Sterically-crowded pyrroles

The syntheses in good yields of a considerable number of moderately to severely sterically-crowded 1,2,5-trisubstituted pyrroles by means of modifications of the Knorr-Paal condensation are described. Notwithstanding reports to the contrary, the classical procedure; succeeded in moderately sterically-crowded cases, e.g., R² = R⁵ = Me, R¹ = i-Pr, Me₂CHMeCH-, Me₂N-, Ph₂N-, 9-carbazolo, subst. 4-morpholino, 1-piperazino, 1-piperidino, etc. A product-water azeotroping procedure succeeded in more severely sterically-crowded cases e.g., R² = R⁵ = Et, R¹ = i-Pr; R² = R⁵ = Me, R¹ = Me₃CCH₂-, R² = R⁵ = i-Pr, R¹ = PhCH₂-. Highly sterically-crowded pyrroles, e.g., R² = R⁵ = Me, R¹ = t-Bu, adamantyl, PhCH₂CMe₂; R² = R⁵ = Et, R¹ = t-Bu; R¹ = R² = R⁵ = i-Pr; R² = R⁵ = t-Ru, R¹ = Me, Et, PhCH₂-, were obtained using titanium tetrachloride as a Lewis acid catalyst and as a water scavenger. Infrared spectra of the reacting mixtures provided evidence pertinent to the mechanism of the reaction, which is discussed.     

Alkali Metal- and Ammonium Picrate Extraction and Complex Forming Capabilities of d-Glucose and d-Mannose-based Lariat Ethers

The alkali metal- and ammonium picrate extracting ability of d-glucose- and d-mannose-based 15-crown-5 ethers and related lariat ethers was investigated in dichloromethane – water system. A heteroatom was waried in the crown ether containing a 4,6-O-benzylidene-α-d-glucopyranoside unit 6, (X=O), 2 (X=S) and 8a (X=NH). Extracting ability of the latter species (8a) was excellent (97–99%) in regard of all cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and NH₄⁺) examined, it was not, howewer, selective. Introduction of a side arm on the nitrogen atom of 8a decreased the extracting ability, but increased the selectivity. In this series of compounds (8b–f, 4), 4 with a pyridylethyl substituent allowed the extraction of sodium picrate in 72%. The glucose-based macrocycles 8a, 8e and 8f formed a stronger complex with the cations examined than the mannose-based analogues 9a, 9e and 9f, that can be explained by the all-gauche conformation of the former ones. It was pointed out that in the case of crowns with tertiary amine moieties, the basicity increases the quantity of the picrates extracted. According to complex forming measurements by FAB-MS, the best sodium ion selectivity was achieved by the γ-hydroxypropyl substituted lariat ether (8e). Possible structures of the complexes formed by the two types of monosacharides with sodium cation were evaluated by molecule modelling calculations.     

On-line delta(18)O measurement of organic and inorganic substances

A method for the automated sample conversion and on-line oxygen isotope ratio (delta(18)O) determination for organic and inorganic substances is presented. The samples are pyrolytically decomposed at 1400 degrees C in the presence of nickelized graphite. With the system presented organic as well as inorganic samples such as nitrates, sulphates and phosphates of 50-100 &mgr;g O can be analyzed for their delta(18)O values with a standard deviation usually better than 0.5 per thousand. Additionally, carbon isotope ratios of organic substances and nitrogen isotope ratios of inorganic nitrogenous compounds are available in the same sample run. Data for international and some inter-laboratory reference materials are presented to show the accuracy and reliability of the method. The effect of some additives on the CO yield was checked for substances which do not pyrolyze completely. Copyright 1999 John Wiley & Sons, Ltd.     

Vapor pressure and sublimation rate of molecular crystals: role of internal degrees of freedom

It is a common practice to approximate the desorption rate of atoms from crystal surfaces with an expression of the form nueff exp(-DeltaE/kBT), where DeltaE is an activation barrier to desorb and nueff is an effective vibrational frequency approximately 10(12) s(-1). For molecular solids, however, such an approximation can lead to a many orders of magnitude underestimation of vapor pressure and sublimation rates due to neglected contributions from molecular internal degrees of freedom. Here, we develop a simple working formula that yields good estimates for a general molecular (or atomic) solid and illustrate the approach by computing equilibrium vapor pressure of three different molecular solids and an atomic solid, as well as the desorption rate of a foreign (inhibitor) molecule from the surface of a molecular solid.     

