Gas-phase reactions of ClMn(H2O)+ with polar and nonpolar hydrocarbons in a mass spectrometer

Article abstract of DOI:10.1021/ja073270r

The gas-phase reactions of ClMn(H₂O)⁺ with a variety of volatile and nonvolatile, saturated and unsaturated hydrocarbons have been examined by using Fourier transform ion cyclotron resonance mass spectrometry (FT/ICR). The ClMn(H₂O)⁺ ion reacts rapidly by exclusive H₂O ligand displacement with all the hydrocarbons studied, including highly branched alkanes that usually fragment upon ionization. These observations are rationalized on the basis of the electronic structure of ClMn⁺. Collision-activated dissociation of the product ions provides structural information which promises to allow the distinction and structural characterization of isomeric hydrocarbons. These findings suggest that the ClMn(H₂O)⁺ ion is a highly promising chemical ionization reagent for mass spectrometric characterization of hydrocarbons, including those that commonly exist in petroleum. Copyright

Full text of DOI:10.1021/ja073270r

Published on Web 07/11/2007
Gas-Phase Reactions of ClMn(H2O)+ with Polar and Nonpolar Hydrocarbons
in a Mass Spectrometer
Penggao Duan, Mingkun Fu, David S. Pinkston, Steven C. Habicht, and Hilkka I. Kentta¨maa*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
Received May 8, 2007; E-mail: hilkka@purdue.edu
Table 1. Products Formed in Reactions of ClMn(H2O)+ with a
Variety of Compounds Typical of Those Present in Petroleum
The ability to use mass spectrometry in the characterization of
nonpolar hydrocarbons and their mixtures, such as synthetic
saturated hydrocarbon polymers and the nonpolar components of
petroleum, is highly desirable because this method yields both
structural as well as molecular weight (MW) information.1 However,
saturated hydrocarbons are notoriously difficult to analyze by mass
spectrometry owing to a lack of ionizable functional groups. The
few methods that are known to afford ionization of these compounds
tend to cause dissociation which results in the loss of MW
information.1 In the search for better ionization methods for
hydrocarbons, the reactivity of bare and ligated gas-phase transition
metal ions (e.g., Fe+, Ni+, Ag+, FeCH3+, and CpCo+; Cp ) η5-
cyclopentadienyl) has attracted interest.2-9 The CpCo+ ion has been
reported to ionize most hydrocarbons without excessive fragmenta-
tion. It reacts with many alkanes by C-H bond insertion,
predominantly yielding adduct ions that have lost one or two
molecules of H2.4c,6-8 These product ions provide MW information
although the extent of unsaturation may be difficult to determine
(e.g., cyclohexene loses H2 and cyclohexane loses 2H2, thus yielding
the same product ion). Also, loss of cyclopentadiene (CpH),
sometimes accompanied by an additional H2 loss, has been observed
for higher MW alkanes (CnH2n+2, n > 20).4c Moreover, some
alkanes undergo C-C bond cleavages upon reactions with CpCo+
•
(e.g., loss of C2H4, C3H6, C4H8, or CH3 ).4a,8 Hence, a less aggressive
reagent ion is desirable. We report here on the gas-phase reactions
of such an ion, a ligated water cluster of Mn+: ClMn(H2O)+.
The ClMn(H2O)+ ions were generated by electron ionization (30
eV electron energy, 7 µA emission current, 80 ms ionization time)
of MnCl(CO)5 (synthesized10 from Mn2(CO)10 by reaction with Cl2
in CCl4) in the presence of H2O vapor in one cell of a Finnigan
dual-cell Fourier transform ion cyclotron resonance mass spec-
trometer (FT-ICR). The ions were transferred into the other cell
and isolated before reactions with neutral compounds. Volatile
hydrocarbons were introduced via an adjustable leak valve.
Nonvolatile hydrocarbons were dissolved in tetrahydrofuran, elec-
trospray-deposited on a titanium foil, and evaporated by using laser-
induced acoustic desorption11 (LIAD) as described earlier.11e,f
The isolated ClMn(H2O)+ ions were allowed to react with a
a Introduction via LIAD. b Introduction via an adjustable leak valve.
variety of hydrocarbons typical of those that commonly exist in
petroleum. Remarkably, every compound studied yields only one
disruption of this stable half-filled shell.13 Hence, H2O in ClMn-
product ion (Table 1). This is true even for those compounds that
(H2O)+ is weakly bound and should be readily replaced by many
molecules. Indeed, this is the only reaction observed for the
hydrocarbons studied. Further reactions within the ClMn+/
hydrocarbon complex do not take place because the ground-state
ClMn+ is not particularly reactive toward hydrocarbons for the
reasons discussed above. The only reaction of ClMn+ with small
hydrocarbons reported in the literature is a slow replacement of
the Cl atom (i.e., most of the ClMn+/alkane complexes dissociate
to ClMn+ and alkane, as expected).13a This reaction does not occur
for ClMn(H2O)+ because H2O loss lowers the energy of the system.
undergo skeletal fragmentation upon reactions with CpCo+ (e.g.,
5-R-cholestane and tetrahydrofuran).8 The product ion is formed
by replacement of H2O in ClMn(H2O)+ by the hydrocarbon.
The H2O replacement reaction is fast. For example, the reaction
efficiencies12 for benzene and 2,2,4,4-tetramethylpentane are 70%
and 31%, respectively. This can be rationalized on the basis of the
electronic structure of ClMn+.13a The Mn+-Cl bond involves the
one readily available electron (4s1) in Mn+ (ground electronic state
3d54s1), leaving the rest of the valence electrons in Mn+ in a high-
spin d5 configuration.13b Additional bond formation requires the
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10.1021/ja073270r CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Finally, a mixture of tetracosane (a linear alkane), 5-R-cholestane
(a cyclic alkane), squalane (a branched alkane), coronene (a
polyaromatic hydrocarbon), and 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (a N-heteroaromatic compound) (all in equimolar
ratios) was evaporated by LIAD and allowed to react with ClMn-
(H2O)+. The resulting mass spectrum shows the H2O replacement
product for every component of the mixture (Figure 1, bottom).
While the relative product ion abundances do not exactly match
the relative molar concentration of each mixture component, they
are still remarkably close when considering the fact that the
compositions, structures, and volatilities of the compounds vary
widely. In sharp contrast, electrospray ionization (ESI; performed
on a Finnigan linear quadrupole ion trap (LTQ) mass spectrometer)
only reveals the presence of the most polar mixture component,
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Figure 1, top).
In conclusion, ClMn(H2O)+ efficiently ionizes various types of
hydrocarbons, both polar and nonpolar, as well as other compounds
typically present in petroleum, to exclusively form pseudomolecular
ions (adduct-H2O). Even highly branched hydrocarbons yield solely
this product ion when exposed to ClMn(H2O)+. Collisional activa-
tion of these product ions yields structural information on the
hydrocarbons and may allow distinction and identification of
isomeric hydrocarbons.
Acknowledgment. Professors Peter B. Armentrout and Helmut
Schwarz are thanked for insightful discussions regarding metal ion
chemistry.
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