Hunting dimers

Abstract

HBCBH and HAlCAlH are related electrondeficient molecules, predicted as local minima on their respective potential energy surfaces. Different views of their electronic structure-as allenes, carbenes, or carbones-prompted two research groups to a friendly competition exploring the predilection of these small molecules to dimerize. Many such dimers emerged from the calculations, some with large enthalpies favoring dimerization. Most interesting among these is an octahedral didehydrocarborane (or alane). The menagerie of (HECEH) 2 , E = B, Al structures obtained shows great variety in its bonding patterns. That variety is instructive, for it points to the necessity of examining dimerization for any theoretically postulated metastable molecule. We also learn of the continuing utility of simple Lewis structures, the acid-base reactivity of carbones and carbenes, and the dominant role of electron deficiency in borane and alane chemistry.

Nearly Degenerate Isomers of C(BH)2: Cumulene, Carbene, or Carbone?

“…The process is shown schematically in Figure 6b, which depicts an initial carbone HB→C←BH reference structure, whose carbon σ lone pair is subsequently donated into the empty in‐plane p AOs of boron. An alternative view is provided by a cyclic C(BH) 2 reference structure with an “inverted” carbene54 configuration that donates charge into the vacant carbon σ lone‐pair orbital (Figure 6c) 55. Both perspectives come to the same conclusion: the HOMO−1 in bent C(BH) 2 enhances carbon–boron bonding, it yields partial boron–boron bonding, and it has some (reduced) σ lone‐pair character.…”

Global structure of C2B4H4: hypercloso or not

“…To our surprise, in a recent computational study, 04 was predicted to have a perfect D 4h -cage at the BP86/def2-TZVPP level. 7 So, we performed extra optimizations for isomer 04 using eight exchangecorrelation functionals, e.g., BP86, X3LYP, M062X, WB97XD, B3PW91, PBEPBE, PBEPW91 and HSEH1PBE with aug-cc-pVTZ basis set. Table 2 lists the energy difference at the CCSD(T)/augcc-pVTZ + ZPEC level based on various DFT geometries between the global minimum and the hypercloso cage.…”

“…Here, through extensive isomeric searching, we report an abnormal example C 2 B 4 H 4 , whose global minimum is a ribbon-like isomer at the CCSD(T) aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level, instead of the octahedrallike hypercloso species that was revealed in a recent theoretical study. 7 The detailed calculation strategy, results and discussions can be found in the following sections.…”

A flat carborane with multiple aromaticity beyond Wade–Mingos’ rules

“…In this context, the group of Frenking and Hoffmann reported the theoretical studies on a C 2 B 4 H 4 cluster, and concluded that the favored structure is the hypercloso octahedron I involving two capped carbon atoms (Fig. 1a) 21 . By contrast, Ding and coworkers proposed that the global minima of the C 2 B 4 H 4 cluster is a ribbon-like isomer II, which lies 16.0 kcal mol −1 lower in energy than I 22 .…”