# The N‐Bromo‐Hammick Intermediate

## TL;DR

Scientists identified a unique chemical intermediate with a special electronic structure that allows it to convert into a more stable form without bending.

## Contribution

The discovery of the N-bromo-Hammick intermediate's σ²σ*⁰ electronic structure and planar isomerization mechanism.

## Key findings

- The N-bromo-Hammick intermediate is a singlet carbene with a σ²σ*⁰ electronic structure.
- It isomerizes to bromopyridine via a planar transition state, avoiding structural distortion.
- Its reaction with hydrogen is blocked by a small but significant energy barrier at low temperatures.

## Abstract

We report the characterization of bromopyridin‐2‐ylidene, the N‐bromo‐Hammick intermediate, isolated in solid neon at 4.4 K. Computations (B2PLYP, NEVPT2), IR, and UV/vis spectroscopy characterize this intermediate as a singlet carbene with a vacant σ*‐type frontier orbital. This unusual electronic structure (σ2σ*0) allows the carbene to isomerize to its more stable pyridine tautomer via a planar transition state, avoiding the deviation from planarity seen in other singlet carbenes. The compound's reaction with molecular hydrogen faces a minor but prohibitive barrier (8.4 kcal mol−1) at the low temperatures of the experiment.

Spectroscopic and computational analysis uncovers the σ2σ*0 electronic structure of the N‐bromo‐Hammick intermediate, a singlet N‐heterocyclic carbene isolated in solid neon, whose back‐isomerization to bromopyridine proceeds via a planar transition state.