Modelling carbocationic polymerizations: Kinetics of the reactions of carbocations with alkenes

Reactions of carbocations with olefins and related π-nucleophiles follow second order kinetics, first order with respect to carbocation and first order with respect to olefin. The rate constants are equal for paired and non-paired ions and independent of the nature of the negative counter-ions. Rate constants k < 10⁷-10⁸ L mol⁻¹ s⁻¹ can be calculated by lg k(20 °C) = s (N+E), where E represents the strengths of the electrophiles, while nucleophiles are characterized by the slope parameter s and the nucleophilicity parameter N. These parameters can be used for selecting initiators for carbocationic polymerizations and for designing copolymers.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

Some factors that influence the steady state in steady-state free precession

Steady-state free precession (SSFP) and particularly the spatially periodic magnetization response with wavelength lambda that results in the presence of an applied gradient is discussed. The maximum SSFP magnetization does not always occur at the Larmor frequency but rather depends on both the phase cycling of the rf pulses and on the rf tip angle. The slow flow sensitivity of SSFP also depends on lambda. Experimental data of the flow sensitivity is shown to be parameterized by a dimensionless dephasing parameter phi defined as the ratio of the distance traveled by a spin in one cycle of the SSFP sequence as compared to lambda. An example of the creation of a moving reference frame with the SSFP sequence is given. The modulation in image intensity that results when the gradients are nearly but not exactly compensated and lambda becomes large (i.e., tens of pixels long) is demonstrated. The pixel length must be an integral number of lambda's in order to have a uniform image intensity from a uniform phantom.     

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

We describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in vivo hyperpolarized ³He imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine, including asthma and obesity. The imager uses a bi-planar B₀ coil design that produces an optimized 65 G (6.5 mT) magnetic field for ³He MRI at 210 kHz. Three sets of bi-planar coils produce the x, y, and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RF coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D ³He images with up to 2.8 mm resolution, and present initial 2D and 3D ³He images of human lungs in both supine and upright orientations. ¹H MRI can also be performed for diagnostic and calibration reasons.     

XeNA: An automated ‘open-source’ Xe hyperpolarizer for clinical use

Here we provide a full report on the construction, components, and capabilities of our consortium’s “open-source” large-scale (~ 1 L/h) ¹²⁹Xe hyperpolarizer for clinical, pre-clinical, and materials NMR/MRI (Nikolaou et al., Proc. Natl. Acad. Sci. USA, 110, 14150 (2013)). The ‘hyperpolarizer’ is automated and built mostly of off-the-shelf components; moreover, it is designed to be cost-effective and installed in both research laboratories and clinical settings with materials costing less than $125,000. The device runs in the xenon-rich regime (up to 1800 Torr Xe in 0.5 L) in either stopped-flow or single-batch mode—making cryo-collection of the hyperpolarized gas unnecessary for many applications. In-cell ¹²⁹Xe nuclear spin polarization values of ~ 30%–90% have been measured for Xe loadings of ~ 300–1600 Torr. Typical ¹²⁹Xe polarization build-up and T₁ relaxation time constants were ~ 8.5 min and ~ 1.9 h respectively under our spin-exchange optical pumping conditions; such ratios, combined with near-unity Rb electron spin polarizations enabled by the high resonant laser power (up to ~ 200 W), permit such high P_Xe values to be achieved despite the high in-cell Xe densities. Importantly, most of the polarization is maintained during efficient HP gas transfer to other containers, and ultra-long ¹²⁹Xe relaxation times (up to nearly 6 h) were observed in Tedlar bags following transport to a clinical 3 T scanner for MR spectroscopy and imaging as a prelude to in vivo experiments. The device has received FDA IND approval for a clinical study of chronic obstructive pulmonary disease subjects. The primary focus of this paper is on the technical/engineering development of the polarizer, with the explicit goals of facilitating the adaptation of design features and operative modes into other laboratories, and of spurring the further advancement of HP-gas MR applications in biomedicine.