Dedicated to Professor William J. Evans on the occasion of his 60th birthdayThe continuing quest for molecular complexes of lowoxidation-state lanthanides (other than the well-known samarium(II), europium(II), and ytterbium(II)) has produced significant achievements.[1] The majority of such compounds are derivatives of neodymium, dysprosium, and thulium, [2] obtained from the corresponding molecular iodides (often prepared in situ): [NdI 2 (thf) [2c,d] or by the reduction of an appropriate Ln III derivative. [2e,f] The present state of lanthanum, cerium, praseodymium, and gadolinium redox chemistry was described by S. A. Cotton: "None of these metals exhibits a stable + 2 state in any of its compounds and … they are unlikely to form stable compounds in this state." [3] Thus, the isolation of room-temperature-stable solid La II compounds is a very unexpected and significant breakthrough in the chemistry of lanthanides.Related to the subject of this study is a large group of inorganic materials: reduced or metal-rich rare-earth-metal halides, [4] including LaI 2 (featuring an extended structure with La 5d1 electrons delocalized on a conduction band), in which a lanthanum atom can be considered as being in the + 2 oxidation state.[4b] These compounds are better treated in a materials science context, where the metal valence [5,6] rather than the oxidation state is of primary importance.Previously [8] The latter featured redox ambiguity, which was also found in cerocene [9] and bis(pentalene)-cerium, [10] in which a metal-bound ligand had readily accessible adjacent oxidation states. For the binuclear arenebridged complexes, three different metal-ligand formal charge distributions are possible: Ln 2+

The salts [emim]X ( X = Br or I, emim = 1 -ethyl-3-methylimidazolium) and [emim] [AIBr,] have been prepared as solid-state models of the [emim]X-AIX, ( X = CI or Br) ionic liquid systems. All three have been characterised crystallographically. The salts [emim]X ( X = Br or I) are isomorphous and are composed of ions in an extended hydrogen-bonded network. The implications of these results for both the structure and solvent properties of the ionic liquids are discussed.

Crystals of 1,2 dilauroyl-DL-phosphatidylethanolamine: acetic acid are monoclinic with a = 46.2, b = 7.77, c = 9.95 A, j3 = 92.00; space group P21/c. The structural analysis, based on the visual estimates of 1467 reflection intensities, was achieved by direct methods, and least squares analysis convergence was to R1 = 0.28. There are marked differences between the observed molecular conformation and those that have been predicted theoretically. The mean planes containing the lipid chains are essentially parallel to one another; the phosphodiester moiety has a double gauche conformation, while intermolecular hydrogen bonding modifies the conformation that could be anticipated for an isolated phosphatidylethanolamine molecule. The intermolecular packing produces the classical lipid bilayer structure, adjacent lipid bilayers being separated by acetic acid molecules of crystallization. The hydrocarbon chain packing can be considered either as a quasi-hexagonal type or as a complex orthorhombic subcell arrangement. One-dimensional electron density profiles across the lipid bilayer at increasing resolution clearly demonstrate the origin of features present on the low resolution profiles of both model and natural membranes.Phospholipids, glycolipids, and glycosphingolipids play a key structural and functional role in animal, plant, and bacterial cell membranes (1). Furthermore, phospholipids are important molecular components of the serum lipoproteins responsible for fat transport in the body (2) and of bile secreted by the liver where, together with bile salts, they solubilize cholesterol in a complex, aqueous, mixed micellar system (3).Although much information is available on the behavior of phospholipids in water (4, 5), the mutual interactions of different lipid classes, particularly phospholipid-cholesterol (6) (14), and sugars (15), which determine the different lipid classes, have been described. From this structural information, conformational analyses of different phospholipids have been made and, on this basis, the most likely structures have been predicted (12,16,17).In this paper we describe a single-crystal analysis of a phospholipid, the synthetic compound 1,2 dilauroyl-DL-z HsOJI-OCHsCHr-NHa O_ A knowledge of the molecular conformation and intermolecular packing of phosphatidylethanolamine and other phospholipids will allow a more detailed interpretation of the x-ray diffraction data derived from lipid-water, lipid-protein, and natural membrane systems.

