2,4‐Di­methoxy­benzoic acid and 2,5‐di­methoxy­benzoic acid

The title compounds (both C₉H₁₀O₄) have nearly planar structures, and the methyl and/or carboxylic acid groups lie out of the molecular plane, as dictated by steric interactions. 2,5‐Di­methoxy­benzoic acid (2,5‐DMBA) forms an unusual intramolecular hydrogen bond between the carboxylic acid group and the O atom of the methoxy group in the 2‐position [O⋯O = 2.547 (2) Å and O—H⋯O = 154 (3)°]. 2,4‐DMBA forms a typical hydrogen‐bond dimer with a neighboring mol­ecule.     

High-field cross polarization NMR from laser-polarized xenon to surface nuclei

A method for NMR investigations of surface nuclei using cross polarization from optically polarized xenon (OPCP) is described. We find this methodology results in enhancement factors of approximately 10³ upon application to surface protons. The dynamics of¹²⁹Xe transfer to protons is examined in some detail, including the time, temperature, and multiple contact dependences of signal intensities. Furthermore, we discuss the sensitivity of the transfer process to spatial diffusion. Finally, we report on application of the OPCP experiment to a low total surface area sample.     

Correlations of relative sensitivities in gas chromatography electron ionization mass spectrometry with molecular parameters

The relative molar sensitivities for a number of compounds having a variety of functional groups were obtained in gas chromatography electron ionization mass spectrometry. Comparable results were obtained with a quadrupole and with a magnetic mass spectrometer. The present relative molar sensitivities are in good agreement with relative ionization cross sections obtained by different techniques and different instruments for a variety of compounds with molecular weights below about 200 u. For compounds of higher molecular weight, the present experimental sensitivities are significantly larger than estimates extrapolated from earlier data. The relatively molar sensitivities correlate well with molecular polarizability.     

Studies in chemical ionization mass spectrometry: Aryl Ketones

The CI mass spectra of aryl ketones, πCOR, were studied and found to give primarily [M + 29]⁺, [M + 1]⁺, [M ? 1]⁺, [πCO]⁺ and [RCO]⁺ ions. The major change in the spectra with increasing length of the aliphatic side chain was an increase in the [M ? 1]⁺/[M + 1]⁺ ratio. Increasing sample size was reflected primarily in the formation of [2M + 1]⁺ ions and a decrease in [M + 1]⁺ ions. Small amounts of water in the reactant gas reduced the extent of fragmentation action.     

Determination of theophylline in plasma by oxidation at the stationary carbon-paste electrode

A method for the determination of the concentration of theophylline in plasma at therapeutic levels is described. The method is based on the oxidation of theophylline at the stationary carbon-paste electrode and involves a simple extraction procedure and differential pulse voltammetry. Possible interferences are discussed and typical calibration curves are presented.     

Crystal structure and physical characterization of neotame methanol solvate

The crystal structure of the methanol solvate (empirical formula: 2C₂₀H₃₀N₂O₅·3CH₃OH) of a new dipeptide sweetener, neotame (N-(3,3-dimethylbutyl)-L--aspartyl-L-phenylalanine 1-methyl ester), has been determined. Crystal data: a = 9.8989(1), b = 18.1331(1), c = 27.5725(1) Å, orthorhombic, space group P2₁2₁2₁, with Z = 4. Each unit cell includes 8 neotame and 12 methanol molecules. Disorder exists in one neotame molecule and one methanol molecule. The crystals were characterized by the following techniques: hot-stage microscopy (HSM), Karl-Fischer titrimetry (KFT), powder X-ray diffractometry (PXRD), differential scanning calorimetry (DSC), thermogravimetry (TGA), ¹³C solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Under HSM at a heating rate of 10°C/min in silicone oil, the sample melts at 64–84°C and liberates bubbles at 71–86°C. DSC in open pans shows two overlapping endotherms at 56 and 71°C, probably due to melting and desolvation, respectively. TGA in open pans shows 5.9% weight loss due to desolvation below 70°C. Under house vacuum (23 mm Hg) over phosphorus pentoxide at 23°C, the methanol solvate produces pure amorphous anhydrate, which converts to crystalline neotame monohydrate in the presence of moisture.     

Minimum-weight two-connected spanning networks

We consider the problem of constructing a minimum-weight, two-connected network spanning all the points in a setV. We assume a symmetric, nonnegative distance functiond(·) defined onV × V which satisfies the triangle inequality. We obtain a structural characterization of optimal solutions. Specifically, there exists an optimal two-connected solution whose vertices all have degree 2 or 3, and such that the removal of any edge or pair of edges leaves a bridge in the resulting connected components. These are the strongest possible conditions on the structure of an optimal solution since we also show thatany two-connected graph satisfying these conditions is theunique optimal solution for a particular choice of canonical distances satisfying the triangle inequality. We use these properties to show that the weight of an optimal traveling salesman cycle is at most 4/3 times the weight of an optimal two-connected solution; examples are provided which approach this bound arbitrarily closely. In addition, we obtain similar results for the variation of this problem where the network need only span a prespecified subset of the points.     

Flexible supply contracts under price uncertainty

This article develops supply contracts covering environments with changing prices. We investigate characterization properties of the price processes, while considering costs and discount factors. We determine expressions of the contract’s expected low price and its second moment for a given horizon. We then employ these expected price and second moment values to identify an expected optimum time before the contract expires at which the lowest price occurs. Simulation experiments verify our analysis, and they illustrate how the optimum purchase time decreases as the drift term increases.