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

TL;DR

This study trains a combined experimental-theoretical framework to dissect how nitrogen placement in PANHs influences dication fragmentation under keV ion impact. Using ion-ion coincidence mass spectrometry, the authors quantify BRs for H-loss, C$_2$H$_2$-loss, and HCN-loss channels in Q$^{2+}$ and IQ$^{2+}$ and compare with naphthalene to isolate nitrogen-specific effects. Computational PES and MD simulations reveal isomerization to seven-membered azulene-like structures preceding loss, with HCN-loss being energetically favored and dominating across conditions; delayed fragmentation reveals metastable intermediates in the fragmentation pathways. The internal-excitation dependence (O$^+$ vs O$^{6+}$) reverses BR ordering between H-loss and C$_2$H$_2$-loss and underscores the role of multistep fragmentation in PANHs. The results imply PANHs have distinct fragmentation behavior and potentially shorter lifetimes in Titan-like environments, highlighting the relevance of nitrogen-containing PAHs in astrochemical contexts and motivating further systematic studies of PANH dissociation and association processes.

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.

Figures (9)

• Figure 1: Structures of naphthalene (C$_{10}$H$_8$, 128 au), quinoline and isoquinoline (C$_{9}$H$_7$N, 129 au).
• Figure 2: Total time-of-flight (TOF) mass spectra of cationic products observed in the interaction of neutral target molecules quinoline, isoquinoline and naphthalene with (a) 7 keV O$^+$ and (b) 48 keV O$^{6+}$, projectiles. Each spectrum is normalized by its total ion yield. The water contribution arises from the background. Insets show magnified views of the indicated regions.
• Figure 3: Selected regions from the coincidence map, where two fragment ions are detected in coincidence, corresponding to different neutral-loss dissociation channels of the quinoline dication (a)-(c) and isoquinoline dication (d)-(f) produced under 48 keV O$^{6+}$ impact. For reference, some of the ion pairs are labeled with the associated neutral-loss (green), and the dominant neutral-loss channel in each category is highlighted in red. Numbers in circles mark diagonal tail features arising from delayed fragmentation. Intensities are plotted on a logarithmic scale.
• Figure 4: Scheme summarizing isomerization and subsequent neutral-loss dissociation pathways of the quinoline and isoquinoline dications. Blue and orange arrows connect isomerization structures of Q$^{2+}$ and IQ$^{2+}$, respectively and dotted green arrows indicate the neutral-loss channels. Highest barrier in each step is mentioned along the arrows. Energies are given in eV and are relative to neutral quinoline and isoquinoline.
• Figure 5: Potential energy surfaces (PES) are summarized for a loss of HCN (top) and C$_2$H$_2$ (bottom) from IQ$^{2+}$ for its both possible pathways.