# Reactive MD Screening of Antioxidants for Substituent-Dependent Phenoxyl Radical Stability

## TL;DR

This paper uses simulations to study how the structure of phenolic antioxidants affects their stability, which is crucial for their performance in lubricants.

## Contribution

The study introduces a reactive molecular dynamics approach to screen antioxidants based on substituent effects on radical stability.

## Key findings

- Strong hydrogen bonding and steric hindrance around the phenoxyl oxygen decrease reaction rates, increasing radical stability.
- Faster diffusion increases reaction rates, reducing radical stability.
- A multivariate model shows hydrogen bonding is the main factor in radical stability at low reaction rates.

## Abstract

Oxidation limits
the performance and lifetime of lubricants, and
phenolic antioxidants are commonly used to slow this process by scavenging
hydrocarbon peroxyl radicals. The performance of phenolic antioxidants
is largely determined by the stability of the antioxidant radical
that remains after hydrogen donation. To explore the relationship
between antioxidant chemical structure and radical stability, we used
REACTER-based reactive molecular dynamics simulations to model the
reverse hydrogen transfer reaction from polyalphaolefin hydroperoxides
to phenoxyl radicals. Simulations were run for 718 distinct single-ring
phenoxyl radicals with varied substituent types and positions in a
polyalphaolefin hydroperoxide environment. Reaction rates were obtained
from the time evolution of hydrogen transfer events, where lower reaction
rates correspond to higher radical stability and better antioxidant
performance. Analysis of diffusivity, hydrogen bonding, and steric
hindrance showed that strong hydrogen bonding and high steric hindrance
around the phenoxyl oxygen atom decreased the reaction rate, while
faster diffusion increased it. A multivariate linear model confirmed
that hydrogen bonding was the dominant contributor to radical stability
in the low reaction rate region. These results highlight the molecular
features that influence antioxidant behavior and demonstrate that
reactive simulations offer an efficient route for screening and designing
antioxidant molecules.