Molecular Mass Determination of Saturated Hydrocarbons: Reactivity of 5- ؉ Cyclopentadienylcobalt Ion (Cpco• ) and Linear Alkanes up to C-30
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Molecular Mass Determination of Saturated Hydrocarbons: Reactivity of 5- ؉ Cyclopentadienylcobalt Ion (CpCo• ) and Linear Alkanes up to C-30 H. C. Michelle Byrd* and Charles M. Guttman National Institute of Standards and Technology, Polymers Division, Gaithersburg, Maryland, USA Douglas P. Ridge Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, USA ⅐ϩ The present study demonstrates the feasibility of the 5-cyclopentadienylcobalt ion (CpCo ) as a suitable cationization reagent for saturated hydrocarbon analysis by mass spectrometry. ⅐ϩ Ion/molecule reactions of CpCo and three medium chain-length n-alkanes were examined using Fourier-transform ion cyclotron resonance mass spectrometry. Second-order rate con- stants and reaction efficiencies were determined for the reactions studied. Loss of two hydrogen molecules from the CpCo-alkane ion complex was found to dominate all reactions (Ն80%). Furthermore, this dehydrogenation reaction efficiency increases with increasing chain ⅐ϩ length. These preliminary results suggest that the CpCo ion may be a promising cationiza- tion reagent of longer chain saturated hydrocarbons and polyolefins. (J Am Soc Mass Spectrom 2003, 14, 51–57) © 2003 American Society for Mass Spectrometry aturated hydrocarbon polymers, polyethylene, Ideally, ionization of saturated polyolefins for MMD polypropylene, and their derivatives, are the most determination requires that (1) chemical modification of Swidely used of all synthetic polymers. Their mo- the hydrocarbon occurs inside the mass spectrometer, lecular structure, chemical composition, and the molec- (2) the hydrocarbon does not require the presence or ular-mass distribution (MMD) are critical in determin- addition of a functional group for ionization to occur, ing performance properties. The potential for rapid and (3) the predominant reaction pathway leads to a prod- direct measurement of single-chain composition and uct characteristic of the original hydrocarbon, and (4) MMD makes mass spectrometry (MS) especially attrac- minimal side reactions and chain fragmentation would tive to the polymer industry [1]. However, using MS occur reducing complications with accurate molecular analysis for the determination of MMDs requires the mass determination. Currently, there are two main formation of intact macromolecular ions in the gas strategies for tackling the polyolefin ionization chal- phase, which has proven to be quite difficult for satu- lenge: (1) chemical derivatization of the polyolefin and rated hydrocarbon polymers (commonly referred to as (2) “bare” (atomic) transition metal cationization in the polyolefins). gas phase [8–15]. Neither meet the above four criteria. Polar polymers, such as polyesters, polyglycols, and Several research groups have performed the chemi- polyamides, have functional groups that can be readily cal derivatization necessary for ionization by either ϩ ϩ cationized, usually by Na or K ions [1–7]. Nonpolar chemically modifying the polymer to an organic salt or polymers, such as polystyrenes and polybutadienes, chemically attaching a functional group that can be may be cationized via interactions between their pi- readily ionized by a cation or electron attachment or ϩ electrons and transition metal ion d-orbitals, e.g., Ag deprotonation [8–12]. The main disadvantage to the ϩ or Cu ions [2, 4–7]. However, saturated hydrocarbon chemical modification methods is that the polymer polymers lack a suitable ionizable site on the aliphatic must contain a reactive end group, typically a vinyl or chain and therefore require another method of ioniza- halide. Some but not all commercially available poly- tion. ethylenes contain vinyl-terminated chains which are formed by chain transfer termination during polymer- ization. The reactive vinyl group is chemically modified Published online December 5, 2002 to form the desired functionality. However, this vinyl- * Address reprint requests to Dr. H. C. M. Byrd, NIST, 100 Bureau Drive, Polymers Division Bldg. 224, Rm. B-320, Gaithersburg, MD 20899, USA. terminated series may not be representative of the main E-mail: [email protected] polymer series and may introduce a mass bias upon © 2003 American Society for Mass Spectrometry. Published by Elsevier Science Inc. Received August 19, 2002 1044-0305/03/$20.00 Revised October 31, 2002 doi:10.1016/S1044-0305(02)00815-2 Accepted October 31, 2002 52 BYRD ET AL. J Am Soc Mass Spectrom 2003, 14, 51–57 ⅐ϩ chemical derivatization. Furthermore, the derivatiza- C™C bonds [20, 21]. Reaction products consist of CpCo tion process requires at least two days for synthesis and bound to a hydrocarbon formed by eliminating hydro- purification costing time and expense. gen molecules from the reactant alkane. The carbon Gas-phase studies have shown that many first-row number of the reactant alkane is thus preserved in the transition