Jean H. Futrell

Tandem Mass Spectrometer for Study of Ion‐Molecule Reactions

An instrument for the study of ion‐neutral interactions involving momentum transfer to the product ions has been constructed. It consists of a double focusing mass spectrometer which serves as an ion source for a collision chamber connected to a second double focusing mass spectrometer. This instrument provides both mass and energy resolution of the impacting ion beam and the in‐line configuration maximizes collection of momentum transfer products, as from ion‐molecule reactions. Ion beams of experimentally useful intensity are produced over the energy range 0.2 to 100 eV with an energy spread of 0.3 eV.

Ion—Molecule Reactions of Methane

The ionic reactions of gaseous methane have been investigated in a tandem mass spectrometer. The reactions of each primary ion, CH4+, CH3+, CH2+, CH+, C+, have been observed at 0.3 eV and several secondary‐ion reactions have also been studied. An investigation of the effects of translational energy on many of these reactions demonstrated the efficient conversion of translational energy into internal energy. Evidence for randomization of the intermediate [C2HxD7−x+] ion in the methyl‐ion—methane reaction is presented. Isotope effects in hydrogen‐transfer reactions have also been investigated as a function of kinetic energy. The results are interpreted in terms of a transition from a complex model of reaction to a stripping model at higher energies.

Mass‐Spectrometric Study of Ion—Molecule Reactions of Ethanol and Methanol

The ion—molecule chemistry of variously labeled ethanols has been investigated in a single‐stage high‐pressure mass spectrometer at various field strengths and ionizing voltages. The results have been correlated with data obtained from a tandem apparatus having a primary‐beam mass—energy selection capability. The interactions of selected ions have been investigated at low impacting energies with this instrument. Proton transfer and proton transfer followed by dissociation are the major reaction paths observed. The formation of higher‐order aggregates corresponding to solvated proton structures has also been studied at elevated pressures and consectutive ion—molecule chemistry has also been investigated in the tandem instrument. Some related experiments with variously labeled methanols are also reported. A complete analysis of the bimolecular ion—molecule chemistry of ethanol including the stereospecificity of the various reactions is presented.

Ion—Molecule Reactions in Methanol and Ethanol

Proton‐transfer reactions in the methanol and ethanol systems have been investigated by the techniques of conventional mass spectrometry. Using isotopically substituted methanols, relative transfer probabilities from the carbon and hydroxyl positions of the parent molecule ion are found to be 1.00 and 0.81, respectively. The reaction involving CH2OH+ has also been investigated quantitatively. In ethanol, only C2H4OH+ is found to participate significantly in proton transfer processes. Other secondary ions formed in the high‐pressure mass spectrum of methanol and ethanol are reported, as well as the rate constants for the various proton‐transfer processes.

Reactions of C3H6+ with C3 and C4 Paraffins

Rate constants for H− and H2− transfer from C3 and C4 paraffins to propylene and cyclopropane molecular ions have been determined for ions of low translational energy at the ionization threshold of C3H6+. The results indicate that an increase in KH− is accompanied by a compensating decrease in KH2− and vice versa, such that the total reactivity of C4 structural isomers is invariant. By using selectively labeled compounds, it was possible to derive transfer probabilities for the various substituent sites on the normal butane carbon skeleton. Evidence is also presented which suggests that cyclopropane molecular ions do not assume the propylene‐ion structure prior to or during reaction with C4 paraffins. The results of the present investigation are compared with some complementary data obtained from radiolytic and photolytic systems at much higher pressures.

Dissociative Charge‐Exchange Reactions of Rare‐Gas Ions with Propane

Dissociative charge‐exchange reactions of rare‐gas ions with propane and with partially deuterated propanes have been investigated using a modified commercial mass spectrometer. The experimental results for propane are in good agreement with data previously obtained using much more sophisticated instrumentation. Charge‐exchange spectra obtained with 2,2‐dideuteropropane and with 1,1,1,3,3,3‐hexadeuteropropane permit the determination of details of the fragmentation mechanism. The data suggest that both s‐propyl and n‐propyl ions are formed by charge exchange, but that the latter is a relatively unimportant process. Ethylene is formed chiefly by 1,3 elimination of methane from the molecule ion, but 1,2 elimination becomes relatively more important with increasing recombination energy of the rare‐gas ion. In contrast to observations on the vacuum uvphotolysis of deuterated propanes, the present data indicate that elimination of molecular hydrogen as HD is of substantial importance in the over‐all ionic decomposition of both compounds.

