Predicting Competitive and Non-Competitive Torquoselectivity in Ring-Opening Reactions using QTAIM and the Stress Tensor

TL;DR

This study introduces a vector-based bond representation within QTAIM to predict ring-opening reaction preferences, offering a new criterion that better aligns with experimental outcomes than traditional activation energy analysis.

Contribution

The paper presents a novel vector-based bond-path framework set in QTAIM for predicting reaction selectivity, challenging the reliance on activation energies.

Findings

Longer path lengths predict inward or outward ring opening.

The new criterion accurately classifies reactions as competitive or non-competitive.

Activation energies are less reliable for determining reaction competitiveness.

Abstract

We present a new vector-based representation of the chemical bond referred to as the bond-path frame-work set $\mathbb{B} = {p, q, r}$, where $p$, $q$ and $r$ represent three paths with corresponding eigenvector-following path lengths $\mathbb{H}^{*},\mathbb{H}$ and the bond-path length from the quantum theory of atoms in molecules (QTAIM). We find that longer path lengths $\mathbb{H}$ of the ring-opening bonds predict the preference for the transition state inward (\textbf{TSIC}) or transition state outward (\textbf{TSOC}) ring opening reactions in agreement with experiment for all five reactions \textbf{R1-R5}. Competitiveness and non-competitiveness have traditionally been considered using activation energies. The activation energy however, for \textbf{R3} does not satisfactorily determine competitiveness or provide consistent agreement with experimental yields. We choose a selection of five competitive and non-competitive reactions; methyl-cyclobutene (\textbf{R1}), ethyl-methyl-cyclobutene (\textbf{R2}), iso-propyl-methyl-cyclobutene (\textbf{R3}), ter-butyl-methyl-cyclobutene (\textbf{R4}) and phenyl-methyl-cyclobutene (R5). Therefore, in this investigation we provide a new criterion, within the QTAIM framework, to determine whether the reactions \textbf{R1-R5} are competitive or non-competitive. We that find \textbf{R2}, \textbf{R3} and \textbf{R5} are competitive and \textbf{R1} and \textbf{R4} are non-competitive reactions in contrast to the results from the activation energies, calling into question the reliability of activation energies.