Gas-phase Ion–Molecule Reactions of Neutral C60With Alkyl Methyl Ethers Under ChemicalIonization Conditions

Li Ma, Ziyang Liu, Yaping Xu, Weijie Wang, Xinghua Guo and Shuying Liu*Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China

Gas-phase ion–molecule reactions of buckminsterfullerene (C60) with the ion systems generated from theself-chemical ionization of alkyl methyl ethers (CH3OR, R = n-C2H5, n-C3H7, n-C4H9) were studied in theion source of a mass spectrometer. The adduct cation [C60C2H5O]� and protonated molecule [C60 H]� wereobserved as the major products. The former adduct ion was produced by the reactions of C60 with themethoxymethyl ion [CH3OCH2]

�, and the latter resulted from the proton transfer reactions fromprotonated alkyl methyl ethers to C60. It is suggested that the [3�2] cycloadduct to a 6–6 bond of C60 (a C–Cbond common to two annulated six-membered rings) is the most favorable structure among the probableisomers of [C60C2H5O]�.# 1998 John Wiley & Sons, Ltd.

Received 26 February 1998; Revised 8 April 1998; Accepted 11 April 1998

C60, the new allotrope of carbon, has received moreattention1–5 than any other scientific substance in historyafter the success in macroscopic fullerene synthesis andisolation.6 The addition of molecules to C60 in thecondensed-phase have been extensively studied, e.g. theaddition of azides and azomethine ylides to C60,

7,8 thephotochemical cycloadditions of C60 with silylene,9,10

cyclic 1,3-diones,11 and the highly symmetric sixfold[4� 2] thermochemical cycloaddition of C60 with 1,3-butadiene etc.12 In parallel with the development ofcondensed-phase fullerene chemistry, the gas-phase ion–molecule reactions of fullerenes have made rapid progress,Bohme and his co-workers have thoroughly investigated thegas-phase reactions of fullerene cations C60

.�, C602�, C60

.3�

with water, alcohols, ethers, aldehydes, ketones, carboxy-licacids, esters and nitriles, etc., using the selected-ion flowtube (SIFT) technique.13–17They observed that the reactionsinitiated by doubly charged and triply charged C60 cationsusually produced single or multiple adducts, but thereactions with singly charged C60

.� were rare. In contrastto Bohme’s findings, we have reported the reactions ofneutral C60 with the ion systems of small organiccompounds generated under chemical ionization conditions,and found that neutral C60 can easily react with variousfragment ions (or molecular ions).18–22As a result, the two-fold reactivity of C60, ‘superalkenity’ and ‘superaromati-city’, in the gas-phase has been confirmed extensively.Therefore, the investigations on the reactivity of fullerenesin the gas-phase benefit the understanding of reactivity inthe condensed-phase. In this communication, we report theresults of the gas-phase ion–molecule reactions of C60 withthe ions derived from the self-chemical ionization (CI) ofalkyl methyl ethers in the ion source of a mass spectrometer.

EXPERIMENTAL

All reactions were performed using a VG-Quattro triple-quadrupole mass spectrometer (Fisons Instruments, Man-chester, UK). Methyl-ethyl ether, methyl-n-propyl ether andmethyl-n-butyl ether were injected into the ion sourcethrough the reference gas inlet before C60 was introduced.The narrow CI slit was used in order to afford a high enoughpressure for ion–molecule reactions, and the gauge readingsof the ion source housing pressure were maintained ataround 1.2� 10ÿ2 Pa. The ion gauge is located outside theionization cell, but the real pressure inside the cell is about1000 times greater.23 C60 was introduced into the ion sourcevia the desorption chemical ionization (DCI) probe. Thesolution of C60 was deposited on the DCI filament, whichwas ramped up to 1200 mA while operating. The electronenergy was 100 eV, the ion source was operated at 473 K,and a unit-mass resolving power atm/z1000 was employedfor recording mass spectra. The fullerenes were produced bythe arc-discharge method6 in our laboratory and separatedby column chromatography. The purity of C60 wasconfirmed to be more than 99.9% using UV/visible andmass spectra. Ethers were synthesized24 by means ofliterature methods, and their purity was determined bymass spectrometry.

