How do Nanohoops Exercise Their Strain in [5]Helicene Racemization?

TL;DR

This paper explores how strain in carbon nanohoops affects the racemization of [5]helicene, revealing that larger nanohoops increase the molecule's configurational stability.

Contribution

The study establishes a direct relationship between nanohoop strain and [5]helicene racemization through experimental and computational methods.

Findings

The configurational stability of [5]helicene increases with larger nanohoop size.

Activation free energies for racemization range from 25 to 29 kcal mol–1.

DFT calculations confirm the strain-racemization relationship in helicene-embedded nanohoops.

Abstract

The effect of strain
in curved carbon nanohoops has a major effect
on their optoelectronic properties. How this strain affects their
dynamics and reactivity is, however, far less documented in the literature.
Here, we investigate the effect of strain on racemization of [5]­helicene,
which we embedded in three different [m]­cycloparaphenylenes
(m = 5–7). [5]­Helicene is a prototypical organic
molecule with helical chirality that racemizes at room temperature.
We synthesized and separated the enantiomers of the strained helicene-paraphenylene
macrocycles, and we investigated their racemization as a function
of temperature. Our results revealed that the configurational stability
of the [5]­helicene increases with increasing nanohoop size, with activation
free energies ranging from 25 to 29 kcal mol–1.
The combination of the experimental data and DFT calculations established
a clear relationship between the strain and racemization in helicene-embedded
nanohoops.

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