Absolute proton affinity and basicity of the carbenes CH2, CF

2,

FCOH, and ÑuorenylideneCCl2, C(OH)

2, CPh

2

Josefredo R. Pliego Jr. and Wagner B. De Almeida*de Computacional e Modelagem Molecular, Departmento de ICEx,L aborato� rio Qu•�mica Qu•�mica,

UFMG, Belo Horizonte, MG, 31270-901 Brazil

Ab initio molecular orbital calculations were performed in order to determine the absolute proton affinity and basicity of somecarbenes. For the species and FCOH, the G2(MP2) method was utilized, and we have obtained theCH2 , CF2 , CCl2 C(OH)2 ,values 207.0, 177.4, 209.6, 217.3 and 199.9 kcal mol~1, respectively, for the absolute proton affinities. For and Ñuo-CPh2renylidene the calculation was performed at the HF/DZ] (P)//HF/DZ and MP2/DZ//HF/DZ levels of theory. For weCPh2have obtained an absolute proton affinity of 275.0 kcal mol~1 and, for Ñuorenylidene, the value is 272.4 kcal mol~1. Theimplication of these results for the carbene reaction mechanism with OH groups is discussed.

Introduction

Carbenes are molecules of general formula CXY, in which thecarbon atom has a valency of two. They are reactive interme-diates, and in general they have a short lifetime. The study ofthe chemistry of these species requires special experimentaltechniques, in order to detect their formation and reactions.Alternatively, carbenes may be studied by ab initio methods.Some carbenes are small molecules, so that a high level oftheory can be applied to elucidate the chemistry of thesespecies. The electronic structure of carbenes permit them toact as electrophiles or nucleophiles. In the singlet state, theyhave a vacant n orbital and a doubly occupied p orbital, sothey can receive an electron pair. Also, in protic solvents suchas water or alcohols they can capture protons to form carbo-cations.

The reaction mechanisms of carbenes with hydroxy groupshave been controversial for a long time.1h13 Three generalmechanisms were proposed for their reaction with alcohols :1,2(a) proton transfer from the hydroxy group to the carbene,followed by addition of the ion pairs, (b) ylide formation fol-lowed by proton migration to form the corresponding ether,(c) direct insertion of the carbene into the OwH bond.Scheme 1 shows these three pathways.

Experimental studies indicate that all three mechanisms arepossible, depending on the nature of X and Y in CXY. Bethellet al.3 have studied the reaction of with methanol andCPh2tert-butyl alcohol in acetonitrile, and based on the analysis ofproduct isotopic e†ect, they concluded that in this casemechanism (b) is operating. Kirmse et al.4 have performedexperiments to determine the reaction mechanism of MeOHwith cyclopentadienylidene and cycloheptatrienylidene. Theirresults led to the conclusion that the Ðrst reacts via path (b)and the second via path (a).

The reaction of Ñuorenylidene with ROH was studied by

Scheme 1

Zupancic et al.2 using laser spectrophotometric techniques,and by measuring kinetic and product isotopic e†ects inMeCN solution. They concluded that the mechanism is not aone-step process, and suggested that there is ylide formation,i.e., path (b). A more recent study by Kirmse et al.6 has shownthat Ñuorenylidene is not protonated in the solvent 1,1,1,3,3,3-hexaÑuoropropan-2-ol, which has However,pKa \ 9.3.Kirmse et al. detected the formation of a diarylcarboniumcation by protonation of a diarylcarbene, and concluded thatit reacts with in acetonitrileÈwater solution by path (a),H2Owhich is in apparent conÑict with the results of Bethell et al.3for diphenylcarbene. In a study of diphenylcarbene reactionswith in acetonitrileÈwater solution and with methanol,H2Oethanol and propan-2-ol using laser Ñash photolysis,Chateauneuf7 detected the diphenylcarbonium cation. Theseresults led to the conclusion that mechanism (a) is operating,also in apparent conÑict with BethellÏs Ðrst studies. Studies8were performed involving the carbenes and FCOMeC(OMe)2with several alcohols and acetic acid. The reaction rate con-stants were measured, and it was observed that FCOMe ismuch less reactive than It was suggested that theC(OMe)2 .carbene reacts by proton transfer (a), while theC(OMe)2carbene FCOMe reacts by direct insertion (c).

