Nearly degenerate isomers of C(BH)₂: Cumulene, carbene, or carbone?

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 0.02kcal mol^-1 and a corresponding barrier of only 1.89kcal mol^-1. Quantum tunneling computations reveal interconversion of the two isomers on a timescale much less than 1s even at 0K. 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.