HOMO–LUMO Gaps and Molecular Structures of Polycyclic Aromatic Hydrocarbons in Soot Formation

Yabei Xu,Andrés Fuentes,Dongping Chen,Qingzhao Chu

doi:10.3389/fmech.2021.744001

Abstract

A large number of PAH molecules is collected from recent literature. The HOMO-LUMO gap value of PAHs was computed at the level of B3LYP/6-311+G (d,p). The gap values lie in the range of 0.64–6.59 eV. It is found that the gap values of all PAH molecules exhibit a size dependency to some extent. However, the gap values may show a big variation even at the same size due to the complexity in the molecular structure. All collected PAHs are further classified into seven groups according to features in the structures, including the types of functional groups and the molecular planarity. The impact of functional groups, including –OH, –CHO, –COOH, =O, –O– and –CnHm on the bandgap is discussed in detail. The substitution of ketone group has the greatest reduction on the HOMO-LUMO gap of PAH molecules. Besides functional groups, we found that both local structure and the position of five-member rings make critical impacts on the bandgap via a detailed analysis of featured PAHs with unexpected low and high gap values. Among all these factors, the five-member rings forming nonplanar PAHs impact the gap most. Furthermore, we developed a machine learning model to predict the HOMO-LUMO gaps of PAHs, and the average absolute error is only 0.19 eV compared with the DFT calculations. The excellent performance of the machine learning model provides us an accurate and efficient way to explore the band information of PAHs in soot formation.

Highlights

Polycyclic aromatic hydrocarbons (PAHs) generated from the incomplete combustion of hydrocarbon fuels are accepted as the precursors of soot
The gap values cluster in the range of 20–50 carbon atoms, and the variation of the same sized PAHs in the gap values can be as large as ∼2 eV
Among all the functional groups, the ketone group substitution has the greatest reduction on the HOMOLUMO gap value, while the –CH3, –OH and –COOH substitution has a relatively small influence on the gap value

Summary

Introduction

Polycyclic aromatic hydrocarbons (PAHs) generated from the incomplete combustion of hydrocarbon fuels are accepted as the precursors of soot. The identification of PAHs and their structures is critical to interpret their growth mechanism, which is the basis for the reduction of the soot emission (Wang and Chung, 2019). With the application of novel measurement methods, recent researchers have made important progress in the investigation of the key process in soot formation by identifying the potential intermediates. Johansson and his coworkers proposed a radical-assisted PAH growth mechanism supported by the aerosol mass spectrometry measurements (Johansson et al, 2018). The abundance and relevance of these identified PAHs to soot formation is unknown, it was the first time that the configurations of large PAHs are confirmed in measurements. Adamson (Adamson et al, 2018) detected

Methods

Results

Conclusion