programs.20

Results

The computational absolute enthalpies for all the moleculesincluded in reactions (4) to (19), as well as the available experi-mental values, are given in Table 1. We have included also theatoms, to estimate the errors produced in the computation ofthe enthalpies of formation using atomization reactionsinstead of the isodesmic ones.The first important result concerns the enthalpy of forma-

tion of ethanethial S-oxide, 6, obtained from reactions (8)and (9). As one can see in Table 2, the results obtained atthe CBS-QB3 level differ only by 1.3 kJ mol�1. Therefore,one can say that the results obtained from these two differentreactions agree quite well. However, one has to keep in mindthat there is a common factor in both reactions: the experimen-tal enthalpy of reaction of ethanethial, 5. We will have more tosay about this subject later. Incidentally, notice that theB3PW91 results for these reactions are in line with the CBSones, although the B3LYP result for reaction (9) is a bit off.In order to discuss trends and not individual values, we willuse the averages of the enthalpy of formation obtained witheach method for all the reactions considered. These valuesare also listed in Table 2.

Once the results for propanethione were obtained usingreaction (10), all the other results follow easily from the restof the reactions. The precision of the determinations (as 2s)is always below 4 kJ mol�1.The second important point concerns the accuracy of the

determinations and this is related, in turn, to the anchors wehave chosen for the theoretical estimations. As we said in theprevious section, one way to gauge the accuracy of the deter-minations is to connect the thioaldehydes and thioketones withaldehydes and ketones, whose experimental enthalpies of for-mation are determined with better accuracy. The simplest ofthese reactions is reaction (15) from which the enthalpy of for-mation of thioformaldehyde can be derived. Experimentally,this value was determined by Roy and McMahon in 198222

and by Ruscic and Berkowitz in 1993.23 The first value wasmuch too low (90� kJ mol�1) while the one accepted nowa-days is 118� 8.4 kJ mol�1, as reported in Table 1. The experi-mental enthalpies of formation of the other species in reaction(15) are well known14,21 and combined with the enthalpy ofreaction calculated at the three different computational levels,give an average theoretical estimation of 113.2� 4.2 kJ mol�1,in good agreement with the experimental result. Notice thatthe CBS-QB3 estimation is practically the same regardless ofwhether one uses the isodesmic or the atomization reactions.A startling result was obtained from reactions (16) and (17)

however. The average enthalpy of formation derived for ethan-ethial, 5, is 70.4� 4.2 kJ mol�1, clearly outside the error of theexperimental determination, 50� 8 kJ mol�1. Should we preferthe experimental determination or the computationally derivedvalue? In our opinion, the experimental determination is prob-ably wrong on two counts. First, the experimental set-up (ioncyclotron resonance spectrocopy) and year of determination(1983, Butler and Baer24) are analogous to the first experimen-tal determination of the enthalpy of formation of H2CS (1982,Roy and McMahon22) which was much too low. Second, theindependent values determined through the use of reactions(16) and (17) are in agreement, which would not be the caseif the experimental enthalpy of formation 50� 8 kJ mol�1 werecorrect. In fact, if this were the case, reaction (17) could beused to determine the enthalpy of formation of H2CS, giving94.7 kJ mol�1, more in agreement with the much too low ori-ginal experimental determination of the enthalpy of formationof 2 than with that accepted nowadays. On the strength ofthese arguments then, we propose to accept the CBS-QB3

Table 1 Absolute enthalpies (in Eh) and experimental enthalpies of formation (in kJ mol�1) of the specied involved in the reactions studied

Species B3LYP/6�311++G(3df,2pd) B3PW91/6�311++G(3df,2pd) CBS-QB3 Experimentala

H �0.499897 �0.501705 �0.497457 217.998� 0.006

C �37.855111 �37.833139 �37.783025 716.68� 0.45

O �75.088554 �75.057303 �74.987633 249.18� 0.10

S �398.132138 �398.077503 �397.655005 277.17� 0.15

Propanethial S-oxide, 1 �591.323832 �591.195718 �590.5268

Thioformaldehyde, 2 �437.483009 �437.412301 �436.934273 118.� 8.4

Propanethial, 3 �516.089422 �515.989591 �515.392112

Methanethial S-oxide, 4 �512.72026 �512.621317 �512.070537 �29.7� 6.0b

Ethanethial, 5 �476.792441 �476.707428 �476.168061 50.� 8.

