Dicopper(II) complexes of a new di-para-xylyldioxatetraazamacrocycle and cascade species with dicarboxylate anions: thermodynamics and structural properties

The new dioxatetraazamacrocycle (L-1) was synthesized by a 2 + 2 condensation and characterized. Stability constants of its copper(II) complexes were determined by spectrophotometry in DMSO at 298.2 K in 0. 10 mol dm(-3) KClO4. Mainly dinuclear complexes are formed and the presence of mononuclear species is dependent on the counterion (Cl- or ClO4-). The association constants of the dinuclear copper(II) complexes with dicarboxylate anions [oxalate (oxa(2-)), malonate (mal(2-)), succinate (suc(2-)), and glutarate (glu(2-))] were also determined by spectrophotometry at 298.2 K in DMSO, and it was found that values decrease with an increase of the alkyl chain between the carboxylate groups. X-Band EPR spectra of the dicopper(II) complexes and of their cascade species in frozen DMSO exhibit dipole-dipole coupling, and their simulation, together with their UV-vis spectra, showed that the copper centres of the complexes in solution had square pyramidal geometries though with different distortions. From the experimental data, it was also possible to predict the Cu...Cu distances, the minimum being found at 6.4 angstrom for the (Cu2LCl4)-Cl-1 complex and then successively this distance slightly increases when the chloride anions are replaced by dicarboxylate anions, from 6.6 angstrom for oxa(2-) to 7.8 for glu(2-). The crystal structures of the dinuclear copper cascade species with oxa(2-) and suc(2-) were determined and showed one anion bridging both copper centres and Cu...Cu distances of 5.485(7) angstrom and 6.442(8) angstrom, respectively.

Cascade dicopper architectures of a dibenzodioxatetraazamacrocycle

The dibenzodioxatetraazamacrocycle [26]pbz(2)N(4)O(2) was characterised by single crystal X-ray diffraction and the protonation constants of this compound and the stability constants of its copper(II) and lead(II) complexes were determined by potentiometry in water at 298.2 K in 0.10 mol dm(-3) in KNO3. Mono- and dinuclear complexes were found for both metal ions, the dinuclear complexes being the main species in the 5-7.5 pH range for copper(II) and 7.5-8.5 for lead(II). As expected the values of the stability constants for the copper(II) complexes are lower than those for related macrocycles containing only nitrogen atoms. The presence of mono- and dinuclear copper complexes was also confirmed by electrospray ionization mass spectrometry. These results suggest that the symmetric macrocyclic cavity of [26]pbZ(2)N(4)O(2) has enough space for the coordination of two metal ions. Additionally, NMR spectroscopy showed that the dinuclear complex of lead(II) has high symmetry. The equilibrium constants of the dinuclear copper(II) complexes and dicarboxylate anions (oxalate, malonate and succinate) were also determined in 0.10 mol dm-3 aqueous KNO3 solution. Only species containing one anion, Cu(2)H(h)LA((2+h)), were found, strongly suggesting that the anion bridges the two copper(II) ions. The binding constants of the cascade species formed by [Cu-2[26]pbZ(2)N(4)O(2)(H2O)(4+) with dicarboxylate anions decrease with the increase in length of the alkyl chain of the anion, a fact which was attributed to a higher conformational energy necessary for the rearrangement of the macrocycle to accommodate the larger anions bridging the two copper(II) centres. The variation of the magnetic susceptibility with temperature Of [Cu-2(H-2[26]pbz(2)N(4)O(2))(oxa)(3)]-4H(2)O and [Cu-2([26]pbz(2)N(4)O(2))(suc)Cl-2] were measured and the two complexes showed different behaviour. (c) 2007 Elsevier Ltd. All rights reserved.

