Carbocation Stability and Spectroscopy in Organic Chemistry

Carbocations are pivotal intermediates in a wide range of organic reactions, characterised by a positively charged carbon centre with an empty p orbital. Their stability is governed by electronic effects such as hyperconjugation, resonance and inductive donation, as well as by the nature of accompanying counterions. Advances in spectroscopic and crystallographic techniques have enabled the direct characterisation of transient and stabilised carbocations, shedding light on charge distribution, bonding motifs and dynamic rearrangements. Detailed infrared and NMR spectroscopic analyses, complemented by X-ray diffraction and quantum chemical calculations, have revealed subtle interactions within cationic frameworks, including non-classical bonds and inverse hydrogen-bonding motifs. Insights into substituent effects, solvent interactions and solid-state behaviour have enriched our understanding of reaction mechanisms in catalysis, polymerisation and complex molecule synthesis, informing the design of new reagents and functional materials.

Research from Nature Portfolio

Recent studies have uncovered a novel bonding interaction in adducts of vinyl carbocations and alkenes. By combining X-ray diffraction and infrared spectroscopy, vinyl cations paired with inert carborane counterions were shown to form stable monosolvates with alkenes, featuring an unprecedented =C+···Hδ−–Cδ+= linkage. The quasi-symmetrical arrangement exhibits a short C+···Cδ+ distance akin to proton disolvates, yet differs in electron-density distribution: an electron-rich hydrogen atom from the alkene donates to the electron-deficient carbocation centre. This inverse hydrogen bond challenges classical bonding paradigms and offers a refined model for carbocation–substrate interactions in solution and solid phases.

Carbocation Stability and Spectroscopy in Organic Chemistry publication trend

The graph below shows the total number of articles in carbocation stability and spectroscopy in organic chemistry across all publications each year (not limited to Nature Index journals).

Technical terms

Carbocation: A positively charged carbon species with three bonds and an empty p orbital.

Hyperconjugation: Delocalisation of σ-electron density from adjacent C–H or C–C bonds into an empty p orbital to stabilise a carbocation.

Counterion: A negatively charged ion that balances the positive charge of a carbocation in a crystalline or solution-phase salt.

Vinyl carbocation: A carbocation in which the positive charge resides on an sp2-hybridised carbon atom of an alkene.

Infrared spectroscopy: A technique measuring molecular vibrations to deduce functional groups and bonding environments.

Inverse hydrogen bond: An interaction where an electron-rich hydrogen atom engages a positively charged carbon centre, in contrast to classical hydrogen bonding.

References

1. Spontaneous Transition of Alkyl Carbocations to Unsaturated Vinyl-Type Carbocations in Organic Solutions. International Journal of Molecular Sciences (2023).
2. A new type of C+⋯Hδ−(C=) bond in adducts of vinyl carbocations with alkenes. Scientific Reports (2024).
3. Substitution of H Atoms in Unsaturated (Vinyl-Type) Carbocations by Cl or O Atoms. International Journal of Molecular Sciences (2023).
4. The Chloronium Cation [(C2H3)2Cl+] and Unsaturated C4-Carbocations with C=C and C≡C Bonds in Their Solid Salts and in Solutions: An H1/C13 NMR and Infrared Spectroscopic Study. International Journal of Molecular Sciences (2022).