Calibration method at the N K‐edge using interstitial nitrogen gas in solid‐state nitrogen‐containing inorganic compounds

The standard method of soft X‐ray beamline calibration at the N K‐edge uses the ν = 0 peak transition of gas‐phase N₂. Interstitial N₂ gas trapped or formed within widely available solid‐state ammonium‐ and amine‐containing salts can be used for this purpose, bypassing gas‐phase measurements. Evidence from non‐nitrogen‐containing compounds (KH₂PO₄) and from He‐purged ammonium salts suggest that production of N₂ gas is through beam‐induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.     

The effect of optical bias on grating formation dynamics in photorefractive BSO

We present experimental results describing the influence of optical bias on the transient characteristics of gratings formed in photorefractive BSO. A transient peak is evident, and the properties of this peak can be controlled by suitable adjustment of the relative intensities of the optical beams involved.     

Incompatibility of the Schrödinger equation with Langevin and Fokker-Planck equations

Quantum mechanics posits that the wave function of a one-particle system evolves with time according to the Schrödinger equation, and furthermore has a square modulus that serves as a probability density function for the position of the particle. It is natural to wonder if this stochastic characterization of the particle's position can be framed as a univariate continuous Markov process, sometimes also called a classical diffusion process, whose temporal evolution is governed by the classically transparent equations of Langevin and Fokker-Planck. It is shown here that this cannot generally be done in a consistent way, despite recent suggestions to the contrary.     

Differences in the low-energy collision-activated dissociation of carboxylate anions from structurally similar prostaglandins: E2, F2α, D2, and DHKF2α

Some intriguing discoveries were made concerning the collision-activated dissociation behavior of the derivatized carboxylate anions of PGE₂ and PGF_2α. The carboxylate anion [MPFB]^? formed from electron-capture negative chemical ionization of the pentafluorobenzyl ester-trimethylsilyl derivative of PGF_2α showed little fragmentation under typical collision gas pressures and energies (<2.0 mtorr N₂ and <20 eV). In contrast, the daughter spectra of the carboxylate anion of the methoxime-pentafluorobenzyl ester-trimethylsilyl derivative of PGE₂ produced many intense fragments under the same conditions.     

Thermodynamic quantities for the ionization of nitric acid in aqueous solution from 250 to 319°C

The aqueous reaction, HNO₃(aq)=H⁺+NO ₃ ^– was studied as a function of ionic strength I at 250, 275, 300 and 319°C using a flow calorimeter and the equilibrium constant K and enthalpy change (H) at I=0 were determined. Using these experimental values, equations describing logK, H, the entropy change S and the heat capacity change C _p of reaction at I=0 and temperatures from 250 to 319°C were derived. The increasing importance of ion association as temperature rises was discussed. The use of an equation containing identical numbers of positive and identical numbers of negative charges on both sides of the equal sign (isocoulombic reaction principle) was applied to the logK values reported here and to those determined by others. The resulting plot of logK for the isocoulombic reaction vs. 1/T was fairly linear which supports the postulate that the principle is a useful technique for the extrapolation of logK values from low to high temperatures.Presented at the Second International Symposium on Chemistry in High Temperature Water, Provo, UT, August 1991.     

Core distortions in metal atoms and the use of effective core potentials

The recent experimental determination of the geometry of Ti(CH₃)₂Cl₂ shows it to be inconsistent with the VSEPR model, a result not uncommon for molecules containing transition metal atoms. The valence shell charge concentrations (CCs) that appear as maxima in L(r)=−²ρ(r), provide a physical basis for the VSEPR model of molecular geometry for main group molecules. The same model accounts for the geometry of transition metal molecules with the proviso that the CCs are formed within the outer shell of the core of the metal atom, as defined by the shell structure of L(r). This observation appears to be in conflict with calculations for Ti(CH₃)₂Cl₂ showing that its geometry can be predicted using an effective core potential for the metal atom, a procedure that would appear to preclude the presence of core distortions. The apparent contradiction is resolved by distinguishing between the definition of the core using L(r) and one based on the orbital model.     

An electron localization function study of the geometry of d(0) molecules of the period 4 metals Ca to Mn

We have studied the geometry of the formally d(0) MX(n)() (X = F, H, CH(3) and O; n = 2-6) molecules of the period 4 metals from Ca to Mn by studying the topology of the electron localization function (ELF) in order to try to understand why many of these molecules have non-VSEPR geometries. The quantitative analysis of the core basin population shows that it is always larger than its conventional value (18) because, in the LCAO-MO scheme, the 3d basis functions centered on the metal noticeably contribute to the electron density within the core region associated with the M shell. Therefore, the density available to form the bonds is less than Z(M) - 18, the value adopted in electron counts. Under the influence of the ligands, these electrons cause the core to lose its spherical symmetry by the formation of opposite-spin pair localization basins, which in turn influence the geometry of the ligands if the interaction of the ligands with the core is sufficiently strong. All of the ligands considered in this study, except F, interact with the core sufficiently strongly to give non-VSEPR geometries, which we have rationalized on the basis of the ELF topology.