7 research outputs found
Nonamethylcyclopentyl Cation Rearrangement Mysteries Solved
The <i>C</i><sub>1</sub> nonamethylcyclopentyl cation minimum undergoes complete methyl scrambling in SbF<sub>5</sub> with a 7 kcal/mol barrier. This corresponds to the rate-limiting conformational interconversion of enantiomeric hyperconjomers via a <i>C</i><sub>s</sub> transition structure (above right). A remarkable, more rapid, second process only exchanges methyls within sets of four and five (blue and red, see above), as has been observed experimentally at low temperatures. The computed ∼2 kcal/mol barrier involves a <i>C</i><sub><i>s</i></sub> [1s,2s] sigmatropic methyl shift transition structure (above left)
Correlation Effects on the Relative Stabilities of Alkanes
The “alkane branching effect”
denotes the fact that
simple alkanes with more highly branched carbon skeletons, for example,
isobutane and neopentane, are more stable than their normal isomers,
for example, <i>n</i>-butane and <i>n</i>-pentane.
Although <i>n</i>-alkanes have no branches, the “kinks”
(or “protobranches”) in their chains (defined as the
composite of 1,3-alkyl–alkyl interactionsincluding
methine, methylene, and methyl groups as alkyl entitiespresent
in most linear, cyclic, and branched alkanes, but not methane or ethane)
also are associated with lower energies. Branching and protobranching
stabilization energies are evaluated by isodesmic comparisons of protobranched
alkanes with ethane. Accurate ab initio characterization of branching
and protobranching stability requires post-self-consistent field (SCF)
treatments, which account for medium range (∼1.5–3.0
Å) electron correlation. Localized molecular orbital second-order
Møller–Plesset (LMO-MP2) partitioning of the correlation
energies of simple alkanes into localized contributions indicates
that correlation effects between electrons in 1,3-alkyl groups are
largely responsible for the enhanced correlation energies and general
stabilities of branched and protobranched alkanes
Evaluation of Triplet Aromaticity by the Isomerization Stabilization Energy
The many manifestations of aromaticity have long fascinated both experimentalists and theoreticians. Due to their degenerate half-filled MOs, triplet [<i>n</i>]annulenes with 4<i>n</i> π-electrons are also aromatic, but the degree of their stabilization has been difficult to quantify. The isomerization stabilization energy (ISE) method has been applied to evaluate the triplet aromaticity. The reliability of this approach is indicated by the strong correlation of the ISE results with NICS(1)<sub><i>zz</i></sub>, a magnetic indicator of triplet state aromaticity
Substituent Effects on “Hyperconjugative” Aromaticity and Antiaromaticity in Planar Cyclopolyenes
Computed aromatic stabilization energies (ASEs) and dissected nucleus independent chemical shifts (NICS<sub><i>πzz</i></sub>) quantify the effect of hyperconjugation on the (anti)aromaticities of the planar conformations of three, five, seven, and nine membered (C<sub><i>n</i></sub>H<sub><i>n</i></sub>)CR<sub>2</sub> (R = H, SiH<sub>3</sub>, F) rings. CH<sub>2</sub> and especially C(SiH<sub>3</sub>)<sub>2</sub> groups supply two “pseudo” π electrons hyperconjugatively along with the olefinic π electrons in the ring, whereas a CF<sub>2</sub> group acts like a partially vacant p orbital. Following the Hückel rule, compounds with 4<i>n</i>+2 (or 4<i>n</i>) pseudo π electrons are “hyperconjugatively” aromatic (or antiaromatic)
Why Cyclooctatetraene Is Highly Stabilized: The Importance of “Two-Way” (Double) Hyperconjugation
Despite its highly nonplanar geometry, the tub-shaped <i>D</i><sub>2<i>d</i></sub> cyclooctatetraene (COT)
minimum is
far from being an unconjugated polyene model devoid of important π
interactions. The warped skeleton of <i>D</i><sub>2<i>d</i></sub> COT results in the large stabilization (41.1 kcal/mol)
revealed by its isodesmic bond separation energy (BSE). This originates
largely from the “two-way” hyperconjugation, back and
forth across the C–C single bonds, between the CC/CH σ(σ*)
and the CC (π*)π orbitals. These hyperconjugative
effects compensate for the substantial, but not complete, loss of
π conjugation upon ring puckering. C–C single bond rotation
of 1,3-butadiene involves a similar interplay between π conjugation
and “two-way” double hyperconjugation and serves as
a simple model for the inversion of <i>D</i><sub>2<i>d</i></sub> to <i>D</i><sub>4<i>h</i></sub> COT. The perpendicular rotational transition states of many other
systems, e.g., the allyl cation, styrene, biphenyl, and ethene, are
stabilized similarly by “two-way” hyperconjugation
A Hückel Theory Perspective on Möbius Aromaticity
Heilbronner’s Hückel molecular orbital treatment of Möbius 4n−π annulenes is revisited. When uneven twisting in π-systems of small Möbius rings is accounted for, their resonance energies become comparable to iso-π-electronic linear alkenes with the same number of carbon atoms. Larger Möbius rings distribute π-twisting more evenly but exhibit only modest aromatic stabilization. Dissected nucleus independent chemical shifts (NICS), based on the LMO (localized molecular orbital)–NICS(0)<sub>π</sub> index confirm the magnetic aromaticity of the Möbius annulenes considered
Why Do Two π‑Electron Four-Membered Hückel Rings Pucker?
Notwithstanding their two (i.e., 4<i>n</i> + 2) π electrons, four-membered ring systems, <b>1</b>–<b>4</b>, favor puckered geometries (<b>1a</b>–<b>4a</b>) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C<sub>4</sub>F<sub>4</sub><sup>2+</sup>) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH<sub>3</sub> in C<sub>4</sub>(SiH<sub>3</sub>)<sub>4</sub><sup>2+</sup>) enhance hyperconjugation and increase the ring inversion barriers substantially