Original scientific paper

https://doi.org/10.5562/cca2770

Multireference Configuration Interaction Methods – An Application to the Valence Isomerism in Cyclobutadieno-p-benzoquinone and its Diprotonated Form

Mario Vazdar ; Laboratory for Physical Organic Chemistry, Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, POB 180, Zagreb, Croatia
Mirjana Eckert-Maksić ; Laboratory for Physical Organic Chemistry, Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, POB 180, Zagreb, Croatia

Abstract

Multireference averaged quadratic coupled cluster (MR-AQCC) calculations for cyclobutadieno-p-benzoquinone indicate that valence bond isomers 1a and 1b can exist as distinct species. The energy barrier height for their interconversion are 4.6 and 4.5 kcal mol−1, respectively, what is by ca. 2 kcal mol−1 lower than in the parent cyclobutadiene, implying that they could perhaps exist only under extreme conditions, namely at very low temperatures. For double protonated cyclobutadieno-p-benzoquinone, the CASSCF calculations erroneously predict existence of two valence isomers, 2a and 2b, whereas the MR-AQCC calculations reveal that geometry of the double protonated species could be best described by structure 2b. This nicely illustrates the crucial role of dynamic correlation and the need for using a highly-correlated theoretical method including geometry optimization in studied molecules.

Keywords

cyclobutadieno-p-benzoquinone; multrireference calculations; valence isomerism; energy barriers

Hrčak ID:

151220

URI

https://hrcak.srce.hr/151220

Publication date:

31.12.2015.

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