Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene

✍ Scribed by Michael J. Bearpark; Fernando Bernardi; Massimo Olivucci; Michael A. Robb

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 600 KB
• Volume: 60
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1996)60:1<505::aid-qua49>3.0.co;2-7

✦ Synopsis

The So and S, 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 So, a C,,, minimum (with alternating single and double bonds) and a D Z h transition structure; and on S,, a D,, minimum and an adjacent S , / S , conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the So and S , 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 So transition structure is an open-shell B,, singlet, and the VB ground state is dominated by a spin-coupling which transforms as Big. A C,, minimum and a D,, transition structure were located on the CASSCF S, potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S, and S, states of pentalene are shown to be reverse of the S, and S, states of benzene.