Symmetry-Induced Topological Obstructions in Woodward-Hoffmann Forbidden Reactions: A Molecular Qubit Representation.
5 affiliations
- 1Department of Foundational Sciences (Mathematics), Ramaiah Institute of Science and Management, Bengaluru560064, India.
- 2Department of Chemistry, SRM University-AP, Amaravati522240, India.
- 3School of Mathematics and Natural Sciences, Chanakya University, Bengaluru562165, India.
- 4Department of Chemistry and Biomedical Sciences, Linnaeus University, KalmarSE-39182, Sweden.
- 5Department of Mathematics and Mathematical Statistics, Umeå University, UmeåSE-90187, Sweden.
Abstract
Woodward-Hoffmann (WH) rules for electrocyclic reactions remain to date one of the most successful predictive tools in theoretical organic chemistry. At the heart of these rules is the simple idea that preserving bonding continuity in the highest occupied molecular orbital (HOMO) confers a kinetic advantage compared to a pathway that involves bonding disruption. However, this interpretation has been challenged by a few examples of symmetry-forbidden reactions that exhibit similar or lower kinetic barriers than corresponding allowed pathways. In this study, we develop a generalization of the WH rules through two complementary group-theoretic formulations: the Unitary Evolution Inaccessibility Theorem (UEIT) and the Topological Obstruction to Bonding Disruption Theorem (TOBDT). UEIT establishes that symmetry-forbidden reactions correspond to dynamical inaccessibility between the initial and target states under symmetry-preserving unitary evolution, while the TOBDT demonstrates that disruption in bonding continuity in a forbidden reaction constitutes a symmetry-induced topological obstruction. Within the frontier orbital subspace, the TOBDT is realized through a molecular qubit framework in which the orbital mixing angle Θ serves as a geometric descriptor of topological obstruction. This framework is verified through DFT and CASSCF calculations on four prototype 6π-electrocyclization reactions spanning diradicaloid and zwitterionic classes of transition states (TSs). Our results demonstrate that ΔΘ─the change in Θ between the reactant and the TS─correctly distinguishes diradicaloid from zwitterionic pathways. Additionally, computed nucleus-independent chemical shift (NICSzz) profiles confirm the antiaromatic character of diradicaloid TSs and the nonaromatic character of zwitterionic TSs, suggesting that the orbital symmetry restrictions may not be applicable in the latter.
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BibTeX
@article{repaka2026symmetryinduced,
title = {{Symmetry-Induced Topological Obstructions in Woodward-Hoffmann Forbidden Reactions: A Molecular Qubit Representation.}},
author = {Subha Sandeep Repaka and Thillaiarasi Sukumar and Bhagyashri Devaru Bhat and Ran Friedman and Baswanth Oruganti},
journal = {The journal of physical chemistry. A},
year = {2026},
month = sep,
doi = {10.1021/acs.jpca.6c04534},
url = {https://europepmc.org/article/MED/42917530},
note = {CC BY},
}
RIS
TY - JOUR
TI - Symmetry-Induced Topological Obstructions in Woodward-Hoffmann Forbidden Reactions: A Molecular Qubit Representation.
AU - Subha Sandeep Repaka
AU - Thillaiarasi Sukumar
AU - Bhagyashri Devaru Bhat
AU - Ran Friedman
AU - Baswanth Oruganti
PY - 2026
DA - 2026/09/01
JO - The journal of physical chemistry. A
DO - 10.1021/acs.jpca.6c04534
UR - https://europepmc.org/article/MED/42917530
AB - Woodward-Hoffmann (WH) rules for electrocyclic reactions remain to date one of the most successful predictive tools in theoretical organic chemistry. At the heart of these rules is the simple idea that preserving bonding continuity in the highest occupied molecular orbital (HOMO) confers a kinetic advantage compared to a pathway that involves bonding disruption. However, this interpretation has been challenged by a few examples of symmetry-forbidden reactions that exhibit similar or lower kinetic barriers than corresponding allowed pathways. In this study, we develop a generalization of the WH rules through two complementary group-theoretic formulations: the Unitary Evolution Inaccessibility Theorem (UEIT) and the Topological Obstruction to Bonding Disruption Theorem (TOBDT). UEIT establishes that symmetry-forbidden reactions correspond to dynamical inaccessibility between the initial and target states under symmetry-preserving unitary evolution, while the TOBDT demonstrates that disruption in bonding continuity in a forbidden reaction constitutes a symmetry-induced topological obstruction. Within the frontier orbital subspace, the TOBDT is realized through a molecular qubit framework in which the orbital mixing angle Θ serves as a geometric descriptor of topological obstruction. This framework is verified through DFT and CASSCF calculations on four prototype 6π-electrocyclization reactions spanning diradicaloid and zwitterionic classes of transition states (TSs). Our results demonstrate that ΔΘ─the change in Θ between the reactant and the TS─correctly distinguishes diradicaloid from zwitterionic pathways. Additionally, computed nucleus-independent chemical shift (NICSzz) profiles confirm the antiaromatic character of diradicaloid TSs and the nonaromatic character of zwitterionic TSs, suggesting that the orbital symmetry restrictions may not be applicable in the latter.
SN - 1089-5639
SN - 1520-5215
ER -