Geodetic mass balance of Azarova glacier, Kodar Mountains, eastern Siberia, and its links to observed and projected climatic change

The Kodar Mountains in eastern Siberia accommodate 30 small, cold-based glaciers with a combined surface area of about 19 km2. Very little is known about these glaciers, with the first survey conducted in the late 1950s. In this paper, we use terrestrial photogrammetry to calculate changes in surface area, elevation, volume and geodetic mass balance of the Azarova Glacier between 1979 and 2007 and relate these to meteorological data from nearby Chara weather station (1938-2007). The glacier surface area declined by 20±6.9% and surface lowered on average by 20±1.8 m (mean thinning: 0.71 m a-1) resulting in a strongly negative cumulative and average mass balance of -18±1.6 m w.e. and -640±60 mm w.e.a-1 respectively. The July-August air temperature increased at a rate of 0.036oC a-1 between 1979 and 2007 and the 1980-2007 period was, on average, around 1oC warmer than 1938-1979. The regional climate projections for A2 and B2 CO2 emission scenarios developed using PRECIS regional climate model indicate that summer temperatures will increase in 2071–2100 by 2.6-4.7°C and 4.9-6.2°C respectively in comparison with 1961–1990. The annual total of solid precipitation will increase by 20% under B2 scenario but decline by 3% under A2 scenario. The length of the ablation season will extend from July–August to June-September. The Azarova Glacier exhibits high sensitivity to climatic warming due to its low elevation, exposure to comparatively high summer temperatures, and the absence of a compensating impact of cold season precipitation. Further summer warming and decline of solid precipitation projected under the A2 scenario will force Azarova to retreat further while impacts of an increase in solid precipitation projected under the B2 scenario require further investigation.

Organocatalytic enantioselective construction of nitrocyclohexanes containing multiple chiral centres via a cascade reaction

The stereoselective construction of complex molecules with multiple stereogenicity in a single step represents an extremely useful, but challenging approach to complexity in chemical synthesis. The development of organocatalytic cascade processes has proven useful in these studies, but reports where four or more stereocentres are created in a single step from just two achiral reagents are rare. Herein we report the development of a novel asymmetric domino Michael-Michael reaction between nitrohex-4-enoates and nitro-olefins to generate cyclohexanes of high complexity, including one with a quaternary centre, and one with five contiguous stereocentres. This methodology provides access to a range of useful nitrocyclohexane derivatives, including a novel class of a-lycorane-like structures.

The relationship between the North American summer monsoon, the Rocky Mountains and the North Pacific subtropical anticyclone in HadAM3

In this study the relationship between the North American monsoon, the Californian sea surface temperature (SST) cold pool, the Rocky Mountains and the North Pacific subtropical anticyclone is investigated using the Hadley Centre's atmospheric climate model, HadAM3. In 1996 Hoskins hypothesized that heating in the North American monsoon might be important for the maintenance of the summertime North Pacific subtropical anticyclone, since the monsoon heating may induce descent to the north-west of the monsoon in the descending eastern flank of the subtropical anticyclone. This descent is further enhanced by radiative cooling and is associated with equatorward surface winds parallel to the western coast of North America. These equatorward winds induce oceanic upwelling of cold water and contribute to the formation of the Californian SST cold pool, which may feed back on the anticyclone by further suppressing convection and inducing descent. More recently, Rodwell and Hoskins also investigated the global summer monsoon–subtropical anticyclone relationship. They examined the role that mountains play in impeding the progress of the low-level mid-latitude westerlies, either deflecting the westerlies northwards where they ascend along the sloping mid-latitude isentropes or deflecting them southwards forcing them to descend along the isentropes. In particular, the introduction of the Rockies into a primitive-equation model adiabatically induces descent in the eastern descending flank of the North Pacific subtropical anticyclone. These hypothesized mechanisms have been investigated using HadAM3, focusing on the possible suppression of convection by the Californian SST cold pool, the response of the North Pacific subtropical anticyclone to the strength of the North American monsoon and the ‘blocking’ of the mid-latitude westerlies by the Rocky Mountains. The role of the Rockies is examined by integrating the model with modified orography for the Rocky Mountains. Changing the height of the Rockies alters the circulation in a way consistent with the mechanism outlined above. Higher Rocky mountains force the westerlies southwards, inducing descent in the eastern flank of the subtropical anticyclone as the air descends along the sloping isentropes. The relationship between the North American monsoon and the North Pacific subtropical anticyclone is investigated by suppressing the monsoon in HadAM3. The suppression of the monsoon is accomplished by increasing the surface albedo over Mexico, which induces anomalous ascent on the eastward flank of the subtropical anticyclone and anomalous polewards surface winds along the western coast of the North American continent, also providing support for the above hypothesis. The removal of the Californian SST cold pool, however, has a statistically insignificant effect on the model, suggesting that in this model the feedback of the SST cold pool on the eastern flank of the anticyclone is weak.