RESULTS AND DISCUSSIONS

Figure 1(a)–(c) show the self-CI mass spectra of methyl-ethyl ether, methyl-n-propyl ether and methyl-n-butyl etherat a source pressure of 1.2� 10ÿ2 Pa, respectively. It can beseen that the ion [CH3OCH2]

� (m/z45), [M� H]� ions (m/z61, 75 or 89), and [Mÿ H]� ions (m/z59, 73 or 87), arepredominant ionic products under the self-CI conditions.With increase of the carbon chain length the relativeabundance of [CH3OCH2]

� (m/z45) is enhanced. As is wellknown, [CH3OCH2]

� is a typical fragment ion produced by-cleavage of alkyl-methyl ethers in mass spectrometry. Inaddition, a series of ions at higherm/zdue to ion–moleculereactions in the ion source are also observed.

*Correspondence to: S.-Y. Liu, Changchun Institute of AppliedChemistry, Chinese Academy of Sciences, Changchun 130022, P.R.China.Contract/grant sponsor: National Natural Science Foundation of P.R.China.Contract/grant sponsor: Changchun Applied Chemistry ResearchCenter.

CCC 0951–4198/98/120790–03 $17.50 # 1998 John Wiley & Sons, Ltd.

RAPID COMMUNICATIONS IN MASS SPECTROMETRYRapid Commun. Mass Spectrom.12, 790–792 (1998)

After C60 was introduced into the ion source, it reactedwith the ion systems mentioned above to form adductcations. The mass spectraare shown in Fig. 2. It is acommoncharacteristicthattheadductions[C60C2H5O]� atm/z 765, [C60�M ÿ H]� at m/z 779, 793 or 807 and[C60CH3]

� at m/z735, resulted from the reactionsof C60

with thefragment ion [C2H5O]�, the[M ÿ H]� ions, andthemethyl cation CH3

�, respectively.It can be seen from Fig. 2 (a)–(c)that the relative

abundancesof the adduction [C60C2H5O]� increase fromethyl-methyl ether to n-butyl-methyl ether, which isconsistent with the relative abundancesof the [C2H5O]�

ion in their self-CI spectra.The relative abundances of

[C60�M ÿ H]� are lower and show no obvious trends,which may be due to the larger steric hindrance of the[M ÿ H]� ions.Therearenoadditionreactionsbetween C60

and[M � H]� although[M � H]� is oneof themajor ions.The [M � H]� ions of alkyl methyl ethersdo reactwith

C60, in a fashion similar to thatof acetonewith C60, andtheprotonatedmolecule [C60H]� is the basepeak.The protonaffinity (PA) of methyl-ethyl ether is 196.4 kcal/mol,25

significantly lower than that of C60 (205.5 kcal/mol)determinedpreviously by Bohme andco-workers16 andbyMcElvanyandCallahan,26 soprotontransferto C60 from theprotonatedethersis facile and the abundancesof [C60H]�

arealmostthe sameasthose of [M � H]�.Among the adducts,we focus our study only on the

structuresof theadduct[C60C2H5O]�. Thepossible isomersof [C2H5O]� ions derived from a variety of organiccompoundshave been the subject of extensively experi-mental and theoretical investigations.27–30 An ab initiotheoretical study by Nobes et al.30 of 16 structures ofcomposition [C2H5O]� identified four energetically low-lying stable isomers, viz. the 1-hydroxyethyl ion[CH3CHOH]�, the methoxymethyl ion [CH3OCH2]

�,vinyloxonium (protonated vinyl alcohol) [CH2CHOH2]

�

andO-protonatedoxirane[CH2CH2OH]�. [CH3OCH2]� is

the predominantfragmentcation derivedfrom a-cleavageof alkyl methyl ethers.