Ethanethial S-oxide, 6 �591.323832 �591.195718 �590.5268

Sulfur dioxide, 7 �548.704936 �548.591685 �548.033595 �296.81� 0.20

Sulfur trioxide, 8 �623.919508 �623.781275 �623.150824 �395.77

Propanethione, 9 �516.098522 �515.999294 �515.401271

Propanethione S-oxide, 10 �591.330854 �591.203402 �590.53419

Carbon monoxide, 11 �113.348438 �113.295885 �113.178661 �110.53� 0.17 �110.53c

Carbon sulfide, 12 �436.25113 �436.177029 �435.710579 280.33 279.78c

Formaldehyde, 13 �114.519532 �114.470052 �114.340319 �115.90

Acetaldehyde, 14 �153.833171 �153.768961 �153.577598 �170.7� 1.5

Acetone, 15 �193.143619 �193.06484 �192.814063 �218.5� 0.59

a Taken from ref. 14 unless noted otherwise. b Determined theoretically, see ref. 4 and 5. c Ref. 21.

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value of 68� 4 kJ mol�1 as the enthalpy of formation of 5 anduse this value for the calculation of the other enthalpies of for-mation. These results are those given in parenthesis in Table 2.One can see that now the values for the enthalpy of formationof propanethione, 9, determined from reactions (10), (18) and(19) are in agreement, which was not the case previously, lend-ing further support to the proposal.Using then the CBS-QB3 averages of the values obtained

with the different reactions (employing the newly proposedvalue of the enthalpy of formation of ethanethial) weconclude the following: DfH

�298(CH3CHSO) ¼ �74.7� 8.4

kJ mol�1; DfH�298(CH3CH2CHSO) ¼ �94.1� 8.4 kJ mol�1;

DfH�298((CH3)2CSO) ¼ �112.1� 8.4 kJ mol�1; DfH

�298(CH3-

CH2CHS) ¼ 50.4� 8.4 kJ mol�1 and DfH�298((CH3)2CS) ¼

28.0� 8.4 kJ mol�1.It is pleasing to observe (Fig. 1) that the trend of the enthal-

pies of formation of thioformaldehyde, ethanethial, propa-nethial and propanethione follow closely the trend of thecorresponding oxo species formaldehyde, acetaldehyde, propa-naldehyde and acetone, lending more support to our methodof estimation. Notice also that the enthalpies of formation cal-culated on the basis of the atomization reactions are somewhaton the low side of those obtained with the isodesmic reactions.However, we deemed it appropriate to include them in theaveraging of the CBS-QB3 results because they are never toofar away from those of the isodesmic reactions. The possibleerror is reasonably accounted for in the large error bar.It can be observed that the averages of the DFT results are

reasonably close to those of the CBS-QB3 method and wouldbe even closer if the atomization energies were not consideredin the CBS-QB3 average. All in all one can say that the use ofthe DFT methods is as good as that of the CBS-QB3 model forthe purpose of these calculations. We believe that this conclu-sion allows one to use DFT methods to extend the model pro-posed here to much larger compounds without too much lossin accuracy.A final important result was obtained in the study of propa-

nethial S-oxide. The geometry of this compound optimized ateither the B3LYP or B3PW91 levels (as we said previously, theCBS-QB3 method uses B3LYP for the geometry optimizationstep) show a marked discrepancy with the experimental C=Sand =CH–CH2– bond lengths derived from the microwavespectrum. Gillies et al.6 determined the geometry of propa-nethial S-oxide employing pulsed-beam Fourier transformmicrowave spectroscopy. They found a (Z)/(E) conformer

Fig. 1 Comparison of enthalpies of formation (in kJ mol�1) for alde-hydes and thioaldehydes.

Table 2 Enthalpies of reaction and derived enthalpies of formation of the species of interest (in kJ mol�1)

Reaction B3LYPa B3PW91a CBS-QB3 Speciesb B3LYPa B3PW91a CBS-QB3

(6) �32.69 �34.15 �25.65 1 �130.94(�88.74) �125.19(�82.99) �136.61(�94.41)

(7) �23.88 �25.58 �24.13 1 �138.97(�96.77) �128.44(�86.24) �132.11(�89.91)

Atomization 1 �98.1

Average 1 �92.8 �84.6 �94.1

(15) 159.82 160.66 163.11 2 115.14 114.30 111.85

Atomization 2 111.7

Average 2 115.1 114.3 111.8

(3) �7.47 �7.60 �4.14 3 5.28(47.48) 13.29(55.49) 9.20(51.40)

(4) 0.05 �0.07 �0.05 3 10.72(52.92) 14.46(56.66) 9.85(52.05)

(5) 52.16 43.48 45.90 3 16.17(58.37) 15.63(57.83) 10.50(52.70)

Atomization 3 45.6

Average 3 52.9 56.7 50.4

(16) 148.76 150.72 153.93 5 71.40 69.44 66.23

(17) �11.06 �9.94 �9.18 5 74.26 73.14 72.38

Atomization 5 65.8

Average 5 72.8 71.3 68.1

(8) �7.52 �7.53 �4.10 6 �90.18(�69.08) �90.17(�69.07) �93.60(�72.50)

