Over the last two decades, quantum chemical correlated methods had been shown to produce highly accurate molecular energies and spectroscopic data. However, recent theoretical studies on the simplest aromatic hydrocarbon, cyclopropenylidene, have shown notable errors in the out-of-plane motions for its carbons engaged in π bonding. Correlated methods are producing intramolecular basis set superposition errors (BSSE) on the two out-of-plane bends (OPBs) in c-C3H2. Such errors have been treated previously by removing the most offending basis functions in the basis set. In this work, standard correlation methods combined with explicitly-correlated methods to producing hybrid potential surfaces are also utilized to address the OPB errors. Additionally, previously defined OPB frequencies are questioned here, and the intramolecular BSSE is hypothesized to be both a method and a basis set effect. The correlated wavefunction’s largest higher-order substitution term comes from a π à π* excitation where the overlap of π electrons artificially stabilizes the OPB. By employing schema to overcome this issue, the symmetric OPB n9 will be observed very close to 775 cm-1 and is predicted to be the second brightest transition for this simple molecule. More work from the quantum chemical community is necessary to better understand how carbons interact with their adjacent atoms in π-bonded systems.

Rovibrational quantum chemical analysis of c-C3H2: Overcoming the failure of correlation for out-of-plane motions

255th National Meeting (2018)

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