Seasonal pattern of regional carbon balance in the central Rocky Mountains from surface and airborne measurements

[1] High-elevation forests represent a large fraction of potential carbon uptake in North America, but this uptake is not well constrained by observations. Additionally, forests in the Rocky Mountains have recently been severely damaged by drought, fire, and insect outbreaks, which have been quantified at local scales but not assessed in terms of carbon uptake at regional scales. The Airborne Carbon in the Mountains Experiment was carried out in 2007 partly to assess carbon uptake in western U.S. mountain ecosystems. The magnitude and seasonal change of carbon uptake were quantified by (1) paired upwind-downwind airborne CO2 observations applied in a boundary layer budget, (2) a spatially explicit ecosystem model constrained using remote sensing and flux tower observations, and (3) a downscaled global tracer transport inversion. Top-down approaches had mean carbon uptake equivalent to flux tower observations at a subalpine forest, while the ecosystem model showed less. The techniques disagreed on temporal evolution. Regional carbon uptake was greatest in the early summer immediately following snowmelt and tended to lessen as the region experienced dry summer conditions. This reduction was more pronounced in the airborne budget and inversion than in flux tower or upscaling, possibly related to lower snow water availability in forests sampled by the aircraft, which were lower in elevation than the tower site. Changes in vegetative greenness associated with insect outbreaks were detected using satellite reflectance observations, but impacts on regional carbon cycling were unclear, highlighting the need to better quantify this emerging disturbance effect on montane forest carbon cycling.

Internal wave drag in stratified flow over mountains on a beta plane

The impact of the variation of the Coriolis parameter f on the drag exerted by internal Rossby-gravity waves on elliptical mountains is evaluated using linear theory, assuming constant wind and static stability and a beta-plane approximation. Previous calculations of inertia-gravity wave drag are thus extended in an attempt to establish a connection with existing studies on planetary wave drag, developed primarily for fluids topped by a rigid lid. It is found that the internal wave drag for zonal westerly flow strongly increases relative to that given by the calculation where f is assumed to be a constant, particularly at high latitudes and for mountains aligned meridionally. Drag increases with mountain width for sufficiently wide mountains, reaching values much larger than those valid in the non-rotating limit. This occurs because the drag receives contributions from a low wavenumber range, controlled by the beta effect, which accounts for the drag amplification found here. This drag amplification is shown to be considerable for idealized analogues of real mountain ranges, such as the Himalayas and the Rocky mountains, and comparable to the barotropic Rossby wave drag addressed in previous studies.

A linear model of gravity wave drag for hydrostatic sheared flow over elliptical mountains

An analytical model of orographic gravity wave drag due to sheared flow past elliptical mountains is developed. The model extends the domain of applicability of the well-known Phillips model to wind profiles that vary relatively slowly in the vertical, so that they may be treated using a WKB approximation. The model illustrates how linear processes associated with wind profile shear and curvature affect the drag force exerted by the airflow on mountains, and how it is crucial to extend the WKB approximation to second order in the small perturbation parameter for these effects to be taken into account. For the simplest wind profiles, the normalized drag depends only on the Richardson number, Ri, of the flow at the surface and on the aspect ratio, γ, of the mountain. For a linear wind profile, the drag decreases as Ri decreases, and this variation is faster when the wind is across the mountain than when it is along the mountain. For a wind that rotates with height maintaining its magnitude, the drag generally increases as Ri decreases, by an amount depending on γ and on the incidence angle. The results from WKB theory are compared with exact linear results and also with results from a non-hydrostatic nonlinear numerical model, showing in general encouraging agreement, down to values of Ri of order one.

