Quantitative Assessment of Aromaticity and Antiaromaticity Utilizing Vibrational Spectroscopy
Dani Setiawan1, Elfi Kraka1, Dieter Cremer1
1Computational and Theoretical Chemistry Group (CATCO), Department of Chemistry, Southern Methodist University , 3215 Daniel Ave., Dallas, Texas 75275-0314, United States.
The Journal of Organic Chemistry
|September 17, 2016
Summary
A new aromaticity index (AI) precisely quantifies π-delocalization in cyclic systems using local vibrational modes. This method offers a more accurate assessment of aromaticity compared to previous models.
Area of Science:
- * Molecular spectroscopy
- * Quantum chemistry
- * Organic chemistry
Background:
- * Vibrational frequencies offer precise molecular electronic structure analysis.
- * Local vibrational modes are ideal for studying molecular properties.
- * Existing methods for assessing aromaticity have limitations.
Purpose of the Study:
- * To introduce a novel aromaticity index (AI) based on local CC stretching force constants.
- * To quantitatively determine the degree of π-delocalization in cyclic conjugated systems.
- * To compare the new AI with the harmonic oscillator model and Clar's rule.
Main Methods:
- * Calculation of local CC stretching force constants.
- * Derivation of a new aromaticity index (AI).
- * Application to 30 mono- and polycyclic conjugated hydrocarbons.
Main Results:
- * The AI accurately describes π-delocalization, including local, peripheral, and all-bond contributions.
- * The harmonic oscillator model inaccurately exaggerates antiaromaticity in 4n π-systems.
- * The AI validates Clar's rule in many cases but corrects it where peripheral delocalization enhances stability.
- * Aromaticity confirmed for [5]-, [6]-, [7]-Circulene and Kekulene with varying delocalization.
Conclusions:
- * Local vibrational mode properties provide an accurate description of π-delocalization.
- * The developed AI offers a reliable quantitative measure of aromaticity.
- * The new AI improves upon existing models by addressing limitations in describing antiaromaticity and local aromaticity.
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