The Mass Spectra of C1–C4 Monodeutero Paraffins

Abstract

The magnetically scanned mass spectra of methane‐d, ethane‐d, propane−1‐d, propane−2‐d, n‐butane−1‐d, n‐butane−2‐d, i‐butane−1‐d, and i‐butane−2‐d, characteristic of 75‐volt ionizing electrons and 75°C are given and compared with the mass spectra of the corresponding hydrocarbons. A qualitative account of the origin of the isotope effects in mass spectra is given. It is shown that in a molecule containing equivalent isotopic species, the molecule‐ion formed by electron impact will preferentially dissociate the lighter isotope and that the mass spectra of symmetrically substituted isotopic species (A′BA′) will be more like the mass spectra of the isotopically pure species (ABA) than are those of the unsymmetrically substituted isotopic species (ABA′). It is further shown that the equality (or lack thereof) of parent ion sensitivities of different isotopic preparations to the parent ion sensitivity of the natural substance is not a criterion of the purity (or lack thereof) of the preparations. Attention is called to the close relation between isotope effects in mass spectra and temperature effects in mass spectra. The two‐parameter empirical theory of isotope effects proposed by Evans, Bauer, and Beach is applied to the mass spectra of the methanes and ethanes and found to be inadequate for the prediction of mass spectra for accurate analysis of isotopic mixtures. It is found, however, that for certain electron impact induced processes, such as RH+ε⁻→R⁺+H+2ε⁻, the effective values of the parameters, Γ and π, are essentially equal for the monodeutero C₁ through C₄ paraffins. It is shown that for the process, RH+ε⁻→R⁺+H+2ε⁻, the specific dissociation probability of secondary hydrogens is greater than that of primary hydrogens by a factor of 4 for propane and 10 for n‐butane, while in isobutane the specific dissociation probability of the tertiary hydrogen is 30 times that of a primary hydrogen. It is found that in certain states of the paraffin molecule‐ions the hydrogens are sufficiently mobile that they undergo very extensive intramolecular redistribution in the short life of the molecule ion (half‐life <0.1–1.0 microseconds) before they dissociate. Such states are of less common occurrence among paraffins than among olefins.