The Alkaline Hydrolysis of Sulfonate Esters: Challenges in Interpreting Experimental and Theoretical Data

Sulfonate ester hydrolysis has been the subject of recent debate, with experimental evidence interpreted in terms of both stepwise and concerted mechanisms. In particular, a recent study of the alkaline hydrolysis of a series of benzene arylsulfonates (Babtie et al., Org. Biomol. Chem. 10, 2012, 8095) presented a nonlinear Brønsted plot, which was explained in terms of a change from a stepwise mechanism involving a pentavalent intermediate for poorer leaving groups to a fully concerted mechanism for good leaving groups and supported by a theoretical study. In the present work, we have performed a detailed computational study of the hydrolysis of these compounds and find no computational evidence for a thermodynamically stable intermediate for any of these compounds. Additionally, we have extended the experimental data to include pyridine-3-yl benzene sulfonate and its N-oxide and N-methylpyridinium derivatives. Inclusion of these compounds converts the Brønsted plot to a moderately scattered but linear correlation and gives a very good Hammett correlation. These data suggest a concerted pathway for this reaction that proceeds via an early transition state with little bond cleavage to the leaving group, highlighting the care that needs to be taken with the interpretation of experimental and especially theoretical data

Homochirality in space – Selective enrichment of chiral molecules on chiral surfaces

Life on Earth, as we know it to day, is inseparable from homochirality. The presence of an enantiomeric excess in carbonaceous chondrites is well documented; among the different hypotheses considered to explain this undisputed fact, we chose to investigate the possibilities open by a selective adsorption on a chiral surface that would engage a process of enantiomeric enrichment. The chiral surface is that of the α-quartz $\left\{ {10\,\overline {10} } \right\}$ and the chiral molécule is lactic acid, HOCH(CH3)COOH. In this theoretical work we rely on numerical simulations based on Density Functional Theory (DFT) that proved efficient in the super-molecule and periodic approaches to this category of phenomena. In view of the different types of adsorption sites, a statistical treatment was necessary; it shows that (S)-lactic acid is preferentially adsorbed with a selectivity of ~0.7 kcal/mol with respect to the (R) isomer

The alkaline hydrolysis of sulfonate esters: challenges in interpreting experimental and theoretical data

Sulfonate ester hydrolysis has been the subject of recent debate, with experimental evidence interpreted in terms of both stepwise and concerted mechanisms. In particular, a recent study of the alkaline hydrolysis of a series of benzene arylsulfonates (Babtie et al., Org. Biomol. Chem.10, 2012, 8095) presented a nonlinear Brønsted plot, which was explained in terms of a change from a stepwise mechanism involving a pentavalent intermediate for poorer leaving groups to a fully concerted mechanism for good leaving groups and supported by a theoretical study. In the present work, we have performed a detailed computational study of the hydrolysis of these compounds and find no computational evidence for a thermodynamically stable intermediate for any of these compounds. Additionally, we have extended the experimental data to include pyridine-3-yl benzene sulfonate and its N-oxide and N-methylpyridinium derivatives. Inclusion of these compounds converts the Brønsted plot to a moderately scattered but linear correlation and gives a very good Hammett correlation. These data suggest a concerted pathway for this reaction that proceeds via an early transition state with little bond cleavage to the leaving group, highlighting the care that needs to be taken with the interpretation of experimental and especially theoretical data

Estimation of aerodynamic parameters under non-neutral stability conditions from Alpilles measurement data

International audienceAerodynamic parameters (displacement height d and roughness length for momentum z(o)), as well as friction velocity u. and temperature scaling parameter T-., were estimated from the Alpilles energy balance measurement data. Air temperature and wind speed measurements at two heights and sensible heat flux data were needed in the estimation. Application of this method to two data sets measured in different ways for the temperature gradients found that the estimation of d and z(o) was applicable only when the temperature gradients were measured differentially. Sensitivity tests showed that the estimated d and z(o) were sensitive to the temperature gradients. However, both of the data sets produced good estimation of it, and T-.. The main advantage of this method is that it is applicable not only under near-neutral conditions but also under strong unstable conditions

Influence of agricultural practices on micrometerological spatial variations at local and regional scales

