Journal of Physical Chemistry A, Vol.122, No.18, 4490-4498, 2018
Natural Bond Orbital Theory of Pseudo-Jahn-Teller Effects
We describe a unified picture of symmetry breaking electronic interactions that are usually described as "pseudo-Jahn-Teller(PJT) effects" and attributed to vibronic coupling but can also be associated with hyperconjugative donor acceptor interactions in the framework of natural bond orbital (NBO) and natural resonance theory (NRT) analysis. We show how NBO/NRT descriptors offer a simplified alternative to the vibronic coupling picture of PJT effects that yields both improved cause effect specificity and chemically enriched understanding of symmetry-breaking phenomena but with no necessary input from ground-state vibrational or excited-state electronic properties. Comparative NBO/NRT vs vibronic coupling analyses of PJT effects are illustrated for two well-known cases: trans-bending in Si2H4 and higher Group-14 homologues of ethylene and chain-kinking in cyclopentadienylideneketene (C5H4CCO) and related cumulene ketones. The conceptual and practical advantages of the NBO-based hyperconjugative approach may be expected to extend to numerous PJT-type symmetry-breaking phenomena throughout the chemical sciences.