A REKS Assessment of the Face‐Diagonal Bond in 1,3‐Didehydrocubane and a Comparison with Benzyne Biradicals

Abstract

Keywords: Density functional calculations / Radicals / Benzynes / Through-space interactions / Through-bond interactionsThe three isomeric cubane biradicals were studied using spin-restricted ensemble-referenced Kohn−Sham (REKS) density functional calculations at the B3LYP/6-31G(d) level of theory. The most stable biradical was found to be orthocubene with the meta-cubene and para-cubene biradicals 4.7 kcal mol −1 and 17.8 kcal mol −1 higher in energy, respectively. The singlet ground states are well separated from their lowest lying triplet states. These singlet-triplet energy differences mostly originate from through-space interactions for

The structures of m-benzyne and tetrafluoro-m-benzyne

“…However, it includes only disconnected three-electron correlation effects so that through-bond interactions of the single electrons are underestimated. 54,55 Accordingly, it exaggerates the importance of forming a C1-C3 bond and wrongly predicts the existence of a bicyclic form. Both basis sets used ͓6-31G͑d , p͒ and cc-pVTZ͔ predict an asymmetric PES with a shoulder at the position of the open form.…”

An approximate diatomics in molecules formulation of generalized valence bond theory

“…The most stable, the singlet 1,2‐cubane is used as a reference. Included for comparison are the spin‐unrestricted B3LYP (UB3LYP) computed energies and the spin‐restricted ensemble‐reference Kohn–Sham (REKS) results of de Viser et al36 All calculations were performed on optimized molecular geometries for their respective methods. The REKS method, developed by Filatov and Shaik,37 represents the total energy and density by weighted sums of single determinant DFT energies and densities.…”

11 Computational organic chemistry

“…Bond-stretch isomers 5a and 5b have been investigated with high levels of multi-reference theory (two state CASSCF and CASPT2). 6 With the inclusion of zero-point vibrational energy, these calculations show that 5b is more stable (by about 10 kJ mol 21 ) and that the two isomers are separated by a barrier of only 2-4 kJ mol 21 . The authors concluded that only 5b would exist experimentally, but they showed that suitable substitution of this structure would lead to a fine-tuning of the preference for either isomer and their subsequent experimental isolation.…”