Relationships between DFT/RRKM Branching Ratios of the Complementary Fragment Ions [C5H5O]+ and [M – C5H5O]+ and Relative Abundances in the EI Mass Spectrum of N‑(2-Furylmethyl)aniline

The energy-dependent branching ratios of the complementary fragment ions [C5H5O]+ and [HC6H4NH]+ ([M – C5H5O]+), originating from the N-(2-furylmethyl)aniline molecular ion, [HC6H4NH–C5H5O]+•, were obtained from Rice–Ramsperger–Kassel–Marcus (RRKM) rate calculations based on density functional theory (DFT) energy profiles. The UB3LYP/6-311G+(3df,2p)//UB3LYP/6-31G(d) level of theory was used to model the competitive reaction mechanisms by which the molecular ion can be fragmented. Initially, eight pairs of products were taken into account, corresponding to the combination of two isomeric structures for each fragment ion and the concomitant radicals, which can be formed by direct dissociations or through some isomerization-fragmentation pathways. A great deal of the obtained pathways was discarded by looking over the kinetic barrier heights and the individual RRKM rate coefficients calculated for all the steps. This way, the potential energy profiles were simplified to only three reaction channels, two pathways to [C5H5O]+ and one to [M – C5H5O]+. The pre-equilibrium and steady-state approximations were then applied to different regions of the remaining potential energy profiles, allowing the branching ratios of the complementary fragment ions to be easily calculated and discriminated among the three rival processes. According to these results, the major fragment ion in the ion source is [C5H5O]+, which is produced as a mixture of two structures, the furfuryl and pyrylium cations, one formed by a direct C–N bond cleavage and the other through an isomerization-fragmentation channel. In turn, the direct fragmentation is the only mechanism to produce [M – C5H5O]+. To confront these results with the available experimental information, the model was broadened out to the 4-substituted analogues [4-R–C6H4NH–C5H5O]+• in which R = F, Br, Cl, CH3, and OCH3, finding excellent correlations of the calculated branching ratios and the relative abundances in the electron ionization mass spectra.