Sample-specific diamagnetic and paramagnetic passive shimming

When homogenizing the static magnetic field over extended in vivo volumes, significant residual inhomogeneity can remain after spherical harmonic shim optimization. This is due to the low spatial orders of shims available on in vivo MR systems and the presence of higher-order inhomogeneity in the vicinity of anatomic air cavities. Mediation of this problem through the development of higher-order spherical harmonic shims is severely impeded by bore space limitations. Sample-specific passive shims are not limited to low-order spatial compensation and offer an alternative means to increased homogenization. Here, we present a novel construction protocol for sample-specific passive shims comprised of both diamagnetic (bismuth) and paramagnetic (zirconium) materials. A prototype shim is constructed and shown to significantly homogenize the mouse brain at 9.4 T. Further homogenization capabilities are simulated through alteration of the shim construction.     

Spectral Changes Due to Performance Environment in Singers, Nonsingers, and Actors

From postrecording interviews of professional singers, it was hypothesized that recording environments, i.e., sound-treated environment versus an auditorium, may induce different vocal behaviors. To test this hypothesis, three groups consisting of nonsingers, singers, and actors were recorded in two different recording environments: a sound-treated booth (IAC) and an auditorium (AUD). Three recordings were obtained from each participant: recording one (IAC) and two (AUD1) required the participants to read in a normal voice; recording three (AUD2) required participants to pretend that they were "performing" before a full house. Results indicated that only the singers and the actors exhibited significant spectral and/or frequency/duration differences from one recording environment to another, with the most dramatic differences exhibited by the singers. It was concluded that the environment in which we record experimental samples from professional voice users, especially singers, should be considered as a variable that can affect results.     

Study on low-cost calibration-free pH sensing with disposable optical sensors

As labor costs become more expensive, less labor-intensive disposable devices have become more ubiquitous. Similarly, the disposable optical pH sensor developed in our lab could provide a convenient yet cost-effective way for pH sensing in processes that require stringent pH control. This optical pH sensor is prepared in uniform individual lots of 100–200 sensors per lot. Calibration is accomplished on a few randomly selected sensors out of each lot. We show that all others in the same lot can then be used directly without requiring individual calibration. In this paper, a calibration model is derived to include all the factors that affect the signal of the disposable sensor. Experimental results show that the derived calibration model fits the experimental data. The readings of 28 randomly selected disposable sensors with 4 sensors from each of the 7 lots show an error less than 0.1 pH units in the useful sensing range of the sensor. The calibration model indicates that if further improvement on precision is desired, more uniform porous material and more advanced coating techniques will be required. When it comes to the effects of the varying coasters, house-made low-cost fluorometers, the variability in the brightness ratio of the blue-to-violet LEDs is the primary reason for the lack of precision. Other factors like LED light intensity distribution, optical properties of the filters and electronics also contribute to the coaster-to-coaster difference, but to a lesser extent. Two different methods for correcting the instrument variations were introduced. After correction, the collective reading errors for all the tested instruments were reduced to less than 0.2 pH units within the sensor's useful sensing range. Based on this result, our lab is currently implementing further improvements in modifying the coasters to equalize the ratios of blue-to-violet LED brightness.     

Evidence of enhanced conjugation in ortho-arylene ethynylenes with transition metal coordination

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.     

Using Kinetic Modeling and Experimental Data to Evaluate Mechanisms in PET-RAFT

Reversible deactivation radical polymerization (RDRP) techniques have become important tools for polymer chemists because they control the structure and are tolerant to functionality. Photoinduced polymerizations have seen a growing interest due to their mild conditions, as well as spatial and temporal control over the polymerization. Among these techniques, photoinduced electron/energy transfer reversible addition–fragmentation chain transfer polymerization (PET-RAFT) is one of the most widely investigated. While PET-RAFT is seen as an increasingly useful tool, there is still much to understand about the mechanism of this process. In particular, there are ongoing questions regarding the kinetic contribution of electron versus energy transfer. In order to better understand the mechanism, this work aims to use kinetic modeling along with experimental data to help determine the likelihood of the proposed mechanisms for the PET-RAFT process using the trithiocarbonate-mediated polymerization of methyl acrylate with fac-tris[2-phenylpyridinato-C²,N]iridium(III) as a photocatalyst. Simulation data show that electron transfer without a corresponding reduction pathway cannot explain the experimental kinetics, while energy transfer offers a good fit to experimental data. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 139–144     

Probing the anomeric effect. The diaminomethylene group: Calculations of N?C?N-containing molecular systems1

A study of systems containing the title moiety is described, with special reference to the anomeric effect. We have calculated ab initio, using Gaussian-80 with the 3-21G basis set, all basic conformations of methylene-diamine (H₂N? CH₂? NH₂) and its N-methyl derivative with full geometry optimization of energy minima and barriers. The structural data thus obtained, were then employed to parameterize Allinger's MM2-80 force field in a procedure similar to that described for oxygen derivatives, including hydrogen-bonding effects and C? N bond shortening in tertiary amines. This modification, termed MM2-AE was then used to calculate larger molecules, including N,N′-di- and N,N,N′,N′-tetramethyl-methylenediamine, various 1,3-diazane systems, and 1,4,5,8-tetraazadecalin derivatives of established (x-ray) structures. The results are discussed in light of their verificative and predictive power and appear to validate MM2-AE as a useful computational procedure.