It hasbeenproved experimentally andtheoretically thatthereactionson a 6–6bondof C60 (a C–Cbondcommontotwo annulatedsix-membered rings) is morefavorable thanon a 5–6 bond of C60 (a C–C bond sharedby a five-membered ring and a six-membered ring).3,5,31 Here wediscussonly the reactionproducts of [C2H5O]� with a 6–6bond of C60 for which there are five possible structuresincluding the�-adduct(e), 1,2 �-adduct(c andd), andthe[2� 2] and[3� 2] cycloadducts(a, b) shown in Scheme1.A semiempirical quantum chemistry calculation with theAM1 methodhadbeenperformedon thepossible structuresto examine their relative stability. The preliminary results

Figure 1. Mass spectrafor the self-CI (a) methyl-ethyl ether (b)methyl-n-propyl (c) methyl n-butyl ether.

Figure 2. Massspectrafor thereactionsof C60 with theion systemsofa, b andc, respectively.

Scheme 1. Possible structuresof [C60C2H5O]� obtained by AMImethod.

# 1998JohnWiley & Sons,Ltd. Rapid Commun.MassSpectrom.12, 790–792(1998)

ION–MOLECULE REACTIONSOF NEUTRAL C60 791

havedemonstratedthat the stability order of the isomericstructures is the following: b(DHf

o = 1088.8 kcal/mol)> d(DHf

o = 1100.5kcal/mol)> c(DHfo = 1109.3kcal/

mol)> e (DHfo = 1110.5kcal/mol)> a(DHf

o = 1120.7kcal/mol). The[3� 2] cycloadduct, in which thefive-memberedring has lower ring strain, is the most stable amongthepossible isomericstructures. Although thereis a hydrogenshift from carbonto oxygen during the formation of the[3� 2] cycloadduct, the cycloaddition reactionshould notbe hindered because hydrogen rearrangements generallyhavesmall energybarriers.In addition,hydrogenrearrange-ments area commonphenomenon in organic andgasphaseion chemistry.24,32Therefore,it is concludedthatthe[3� 2]cycloadductis themost favorablestructure, andthepathwayproposed for the formation of the [3� 2] cycloadduct ispresentedin Scheme 2. Our previousinvestigation on theadducts of C60 with [C2H3O]� supported this conclusion.22

Moreover, the conclusion wassupported by the propertiesthatC60 canreadilyundergothe[3� 2] cycloaddition undersuitable condition in the condensedphaseaswell.33–36

CONCLUSION

We haveinvestigatedthegas-phaseion–molecule reactionsof C60 with the ion systemsof alkyl methyl ethers in thechemical ionization source of a massspectrometer. Theadduct ion [C60C2H5O]�, together with other adductcations, was observed and it is suggested that the [3� 2]cycloadductis themost stablestructure.Unfortunately, MS/MS experimentson these adductions yieldedonly C60

� atlow abundance,underour low-energycollisionconditions.

Acknowledgement

The authors are very grateful to the National Natural ScienceFoundationof P.R.ChinaandChangchunAppliedChemistryResearchCenterfor financial support.

REFERENCES

1. F. Wudl, Acc.Chem.Res.25, 157(1992).2. R. Taylor andD. R. M. Watton,Nature363,685(1993).3. A. Hirsh, TheChemistryof Fullerenes,Thieme,Stuttgart,(1994).4. W. Sliwa, FullereneSci.Technol.3, 243(1995).5. F. DiederichandC. Thilgen,Science271,317(1996).6. W. Kratschmer,L. D. Lamb,K. FostiropoulosandD. R. Huffman,

Nature347,354(1990).