metal ions react with various linear and cyclic ionized product. The reaction does not produce frag- ϩ ϩ saturated aliphatic hydrocarbons [13–17]. Fe ,Co , and mentation of the alkane beyond dehydrogenation. Such ϩ Ni react efficiently with low molecular mass alkanes a reaction might provide the basis for obtaining an (C H ϩ ,nϽ 8) by dehydrogenation (Reactions 1 and accurate representation of the MMD for polyolefins. n 2n 2 ⅐ϩ 2) and dealkylation (Reaction 3) [16]. However, to date, no reactions of CpCo ion with n Ͼ 6 saturated aliphatic hydrocarbons have been reported. ϩ ϩ 3 ϩ ϩ In the present gas-phase study, we report results on M CnH2nϩ2 MCnH2n H2 (R1) ⅐ϩ the CpCo ion reactivity toward three medium chain ϩ ϩ length n-alkanes using Fourier-transform ion cyclotron M ϩ C H ϩ 3 MC H Ϫ ϩ 2H (R2) n 2n 2 n 2n 2 2 resonance mass spectrometry (FT-ICR MS). Pentade- ϩ ϩ 3 ϩ ϩ cane (C-15), eicosane (C-20), and octacosane (C-28) M CnH2nϩ2 MCmH2m CnϪmH2͑mϪn͒ϩ2 serve as lower-molecular mass models of polyethylene. ⅐ϩ (R3) We find that the CpCo ion reacts predominantly by double H2 loss from the CpCo-alkane ion complex for ™ These reactions have provided useful insights into the all three alkanes with no C C bond cleavage observed. reactivity of alkane C™C and C™H bonds towards tran- We observe an increase in reaction efficiency with increasing alkyl chain length. The results suggest that sition metals. The gas phase reactivity of alkene and ⅐ϩ alkyne C™C bonds towards transition metal atomic ions the CpCo ion is a promising cationization reagent for ϩ such as Fe is very selective and has been proposed as larger alkyl and polymeric systems. a means of characterizing molecular structures, e.g., [18, 19]. Allylic C™C bonds, for example, are generally much Experiment more reactive than other C™C bonds in olefins. The ϩ reactivity of alkane C™C bonds towards Fe is not very Samples selective, however. Typically in Reaction 3 all products ϭ ϭ Ϫ Cyclopentadienylcobalt dicarbonyl [CpCo(CO)2], pen- from m 2tom n 1 are observed in significant tadecane (C-15), eicosane (C-20), and octacosane (C-28) amounts. were purchased from the Aldrich Chemical Company More recently, atomic transition metal ion chemistry and used as received. [Certain commercial materials in the gas phase has been pursued as a means of and equipment are identified in this paper in order to ionizing saturated polyolefins for molecular mass de- specify adequately the experimental procedure. In no termination by MS analysis [13–15]. The lack of speci- case does such identification imply recommendation by ™ ficity in the reactivity of alkane C C bond is one of the National Institute of Standards and Technology nor several difficulties revealed in these studies. The reac- does it imply that the material or equipment identified tions of the various atomic transition metal ions with is necessarily the best available for this purpose.] the polyolefins fall under one of two categories. If the Ϫ reactions are slow (ϽϽ10 11 cm3/molecule⅐s) relative to the experiment time frame, no product ions of interest Instrument/Measurements are observed. Alternatively, if the reactions are fast, All reaction rate measurements were carried out using ™ C C bonds in the chain react without much selectivity a 3-tesla dual-cell Fourier transform ion cyclotron reso- and extensive chain fragmentation results. The presence nance mass spectrometer (Extrel FT/MS 2000) of fragmentation products complicates quantitative equipped with a heated multi-sample introductory sys- measurement of simple hydrocarbon chains and will tem, described previously [22, 23]. Figure 1 shows a distort the MMD for saturated hydrocarbon polymers schematic of the FT-ICR MS system and dual cell. The ⅐ϩ [14, 15]. CpCo ions were formed by 70-eV dissociative electron More control of reaction selectivity may be achieved ionization of CpCo(CO)2 leaked into cell 1. The neutral by using ligated-metal ion chemistry. For example, a alkane was introduced into cell 2 by either a heated ϩ ϩ ϩ recent study showed that while Fe ,Co , and Ni all batch inlet or solids probe. ϩ react efficiently with alkanes, CpM ions (Cp ϭ 5- During the electron ionization event, ions are also cyclopentadienyl) react with ethane (producing formed from the alkane in cell 2 (Figure 1). To reduce ϩ ϭ ϩ CpMC2H4 ) only for M Co and Ni [20]. The CpFe ion complications from interfering reactions, ions generated ϭ Ͻ is relatively unreactive [kCo:kNi:kFe 1.00:0.40:( .01)] from the neutral alkane were removed from cell 2 by [20]. applying Ϫ2 V potential to the trapping plate for 5 ms ϩ ⅐ϩ The CpCo ion is of special interest. Not only is it the prior to ion transfer. The CpCo ions were transferred ϩ most reactive of the CpM ions, but with small alkanes from cell 1 into cell 2 by grounding the conductance (n ϭ 2–6) it reacts exclusively with C™H bonds and not limit for approximately 240 s (see Figure 1). After J Am Soc Mass Spectrom 2003, 14, 51–57 REACTIONS OF CpCOϩ WITH ALKANES 53 spectrum, and (Figure 2c) the resultant reaction spec- trum.