Pneumatochemical impedance spectroscopy. 1. Principles

Pneumatochemical impedance spectroscopy (PIS) is the indirect transposition of electrochemical impedance spectroscopy (EIS) to solid-gas reactions. In PIS analysis, an analogy is made between pressure and electrical potential on one hand and gas flow and electric current on the other hand, and pneumatochemical transfer functions are derived from gas-phase measurements. Potentially, this spectroscopy can be used to analyze the dynamics of any solid-gas system including adsorption (surface) and absorption (bulk) phenomena, gas (H2) permeation across metallic membranes, and electrocatalysis in gaseous fuel cells. Hydrogen absorption by intermetallic compounds (IMCs), a process of great practical interest for hydrogen storage applications, is more specifically considered in this paper, and the kinetic equations derived in this work pertain only to this case. Whereas classical electrochemical impedance measurements are performed using an harmonic analyzer and monochromatic potential (potentiostatic mode) or current (intentiostatic mode) perturbations, PIS investigation of the dynamics of IMC-H2(g) systems is more conveniently performed using Sievert's-type gas distribution apparatus (SGDA) and polychromatic pressure perturbation signals. This is first because monochromatic isothermal pressure modulations cannot be easily obtained experimentally over the frequency domain of interest and, second, because most IMC-H2(g) systems exhibit strongly nonlinear behaviors in two-phase domains (hysteresis), and this proscribes harmonic analysis. A further benefit of using SGDA and nonharmonic perturbations is that kinetic and thermodynamic information are collected simultaneously during the same experiment. The measurement and modeling of the pneumatochemical transfer functions associated with IMC-H2(g) systems, both in solid solution and two-phase domains, are discussed in this paper which is organized in two parts. The principles of PIS analysis, based on the theory of linear and time-invariant systems, are presented in the first part. The dynamics of hydrogen sorption by metals and IMCs is analyzed in the second part, where a detailed analysis of the multistep reaction paths involved in sorption mechanisms is proposed.     

Assessing the transfer of pesticides to the atmosphere during and after application. Development of a multiresidue method using adsorption on Tenax and thermal desorption-GC/MS

An air sampling and analytical method based on adsorption on porous polymer (Tenax TA) followed by automatic thermal desorption (ATD) and GC/MS analysis was developed for ten pesticides commonly used on major crops in Britanny and some of their metabolites in air (from spray drift and volatilisation transfer processes): alachlor, atrazine (and two major degradation products: deethylatrazine and deisopropylatrazine), carbofuran, cyprodinil, epoxyconazole, iprodione (and 3,5-dichloroaniline), lindane (and -HCH, its isomer), metolachlor, terbuconazole and trifluralin. This method was established with special consideration for optimal thermal desorption conditions, linear ranges, limits of detection and quantification. Moreover, collection efficiencies of Tenax TA at room temperature were examined. This method was then applied to the determination of ambient pesticide levels during the spraying season at a rural area. The method was also applied to determine the vertical gradient of alachlor concentrations on a treated maize parcel to evaluate volatilisation fluxes.     

Transition metal-free addition of ketones or nitriles to 1,3-dienes

The simpler the better (or going green): The first examples of the catalytic addition of 1,3-dienes to simple ketones or nitriles are described. These reactions can be effected on a kilogram scale, representing the shortest access featuring a perfect atom economy to molecules of interest in the perfume industry.     

Ground state of the kagomé-like S=1/2 antiferromagnet volborthite Cu3V2O7(OH)2 x 2H2O

The volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2 K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4 K +/- 0.2 K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% corresponds to a well defined short-range order, possibly of the sqrt(3) x sqrt(3) type. While the freezing involves all the Cu2+ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.