Linear free energy relationships in CN bond dissociations in molecular ions of 4‐substituted N‐(2‐furylmethyl)anilines in the gas phase

Abstract

The substituent effect on the reactivity of the C-N bond of molecular ions of 4-substituted N-(2-furylmethyl)anilines toward two dissociation pathways was studied. With this aim, six of these compounds were analyzed by mass spectrometry using electron ionization with energies between 7.8 and 69.9 eV. Also, the UB3LYP/6-31G (d,p) and UHF/6-31G (d, p) levels of theory were used to calculate the critical energies (reaction enthalpies at 0 K) of the processes that lead to the complementary ions [C(5)H(5)O](+) and [M - C(5)H(5)O](+), assuming structures that result from the heterolytic and homolytic C-N bond cleavages of the molecular ions, respectively. A kinetic approach proposed in the 1960s was applied to the mass spectral data to obtain the relative rate coefficients for both dissociation channels from ratios of the peak intensities of these ions. Linear relationships were obtained between the logarithms of the relative rate coefficients and the calculated critical energies and other thermochemical properties, whose slopes showed to be conditioned by the energy provided to the compounds within the ion source. Moreover, it was found that the dissociation that leads to [C(5)H(5)O](+) is a process strongly dependent upon the electron withdrawing or donating properties of the substituent, favored by those factors that destabilize the molecular ion. On the contrary, the dissociation that leads to [M - C(5)H(5)O](+) is indifferent to the polar electronic effects of the substituent. The abundance of both products was governed by the rule of Stevenson-Audier, according to which the major ion is the one of less negative electronic affinity.