On the Bond‐Stretch Isomerism in the Benzo[1,2:4,5]dicyclobutadiene System—An ab initio MR‐AQCC Study

Abstract

Bond-stretch isomerism in benzo[1,2:4,5]dicyclobutadienle (BDCB) has been investigated using the MR-AQCC/6-31G(d) method, a high-level multireference ab initio approach including size-extensivity corrections. The applied theoretical approach includes both nondynamical and dynamical electron correlation effects. Full MR-AQCC geometry optimizations of localized (1) and delocalized (3) isomers as well as the transition structure (TS) have been determined using D2h, symmetry restriction. The calculations show that both isomers are approximately of equal stability separated by a barrier with a height of about 5 kcal mol(-1). Thus, the present results strongly indicate that benzof[1,3:4,5]dicyclobutadiene is a very good candidate for an organic compound exhibiting bond-stretch isomerism, since isomers 1 and 3 correspond to true minima on the double-well potential energy surface, which are separated by a sufficiently high barrier. It is particularly important to emphasize that isomer 3 represents a realization of the highly elusive quasi-[10]annulene.

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

“…[13] This will give us opportunity to compare the results with those of cylobutadiene obtained at the same level of theory and thus allow reliable estimate of annelation effect on the electronic structure and π-bond shifting in the parent molecule. We should mention at this point that practically identical results for automerization of cyclobutadiene were obtained using MR-CISD+Q method, [13] but for the sake of comparison with previously reported MR-AQCC results for variety of cyclobutadiene derivatives studied in our group, [21,23] only MR-AQCC calculations will be performed here. In addition to the study of π-bond shifting in the neutral cyclobutadieno-p-benzoquinone, we shall briefly address effect of diprotonation of the quinone ring on the overall geometry of molecule.…”

“…This feature is of the outmost importance in the study of the bond-stretch isomerization where the transition structure for isomerization cannot be properly described by the single reference methods. [13,21,23] Apart from that, MRCI methods are frequently used for calculating static electronic properties, such as vertical and adiabatic excitation energies or transition dipole moments, [41][42][43][44] and dynamical aspects of a fate of photoexcited molecules. [45][46][47][48][49][50][51] Finally, it should be stressed that use of MCSCF, and especially MRCI methods, is often computationally too expensive which limits their application to relatively small molecules containing no more than 10−15 heavy atoms.…”

“…[12] The state specific multireference BrillouinWigner CC (MR BWCC) theory gives 4.5 kcal mol −1 , [18] whereas the recent block correlated coupled cluster CAS-BCCC4 method yields 6.2 kcal mol −1 . [19] Except for cyclobutadiene MR-AQCC calculations were also performed on derivatives of CBD, such as benzo [1,2:4,5]dicyclobutadiene, [21,22] and cyano substituted CBDs. [23] For all these molecules isomerization barrier compared to parent CBD was found to be somewhat lower, being around 5 kcal mol −1 .…”

[2.2.2]Propellane Isomerization by Grob Rearrangement: An Ab Initio MR‐AQCC Study

“…[28,29] The availability of analytical gradients [30][31][32] for the MR-AQCC method allows geometry optimization of equilibrium structures and location of the transition states at a very high level of theory thus enabling reliable results to be obtained. The www.eurjoc.orgPople 6-31G(d) basis set, [33] which represents a fair compromise between quality and flexibility, [34] was used in all the calculations reported herein.…”

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.…”