Theoretical studies have also been performed for somesystems. Pople et al. studied the potential-energy surface forthe reaction at the MP4/6-31G*//HF/6-31G*H2O] CH2level, and they found that the reaction proceeded without anenergy barrier, by direct insertion.9 A correlated CASSCFstudy by Walch also supported this result.10 However, a morerecent higher level calculation by Gonzalez et al.11 shows thatthe ylide can be formed, with a stabilizationH2OwCH2energy of 6.37 kcal mol~1, and an energy barrier to protontransfer of just 1.43 kcal mol~1. Therefore, the ylide has a veryshort lifetime. Gonzalez et al. have also used the PCM modelto include the solvent e†ect (using the relative permittivity ofwater), and they observed that the energy barrier increased to7.49 kcal mol~1, which suggests that in solution the ylide cansurvive for a longer time.

The system was also studied12 using ab initioH2OwCCl2methods. The formation of an H-bonded complex wasobserved, which had a dissociation energy of 2.4 kcal mol~1,and a Ðrst order transition state of structure similar to anylide, but with a long CwO bond (2.971 characteristic of aA� )weakly bound complex. In a complementary theoreticalstudy,13 it was proposed that inserts into the OwHCCl2bond of water by the direct mechanism (c), with an activationenergy of 13.4 kcal mol~1.

J. Chem. Soc., Faraday T rans., 1997, 93(10), 1881È1883 1881

Publ

ishe

d on

01

Janu

ary

1997

. Dow

nloa

ded

on 2

8/10

/201

4 14

:57:

30.

View Article Online / Journal Homepage / Table of Contents for this issue

Table 1 Absolute proton affinity (*H) and basicity (*G) at 298.15 Kfor some carbenes and for and obtained by the G2(MP2)H2O NH3methoda

molecule *H/kcal mol~1 *G/kcal mol~1

CH2 207.0 (206.3^ 0.8)b 200.1CF2 177.4 (172 ^ 2)c 170.1CCl2 209.6 (209.6^ 2.0)d 202.2C(OH)2 217.3 210.1FCOH 199.9 192.2H2O 164.5 (167.2^ 1.8)e 157.8NH3 204.0 (203.6^ 1.3)f 196.6

a Experimental valves are in parentheses. b Ref. 16. c Ref. 17. d Ref. 18.e The value reported in ref. 19 is 165.8^ 1.8. We have includedthermal corrections using theoretical data. fRef. 20.

In view of these results knowledge of the absolute protonaffinities of carbenes is necessary in order to understand betterand to predict the chemical reaction mechanism of thesespecies. In this article, we report theoretical calculations of *H(absolute proton affinity) and *G (absolute basicity) forproton capture by the following carbenes : CH2 , CF2 , CCl2 ,

FCOH, and Ñuorenylidene.C(OH)2 , CPh2

CalculationThe Ðrst Ðve carbenes mentioned above are small species, sohigh level ab initio calculations may be carried out. We haveused the G2(MP2) method of Pople et al.14 to compute theabsolute proton affinity (*H) and absolute basicity (*G) ofthese carbenes, where *H and *G are deÐned for the process :

HCXY`] CXY] H`

The G2(MP2) method consists of the following sequence. (1)The geometry and frequency analysis is performed at the HF/6-31G* level. (2) More reÐned geometry is obtained by opti-mization at the MP2/6-31G* level. (3) Single point calculationis performed at the QCISD(T)/6-311G** and MP2/6-311 ] G(3df,2p) levels of theory. (4) Additivity of correlationenergy is used to obtain an e†ective QCISD(T)/6-311 ] G(3df,2p) level of calculation. The G2(MP2) enthalpy is then :

