Field Ionization Mass Spectrometric and Quantum Chemical Analysis of Alkyl Fragment Ions Produced by Field Dissociation of n-Heptane, 2-Methyl Hexane, and 2-Methyl Heptane

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Abstract
The alkyl fragment ions (CnH2n+1)+, produced by field dissociation of n-heptane, 2-methyl hexane, and 2-methyl heptane, have been examined by using a field ionization time-of-flight mass spectrometer, which enables the detection of fragment ions by the rapid field dissociation (10-14 to 10-12 s) and slow (10-6 to 10-5 s) time windows. A rigorous analysis of the intensity and stability of the alkyl fragment ions was conducted by employing density functional theory (DFT) calculations. The observed order of intensity for the complementary pair of alkyl fragment ions m1+ and m2+ arising from the C-C σ-bond cleavage of the molecular ion M+• was found to be consistent with the order of the hyperconjugative stabilization energy, as determined by natural bond orbital analysis. However, the observed order of fragment ion intensity could not be explained by Stevenson's rule, hydride ion affinity, and energy barrier as a kinetic parameter. According to the DFT analysis, it was suggested that the field dissociation of the C-C σ-bond is accompanied by the partitioning of the total charge and spin density of the M+• ion into fragment ions m1+ and m2+. The singly occupied molecular orbital of the molecular ions predicted the C-C bonds that underwent preferential cleavage, as evidenced by the extension of bond length of the C-C bonds. This is especially evident in the formation of the fragment ion (C2H5)+ at m/z 29 of n-heptane and the secondary carbocation fragment (CH3CHCH3)+ at m/z 43 of 2-methyl hexane and 2-methyl heptane.

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