ESR Studies of Phosphorescent Corannulene; Evidence for Pseudorotation

Electron spin resonance (ESR) spectra have been recorded for the lowest triplet state of corannulene. The experiments were performed by ultraviolet irradiation of the molecule in rigid glass solutions at 77 and 15°K. In the high‐field (Δm  =  1)(Δm=1) region the line shape and line positions of the absorptions associated with the XX and YY magnetic axes were strongly temperature dependent. From an analysis of the spectra the following conclusions were drawn: Corannulene in its first triplet state has a configuration of lowest energy which is not fivefold symmetric; at 15°K corannulene gives in the high‐field region the spectrum of a molecule “frozen” in conformations of less than threefold symmetry; tunneling between equivalent conformations at 77°K is believed to occur at a rate of about 109 Hz.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69781/2/JCPSA6-52-11-5656-1.pd

ESR studies of the triplet state of [n.n] paracyclophanes

ESR experiments were performed on the triplet state of randomly oriented paracyclophanes in a variety of rigid glasses at temperatures ranging from 103 to 15°K. Spectra were recorded for [2.2]; [3.3]; (4,7,12,15)‐tetramethyl [2.2]‐paracyclophane and stagger‐ring paracyclophane. For all the samples except stagger ring, a four‐ringed paracyclophane, only the Hmin feature was observed from which D*, the root‐mean‐square zero‐field splitting, was calculated. For stagger ring the triplet spectrum has two features in the Δms = 1Δms=1 region in addition to the Hmin feature. From these, the zero‐field splittings, D and E, were calculated. The triplet spectra for the paracyclophanes show that there is strong transannular interaction with electron delocalization over all benzene rings. There is evidence for strongly coupled intramolecular exciton effects. The effect of increasing the inter‐ring separation from [2.2] paracyclophane to [3.3] paracyclophane is to decrease the transannular interaction. The effect of methyl substitution is to increase transannular effects relative to the parent compound. Transannular interactions in stagger ring are greater than in [2.2] paracyclophane despite the increased electron delocalization possible through the introduction of more than two rings. The large value of E for stagger ring represents a significant deviation from axial symmetry for the zero‐field‐splitting tensor and indicates that the methylene bridges, the methyl substituents, or the ring distortion, may make important contributions to the electronic distribution of the triplet state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70725/2/JCPSA6-58-2-420-1.pd

The Long Term Response of Birds to Climate Change: New Results from a Cold Stage Avifauna in Northern England

The early MIS 3 (55–40 Kyr BP associated with Middle Palaeolithic archaeology) bird remains from Pin Hole, Creswell Crags, Derbyshire, England are analysed in the context of the new dating of the site’s stratigraphy. The analysis is restricted to the material from the early MIS 3 level of the cave because the upper fauna is now known to include Holocene material as well as that from the Late Glacial. The results of the analysis confirm the presence of the taxa, possibly unexpected for a Late Pleistocene glacial deposit including records such as Alpine swift, demoiselle crane and long-legged buzzard with southern and/or eastern distributions today. These taxa are accompanied by more expected ones such as willow ptarmigan /red grouse and rock ptarmigan living today in northern and montane areas. Finally, there are temperate taxa normally requiring trees for nesting such as wood pigeon and grey heron. Therefore, the result of the analysis is that the avifauna of early MIS 3 in England included taxa whose ranges today do not overlap making it a non-analogue community similar to the many steppe-tundra mammalian faunas of the time. The inclusion of more temperate and woodland taxa is discussed in the light that parts of northern Europe may have acted as cryptic northern refugia for some such taxa during the last glacial. These records showing former ranges of taxa are considered in the light of modern phylogeographic studies as these often assume former ranges without considering the fossil record of those taxa. In addition to the anomalous combination of taxa during MIS 3 living in Derbyshire, the individuals of a number of the taxa are different in size and shape to members of the species today probably due to the high carrying capacity of the steppe-tundra