From Neutral Aniline to Aniline Trication: A Computational and Experimental Study.

We report density functional theory computations and photoionization mass spectrometry measurements of aniline and its positively charged ions. The geometrical structures and properties of the neutral and singly, doubly, and triply positively charged aniline are computed using density functional theory with the generalized gradient approximation. At each charge, there are multiple isomers closely spaced in total energy. Whereas the lowest energy states of both neutral and cation have the same topology CH-NH, the dication and trication have the CNH-CH topology with the nitrogen atom in the meta- and para-positions, respectively. We compute the dissociation pathways of all four charge states to NH or NH and NH or NH, depending on the initial charge of the aniline precursor. Dissociation leading to the formation of NH (from the neutral and cation) and NH (from the dication and trication) proceeds through multiple transition states. On the contrary, the dissociation of NH (from the neutral and cation) and NH (from the dication and trication) is found to proceed without an activation energy barrier. The trication was found to be stable toward abstraction on NH and NH by 0.96 and 0.18 eV, respectively, whereas the proton affinity of the trication is substantially higher, 1.98 eV. The mass spectra of aniline were recorded with 1300 nm, 20 fs pulses over the peak intensity range of 1 × 10 to 3 × 10 W cm. The analysis of the mass spectra suggests high stability of both dication and trication to fragmentation. The formation of the fragment NH and NH ions is found to proceed via Coulomb explosion.