Combined experimental and theoretical study on photoionization cross sections of benzonitrile and o/m/p-cyanotoluene

Jiabin Huang,Feng Zhang,Guangjun Tian,Can Huang,Jiuzhong Yang,Qifeng Hou,Xiaoqing Wu

doi:10.1063/5.0053119

Abstract

Photoionization cross sections (PICSs) for the products of the reaction from CN with toluene, including benzonitrile and o/m/p-cyanotoluene, were obtained at photon energies ranging from ionization thresholds to 14eV by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Theoretical calculations based on the frozen-core Hartree-Fock approximation and Franck-Condon simulations were carried out to cross-verify the measured PICS. The results show that the photoionization cross sections of benzonitrile and cyanotoluene isomers are similar. The generalized charge decomposition analysis was used to investigate the components of the highest occupied molecular orbital (HOMO) and HOMO-1. It was found that the HOMO and HOMO-1 of benzonitrile and cyanotoluene isomers are dominated by the features of the benzene ring, indicating that the substitution of CN and methyl has a minor influence on the PICS of the studied molecules. The reported PICS on benzonitrile and cyanotoluene isomers in the present work could contribute to the near-threshold PIMS experiments and determine the ionization and dissociation rates in interstellar space for these crucial species. The theoretical analysis on characteristics of molecular orbitals provides clues to estimating the PICS of similar substituted aromatic compounds.

Highlights

The maximum of Franck-Condon factor appears near the threshold ionization energy, it decreases with the photon energy (Figure S3-S4 in the supplementary materials)
It was revealed that the photoionization of HOMO-1is involved in the Franck-Condon region for benzonitrile and cyanotoluene isomers, and deeper orbitals have contributions with increasing the photon energy by ~2 eV higher than the threshold
To study the effect of cyano radical (CN) and methyl substitution on the molecular orbitals of aromatic compounds, we used the generalized charge decomposition analysis method to investigate the composition of the orbitals

Summary

Introduction

Toluene has been suggested as an important aromatic compound in the atmosphere of Titan.[1,2,3] The reaction of toluene with cyano radical (CN) is a crucial channel to incorporate nitrogen into Titan’s molecular weight growth model,[4] which leads to the brown appearance of Tit an’s atmosphere.[5,6,7] Previous studies have shown very fast rate coefficients between CN and unsaturated compounds including aromatics at low temperatures.[8, 9] Recently, Messinger et al.[10] investigated this reaction by the pulsed laser photolysis–laser-induced fluorescence (PLP-LIF) technique and quantum chemical calculations, indicating that the rate constants of CN + toluene between 15 and 294 K are independent of temperature, with an average value of (4.1 ± 0.2) × 10-10 cm[3] molecule-1 s-1, at the total gas density from 2.7 to 10.5 × 1016 cm-3. CN adds to the benzene ring removes a hydrogen atom to generate ortho, meta, or parasubstituted products It may substitute the methyl group to generate benzonitrile.[12,13,14] In recent years, benzonitrile, as one of the simplest aromatic derivatives, has detected in the cold-core Taurus Molecular Cloud 1 (TMC-1). It provides an important reference for the formation mechanism of PAHs (polycyclic aromatic hydrocarbons) in the ISM (interstellar medium).[15,16] No evidence of benzonitrile formation was observed in the study of Trevitt et al The lack of photoionization cross-sections (PICS) of various cyanotoluene isomers has hindered the identification of isomers in the SVUV-PIMS experiment.[11]

Objectives

Methods

Results

Conclusion