Why Are 5-Thioglycopyranosyl Donors More Axially Selective than their Glycopyranosyl Counterparts? A Low and Variable Temperature NMR Spectroscopy and Computational Study.

5-Thioglycopyranosyl donors differ in reactivity and selectivity from simple glycopyranosyl donors. An extensive study has been conducted on the nature and stability of the reactive intermediates generated on the activation of per--acetyl and per--methyl 5-thioglucopyranosyl donors and the corresponding glucopyranosyl donors. Variable temperature nuclear magnetic resonance (NMR) studies with per--methylated or per--acetyl glycosyl sulfoxides and trichloroacetimidates on activation with trifluoromethanesulfonic anhydride or trimethylsilyl triflate are reported.

Alkyne dichotomy and hydrogen migration in binuclear cyclopentadienylmetal alkyne complexes.

The structures and energetics of the binuclear cyclopentadienylmetal alkyne systems CpMCR (M = Ni, Co, Fe; R = Me and NMe) have been investigated using density functional theory. For the CpMC(NMe) (M = Ni, Co, Fe) systems the relative energies of isomeric tetrahedrane CpM(alkyne) structures having intact alkyne ligands and alkyne dichotomy structures CpM(CNMe) in which the C[triple bond, length as m-dash]C triple bond of the alkyne has broken completely to give separate MeNC units depending on the central metal atoms. For the nickel system CpNiC(NMe) as well as the related nickel systems CpNi(MeCNMe) and CpNiCMe the tetrahedrane structures are clearly preferred energetically consistent with the experimental syntheses of several stable CpNi(alkyne) complexes.

Linear-Scaling Local Natural Orbital-Based Full Triples Treatment in Coupled-Cluster Theory.

We present an efficient, asymptotically linear-scaling implementation of the canonically coupled-cluster method with singles, doubles, and full triples excitations (CCSDT) method. We apply the domain-based local pair natural orbital (DLPNO) approach for computing CCSDT amplitudes. Our method, called DLPNO-CCSDT, uses the converged coupled-cluster amplitudes from a preceding DLPNO-CCSD(T) computation as a starting point for the solution of the CCSDT equations in the local natural orbital basis.

Geographic and Biological Drivers Shape Anthropogenic Extinctions in the Macaronesian Vascular Flora.

Whether species extinctions have accelerated during the Anthropocene and the extent to which certain species are more susceptible to extinction due to their ecological preferences and intrinsic biological traits are among the most pressing questions in conservation biology. Assessing extinction rates is, however, challenging, as best exemplified by the phenomenon of 'dark extinctions': the loss of species that disappear before they are even formally described. These issues are particularly problematic in oceanic islands, where species exhibit high rates of endemism and unique biological traits but are also among the most vulnerable to extinction.