Influence of Ions on Aqueous Acid–Base Reactions

We study the effects of bromide salts on the rate and mechanism of the aqueous proton/deuteron‐transfer reaction between the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) and the base acetate. The proton/deuteron release is triggered by exciting HPTS with 400 nm femtosecond laser pulses. Probing the electronic and vibrational resonances of the photoacid, the conjugate photobase, the hydrated proton/deuteron and the accepting base with femtosecond visible and mid‐infrared pulses monitors the proton transfer. Two reaction channels are identified: 1) direct long‐range proton transfer over hydrogen‐bonded water bridges that connect the acid and base and 2) acid dissociation to produce fully solvated protons followed by proton scavenging from solution by acetate. We observe that the addition of salt affects the long‐range reaction pathway, and reduces both the rate at which protons are released to solution by HPTS and the rate at which solvated protons are scavenged from solution by acetate. We study the dependence of these effects on the nature and concentration of the dissolved salt.     

Effect of Confinement on Proton‐Transfer Reactions in Water Nanopools

Proton transfer from the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) to water is studied in reverse micelles with ionic (AOT=sodium dioctyl sulfosuccinate) and non‐ionic (BRIJ‐30=polyoxyethylene(4)lauryl ether) surfactants. The dynamics are studied by probing the transient electronic absorption and transient vibrational absorption, both with sub‐picosecond resolution. The reverse micelle sizes range from approximately 1.6 to 5.5 nm in diameter. For both surfactants it is found that the rate of proton transfer decreases with decreasing reverse micelle size, regardless of surfactant. In addition, for AOT reverse micelles, a fraction of the photoacid molecules exhibit non‐radiative decay, preventing proton transfer.     

Quantitive binding constants of H(+)-selective chromoionophores and anion ionophores in solvent polymeric sensing membranes

The binding properties of neutral or charged chromoionophores and anion ionophores in solvent polymeric membranes were characterized in situ by the so-called sandwich membrane method. Acidity constants (pK(a)) of eight chromoionophores (ETH 5294, ETH 2439, ETH 5350, ETH 5418, ETH 5315, ETH 7061, ETH 7075, ETH 2412) were measured in bis(2-ethylhexyl)sebacate (DOS) and o-nitrophenyloctylether (NPOE) plasticized poly(vinyl chloride) (PVC) membranes commonly used in optical and potentiometric ion sensors. The pK(a) values of all chromoionophores in DOS membranes are by 2-3 orders of magnitude smaller than in NPOE membranes. The weak alkali metal ion binding properties with neutral H(+)-chromoionophore and anion binding with electrically charged chromoionophores were also studied quantitatively. The complex formation constants of the commercially available Co(III)cobyrinate nitrite ionophore and the organomercury chloride ionophore, ETH 9009, were also measured. The very low stability constant observed for ETH 9009 (logbeta(2)=3.60+/-0.03 in PVC-DOS and 3.61+/-0.01 in PVC-NPOE) was explained by the decomposition of the ionophore in contact with chloride samples. On the other hand, the electrically charged nitrite ionophore showed strong complexation with nitrite ions, with logbeta=10.58 and 10.59 in DOS and NPOE membranes, respectively. In contrast to cation ionophores, the stability constant of the NO(2)(-) ionophore does not change with different plasticizers.     

Optical chloride sensor based on [9]mercuracarborand-3 with massively expanded measuring range

Bulk optodes for monovalent ions typically exhibit a relatively narrow measuring range of about two-orders of magnitude. Here, the measuring range was expanded to about six-orders of magnitude concentration change by optimizing a fluorescent chloride optical sensing film based on a plasticized poly(vinyl chloride) film incorporating the halide-selective ionophore [9]mercuracarborand-3 and the H⁺-chromoionophore 9-dimethylamino-5-[4-(15-butyl-1,13-dioxo-2,14-dioxanonadecyl) phenyl-imino]benzo[a]phenox-azine (ETH 5418). This was achieved with the recently established two-step ionophore binding mechanism: the expanded sensing range sequentially makes use of the 1:2 and 1:1 binding stoichiometries between ionophore to chloride. The relevant complex formation constants were found here as log K₁=9.1 and log K₂=2.7, which are consistent with previously reported values. The selectivity of this optode is very good, with an excellent discrimination of thiocyanate and lipophilic anions such as salicylate, nitrate and perchlorate. As with earlier work, the main interferences are the other halides bromide and iodide.     

The Interaction of Water with Free Mn4O4+ Clusters: Deprotonation and Adsorption‐Induced Structural Transformations

As the biological activation and oxidation of water takes place at an inorganic cluster of the stoichiometry CaMn₄O₅, manganese oxide is one of the materials of choice in the quest for versatile, earth‐abundant water splitting catalysts. To probe basic concepts and aid the design of artificial water‐splitting molecular catalysts, a hierarchical modeling strategy was employed that explores clusters of increasing complexity, starting from the tetramanganese oxide cluster Mn₄O₄⁺ as a molecular model system for catalyzed water activation. First‐principles calculations in conjunction with IR spectroscopy provide fundamental insight into the interaction of water with Mn₄O₄⁺, one water molecule at a time. All of the investigated complexes Mn₄O₄(H₂O)_n⁺ (n=1–7) contain deprotonated water with a maximum of four dissociatively bound water molecules, and they exhibit structural fluxionality upon water adsorption, inducing dimensional and structural transformations of the cluster core.     

Nitrite-selective microelectrodes

The development of nitrite-selective liquid membrane microelectrodes based on a synthetic charged ionophore is described. The addition of potassium tetrakis(4-chlorophenyl)borate and poly(vinyl chloride) to the membrane phase is essential to lower the ohmic resistance and to prolong the lifetime of the microelectrodes. The detection limits for sodium nitrite solutions without and with an ion background of 0.1M chloride are 10(-5.1) and 10(-4.2)M, respectively. The comparison with macroelectrodes shows that the miniaturization reduces, to some extent, the selectivity and the slopes of the EMF response functions.     

On the orientational relaxation of HDO in liquid water

We use femtosecond mid-infrared pump-probe spectroscopy to study the orientational relaxation of HDO molecules dissolved in H2O. In order to obtain a reliable anisotropy decay we model the effects of heating and correct for these effects. We have measured the reorientation time constant of the OD vibration from 2430 to 2600 cm(-1), and observe a value of 2.5 ps that shows no variation over this frequency interval. Our results are discussed in the context of previous experiments that have been performed on the complementary system of HDO dissolved in D2O.     

Upper critical fields of single-crystalline and polycrystalline Ca-Sr-Bi-Cu-O compounds

The ac resistivity of a 110 K phase multiphase polycrystalline Ca-Sr-Bi-Cu-O compound and an 85 K phase single-crystalline Ca_0.9Sr_2.1Cu_2.0O_{8 +} has been measured in various magnetic fields up to 8 T. Values forB _c ^2/ (0) of 71.5 T and forB _c2 (0) of 542 T are found for the 85 K phase sample. A value forB _c2(0) of 57.9 T is estimated for the 110K phase compound.     

Temperature dependence of the Mössbauer parameters of the Fe−Ni phases in the Santa Catharina meteorite

The temperature variation in the range 8–760K of the hyperfine parameters of the Fe−Ni phases in the Santa Catharina meteorite has been determined. It is suggested that the disordered 50–50 Fe−Ni phase actually consists of two distinct fractions, i.e. a completely disordered phase and one with intermediate long-range ordering parameter. The single-line subspectrum of the 28%-Ni phase was found to display magnetic ordering below approximately 25K.