Resonant gravity-wave drag enhancement in linear stratified flow over mountains

High-drag states produced in stratified flow over a 2D ridge and an axisymmetric mountain are investigated using a linear, hydrostatic, analytical model. A wind profile is assumed where the background velocity is constant up to a height z1 and then decreases linearly, and the internal gravity-wave solutions are calculated exactly. In flow over a 2D ridge, the normalized surface drag is given by a closed-form analytical expression, while in flow over an axisymmetric mountain it is given by an expression involving a simple 1D integral. The drag is found to depend on two dimensionless parameters: a dimensionless height formed with z_1, and the Richardson number, Ri, in the shear layer. The drag oscillates as z_1 increases, with a period of half the hydrostatic vertical wavelength of the gravity waves. The amplitude of this modulation increases as Ri decreases. This behaviour is due to wave reflection at z_1. Drag maxima correspond to constructive interference of the upward- and downward-propagating waves in the region z < z_1, while drag minima correspond to destructive interference. The reflection coefficient at the interface z = z_1 increases as Ri decreases. The critical level, z_c, plays no role in the drag amplification. A preliminary numerical treatment of nonlinear effects is presented, where z_c appears to become more relevant, and flow over a 2D ridge qualitatively changes its character. But these effects, and their connection with linear theory, still need to be better understood.

Deriving global flood hazard maps of fluvial floods through a physical model cascade

Global flood hazard maps can be used in the assessment of flood risk in a number of different applications, including (re)insurance and large scale flood preparedness. Such global hazard maps can be generated using large scale physically based models of rainfall-runoff and river routing, when used in conjunction with a number of post-processing methods. In this study, the European Centre for Medium Range Weather Forecasts (ECMWF) land surface model is coupled to ERA-Interim reanalysis meteorological forcing data, and resultant runoff is passed to a river routing algorithm which simulates floodplains and flood flow across the global land area. The global hazard map is based on a 30 yr (1979–2010) simulation period. A Gumbel distribution is fitted to the annual maxima flows to derive a number of flood return periods. The return periods are calculated initially for a 25×25 km grid, which is then reprojected onto a 1×1 km grid to derive maps of higher resolution and estimate flooded fractional area for the individual 25×25 km cells. Several global and regional maps of flood return periods ranging from 2 to 500 yr are presented. The results compare reasonably to a benchmark data set of global flood hazard. The developed methodology can be applied to other datasets on a global or regional scale.

Using the significant dust deposition event on the glaciers of Mt. Elbrus, Caucasus Mountains, Russia on 5 May 2009 to develop a method for dating and "provenancing" of desert dust events recorded in snow pack

A significant desert dust deposition event occurred on Mt. Elbrus, Caucasus Mountains, Russia on 5 May 2009, where the deposited dust later appeared as a brown layer in the snow pack. An examination of dust transportation history and analysis of chemical and physical properties of the deposited dust were used to develop a new approach for high-resolution “provenancing” of dust deposition events recorded in snow pack using multiple independent techniques. A combination of SEVIRI red-green-blue composite imagery, MODIS atmospheric optical depth fields derived using the Deep Blue algorithm, air mass trajectories derived with HYSPLIT model and analysis of meteorological data enabled identification of dust source regions with high temporal (hours) and spatial (ca. 100 km) resolution. Dust, deposited on 5 May 2009, originated in the foothills of the Djebel Akhdar in eastern Libya where dust sources were activated by the intrusion of cold air from the Mediterranean Sea and Saharan low pressure system and transported to the Caucasus along the eastern Mediterranean coast, Syria and Turkey. Particles with an average diameter below 8 μm accounted for 90% of the measured particles in the sample with a mean of 3.58 μm, median 2.48 μm. The chemical signature of this long-travelled dust was significantly different from the locally-produced dust and close to that of soils collected in a palaeolake in the source region, in concentrations of hematite. Potential addition of dust from a secondary source in northern Mesopotamia introduced uncertainty in the “provenancing” of dust from this event. Nevertheless, the approach adopted here enables other dust horizons in the snowpack to be linked to specific dust transport events recorded in remote sensing and meteorological data archives.

