A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB₁₂^– Clusters (M = Co, Rh, Ir)

Strong binding of carbon monoxide (CO) and dinitrogen (N₂) by MB₁₂^– (M = Co, Rh, Ir) clusters results in a spinning umbrella-like structure. For OCMB₁₂^– and NNMB₁₂^– complexes, the bond dissociation energy values range within 50.3–67.7 kcal/mol and 25.9–35.7 kcal/mol, respectively, with the maximum value obtained in Ir followed by that in Co and Rh analogues. COMB₁₂^– complex is significantly less stable than the corresponding C-side bonded isomer. The associated dissociation processes for OCMB₁₂^– and NNMB₁₂^– into CO or N₂ and MB₁₂^– are highly endergonic in nature at 298 K, implying their high thermochemical stability with respect to dissociation. In OCMB₁₂^– and NNMB₁₂^– complexes, the C–O and N–N bonds are found to be elongated by 0.022–0.035 Å along with a large red-shift in the corresponding stretching frequencies, highlighting the occurrence of bond activation therein toward further reactivity due to complexation. The obtained red-shift is explained by the dominance of L←M π-back-donation (L = CO, OC, NN) over L→M σ-donation. The binding of L enhances the energy barrier for the rotation of the inner B₃ unit within the outer B₉ ring by 0.4–1.8 kcal/mol, which can be explained by a reduction in the distance of the longest bond between inner B₃ and outer B₉ rings upon complexation. A good correlation is found between the change in rotational barrier relative to that in MB₁₂^– and the energy associated with the L→M σ-donation. Born–Oppenheimer molecular dynamics simulations further support that the M-L bonds in the studied systems are kinetically stable enough to retain the original forms during the internal rotation of inner B₃ unit

Nonclassical 21-Homododecahedryl Cation Rearrangement Revisited

The degenerate rearrangement in the 21-homododecahedryl cation (<b>1</b>) has been studied via density functional theory computations and Born–Oppenheimer Molecular Dynamics simulations. Compound <b>1</b> can be described as a highly fluxional hyperconjugated carbocation. Complete scrambling of <b>1</b> can be achieved by the combination of two unveiled barrierless processes. The first one is a “rotation” of one of the six-membered rings via a 0.8 kcal·mol^–1 barrier, and the second one is a slower interconvertion between two hyperconjomers via an out-of-plane methine bending (Δ<i>G</i>^⧧ = 4.0 kcal·mol^–1)

Carbo-Cages: A Computational Study

Inspired by their geometrical perfection, intrinsic beauty, and particular properties of polyhedranes, a series of carbo-cages is proposed in silico via density functional theory computations. The insertion of alkynyl units into the C–C bonds of polyhedranes results in a drastic lowering of the structural strain. The induced magnetic field shows a significant delocalization around the three-membered rings. For larger rings, the response is paratropic or close to zero, suggesting a nonaromatic behavior. In the carbo-counterparts, the values of the magnetic response are shifted with respect to their parent compounds, but the aromatic/nonaromatic character remains unaltered. Finally, Born–Oppenheimer molecular dynamics simulations at 900 K do not show any drastic structural changes up to 10 ps. In the particular case of a carbo-prismane, no structural change is perceived until 2400 K. Therefore, although carbo-cages have enthalpies of formation 1 order of magnitude higher than those of their parent compounds, their future preparation and isolation should not be discarded, because the systems are kinetically stable, explaining why the similar systems like carbo-cubane have already been synthesized