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Properties of Enantiomers and Optical Activity

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It is essential to understand the difference between chiral and achiral interactions and the implications thereof in optical activity and their applications. Just as our feet, which are chiral, interact uniquely with chiral objects, such as a pair of shoes, but identically with achiral socks, enantiomers of a molecule exhibit different properties only when they interact with other chiral media. An example of a significant implication from this facet is the phenomenon known as optical activity,...

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Activity is the measure of the effective concentration of the species in solution. It can be expressed as the product of the molar concentration of the species and its activity coefficient. The activity coefficient is a dimensionless quantity and depends on the total ionic strength of the solution.
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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The effective concentration of a species in a solution can be expressed precisely in terms of its activity. Activity considers the effect of electrolytes present in the vicinity of the species of interest and depends on the ionic strength of the solution. The activity of a species is expressed as the product of molar concentration and the activity coefficient of the species.
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Related Experiment Video

Updated: Jul 29, 2025

Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Published on: November 21, 2019

The fundamental relation between electrohelicity and molecular optical activity.

Marc H Garner¹, Clemence Corminboeuf¹

¹Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering, Ecole Polytechnique federale de Lausanne (EPFL), 1015 Lausanne, Switzerland. marc.garner@epfl.ch.

Physical Chemistry Chemical Physics : PCCP

|May 26, 2023

Summary

This summary is machine-generated.

Electrohelicity, the helical nature of molecular orbitals, influences optical activity in some molecules like allene. However, this connection is molecule-dependent, not a universal principle for enhancing chiroptical response.

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Published on: May 27, 2020

Area of Science:

Molecular Orbital Theory
Chiroptical Spectroscopy
Organic Chemistry

Background:

Electrohelicity, characterized by helical frontier molecular orbitals (MOs), appears in molecules with reduced symmetry, leading to optical activity.
Electrohelicity has been proposed as a design strategy to enhance chiroptical responses.
Understanding the fundamental link between electrohelicity and optical activity is crucial for molecular design.

Purpose of the Study:

To investigate the fundamental relationship between electrohelicity and optical activity in various molecular systems.
To determine the origin of electric and magnetic transition dipole moments for π-π* transitions in relation to MO helicity.
To explore the potential for designing molecules with enhanced chiroptical properties based on electrohelicity.

Main Methods:

Theoretical examination of the electric and magnetic transition dipole moments for π-π* transitions.
Analysis of molecular orbital (MO) helicity in allene, butatriene, tolane, and spiropentadiene.
Computational studies to correlate MO helical character with chiroptical response.

Main Results:

The helical character of MOs directly drives optical activity in allene, enabling the design of molecules with enhanced chiroptical response.
While MO helicity contributes to optical activity in non-planar butatriene, no correlation was found in tolane.
The optical activity of spiropentadiene originates from π-system mixing, not MO helicity.

Conclusions:

The connection between electrohelicity and optical activity is highly molecule-specific.
Electrohelicity is not a universal principle for optical activity but offers insights for enhancing chiroptical responses.
Understanding the helical nature of electronic transitions is key to designing molecules with improved chiroptical properties.