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Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene
Author(s) -
Bearpark Michael J.,
Bernardi Fernando,
Olivucci Massimo,
Robb Michael A.
Publication year - 1996
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1996)60:1<505::aid-qua49>3.0.co;2-7
Subject(s) - pentalene , conical intersection , chemistry , valence bond theory , potential energy , ground state , singlet state , excited state , potential energy surface , atomic physics , molecule , molecular physics , physics , molecular orbital , organic chemistry
The S 0 and S 1 potential energy surfaces of pentalene were studied using MMVB—a hybrid force‐field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S 0 , a C 2h minimum (with alternating single and double bonds) and a D 2h transition structure; and on S 1 , a D 2h minimum and an adjacent S 1 / S 0 conical intersection. A VB exchange density matrix (which is independent of the choice of the spin‐coupled basis) was used to rationalize the S 0 and S 1 surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S 0 transition structure is an open‐shell B 1x singlet, and the VB ground state is dominated by a spin‐coupling which transforms as B 1g . A C 2v minimum and a D 2h transition structure were located on the CASSCF S 2 potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S 1 S 2 states of pentalene are shown to be reverse of the S 1 and S 2 states of benzene. © 1996 John Wiley & Sons, Inc.