Can the clusters Zr6O4(OH)4(OOCR)12 and [Zr6O4(OH)4(OOCR)12]2 be converted into each other?

Upon reaction of zirconium alkoxides, Zr(OR)4, with carboxylic acids, the clusters Zr6O4(OH)4(OOCR)12 (Zr6) or [Zr6O4(OH)4(OOCR)12]2 (Zr12) were obtained, depending on the employed carboxylic acid. The structures of Zr12 clusters with acetate, propionate, vinyl acetate or 3,3\u2032-dimethylacrylate ligands were determined by single-crystal X-ray diffraction, as well as that of methacrylate/acetate and methacrylate/propionate mixed-ligand clusters. The structure of the hafnium cluster [Hf6O4(OH)4(acetate)12]2 is also reported for comparison. The Zr12 clusters are structurally related to the Zr6 clusters since they are composed of two Zr6 sub-units which are bridged by four carboxylate ligands. In each compound, carboxylic acid molecules interact with the cluster by hydrogen bonding to part of the \u3bc3-OH ligands. The clusters are highly dynamic in solution, due to site exchange of the carboxylate ligands; low-temperature NMR spectra of the clusters correspond to the solid-state structures. The Zr6 and Zr12 clusters can be clearly distinguished by their solution NMR spectra. Reactions between selected Zr12 clusters and carboxylic acids led to the conclusion, that only part of the ligands is accessible for ligand exchange reactions. In contrast, all ligands were exchanged when the Zr6 cluster Zr6O4(OH)4(methacrylate)12 was treated with an excess of propionic acid, and Zr6O4(OH)4(propionate)12 was formed. However, the Zr12 cluster [Zr6O4(OH)4(propionate)12]2 was obtained from the reaction of Zr(OBu)4 with propionic acid. The Zr6 and Zr12 clusters thus do not interconvert at the given reaction conditions. (\ua9 Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006

Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel–tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWCL-pd) together were the strongest predictors of day capacitance (r2adj =.44). Vessel–tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse-porous species, VWCL-pd and vessel–ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring-porous species, VWCL-pd, WD and vessel–fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue

Precipitation is not limiting for xylem formation dynamics and vessel development in European beech from two temperate forest sites

We investigated the dynamics of xylem differentiation processes and vessel characteristics in Fagus sylvatica L. to evaluate the plasticity of xylem structures under different environmental conditions. In 2008-10, analyses were performed on microcores collected weekly from two temperate sites: Menina planina (1200 m above sea level (a.s.l.)) and Panska reka (400 m a.s.l.). The duration between the onset and end of major cell differentiation steps and vessel characteristics (i.e., density, VD; mean diameter, MVD; mean area, MVA; and theoretic conductivity area, TCA) were analysed in the first and last quarters of the xylem rings, also in respect of local weather conditions (precipitation, temperature). Although the onset, duration and end of xylem formation phases differed between the two sites, the time spans between the successive wood formation phases were similar. Significant differences in MVD, MVA and TCA values were found between the first and last quarters of xylem increment, regardless of the site and year. Vessel density, on the other hand, depended on xylem-ring width and differed significantly between the sites, being about 30% higher at the high elevation site, in beech trees with 54% narrower xylem rings. Vessel density in the first quarter of the xylem ring showed a positive correlation with the onset of cell expansion, whereas a negative correlation of VD with the cessation of cell production was found in the last quarter of xylem increment. This may be explained by year-to-year differences in the timing of cambial reactivation and leaf development, which effect hormonal regulation of radial growth. No significant linkage between intra-annual weather conditions and conduit characteristics was found. It can thus be presumed that precipitation is not a limiting factor for xylem growth and cell differentiation in beech at the two temperate study sites and sites across Europe with similar weather conditions