Recent experimental and computational work suggests that a significant number of curved aromatic molecules are found in carbon materials such as soot. These curved aromatic molecules contain a substantial dipole moment (4-6.5 debye for the sizes seen in soot molecules containing 10-20 rings). In this paper we employ electronic structure calculations to determine the earliest onset of curvature integration. We found an interplay between the σ bonding inducing curvature and the π bonding resisting curvature with a size of six rings, with at least one being pentagonal, required to curve an aromatic molecule. The interactions between polar curved arenes and other species are then considered with particular focus on nascent soot formation, indicating that heterogeneous, not homogeneous, nucleation could be a significant factor in soot formation.

The formation of carbon nanoparticles in flames involves a nucleation step that remains poorly understood. Experimentally, carbon nuclei formation is known to depend strongly on the electrical aspects of combustion but modes of interaction between charged species in the flame and carbon precursors have yet to be found. We present evidence for flexoelectrically polarised aromatics contributing to carbon nanoparticulate nucleation. We imaged the nascent nanoparticles using high resolution transmission electron microscopy, which revealed that the majority of aromatics in the early carbon nanoparticles are fullerene-like and curved. The curvature induces a significant molecular flexoelectric dipole moment in the polyaromatic hydrocarbons. This electric polarization allows these molecules to strongly interact with chemi-ions produced during combustion, which we demonstrate using electronic structure calculations. The results indicate that the physical interaction between fullerene-like polar aromatics and chemi-ions is critically assisting the nucleation, and opens a new route to reduce pollution and improve flame-produced nanomaterials.

The morphologies of heterogeneous clusters of polycyclic aromatic hydrocarbons (PAHs) are investigated using molecular modelling. Clusters of up to 100 molecules containing combinations of the different sized PAHs circumcoronene, coronene, ovalene, or pyrene are evaluated. Replica exchange molecular dynam

short running title: Soot nucleation using a new potential for curved aromatics keywords: force field, curved polycyclic aromatic hydrocarbon, flexoelectric dipole, ion-induced nucleation, soot formation 2 Abstract A potential able to capture the properties and interactions of curved polycyclic aromatic hydrocarbons (cPAHs) was developed and used to investigate the nucleation behaviour and structure of nascent soot particles. The flexoelectric charge polarisation of cPAHs caused by pentagon integration was included through the introduction of off-site virtual atoms, and enhanced dispersion interaction parameters were fitted. The electric polarisation and intermolecular interactions of cPAHs were accurately reproduced compared to ab initio calculations. This potential was used within molecular dynamics simulations to examine the homogeneous and heterogeneous nucleation behaviour of the cPAH corannulene and planar PAH coronene across a range of temperatures relevant to combustion. The enhanced interactions between cPAHs and potassium ions resulted in significant and rapid nucleation of stable clusters compared to all other systems, highlighting their importance in soot nucleation. In addition, the resulting cPAH clusters present morphologies distinct from the stacked planar PAH clusters.

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