Application of global kinetic models to HMX beta-delta transition and cookoff processes

The reduction of the number of reactions in kinetic models for both the HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) beta-delta phase transition and thermal cookoff provides an attractive alternative to traditional multi-stage kinetic models due to reduced calibration effort requirements. In this study, we use the LLNL code ALE3D to provide calibrated kinetic parameters for a two-reaction bidirectional beta-delta HMX phase transition model based on Sandia instrumented thermal ignition (SITI) and scaled thermal explosion (STEX) temperature history curves, and a Prout-Tompkins cookoff model based on one-dimensional time to explosion (ODTX) data. Results show that the two-reaction bidirectional beta-delta transition model presented here agrees as well with STEX and SITI temperature history curves as a reversible four-reaction Arrhenius model yet requires an order of magnitude less computational effort. In addition, a single-reaction Prout-Tompkins model calibrated to ODTX data provides better agreement with ODTX data than a traditional multistep Arrhenius model and can contain up to 90% fewer chemistry-limited time steps for low-temperature ODTX simulations. Manual calibration methods for the Prout-Tompkins kinetics provide much better agreement with ODTX experimental data than parameters derived from differential scanning calorimetry (DSC) measurements at atmospheric pressure. The predicted surface temperature at explosion for STEX cookoff simulations is a weak function of the cookoff model used, and a reduction of up to 15% of chemistry-limited time steps can be achieved by neglecting the beta-delta transition for this type of simulation. Finally, the inclusion of the beta-delta transition model in the overall kinetics model can affect the predicted time to explosion by 1% for the traditional multistep Arrhenius approach, and up to 11% using a Prout-Tompkins cookoff model.     

The proton momentum distribution in strongly H-bonded phases of water: a critical test of electrostatic models

Water is often viewed as a collection of monomers interacting electrostatically with each other. We compare the water proton momentum distributions from recent neutron scattering data with those calculated from two electronic structure-based models. We find that below 500 K these electrostatic models, one based on a multipole expansion, which includes the polarizability of the monomers, are not able to even qualitatively account for the sizable vibrational zero-point contribution to the enthalpy of vaporization. This discrepancy is evidence that the change in the proton well upon solvation cannot be entirely explained by electrostatic effects alone, but requires correlations of the electronic states on the molecules involved in the hydrogen bonds to produce the observed softening of the well.     

Model selection bias and Freedman’s paradox

In situations where limited knowledge of a system exists and the ratio of data points to variables is small, variable selection methods can often be misleading. Freedman (Am Stat 37:152–155, 1983) demonstrated how common it is to select completely unrelated variables as highly “significant” when the number of data points is similar in magnitude to the number of variables. A new type of model averaging estimator based on model selection with Akaike’s AIC is used with linear regression to investigate the problems of likely inclusion of spurious effects and model selection bias, the bias introduced while using the data to select a single seemingly “best” model from a (often large) set of models employing many predictor variables. The new model averaging estimator helps reduce these problems and provides confidence interval coverage at the nominal level while traditional stepwise selection has poor inferential properties.     

Continuum radiation from the mercury arc

Independent, quantitative measurements of continuum radiation from mercury arcs have been made at our separate laboratories and the Abel-inverted emission coefficients analyzed together. Continuum at selected wavelengths (free of atomic lines) between 0.4 and 1.3 μm were included at mercury pressures of 0.1, 1.0 and 2.7 atm. Equilibrium vapor compositions were calculated for the measured radial temperature profiles. The temperature dependence of the continuum emission was used to separate it into components from electron-Hg⁺ recombination, electron-neutral Hg⁰ bremsstrahlung and Hg₂ molecular radiation. Electron-neutral bremsstrahlung is particularly strong because of the large electron scattering cross section of mercury and low fractional ionization making quantitative comparison with theory possible. Our results for this interaction are in good agreement with recent theoretical calculations of S. Geltman at 1.0 μm but ≈3 times the theoretical value at 0.5 μm.