H \ E[QCISD(T)/6-311G**]

]E[MP2/6-311 ] G(3df,2p)][ E(MP2/6-311G**)

]empirical correction for higher correlation level

]zero point energy correction

]thermal correction

For the larger carbenes, and Ñuorenylidene, we per-CPh2formed geometry optimizations at the HF/DZ level, andsingle point calculations at MP2/DZ and HF/DZ] P(d)levels of theory. Although less reliable than G2(MP2), thislevel of calculation is able to produce a reasonable estimate ofabsolute proton affinity. All calculations were performed withthe GAUSSIAN 94 package.15 The results are given in Tables1 and 2.

Table 2 Ab initio absolute proton affinity (*H) and basicity (*G¡) at298.15 K for the carbenes and Ñuorenylidene (units in kcalCPh2mol~1)

CPh2 Ñuorenylidene

*E (HF/DZ) 280.9 275.7*E (HF/DZ] P(d)) 280.7 274.6*E (MP2/DZ//HF/DZ) 283.0 279.8*E (MP2/DZ//HF/DZ)] *EZPEa 273.5 271.0*H 275.0 272.4*G 267.1 264.7

a Zero point energy contribution.

Results and DiscussionThe results show a large range of absolute proton affinityvalues for the carbenes studied. For comparison, the absoluteproton affinities of and are also presented. TheH2O NH3results of the G2(MP2) absolute proton affinities and the cor-responding experimental values are in good agreement, withthe exception of which shows a deviation of ca. 5 kcalCF2mol~1. The G2(MP2) method is very accurate, so we thinkthat the experimental measurement for should be revised.CF2is the least reactive of these carbenes, as is reÑected inCF2its proton affinity, which is small and reasonably close to thatof At the other extreme is the nucleophilic carbeneH2O.

which is more basic than The substitution ofC(OH)2 NH3 .the OH group by Ñuor (FCOH) decreases its basicity substan-tially. This is in accordance with the experimental observ-ations of Moss and co-workers,8 who suggested that C(OMe)2reacts with hydroxy groups by proton transfer, while FCOMereacts by direct insertion. The same behaviour must beexpected for FCOH and C(OH)2 .

The electrophilic carbenes and are highly basic,CH2 CCl2slightly more so than This suggests that the protonationNH3.of these carbenes in aqueous solution is similar to ammonia.In a recent study21 of the cluster, it was shownNH3É É É(H2O)

nthat the formation of the ion pair may occurNH4`É É ÉOH~only if there are at least four water molecules in the cluster,where three water molecules are between the ion pair. So, acluster with just one water molecule only exists as a neutralmonomer. The complex also only exists in theH2OÉ É ÉCCl2neutral form.12 By analogy with ammonia, the dichloro-carbene should occur in aqueous solution as neutral andionized forms, depending on the pH. This suggests that itsreaction with hydroxy groups will depend on the proton con-centration in the medium. More acidic solutions favour proto-nation, followed by addition, whereas less acidic solutionsfavour another mechanism: for it is, direct insertion,13CCl2and for it is ylide formation.11CH2The diphenylcarbene and Ñuorenylidene have very highabsolute proton affinities in all levels of calculation. It ishigher than the absolute proton affinity of the very basic