(9) 52.11 43.55 45.95 6 �101.07(�79.97) �92.51(�71.41) �94.91(�73.81)

Atomization 6 �77.9

Average 6 �74.5 �70.2 �74.7

(10) 1.29 1.04 �2.58 9 �17.22(24.98) �12.68(29.52) �11.98(30.22)

(18) 137.28 140.17 145.37 9 35.08 32.19 26.99

(19) �22.54 �20.50 �17.74 9 37.94 35.90 33.14

Atomization 9 21.5

Average 9 32.7 32.5 28.0

(11) 5.42 5.37 4.70 10 �162.88(�120.68) �149.81(�107.61) �152.19(�109.99)

(12) 14.23 13.95 6.22 10 �154.85(�112.65) �146.56(�104.36) �156.69(�114.49)

(13) 12.93 12.90 8.79 10 �151.99(�109.79) �147.48(�105.28) �150.89(�108.69)

(14) 46.70 38.17 41.25 10 �162.88(�120.68) �149.81(�107.61) �152.19(�109.99)

Atomization 10 �117.5

Average 10 �116.0 �106.2 �112.1

a Obtained using the 6-311++G(3df,2pd) basis set. b Species whose enthalpy of formation is determined using the reaction in the same row.

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ratio of 98/2 favoring then the (Z) structure, as obtained com-putationally. They noted in this paper that they did not havesufficient moment of inertia data to calculate the 33 uniqueatomic coordinates required to completely determine the mole-cular structure. Therefore, they followed an approximate pro-cedure from which they derived the geometrical parametersreported in Table 3. It is immediately striking that the C=Sbond length is much shorter than that found experimentallyfor methanethial and ethanethial S-oxides. The B3PW91 calcu-lations, which agree reasonably well with the experimentaldata for those two species, predict a C=S bond length whichis consistent with the value for the other sulfines and quite abit larger than that determined experimentally. The same istrue with respect to the=CH–CH2– bond length, although inthis case we have no other experimental data with which tocompare. We are then again forced to choose between theexperimental and computational data.In our opinion, the computational result should be trusted

more in this case, than the experimental one. Although micro-wave spectroscopy is normally much more accurate than anycomputationally determined geometry for a medium size mole-cule, in this case there are two pieces of data which alter thebalance. First, the close agreement between the three com-puted C=S bond lengths and the absence in 1 of any funda-mental difference with respect to 4 or 6 is one aspect to takeinto account. Second, the geometry determined on the basisof the observed rotational transitions gives a =C–H bondlength of 1.216 A, clearly anomalous, as properly acknowl-edged by those authors. Using torsional and bond shrinkageparameters, the authors were able to obtain a reasonable CHbond length (1.078 A) at the cost of a general structure whichis even further from the computational one or the similar para-meters in the other sulfines. For these reasons, we think thatthe structure derived from the experimentally observed rota-tional transitions should not be trusted completely and thecomputational results represent a better set of values.

Conclusions

The enthalpy of formation of several organosulfur compounds(thioaldehydes, thioketones and their S-oxides) were deter-mined computationally by coupling a series of isodesmic reac-tions with the CBS-QB3 and DFT calculation of enthalpies.Among these values, the enthalpy of formation of propa-nethial S-oxide, the lachrymatory factor in onions, was deter-mined. In the process, it was shown that the experimentallyaccepted enthalpy of formation of ethanethial is too smalland should probably be corrected, in the same way as theexperimental enthalpy of formation of thioformaldehyde was(and with which the computational estimation agrees well

enough). A new estimation of 68.1� 4.2 kJ mol�1 is proposedfor the enthalpy of formation of ethanethial. Moreover, it wasshown that the structure of propanethial S-oxide, derived fromthe experimental rotational transitions in a microwave spectro-scopy study, is inconsistent with the structure of other sulfinesand in disagreement with the computationally obtained opti-mum structure.

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Table 3 Computational and experimentally derived geometries of the sulfinesa

Parameter

H2CSO CH3CHSO CH3CH2CHSO

Experimental B3PW91b Experimental B3PW91b Experimental B3PW91b

r(C=S) 1.610 1.603 1.618 1.613 1.585(1.578) 1.618

r(S=O) 1.469 1.468 1.477 1.474 1.473(1.473) 1.479

r(=CH–CH3) 1.493 1.482

r(=CH–CH2–) 1.513(1.524) 1.490

r(–CH2–CH3) 1.536(1.544) 1.536

q(CSO) 114.72 115.4 113.9 114.6 113.8(113.8) 114.6

q(CCS) 125.4 126.7 126.7(126.5) 127.4

q(CCC) 112.6(112.4) 112.6

f(CCCS) 118.4(118.3) 121.9

Ref. 25 This work 26 This work 6 This work

a Bond length in A, angles in degrees. b Obtained using the 6-311++G(3df,2pd) basis set.

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