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Individually selecting multi-reference CI and its application to biradicalic cyclizations
B Engels1, M Hanrath, C Lennartz
1Institut für Organische Chemie, Universität Würzburg, Germany.
Computers & Chemistry
|January 12, 2001
Summary
A new algorithm for multi-reference configuration interaction (MR-CI) calculations enables routine analysis of over 10 million configurations. This method clarifies substituent effects on thermal biradical cyclization regioselectivity.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Organic Reaction Mechanisms
Background:
- Accurate theoretical prediction of regioselectivity in complex reactions is crucial for understanding reaction pathways.
- Multi-reference configuration interaction (MR-CI) is a powerful quantum chemical method but computationally demanding.
- Efficient algorithms are needed to extend the applicability of MR-CI to larger systems.
Purpose of the Study:
- To present and validate a novel, efficient algorithm for individually selecting MR-CI calculations.
- To investigate the regioselectivity of thermal biradical cyclizations using the new algorithm.
- To analyze the influence of various substituents on the regioselectivity of these cyclizations.
Main Methods:
- Development of a new MR-CI algorithm utilizing subdivision of molecular orbitals and a specialized cache.
- Application of the algorithm to study the regioselectivity of thermal biradical cyclizations, specifically C2-C7 vs. C2-C6 cyclization.
- Comparison of MR-CI with density functional theory (DFT) and coupled cluster (CC) methods using (Z)-1,2,4 Heptatriene-6-yne as a model system.
- Investigation of substituent effects (H, Ph, t-Bu, NH2) on regioselectivity.
Main Results:
- The new algorithm enables routine calculations with over 10 million selected configuration state functions (CSFs).
- Steric (t-Bu) and electronic (Ph) effects of substituents were rationalized as key factors influencing the switch between C2-C7 and C2-C6 cyclization pathways.
- For NH2 substituents, a significant lowering of the C2-C6 cyclization activation energy was predicted, accompanied by a change in the reaction mechanism.
Conclusions:
- The developed MR-CI algorithm significantly enhances computational efficiency, allowing for routine calculations of unprecedented scale.
- The study provides a detailed mechanistic understanding of substituent effects on biradical cyclization regioselectivity.
- The findings offer valuable insights for predicting and controlling reaction outcomes in organic synthesis.