Theoretical study of electron impact mass spectrometry. I. Ab initio MO study of the fragmentation of n-butane

The fragmentation mechanism of n-butane by low-energy electron bombardment has been studied by means of the ab initio MO method. Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated using the energy gradient technique. The results suggest that the fragmentation to C₁ + C₃ is more favorable than that to C₂ + C₂, when the electron impact energy is at most only a few eV above the ionization threshold. The base peak at m/z 43 has been calculated to be due to the 2-propyl cation. In the course of fragmentation to C₁ + C₃ proton tunneling is expected.