amine, calculated as 233.2 kcal mol~1,22 and(CH3CH2)3Nthat of the nucleophilic carbene imidazol-2-ylidene, calculatedas 257.3 kcal mol~1.23 Therefore, these molecules have a veryhigh tendency to be protonated. The inclusion of a polariza-tion function in the DZ basis set at the HartreeÈFock levelhardly alters the energies for proton capture. The inclusion ofelectron correlation at the MP2 level of theory shows a moresigniÐcant, although small, change. So, it seems that ourmodest calculation is reasonably adequate. Once the carbeneis formed in the singlet state, intersystem crossing to the tripletstate (the fundamental one) can occur. Chateauneuf7 andKirmse et al.6 obtained the transient spectrum of the proto-nated diphenylcarbene, so proton capture competes efficientlywith intersystem crossing. The same result was not obtainedfor Ñuorenylidene. Kirmse et al.6 have performed a laser Ñashphotolysis study of 9-diazoÑuorenylidene in 1,1,1,3,3,3-hexaÑuoropropan-2-ol and have not observed the transientspectrum of the protonated Ñuorenylidene. They concludedthat this carbene has low basicity. However, according to ourresults, Ñuorenylidene is as basic as diphenylcarbene, andshould be protonated rapidly, in conÑict with the results ofKirmse et al.6

Our calculations were carried out for gas phase reactions.In this situation, proton transfer does not occur, because theabsolute proton affinities of hydroxide and alkoxide ions arevery high, [350 kcal mol~1. Also, ion pair formation with thecarbene being protonated seems improbable, as discussedabove. These considerations should be valid for solutions inapolar solvents with low relative permittivity such as pentane.In these cases, reactions could occur by ylide formation or by

1882 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

Publ

ishe

d on

01

Janu

ary

1997

. Dow

nloa

ded

on 2

8/10

/201

4 14

:57:

30.

View Article Online

direct insertion in the hydroxy groups. For diphenylcarbeneand Ñuorenylidene, owing to their very high proton affinity, itis probable that in protic polar solvents, and even in a basicmedium, proton transfer can take place. Nevertheless, it isnecessary to elucidate why the protonation of diphenyl-carbene is observed, and that of the Ñuorenylidene is not.

In recent work13 we suggested that reacts withCCl2 H2Oin basic aqueous solution by direct insertion. We have shownthat the corresponding ylide does not form at all.12 However,in acidic solution the mechanism can occur by carbene proto-nation. We can estimate the transition point between thesetwo mechanisms by making some assumptions. First, thebasicity of is greater than that of in the gas phase.CCl2 NH3In aqueous solution, the ion should be more stableNH4`than because it is a smaller molecular species. So, theHCCl2`di†erence in basicity should diminish in aqueous solution, andwe think that it is reasonable to suppose that for isKb CCl2ca. 10~4, since this constant for is ca. 10~5. Secondly, weNH3suppose that the acidÈbase equilibrium is established, and thatthe rate constant for the reaction of with toHCCl2` H2Oform the corresponding alcohol is 109, the di†usion limit.With these assumptions, we can write the following equations :

KeqCCl2 ] H` A8B HCCl2`

k1HCCl2`] H2O ÈÈÈÕ CHCl2OH ] H`

k2CCl2 ] H2O ÈÈÈÕ CHCl2OH

where l mol~1, l mol~1 s~1 andKeq \ 10~4 k1\ 109 k2 \3 ] 10~2 l mol~1 s~1 (the last value was taken from ref. 13).With these data, we can estimate the H` concentration wherethe inversion occurs by the equation :

[H`]\k2

k1Keq\ 3 ] 10~7 mol l~1

We estimated that above pH B 6.5, the direct insertionmechanism acts, and below this pH the proton transfermechanism takes place.

ConclusionsThe absolute proton affinities of some carbenes were calcu-lated using ab initio methods. The studied species have a rangeof absolute proton affinities from 177.4 to 275.0 kcal mol~1.The more basic carbenes and Ñuorenylidene) should(CPh2react in protic solvents by proton transfer, whereas the lessbasic ones FCOH) should not. The intermediate species(CF2 ,

could abstract a proton in neutral andCH2 , CCl2 , C(OH)2acidic solutions.

would like to acknowledge the support of the ConselhoWeNacional de Desenvolvimento Cienti� Ðco e Tecnolo� gico(CNPq), the de Amparo a Pesquisa do Estado deFundacÓ a8 oMinas Gerais (FAPEMIG), the Programa de Apoio ao

Desenvolvimento Cienti� Ðco e Tecnolo� gico (PADCTÈProc.Number 62.0241/95.0) and the Pro-Reitoria de Pesquisa(PRPq-UFMG).

