3 research outputs found
A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB<sub>12</sub><sup>ā</sup> Clusters (M = Co, Rh, Ir)
Strong
binding of carbon monoxide (CO) and dinitrogen (N<sub>2</sub>) by
MB<sub>12</sub><sup>ā</sup> (M = Co, Rh, Ir) clusters
results in a spinning umbrella-like structure. For OCMB<sub>12</sub><sup>ā</sup> and NNMB<sub>12</sub><sup>ā</sup> complexes,
the bond dissociation energy values range within 50.3ā67.7
kcal/mol and 25.9ā35.7 kcal/mol, respectively, with the maximum
value obtained in Ir followed by that in Co and Rh analogues. COMB<sub>12</sub><sup>ā</sup> complex is significantly less stable
than the corresponding C-side bonded isomer. The associated dissociation
processes for OCMB<sub>12</sub><sup>ā</sup> and NNMB<sub>12</sub><sup>ā</sup> into CO or N<sub>2</sub> and MB<sub>12</sub><sup>ā</sup> are highly endergonic in nature at 298 K, implying
their high thermochemical stability with respect to dissociation.
In OCMB<sub>12</sub><sup>ā</sup> and NNMB<sub>12</sub><sup>ā</sup> complexes, the CāO and NāN bonds are
found to be elongated by 0.022ā0.035 Ć
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 LāM Ļ-back-donation (L = CO, OC, NN) over LāM
Ļ-donation. The binding of L enhances the energy barrier for
the rotation of the inner B<sub>3</sub> unit within the outer B<sub>9</sub> ring by 0.4ā1.8 kcal/mol, which can be explained by
a reduction in the distance of the longest bond between inner B<sub>3</sub> and outer B<sub>9</sub> rings upon complexation. A good correlation
is found between the change in rotational barrier relative to that
in MB<sub>12</sub><sup>ā</sup> and the energy associated with
the LāM Ļ-donation. BornāOppenheimer 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 B<sub>3</sub> 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 BornāOppenheimer 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 ārotationā
of one of the six-membered rings via a 0.8 kcalĀ·mol<sup>ā1</sup> barrier, and the second one is a slower interconvertion between
two hyperconjomers via an out-of-plane methine bending (Ī<i>G</i><sup>ā§§</sup> = 4.0 kcalĀ·mol<sup>ā1</sup>)
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 CāC 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, BornāOppenheimer 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