Quantum Chemical Analysis of Molecular and Fragment Ions Produced by Field Ionization of Methyl Stearate

The formation of molecular and fragment ions observed in the field ionization mass spectrum of methyl stearate has been analyzed on the basis of quantum chemical calculations including time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) analysis. The TDDFT calculations suggest that methyl stearate is ionized via two processes, namely a 7.43 eV excitation and a tunneling effect, while the high electric field of 10¹⁰ V/m enables analyte molecules to ionize at an effective 6 eV lower than the 9.26 eV ionization energy. The NBO analysis suggests that the abundances of aliphatic fragment ions [CH₂]⁺ at m/z 29, 43, and 57 generated in the ionizing cell can be rationalized by hyperconjugation between the sigma (σ)-electrons of sp³ C-H bonds of methyl or methylene groups and the empty p-orbital of the carbocation -CH₂⁺. The C4 periodic methyl ester fragment ions at m/z 115-269 and the complementary McLafferty rearrangement fragment ion at m/z 224 can be explained by metastable ion decay with rearrangement reactions in the ion source.