13C NMR Study of the Acid-Catalyzed Carbonylation of Methyl tert-Butyl Ether (MTBE)

Methyl tert-butyl ether (MTBE) is a widely used additive in oxygenated gasoline that has recently been identified as a potential health threat to the drinking water supply due to leaking underground storage tanks. One alternate use for MTBE is the production of methyl 2,2-dimethylpropanoate (methyl pivalate) via Koch carbonylation chemistry. BF₃/H₂O catalysts are employed in industrial applications of Koch chemistry, but cannot be used for direct ester production because the presence of water in the system leads to the formation of carboxylic acids and lowers the selectivity to esters. Therefore, a BF₃/CH₃OH complex was investigated for the carbonylation of MTBE to avoid this loss in selectivity. This study used ¹³C NMR spectroscopy and ab initio calculations to investigate this carbonylation reaction. NMR results and ab initio calculations suggest a structure for the BF₃/CH₃OH acid which is in agreement with previous studies, and a Hammett acidity value of -4.2 was calculated for BF₃-2.19CH₃OH using the method. It is believed that these are the first reported ab initio calculations on the BF₃/CH₃OH system. NMR results also show that MTBE begins to react between 50 °C and 75 °C to produce oligomers of isobutene when no CO is present and carbonylated species when CO is present.     

From composites to factors: Bridging the gap between PLS and covariance‐based structural equation modelling

Partial least squares (PLS) methods possess desirable characteristics that have led to their extensive use in the field of information systems, as well as many other fields, for path analyses with latent variables. Such variables are typically conceptualized as factors in structural equation modelling (SEM). In spite of their desirable characteristics, PLS methods suffer from a fundamental problem: Unlike covariance‐based SEM, they do not deal with factors, but with composites, and as such do not fully account for measurement error. This leads to biased parameters, even as sample sizes grow to infinity. Anchored on a new conceptual foundation, we discuss a method that builds on the consistent PLS technique and that estimates factors, fully accounting for measurement error. We provide evidence that this new method shares the property of statistical consistency with covariance‐based SEM but, like classic PLS methods, has greater statistical power. Moreover, our method provides correlation‐preserving estimates of the factors, which can be used in a variety of other tests. For readers interested in trying it, the new method is implemented in the software WarpPLS. Our detailed discussion should facilitate the implementation of the method in any numeric computing environment, including open source environments such as R and GNU Octave.     

Computation of through-space NMR shielding effects by aromatic ring-cation complexes: substantial synergistic effect of complexation

The HF-GIAO method in Gaussian 03 was employed to calculate the NMR isotropic shielding values of a diatomic hydrogen probe and to predict the through-space proton NMR shielding increment surfaces above benzene complexed with ammonium, lithium, sodium, potassium, magnesium or calcium ion. The sum of the calculated isotropic shielding values for the proximal hydrogen of a diatomic hydrogen probe over benzene and those calculated at appropriate positions relative to cations were subtracted from the isotropic shielding values calculated for the complexes. The result is a shielding increment for complexation. Complexation results in a synergistic effect on NMR shielding. Enhanced shielding was observed over the π electron cloud of benzene upon complexation with the cations, more than the sum of the separate effects of the aromatic ring and the charge. The results are interpreted in terms of polarization of the π cloud of benzene by the cation and its consequences.     

Team adaptation to electronic communication media: evidence of compensatory adaptation in new product development teams

Prior research on the use of electronic communication media by teams performing complex tasks has led to contradictory findings. Much research has suggested that electronic communication media, due to not incorporating important elements found in face-to-face communication, pose obstacles for communication in comparison with the face-to-face medium. On the other hand, research has also suggested that teams interacting primarily electronically could perform quite well, sometimes even better, than face-to-face teams. A new theoretical framework, which builds on the notion of compensatory adaptation, has recently been advanced to explain these contradictory findings, arguing that (a) electronic communication media do pose obstacles to communication, and (b) individuals working in teams often compensate for obstacles posed by electronic communication media, which sometimes leads to team outcomes that are just as good or even better than those achieved by similar groups interacting primarily face-to-face. This study tests compensatory adaptation theory through a survey of 462 new product development teams, and finds general support for the theory. Important implications for research and practice are also discussed.     

