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

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.

The Influence of Multiple Fluorination on the Ring Opening of [2.2.2]Propellane

“…[9] obtained with multireference averaged quadratic coupled cluster (MR-AQCC) method. [24,25] The latter method was proved to provide accurate predictions of the barriers of several bond-stretch isomerisation reactions, [26][27][28][29] including the [2.2.2]propellane system in which stretching of the central C1-C4 bond is accompanied by inversion in the occupation of frontier orbitals. However, since our study includes calculations of potential energy paths of a number of rather large molecules to keep the frame of calculations within reasonable limits we decided to use less demanding CASSCF and CASPT2 methods.…”