References

1 W. Kirmse, Carbene Chemistry, Academic Press, New York,1964.

2 J. J. Zupancic, P. B. Crase, S. C. Lapin and G. B. Schuster, T etra-hedron, 1985, 41, 1471.

3 D. Bethell, A. R. Newall and D. Whittaker, J. Chem. Soc. B, 1971,23.

4 W. Kirmse, K. Loosen and H. D. Sluma, J. Am. Chem. Soc., 1981,103, 5935.

5 D. Griller, M. T. H. Liu and J. C. Scaiano, J. Am. Chem. Soc.,1983, 104, 5849.

6 W. Kirmse, J. Killan and S. Steenken, J. Am. Chem. Soc., 1990,112, 6399.

7 J. E. Chateauneuf, J. Chem. Soc., Chem. Commun., 1991, 1437.8 X. Du, H. Fan, J. L. Goodman, M. A. Kesselmayer, K. Krogh-

Jespersen, J. A. La Villa, R. A. Moss, S. Shen and R. S. Sheridan,J. Am. Chem. Soc., 1990, 112, 1920.

9 J. A. Pople, K. Raghavachari, M. J. Frish, J. S. Binkley andP. V. R. Schleyer, J. Am. Chem. Soc., 1983, 105, 6389.

10 S. P. Walch, J. Chem. Phys. 1993, 98, 3163.11 C. Gonzalez, A. Restrepo-Cossio, M. Marquez and K. B. Wiberg,

J. Am. Chem. Soc., 1996, 118, 5408.12 J. R. Pliego Jr. and W. B. De Almeida, Chem. Phys. L ett. 1996,

249, 136.13 J. R. Pliego Jr. and W. B. De Almeida, J. Phys. Chem. 1996, 100,

12410.14 L. A. Curtiss, K. Raghavachari and J. A. Pople, J. Chem. Phys.

1993, 98, 1293.15 GAUSSIAN 94, revision D.2, M. J. Frisch, G. W. Trucks, H. B.

Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheese-man, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghava-chari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B.Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara,M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox,J. S. Binkley, D. J. Defrees, J. Baker,, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian, Pittsburgh PA,1995.

16 S. G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D.Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 1988, 17,suppl. 1.

17 J. Vogt and J. L. Beauchamp, J. Am. Chem. Soc., 1975, 97, 6682.18 B. A. Levi, R. W. Taft and W. T. Hehre, J. Am. Chem. Soc., 1977,

99, 8454.19 C. Y. Ng, D. J. Trevor, P. W. Tiedemann, S. T. Ceyer, P. L.

Kronebusch, B. H. Mahan and Y. T. Lee, J. Chem. Phys., 1977,67, 4235.

20 S. T. Ceyer, P. W. Tiedemann, B. H. Mahan and Y. T. Lee, J.Chem. Phys., 1979, 70, 14.

21 C. Lee, G. Fitzgerald, M. Planas and J. J. Novoa, J. Phys. Chem.,1996, 100, 7398.

22 C. Hillebrand, M. Klessinger, M. Eckert-Maksic and Z. B.Maksic, J. Phys. Chem., 1996, 100, 9698.

23 D. A. Dixon and A. J. Arduengo III, J. Phys. Chem., 1991, 95,4180.

Paper 6/08011A; Received 26th November, 1996

J. Chem. Soc., Faraday T rans., 1997, V ol. 93 1883

Publ

ishe

d on

01

Janu

ary

1997

. Dow

nloa

ded

on 2

8/10

/201

4 14

:57:

30.

View Article Online