Performance of a buried radioactive high level waste (HLW) glass after 24 years

A radioactive high level waste glass was made in 1980 with Savannah River Site (SRS) Tank 15 waste. This glass was buried in a lysimeter in the SRS burial ground for 24 years. Lysimeter leachate data was available for the first 8 years. The glass was exhumed in 2004. The glass was predicted to be very durable and laboratory tests confirmed this. Scanning electron microscopy of the glass burial surface showed no significant glass alteration consistent with results of other laboratory and field tests. Radionuclide profiling for alpha, beta, and ¹³⁷Cs indicated that Pu was not enriched in the soil while ¹³⁷Cs and ⁹⁰Sr were enriched in the first few centimeters surrounding the glass. Lysimeter leachate data indicated that ⁹⁰Sr and ¹³⁷Cs leaching from the glass was diffusion controlled.     

Estimation of Nusselt Number in Microchannels of Arbitrary Cross Section with Constant Axial Heat Flux

In many practical instances such as basic design, parametric study, and optimization analysis of thermal systems, it is often very convenient to have closed form relations to obtain the trends and a reasonable estimate of the Nusselt number. However, finding exact solutions for many practical singly connected cross sections, such as trapezoidal microchannels, is complex. In the present study, the square root of cross-sectional area is proposed as the characteristic length scale for Nusselt number. Using analytical solutions of rectangular, elliptical, and triangular ducts, a compact model for estimation of Nusselt number of fully developed, laminar flow in microchannels of arbitrary cross sections with “H1” boundary condition (constant axial wall heat flux with constant peripheral wall temperature) is developed. The proposed model is only a function of geometrical parameters of the cross section, i.e., area, perimeter, and polar moment of inertia. The present model is verified against analytical and numerical solutions for a wide variety of cross sections with a maximum difference on the order of 9%.     

Parameters affecting the resolution and accuracy of 2-D quantitative shear wave images

Time-of-flight methods allow quantitative measurement of shear wave speed (SWS) from ultrasonically tracked displacements following impulsive acoustic radiation force excitation in tissue. In heterogeneous materials, reflections at boundaries can distort the wave shape and confound determination of the wave arrival time. The magnitude of these effects depends on the shear wavelength of the excitation, the kernel size used to calculate the SWS, and the method used to determine the wave arrival time. In this study, we perform a parametric analysis of these factors using finite element modeling of the tissue response and simulated ultrasonic tracking. Two geometries are used, a stiff vertical layer and a stiff spherical inclusion, each in a uniform background. Wave arrival times are estimated using the peak displacement, peak slope of the leading edge, and cross-correlation methods. Results are evaluated in terms of reconstruction accuracy, resolution, contrast, and contrast-to-noise ratio of reconstructed SWS images. Superior results are obtained using narrower excitation widths and arrival time estimators which identify the leading edge of the propagating wave. The optimal kernel size is determined by a tradeoff between improved accuracy for larger kernels at the expense of spatial resolution.     

Unexpected visible‐light‐induced free radical photopolymerization at low light intensity and high viscosity using a titanocene photoinitiator

Most visible‐light photoinitiators are based on electron transfer processes and are comprised of two or more components. These initiators can lose effectiveness in viscous systems because the underlying reactions are diffusion controlled. In this contribution, the visible‐light photoinitiator bis(cyclopentadienyl) bis[2,6‐difluoro‐3‐(1‐pyrryl)phenyl]titanium is characterized for polymerization of viscous systems and low light intensities. This compound absorbs visible light at wavelengths up to 550 nm, and does not rely on diffusion‐controlled electron transfer reactions because it undergoes unimolecular decomposition. In contrast to trends observed for other photoinitiators, the effectiveness of the compound is found to increase markedly with the addition of protonic acids and with increasing system viscosity. For a given concentration of initiator and acid, a remarkably low optimal light intensity for effective polymerization is observed. The origins of these surprising results are discussed in terms of the mechanism of decomposition of the photoinitiator. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013