Decomposition of Tropylium and Substituted Tropylium Ions

Metastable transitions arising from the decomposition of tropylium ions have been investigated in a double‐focusing mass spectrometer. It has been shown that the loss of an acetylene molecule is the only major decomposition process, and metastable peaks in the mass spectrum of toluene‐α‐d3 have been interpreted in terms of this. The relative intensities of the metastable peak and the normal peak at m/e=65 in the mass spectra of several compounds have been shown to correlate with the apparent heat of formation of the C7H7+ ion. Metastable transitions in the mass spectra of several halogenated and alkylated toluene and benzyl compounds have been interpreted in terms of the decomposition of substituted tropylium ions by the elimination of C2H2 or C2HX (X = F, Cl, Br). No evidence could be found for the existence of a pseudotropylium ion in which a CH group is replaced by a N atom in the mass spectra of alkyl pyridines.

Proton‐Transfer Reactions in the Simple Alkanes: Methane, Ethane, Propane, Butane, Pentane, and Hexane

Proton‐transfer reactions from CHO+ and CDO+ to the simple alkanes, methane through hexane, have been investigated in a tandem mass spectrometer. Acetaldehyde is shown to be the molecular source yielding CHO+ reactant with the lowest internal excitation energy. The translational energy dependence observed for the reactions CHO++CH4→CX5++CO, CX5+→CX3++X2 (X=H or D) suggests that the interaction is better represented over the energy range considered as proton stripping rather than as proceeding through an intermediate [CXO+–CX4]* complex. In addition, there is no evidence from isotope effects for a transition in mechanism. A highly simplified model treating the protonated methane species as a collection of closely coupled harmonic oscillators permits application of the classic rate expression for unimolecular decomposition to the dissociation yielding CH3+, and this relation provides a reasonable semiempirical fit to the experimental data. The reaction of CHO+ with ethane at quasithermal energy yields C2H7+...

Ion—Molecule Reactions in Propane

Ion—molecule reactions in propane were studied using mass‐ and energy‐resolved ion beams in a tandem mass spectrometer. The sequence of elementary reactions which lead to C3H8+ and C3H7+ as stable ions was established. Through the use of deuterium‐labeled reagents it was established that the parent ions undergo rapid charge‐exchange reactions but that the resonance hydride transfer reaction is not observed within sensitivity limitations of the instrument (k<10−12 cm3/molecule·sec). The alkene ions C2H4+ and C3H6+ react with propane by H− and H2− transfer reactions and the relative importance of these modes of reaction depends strongly on ion—neutral relative kinetic energy. Interesting rearrangement processes observed for the relatively low‐intensity primary ions C2H3+ and C3H3+ were also studied as a function of ion kinetic energy. In general, atom‐transfer reactions are favored over more complex rearrangement processes as ion velocity is increased and, in certain cases, new reaction channels are opened ...

Ion—Molecule Reactions of Propane Ions with Benzene, 1,3—Butadiene, Hydrogen Sulfide, and Nitric Oxide

Reactions of the principal primary ions from propane, C3H8+, C3H7+, C3H6+, C3H5+, C3H3+, C2H5+, C2H4+, C2H3+, and CH3+ with benzene, 1,3‐butadiene, nitric oxide, and H2S were investigated in a tandem mass spectrometer. Reaction mechanisms are deduced for these systems and it is suggested that a relatively long‐lived reaction complex is involved for most of these reactions. For the reaction of propyl ions with benzene an unstable adduct C9H13+ was observed which could be stabilized by collision and a unimolecular rate constant of 107sec−1 for its decomposition was estimated from its pressure dependence. In the H2S+–C3D8 and C3D8+—H2S systems charge exchange and deuteronation of H2S are competitive reactions, both proceeding via a long‐lived intermediate C3D8H2S+ in which extensive isotopic exchange occurs. The results of this study are correlated with recent radiolysis experiments and it is concluded that these two methods for studing gas‐phase ionic reactions are complementary.