7. M. Prato,Q. ChanLi andF. Wudl, J. Am.Chem.Soc.115,1148(1993).

8. M. MagginiandG. Scorrano,J. Am.Chem.Soc.115,9798(1993).9. T. AkasakaandW. Ando, J. Am.Chem.Soc.115,1605(1993).

10. T. AkasakaandW. Ando, J. Am.Chem.Soc.115,10366(1993).11. A. W. Jensen,A. Khong, M. Saunders,S. R. Wilson and D.

I.Schuster,J. Am.Chem.Soc.119,7303(1997).12. B. Krautler andJ. Maynollo, Angew.Chem.Int. Ed. Engl. 34, 87

(1995).13. V. Baranov,A. C.HopkinsonandD. K. Bohme,J. Am.Chem.Soc.

119,7055(1997).14. S.Petrie,G.Javahery,H. Wincel,J.WangandD. K. Bohme,Int. J.

Mass.Spectrom.Ion Processes138,187(1994).15. G. Javaery,S.Petrie,H. Wincel,J.WangandD. K. Bohme,J. Am.

Chem.Soc.115,6295(1993).16. S. Petrie,G. Javahery,J. Wang,H. Wincel andD. K. Bohme,J.

Am.Chem.Soc.115,6290(1993).17. G. Javahery,S. Petrie,J. Wang,H. Wincel andD. K. Bohme,J.

Am.Chem.Soc.115,9701(1993).18. Z. Liu, G. Hao, X. Guo and S. Liu, Rapid Commun.Mass

Spectrom.9, 213(1995).19. X. Guo, Z. Liu and S. Liu, J. Mol. Struc. (Theochem)340, 169

(1995).20. S.Liu, X. Guo,Z. Liu andJ.Ni, Sciencein China(seriesB) 38(6),

668 (1995).21. X. Guo,Z. Liu andS. Liu, Chin. Chem.Lett. 4, 797(1993).22. X. Guo, Z. Liu, W. Xu and S. Liu, J. Mass.Spectrom.32, 241

(1997).23. A. J. Illies andM. T. Bowers,Anal. Chem.53, 1551(1981).24. T. W. GrahamSolomons,Organic Chemistry,revisedprinting,

Wiley, New York (1978).25. FromJ. Phys.Chem.Ref.Data 13(3) (1984).26. S. W. McEIvany and J. H. Callahan,J. Phys.Chem.95, 6186

(1991).27. H. E. Audier and T. B. McMahon, J. Mass Spectrom.32, 201

(1997).28. L. A. Curtiss,D. J.LucasandJ.A. Pople,J.Chem.Phys.102,3292

(1995).29. C. Dass,Org. MassSpectrom.29, 475(1994).30. R.H. Nobes,W. R.Rodell,W. J.BoumaandRadom,J.Am.Chem.

Soc.103,1913(1981).31. A. Chikama,H. FuenoandH. Fujimoto,J. Phys.Chem.95, 8541

(1995).32. J. H. Futrell, GaseousIon Chemistryand Mass Spectrometry,

Wiley, New York (1986).33. K. I. Guhr,M. D. GreavesandV. M. Rotello,J. Am.Chem.Soc.

116,5997(1994).34. F.Wudl, A. Hirsch,K. C.Khemani,T. Suzuki,P.M. Allemand,A.

Koch, H. Eckert, G. Srdanovand H. M. Webb,ACSSymp.Ser.481,161(1992).

35. M. Prato,T. Suzuki,H. Foroudian,Q. Li, K. C. Khemani,F. Wudl,J.Leonetti,R. D. Little, T. White,B. Richborn,S.YamagoandE.Nakamura,J. Am.Chem.Soc.115,1594(1993).

36. M. Maggini,G. Scorrano,A. Bianco,C. Toniolo,R. P.Sijibesma,F. Wudl andM. Prato,J. Chem.Soc.,Chem.Commun.305(1994).

Scheme2. Possiblepathwayof formationof [3� 2] cycloadductof [C60C2H5O]�.

Rapid Commun.MassSpectrom.12, 790–792(1998) # 1998JohnWiley & Sons,Ltd.

792 ION–MOLECULE REACTIONS OF NEUTRAL C60