Isomer effects on neutral-loss dissociation channels of nitrogen-substituted PAH dications

Abstract

We investigate two nitrogen-containing isomers of polycyclic aromatic hydrocarbons (PAHs), quinoline (Q) and isoquinoline (IQ), of composition C${}_9$H${}_7$N in collisions with 7~keV O${}^{+}$ and 48~keV O${}^{6+}$ projectile ions. Employing ion-ion coincidence mass spectrometry, we determine branching ratios for H-loss, C${}_2$H${}_2$-loss, and HCN-loss dissociation channels of Q${}^{2+}$ and IQ${}^{2+}$. The overall contribution of HCN-loss is found to be the dominant decay channel. A comparison with the results of a parallel experiment on naphthalene, the simplest PAH, reveals that HCN-loss in both isomers has a higher propensity than the analogous C${}_2$H${}_2$-loss of naphthalene. The positional identity of the nitrogen atom in the two isomers mainly manifests in many-body fragmentation of their dications. Potential energy surfaces of Q${}^{2+}$ and IQ${}^{2+}$ are further computed to explore complete fragmentation mechanisms. Parent dications (Q${}^{2+}$ and IQ${}^{2+}$) are identified to isomerize via seven-membered ring structures prior to elimination of C${}_2$H${}_2$ and HCN. While prompt dissociation is the primary pathway, the dominant channel of each neutral-loss class also exhibits delayed fragmentation.

Submitted to The Journal of Chemical Physics (AIP Publishing) on 15-10-2025