A new perspective on transition states: χ1 separatrix

We present a new definition of the transition state for chemical reactions, named the χ(1) separatrix. In contrast to previous transition state definitions which depend on the choice of reaction coordinates, the χ(1) separatrix is defined by choosing a time scale for observation and is connected to exact rate constants in the high friction limit. We demonstrate that this separatrix appears in the isomerization of alanine dipeptide as a stationary population in quasi-equilibrium, without assuming a particular coordinate system or reactant and product surfaces.     

A novel double-peaked spin-glass susceptibility-temperature response in the ternary alloy Fe69Mn26Cr5

We have studied the low-field (B ≦ 10^?2 T) d.c. susceptibility χ of the austenitic stainless-steel alloy Fe₆₉Mn₂₆Cr₅ as a function of the magnetic field B and temperature T. χ(T) shows structure, strong B dependence, and typical irreversible effects. The range of temperatures studied comprises three distinct regions. In the high-temperature region (300 K ≦ T ≦ 380 K) a blunt peak in the susceptibility is noticed at T₂ = 340 K. T₂ was not sensitive to thermal cycling. χ(T) displayed a sharp cusp at T₁ = 200 K. This peak was sensitive to the thermal history of the sample and was strongly suppressed by B. Between T₁ and T₂ a shallow valley with some hysteresis was observed. We interpret this behavior to be due to a low-temperature pure spin-glass phase, a high temperature conventional paramagnetic phase, and coexisting antiferromagnetic and spin-glass phases between T₁ and T₂.     

Numerical and experimental studies of long-range magnetic dipolar interactions

We describe several numerical methods developed to analyze the behavior of spin polarized liquids in the presence of long-range magnetic dipolar interactions and external field gradients. Two of the methods use a discrete lattice of spins. In the first we calculate the magnetic field from the lattice of spins directly, either in the rotating frame, or in the lab frame. In the second method we include the dipolar fields from linear magnetization gradients analytically and calculate the dipolar fields from higher order gradients in Fourier space, where they are a local function of the magnetization. In the third method the magnetization is expanded in a Taylor series and the dipolar fields are calculated analytically for each term. The results of these calculations are compared to experimental data, in which we use two superconducting quantum interference device magnetometers adjacent to a spherical sample of hyperpolarized liquid 129Xe to detect the evolution of magnetization gradients. In particular, we observe an increase by a factor of 100 of the spin dephasing time in a longitudinal magnetic field gradient due to dipolar interactions of the spins. While each of the numerical techniques has certain limitations, they are generally in agreement with each other and with experimental data.     

Zero-field NMR enhanced by parahydrogen in reversible exchange

We have recently demonstrated that sensitive and chemically specific NMR spectra can be recorded in the absence of a magnetic field using hydrogenative parahydrogen induced polarization (PHIP) (1-3) and detection with an optical atomic magnetometer. Here, we show that non-hydrogenative parahydrogen-induced polarization (4-6) (NH-PHIP) can also dramatically enhance the sensitivity of zero-field NMR. We demonstrate the detection of pyridine, at concentrations as low as 6 mM in a sample volume of 250 μL, with sufficient sensitivity to resolve all identifying spectral features, as supported by numerical simulations. Because the NH-PHIP mechanism is nonreactive, operates in situ, and eliminates the need for a prepolarizing magnet, its combination with optical atomic magnetometry will greatly broaden the analytical capabilities of zero-field and low-field NMR.