Individually selecting multi-reference CI and its application to biradicalic cyclizations

In the present review a new algorithm to perform individually selecting MR-CI calculations is discussed. The new algorithm exploits the advantages arising from the subdivision of the molecular orbitals (MOs) into an internal and an external part and avoids the recalculation of representation matrices by the use of a specially designed cache. With the new algorithms we are able to perform calculations including more than 10 million selected configurations state functions (CSFs) as a matter of routine. As an example for the possibilities of the new approach the regioselectivity of thermal biradical cyclizations (C-2-C-7 vs. C-2-C-6 cyclization) is investigated in the second part of the present work. After studying the accuracy of various quantum chemical methods (MR-CH, density functional theory, and coupled cluster approaches) using (Z)-1,2,4 Heptatriene-6-yne as a model system the influence of substituents (R = H, Ph, t-Bu, NH2) on the regioselectivity is investigated. Our results rationalize the switch between the two biradical cyclizations on the basis of mainly steric (t-Bu) or electronic (Ph) substituent effects. Moreover, the results for Ii = NH, predict that the activation energy of the C-2-C-6 cyclization can be, lowered even more. However, a change in the reaction mechanism is found. (C) 2001 Elsevier Science Ltd. All rights reserved.