Evaluating Computational and Structural Approaches to Predict Transformation Products of Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAHs) undergo transformation reactions with atmospheric photochemical oxidants, such as hydroxyl radicals (OH•), nitrogen oxides (NOx), and ozone (O 3 ). The most common PAH-transformation products (PAH-TPs) are nitrated-, oxygenated-, and hydroxylated-PAHs (NPAHs, OPAHs, and OHPAHs, respectively), some of which are known to pose potential human health concerns. We sampled four theoretical approaches for predicting the location of reactive sites on PAHs (i.e., the carbon where atmospheric oxidants attack), and hence the chemoselectivity of the PAHs. All computed results are based on Density Functional Theory (B3LYP/6-31G(d) optimized structures and energies). The four approaches are: 1) Clar's prediction of aromatic resonance structures, 2) thermodynamic stability of all OHPAH adduct intermediates, 3) computed atomic charges (Natural Bond order, ChelpG, and Mulliken) at each carbon on the PAH, and 4) average local ionization energy (ALIE) at atom or bond sites. To evaluate the accuracy of these approaches, the predicted PAH-TPs were compared to published laboratory observations of major NPAH, OPAH, and OHPAH products in both gas-and particlephases. We found that the Clar's resonance structures were able to predict the least stable rings on the PAHs but did not offer insights in terms of which individual carbon is most reactive. The OHPAH adduct thermodynamics and the ALIE approaches were the most accurate when *

Late-Stage Functionalisation of Polycyclic (N-Hetero-) Aromatic Hydrocarbons by Detoxifying CYP5035S7 Monooxygenase of the White-Rot Fungus Polyporus arcularius

“…In fact, the central ring of 6 is 2.5-fold lower than that of the terminal rings [ 52 ], which can be pictured best by Clar’s aromatic π-sextet approach drawing resonance structures with the highest number of π-sextet rings ( Figure S15 ) [ 53 ]. These aromaticity calculations are roughly in agreement with the reactive position of various PAH compounds identified by different prediction methods as well as laboratory data summarised by Cheong et al [ 54 ]. They clearly identified position 9 of both 5 and 6 as the most reactive.…”

“…The presence of 18 as a product of the biotransformation of 5 follows the trend of the laboratory data of Cheong et al [ 54 ], although the formation of 16 as the main product with hydroxylation on the terminal ring suggests that activity can be explained here by the predicted aromaticity ( Figure S15 ). For compounds 3 and 4 , the neighbouring carbon to that in the compound’s bay area (positions 4 and 3, respectively) was classified as the preferred reactive site by laboratory data [ 54 ], and was hence in disagreement with the regioselectivity observed here. Instead, the prediction with Mulliken and the predicted carbon with the lowest hydroxylated PAH adduct stability could explain the product formation catalysed by CYP5035S7.…”

“…However, it is more likely that the similar substrate sizes of 4 and 5 with 6 causes a comparable effect of the substrate orientation in the active site to take effect. Indeed, the lack of activity towards the central 5-membered ring of 4 despite its low aromaticity according to Clar’s aromatic π-sextet is in line with the disregard of the 9-position of 6 [ 54 ]. In light these reactivity explanations, the hydroxylation of the saturated ring of 3 at the activated benzylic position to form 12 [ 31 ] is either a bit of an exception or reveals the true activity of CYP5035S7 purely based on electron density without any steric influence.…”

Formation of Polycyclic Aromatic Hydrocarbon Oxidation Products in α-Pinene Secondary Organic Aerosol Particles Formed through Ozonolysis

“…Ozonolysis of aromatic compounds can lead to different substitutions on the PAH rings 3634 . Thermodynamic calculations suggest that an OH adduction is favorable over carbonyl adductions due to the highly instable epoxide intermediates, both resulting in low-volatility products 37 . PHE ozonolysis has previously been measured to form 2'-formyl[1,1'biphenyl]-2-carboxylate (2'F(1-1'BP)2C) in laboratory studies of PHE bound to the surface of silica particles 22 .…”

Degradation Mechanism of Benzo[a]pyrene Initiated by the OH Radical and 1O2: An Insight from Density Functional Theory Calculations

“…The prediction of reactive site C6 can also be confirmed by the Clar’s aromatic π-sextet approach. 26 The following calculated potential energy surfaces (PESs) of the reaction between • OH and BaP also support this prediction.…”