International audienceSoil-vegetation-atmosphere transfers significantly influence interactions and feedbacks between vegetation and boundary layer, in relation with plant phenology and water status. The current study focused on linking micrometeorological conditions to cultural practices at the local and regional scales (lower than 100km2), over an agricultural region in South Western France. This was achieved considering observation and modelling tools designed for characterising spatial variabilities over land surfaces. These tools were the ASTER high spatial resolution optical remote sensing data, and the SEBAL spatialised surface energy balance model. Surface bidirectional reflectance and brightness temperature were first derived from ASTER data through solar and thermal atmospheric radiative transfer codes, and next used to infer surface radiative properties required for model simulations. Assessing model consistency in terms of air temperature simulations gave satisfactory results when intercomparing against weather station data, although basic model assumptions were not systematically verified in terms of spatial variability. Next, spatialised simulations of evapotranspiration and air temperature were analysed at the regional and local scales, in relation with pedology, land use, and cultural practices. It was shown model estimates were consistent with the considered crops and the related cultural practices. Irrigation appeared as the main factor amongst others (soil, landuse, sowing date) explaining the micrometeorological variability. Although interesting and promising in terms of linking micrometeorological conditions to cultural practices, the results reported here emphasised several difficulties, specially about capturing subfield scale variability and monitoring the considered processes at an appropriate temporal sampling

Monitoring evapotranspiration from remote sensing data for groundwater resources evaluation

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Monitoring evapotranspiration from remote sensing data for groundwater resources evaluation.

International audienceEvapotranspiration (ET) is a fundamental variable of the hydrological cycle which plays a major role on surface water balance and surface energy balance. At local scale ET can be estimated from detailed ground observations (eddy covariance towers), but these measurements are only representative of very limited homogeneous area. When regional information is required, e.g. for monitoring ground water resources, the flux tower measurements cannot be used directly and estimation of ET often relies on estimation from meteorological data through potential evapotranspiration formulas. At regional scale remote sensing provides spatially distributed information for mapping and monitoring ET, but this information is still rarely used for ground water assessment. Indeed, remote sensing estimation of ET suffers several drawbacks. In particular, remote sensing information by itself cannot provide a continuous monitoring of ET because of the presence of clouds and the revisit period of the sensor. Another difficulty originates in the lack of exhaustive evaluation of remote sensed ET since accurate ground measurements are scarce and representative of a limited number of homogeneous areas. This has also for consequence that a large number of methodologies to derive ET were developed with no real possibility of a consistent evaluation. We have developed the EVASPA (EVapotranspiration Assessment from SPAce) tool to monitor ET on a daily basis, together with an evaluation of the associated uncertainties, from remote sensing data in the thermal and the solar domains (Gallego Elvira et al. 2013). This tool combines the estimation of ET from various models and various sources of data, including MODIS sensors, LANDSAT-borne sensors and meteorological information. EVASPA was applied to estimate evapotranspiration over several areas in the products such as MOD16 or analysis of atmospheric-hydrological modeling such as the operational Safran-Isba-Modcou application). The results highlight the potential use of the retrieved ET for calibrating groundwater models (e.g. for estimating aquifer parameters…) or evaluating model inputs (e.g. determination of effective rainfall, identification of irrigated areas…). We also evaluated the impact of the uncertainties in the estimation of ET in the monitoring of ground water. We showed that the main sources of ET uncertainty were related to the uncertainties in incident radiations and surface temperature together with the diversity of ET models. When forced in ground water models, the uncertainties in ET had an impact almost equivalent to the impact of uncertainties in rain inputs. South of France to help in monitoring the water budget of different hydrosystems: superficial aquifer in the Rhône river delta (Camargue), karstic aquifer of the Fontaine de Vaucluse spring system and alluvial aquifer in Limagne with increasing water withdrawing for irrigation. The method was first evaluated against flux tower measurements of evapotranspiration (RMSE between 0.5 and 1 mm/day depending on the ecosystems). When integrated over watershed, ET retrievals were also compared to indirect estimates of evapotranspiration from either water balance and stream flow monitoring or other modelling approaches for time period of more than a decade (these include remote sensing operationa

The Adam Database and its Potential to Investigate High Temporal Sampling Acquisition at High Spatial Resolution for the Monitoring of Agricultural Crops

International audienceThe ADAM project (Assimilation of Spatial Data within Agriculture Models) was aiming at the development and evaluation of methods capable of exploiting high revisit frequency and high spatial satellite observations. The intensive experimental campaign conducted in 2000-2001 over wheat crops yielded a unique database where frequent satellite observations at high spatial resolution in the solar and microwave domains were made concurrently to ground characterization of the soil and the canopy and their functioning. This includes 39 SPOT i mages, 15 Radar (ERS and RadarSat) images, soil permanent characteriz ation, frequent canopy leaf area index, biomass, soil water and nitrogen contents, as measured over 42 elementary sampling units (ESU). This intensive campaign will be followed in 2002 and 2003 by lighter campaigns where a reduced number of ESUs will be sampled, and a reduced number of SPOT scenes will be acquired. The objective of this paper is to present the ADAM experiment and the database generated, along with few examples illustrating its potentials. The ADAM database, freely accessible to the scientific community, is also described. Finally, conclusions are drawn on the problems associated to the exploitation of dense time series of images acquired by different sensors. A discussion is made on the potential exploitation of such a database

Evaluations of the EVASPA tool for mapping EVApotranspiration from SPAce.

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