Accelerated loss of alpine glaciers in the Kodar Mountains, south-eastern Siberia

The recession of mountain glaciers around the world has been linked to anthropogenic climate change and small glaciers (e.g. < 2 km2) are thought to be particularly vulnerable, with reports of their disappearance from several regions. However, the response of small glaciers to climate change can be modulated by non-climatic factors such as topography and debris cover and there remain a number of regions where their recent change has evaded scrutiny. This paper presents results of the first multi-year remote sensing survey of glaciers in the Kodar Mountains, the only glaciers in SE Siberia, which we compare to previous glacier inventories from this continental setting that reported total glacier areas of 18.8 km2 in ca. 1963 (12.6 km2 of exposed ice) and 15.5 km2 in 1974 (12 km2 of exposed ice). Mapping their debris-covered termini is difficult but delineation of debris-free ice on Landsat imagery reveals 34 glaciers with a total area of 11.72 ± 0.72 km2 in 1995, followed by a reduction to 9.53 ± 0.29 km2 in 2001 and 7.01 ± 0.23 km2 in 2010. This represents a ~ 44% decrease in exposed glacier ice between ca. 1963 and 2010, but with 40% lost since 1995 and with individual glaciers losing as much as 93% of their exposed ice. Thus, although continental glaciers are generally thought to be less sensitive than their maritime counterparts, a recent acceleration in shrinkage of exposed ice has taken place and we note its coincidence with a strong summer warming trend in the region initiated at the start of the 1980s. Whilst smaller and shorter glaciers have, proportionally, tended to shrink more rapidly, we find no statistically significant relationship between shrinkage and elevation characteristics, aspect or solar radiation. This is probably due to the small sample size, limited elevation range, and topographic setting of the glaciers in deep valleys-heads. Furthermore, many of the glaciers possess debris-covered termini and it is likely that the ablation of buried ice is lagging the shrinkage of exposed ice, such that a growth in the proportion of debris cover is occurring, as observed elsewhere. If recent trends continue, we hypothesise that glaciers could evolve into a type of rock glacier within the next few decades, introducing additional complexity in their response and delaying their potential demise.

Using the significant dust deposition event on the glaciers of Mt. Elbrus, Caucasus Mountains, Russia on 5 May 2009 to develop a method for dating and provenancing of desert dust events recorded in snow pack.

A significant desert dust deposition event occurred on Mt. Elbrus, Caucasus Mountains, Russia on 5 May 2009, where the deposited dust later appeared as a brown layer in the snow pack. An examination of dust transportation history and analysis of chemical and physical properties of the deposited dust were used to develop a new approach for high-resolution provenancing of dust deposition events recorded in snow pack using multiple independent techniques. A combination of SEVIRI red-green-blue composite imagery, MODIS atmospheric optical depth fields derived using the Deep Blue algorithm, air mass trajectories derived with HYSPLIT model and analysis of meteorological data enabled identification of dust source regions with high temporal (hours) and spatial (ca. 100 km) resolution. Dust, deposited on 5 May 2009, originated in the foothills of the Djebel Akhdar in eastern Libya where dust sources were activated by the intrusion of cold air from the Mediterranean Sea and Saharan low pressure system and transported to the Caucasus along the eastern Mediterranean coast, Syria and Turkey. Particles with an average diameter below 8 μm accounted for 90% of the measured particles in the sample with a mean of 3.58 μm, median 2.48 μm and the dominant mode of 0.60 μm. The chemical signature of this long-travelled dust was significantly different from the locally-produced dust and close to that of soils collected in a palaeolake in the source region, in concentrations of hematite and oxides of aluminium, manganese, and magnesium. Potential addition of dust from a secondary source in northern Mesopotamia introduced uncertainty in the provenancing of dust from this event. Nevertheless, the approach adopted here enables other dust horizons in the snowpack to be linked to specific dust transport events recorded in remote sensing and meteorological data archives.