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

Abstract

The ground electronic state of C(BH)2 exhibits both a linear minimum and a peculiar angle-deformation isomer with a central B-C-B angle near 90°. Definitive computations on these species and the intervening transition state have been executed by means of coupled-cluster theory including single and double excitations (CCSD), perturbative triples (CCSD(T)), and full triples with perturbative quadruples (CCSDT(Q)), in concert with series of correlation-consistent basis sets (cc-pVXZ, X=D, T, Q, 5, 6; cc-pCVXZ, X=T, Q). Final energies were pinpointed by focal-point analyses (FPA) targeting the complete basis-set limit of CCSDT(Q) theory with auxiliary core correlation, relativistic, and non-Born-Oppenheimer corrections. Isomerization of the linear species to the bent form has a minuscule FPA reaction energy of 0.02 kcal mol(-1) and a corresponding barrier of only 1.89 kcal mol(-1). Quantum tunneling computations reveal interconversion of the two isomers on a timescale much less than 1 s even at 0 K. Highly accurate CCSD(T)/cc-pVTZ and composite c~CCSDT(Q)/cc-pCVQZ anharmonic vibrational frequencies confirm matrix-isolation infrared bands previously assigned to linear C(BH)2 and provide excellent predictions for the heretofore unobserved bent isomer. Chemical bonding in the C(BH)2 species was exhaustively investigated by the atoms-in-molecules (AIM) approach, molecular orbital plots, various population analyses, local mode vibrations and force constants, unified reaction valley analysis (URVA), and other methods. Linear C(BH)2 is a cumulene, whereas bent C(BH)2 is best characterized as a carbene with little carbone character. Weak B-B attraction is clearly present in the unusual bent isomer, but its strength is insufficient to form a CB2 ring with a genuine boron-boron bond and attendant AIM bond path.

Unraveling reaction mechanisms by means of Quantum Chemical Topology Analysis

“…The bent isomer displays a BACAB bond angle of 91.14 , in close agreement with the value reported by Frenking et al [108] (90.36 ) at their highest theoretical level used, namely c-CCSDT(Q)/ccpCVQZ(AE). The energy profile along the IRC path is reported in Figure 1 together with the six SSDs found.…”

“…In this domain, consisting of only one point in the IRC that corresponds to the linear isomer, the V(C,B) basins are split into two basins each, with overall populations of 3.8e, becoming apparent the coexistence of r and p interactions between the C atom and the boron atoms due to the multiple character of the CAB bonds. As it can be seen, the ELF analysis renders that the linear C(BH) 2 can be described as a classical cumulene, in agreement with the recent work by Frenking et al [108] The second example accounts for the cycloheptatrienenorcaradiene (CHT-NCD) isomerization, which has attracted much attention for more than 40 years. [110,111] This equilibrium (Scheme 2) includes a valence bond isomerization and a thermally allowed disrotatory electrocyclic cyclopropane ring opening.…”

“…The first example we are going to describe in the present work is the C(BH) 2 isomerization between the bent and the linear forms of the molecule. The two isomers are nearly degenerate, as it was stated by Frenking et al in a very recent work: [108] definitive computations on these species with highlevel theoretical treatments render that the difference in energy between the two isomers is near zero, and that the activation barrier for the interconversion is around 2 kcal/mol. In this process, no forming and breaking of chemical bonds occur although the delineation of factors that control the pair electron reorganization in this carbene on this isomerization is complicated.…”

“…The V(B1,B2) disynaptic basin population is calculated to be 1.05e electrons at this point, while the V(C,B1) and V(C,B2) disynaptic population basins at the bent isomer have 3.29e each. Hence, as proposed by Frenking et al, [108] the bent isomer can be described as a carbene (with some three center CB 2 bonding) and a little carbone character.…”

Bond-bending isomerism of Au2I3−: competition between covalent bonding and aurophilicity

“…This anion is found to have a C 2v bent geometric configuration of I–Au–I–Au–I – and exhibit two near-degenerate isomers that we name as “bond-bending isomers (BBIs)”, similar to BSI, which has not been experimentally established heretofore. 34 , 35 The two BBIs of Au 2 I 3 – reported here differ only in geometry by the bending angle ( Fig. 1 ), owing to different mechanisms of bonding interactions.…”