Related Experiment Video
Updated: Sep 30, 2026

Thermochemical Studies of Ni(II) and Zn(II) Ternary Complexes Using Ion Mobility-Mass Spectrometry
Published on: June 8, 2022
Application of the String Method to Determine the Ambimodal Nature of Transition States in Bifurcation Reactions
Keita Yamaki1, Hayato Yamada1, Daichi Okamoto1
1Department of Chemistry, Saitama University, Saitama, Japan.
Abstract:
Recent quantum chemical studies have revealed that a wide range of organic reactions exhibit post-transition-state bifurcation (PTSB), in which a single transition state (TS) leads directly to two or more distinct products along barrierless downhill pathways. Such TSs are commonly referred to as ambimodal TSs. In contrast, conventional intrinsic reaction coordinate (IRC) calculations typically connect the TS to only one of the possible products and therefore fail to identify alternative non-IRC products. To assess the ambimodal character of TSs in PTSB reactions, we analyzed relaxed pathways connecting a TS directly to a non-IRC product using the double-ended string method. Application of this approach to a total of 33 reaction systems, including pericyclic reactions (among them two trifurcation systems) and non-pericyclic reactions, enabled unambiguous classification of TS-product connections as either a static downhill connection or a static small-barrier connection, depending on whether an energy maximum exists below the TS. The proposed method provides a practical and robust tool for the static topological analysis of PTSB reactions and serves as an effective complement to on-the-fly dynamical trajectory simulations.
Related Concept Videos
Woodward–Hoffmann Selection Rules and Microscopic Reversibility
Transition State Theory
Energy Diagrams, Transition States, and Intermediates
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
SN2 Reaction: Transition State
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...
SN2 Reaction: Stereochemistry
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.

