Charles G. Wade

The Reaction Mechanism for the Organocatalytic Ring-Opening Polymerization of l-Lactide Using a Guanidine-Based Catalyst: Hydrogen-Bonded or Covalently Bound?

We have investigated two alternative mechanisms for the ring-opening polymerization of l-lactide using a guanidine-based catalyst, the first involving acetyl transfer to the catalyst, and the second involving only hydrogen bonding to the catalyst. Using computational chemistry methods, we show that the hydrogen bonding pathway is considerably preferred over the acetyl transfer pathway and that this is consistent with experimental information.

NMR relaxation by the antisymmetric component of the shielding tensor: a longer transverse than longitudinal relaxation time

Abstract The first experimental detection of an antisymmetrical component (σa) in the NMR shielding tensor of a nucleus is reported. The 13C NMR relaxation rates (1/T1 and 1/T2) of the ethylenic carbons in tetrachlorocyclopropene in toluene-d8 at about −90°C are dominated at high fields by shielding tensor modulated by random molecular rotation. We find that T2 is circa 1.1T1 in contrast to that in 13CS2, where σa is zero by symmetry and where the measured T2 is not greater than the maximum (0.857T1) allowed for this case. The present results are in qualitative agreement with previous ab initio IGLO calculations showing that the ethylenic carbons in cyclopropenes have unusually large antisymmetric components in their shielding tensors.

Dynamics of poly(oxyethylene) melts: Comparison of 13C nuclear magnetic resonance spin-lattice relaxation and dielectric relaxation as determined from simulations and experiments

Molecular dynamics simulations of poly(oxyethylene) (POE) melts have been performed using a previously derived quantum-chemistry-based force field. Local chain dynamics for POE melts in the form of 13C nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and NOE values and dielectric relaxation behavior have been determined from molecular dynamics simulations and compared with experimental measurements. 13C NMR T1 and NOE values for the methyl, α, and interior carbons from molecular dynamics simulations are in good agreement with experimental values over a wide range of temperatures. Similar agreement is seen for the dielectric relaxation strength and maximum dielectric loss frequency as a function of temperature. Non-Arrhenius behavior is seen for the correlation times of the P2CH orientational autocorrelation function (OACF), the molecular dipole moment OACF, and the torsional autocorrelation function (ACF). Very close correspondence between the torsional ACF and the molecular dipole mome...

A space-efficient assembly for NMR experiments on anchorage-dependent cells

Abstract A culture assembly is described which can be conveniently used in an NMR spectrometer and which supports anchorage-dependent cells for periods as long as one month. Phosphorus-31 NMR data for chicken embryo fibroblasts obtained with this apparatus are presented.

Power efficient double‐resonance NMR probe for electromagnets

A NMR probe design of high efficiency is presented for a double‐resonance spectrometer. The design uses easily constructed transmission line filters and a bridge box duplexing switch to reduce probe ringing. Circuits are used which allow independent tuning of the 100‐ and 25.1‐MHz circuits in the probe. High strength rf fields are produced in the probe using low power transmitters, and good sensitivity is obtained on carbon.