Photochemical Generation of Allenylidenes from Cyclopropanated Phenanthrenes: An Experimental and Computational Study.

To address the scarcity of generally applicable photochemical routes to allenylidenes in solution, phenanthrene-based sources have been investigated. Specifically, the syntheses of 1-vinylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, 1-(2-phenylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, and 1-(2-methylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, photochemical precursors to propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene have been carried out. Photolysis of these new precursors in olefin traps and benzene afforded the expected cyclopropane adducts of the corresponding allenylidenes. Quantum chemical calculations show that the ground state of all three carbenes is a singlet with a singlet-triplet gap of ∼29, 30, and 33 kcal/mol for propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene, respectively.

Evidence for Suppression of Structure Growth in the Concordance Cosmological Model.

We present evidence for a suppressed growth rate of large-scale structure during the dark-energy-dominated era. Modeling the growth rate of perturbations with the "growth index" γ, we find that current cosmological data strongly prefer a higher growth index than the value γ=0.55 predicted by general relativity in a flat Lambda cold dark matter cosmology. Both the cosmic microwave background data from Planck and the large-scale structure data from weak lensing, galaxy clustering, and cosmic velocities separately favor growth suppression. When combined, they yield γ=0.633_{-0.024}^{+0.025}, excluding γ=0.55 at a statistical significance of 3.7σ. The combination of fσ_{8} and Planck measurements prefers an even higher growth index of γ=0.639_{-0.025}^{+0.024}, corresponding to a 4.2σ tension with the concordance model. In Planck data, the suppressed growth rate offsets the preference for nonzero curvature and fits the data equally well as the latter model. A higher γ leads to a higher matter fluctuation amplitude S_{8} inferred from galaxy clustering and weak lensing measurements, and a lower S_{8} from Planck data, effectively resolving the S_{8} tension.

Photochemical generation and trapping of 3-oxacyclohexyne.

The strained heterocyclic alkyne, 3-oxacyclohexyne, was generated photochemically for the first time using a cyclopropanated phenanthrene precursor, and trapped by cyclopentadienones as Diels-Alder adducts. The precursor initially produced the putative 3-oxacyclopentylidenecarbene that subsequently rearranged to the cycloalkyne. Computational studies indicate that the carbene favors a singlet state, and the barrier for its ring expansion by a 1,2-shift of the carbon proximal to oxygen is lower in energy than the corresponding shift of the distal carbon.