Despite the long history of the Fischer-Tropsch reaction, carbon monoxide has proven remarkably resistant to selective homologation under mild conditions. Here, we find that an organouranium(III) complex induces efficient reductive trimerization of carbon monoxide at room temperature and pressure. The result is a triangular, cyclic C3O(2-)3, or deltate, dianion held between two uranium(IV) units. The bonding within the C3O(2-)3 unit and its coordination to the two U centers have been analyzed by x-ray diffraction and density functional theory computational studies, which show a stabilizing C-C agostic interaction between the C3 core and one U center. Solution nuclear magnetic resonance studies reveal a rapid equilibration of the deltate unit between the U centers.

Major sudden stratospheric warmings (SSWs) are striking phenomena of wintertime stratospheric circulation usually defined as a reversal of zonal mean circulation from westerlies to easterlies. SSWs often have significant impact on tropospheric circulation and cause anomalies in surface climate lasting for up to 2 months. For this reason, dynamics and predictability of SSW receive considerable attention. It is however well‐known that not all SSWs cause significant, long‐lasting impact on the troposphere. In order to explain differences in tropospheric impacts following SSWs, several reasons have been previously proposed, including differences in type of SSW (split or displacement), persistence of stratospheric anomalies, preconditioning of the tropospheric circulation, and whether or not SSW was accompanied by a planetary wave reflection in the stratosphere. Here we address the predictability of tropospheric impacts by SSWs by seeking early precursors of the impacts. We separate midwinter SSWs into two groups: those which are followed by significant, long‐lasting impacts on the tropospheric circulation (defined in terms of anomalous Northern Annular Mode) and those not followed by significant anomalies in the annular mode. We show that SSWs characterised by a more negative Northern Annular Mode index in the lower stratosphere around 150 hPa and enhanced wave activity propagation to the stratosphere during the first few days following the central date have a larger probability of being followed by tropospheric impact, both in reanalyses and in climate model runs. These anomalies play a more important role in the subsequent downward propagation of the signal to the troposphere than the type of SSW: whether it is a split or a displacement, or absorptive or reflective SSW. We propose that using these anomalies as precursors of tropospheric impacts of SSW can enhance climate predictability.

The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by which the zonal-mean evolution of the reference sudden warmings are artificially induced in an ;100-member ensemble spun off from a control simulation. Both reference warmings are taken from a freely running integration of the model. One event is a displacement, the other is a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below.The nudged ensembles exhibit a tropospheric annular mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower-stratospheric annular mode anomalies than with the midstratospheric wind reversal. In addition to the expected synoptic-scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies.Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric nearsurface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response's magnitude is underrepresented.

A novel diagnostic tool is presented, based on polar-cap temperature anomalies, for visualizing daily variability of the Arctic stratospheric polar vortex over multiple decades. This visualization illustrates the ubiquity of extended-time-scale recoveries from stratospheric sudden warmings, termed here polar-night jet oscillation (PJO) events. These are characterized by an anomalously warm polar lower stratosphere that persists for several months. Following the initial warming, a cold anomaly forms in the middle stratosphere, as does an anomalously high stratopause, both of which descend while the lower-stratospheric anomaly persists. These events are characterized in four datasets: Microwave Limb Sounder (MLS) temperature observations; the 40-yr ECMWF Re-Analysis (ERA-40) and Modern Era Retrospective Analysis for Research and Applications (MERRA) reanalyses; and an ensemble of three 150-yr simulations from the Canadian Middle Atmosphere Model. The statistics of PJO events in the model are found to agree very closely with those of the observations and reanalyses.The time scale for the recovery of the polar vortex following sudden warmings correlates strongly with the depth to which the warming initially descends. PJO events occur following roughly half of all major sudden warmings and are associated with an extended period of suppressed wave-activity fluxes entering the polar vortex. They follow vortex splits more frequently than they do vortex displacements. They are also related to weak vortex events as identified by the northern annular mode; in particular, those weak vortex events followed by a PJO event show a stronger tropospheric response. The long time scales, predominantly radiative dynamics, and tropospheric influence of PJO events suggest that they represent an important source of conditional skill in seasonal forecasting.

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