Utility of a tuneless plug and play transmission line probe

In this brief communication we show several examples demonstrating the usefulness of non-resonant transmission line probes. Although rarely seen in the literature and typically limited to zero external field experiments on ferromagnetic materials we have found that this type of probe is perfectly capable of a variety of other nuclear magnetic resonance experiments including cross-polarization.

Suppression of probe background signals via B(1) field inhomogeneity

A new approach combining a long pulse with the DEPTH sequence (Cory and Ritchey, Journal of Magnetic Resonance, 1988) greatly improves the efficiency for suppressing probe background signals arising from spinning modules. By applying a long initial excitation pulse in the DEPTH sequence, instead of a π/2 pulse, the inhomogeneous B(1) fields outside the coil can dephase the background coherence in the nutation frame. The initial long pulse and the following two consecutive EXORCYCLE π pulses function complementarily and prove most effective in removing background signals from both strong and weak B₁ fields. Experimentally, the length of the long pulse can be optimized around odd multiples of the π/2 pulse, depending on the individual probe design, to preserve signals inside the coil while minimizing those from probe hardware. This method extends the applicability of the DEPTH sequence to probes with small differences in B₁ field strength between the inside and outside of the coil, and can readily combine with well-developed double resonance experiments for quantitative measurement. In general, spin systems with weak internal interactions are required to attain efficient and uniform excitation for powder samples, and the principles to determine the applicability are discussed qualitatively in terms of the relative strength of spin interactions, r.f. power and spinning rate.

A 29Si-NMR Investigation of Amorphous Hydrogenated Silicon Nitride

We present the results of a spectroscopic study of amorphous hydrogenated silicon nitride. The nitride was prepared by plasma enhanced chemical vapor deposition and was studied with infrared absorption (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopies. This nitride film shows a particularly interesting distribution of hydrogen: hydrogen atoms are nearly exclusively bound to nitrogen.

Identification of Chemical Growth Mechanisms in Amorphous Semiconductors

Experimental and computational results for three classes of experiments are reported. NMR measurements of phosphorus in amorphous hydrogenated silicon indicate that bonding rearrangements in bulk films are possible. Monte Carlo simulations of film growth demonstrate that surface diffusion is the dominant physical phenomenon responsible for the bulk hydrogen content of a-Si:H films. Finally, the gas phase chemistry of silane, as elucidated by our new “diesel” reactor, is considerably more complicated than many authors assume in models for thin films grown from silane.

Hydrogen Microstructure in Amorphous Semiconductors

The effects of deposition parameters on the H microstructure of plasma deposited amorphous silicon (a-Si:H) and amorphous silicon carbide (a-SiC:H) are measured via multiple quantum nuclear magnetic resonance (MQ NMR). These studies indicate clusters of 5 to 7H atoms exist in a-Si:H films prepared at temperatures ranging from 113 to 324°C. In the range from 270 to 324°C, only these small clusters exist, but lower temperature films also contain larger clusters. Annealing studies indicate H rearranges in a-Si:H prior to evolution. Deposition temperature and annealing temperature have similar effects on H concentration in a-Si:H, but deposition temperature control the density and microstructure of the film. The addition of dopant atoms also affects the H microstructure, with phosphorous causing larger H clusters to form, and boron reducing clustering in a-Si:H films. This perturbation of the film's microstructure suggests that the effects of dopant addition are more complex in amorphous than in crystalline semiconductors. The concentration of carbon atoms also effects H microstructure of a-SiC:H in a complex way. We surmise that H microstructure, rather than H content, determines amorphous semiconductors properties.

Carbon Local Bonding Configurations in Amorphous Hydrogenated Silicon-Carbon Alloys

We present the results of a carbon-13 nuclear magnetic resonance (NMR) study of well-characterized thin films of amorphous hydrogenated silicon carbide. The NMR data detail the distribution of carbon local bonding configurations in films which have carbon-to-silicon ratios less than one. In particular, we show data which clearly identify and quantify non-hydrogenated sp2, or unsaturated, carbon bonding environments.

Nuclear hyperpolarization in solids and the prospects for nuclear spintronics

Nuclear hyperpolarization can be achieved in a number of ways. This article focuses on the use of coupling of nuclei to (nearly) pure quantum states, with particular emphasis on those states obtained by optical excitation in bulk semiconductors. I seek an answer to this question: "What is to prevent the design and analysis of nuclear spintronics devices that use the extremely long-lived hyperpolarized nuclear spin states, and their weak couplings to each other, to affect computation, memory, or informational technology schemes?" The answer, I argue, is in part because there remains a lack of fundamental understanding of how to generate and control nuclear polarization with schemes other than with rf coils.

Active-site motions and polarity enhance catalytic turnover of hydrated subtilisin dissolved in organic solvents

The enzyme subtilisin Carlsberg was surfactant-solubilized into two organic solvents, isooctane and tetrahydrofuran, and hydrated through stepwise changes in the thermodynamic water activity, a(w). The apparent turnover number k(cat)(app) in these systems ranged from 0.2 to 80 s(-1) and increased 11-fold in isooctane and up to 50-fold in tetrahydrofuran with increasing a(w). (19)F NMR relaxation experiments employing an active-site inhibitor were used to assess the dependence of active-site motions on a(w). The rates of NMR-derived fast (k > 10(7) s(-1)) and slow (k < 10(4) s(-1)) active-site motions increased in both solvents upon hydration, but only the slow motions correlated with k(cat). The (19)F chemical shift was a sensitive probe of the local electronic environment and provided an empirical measure of the active-site dielectric constant epsilon(as), which increased with hydration to epsilon(as) approximately 13 in each solvent. In both solvents, the transition state free energy data and epsilon(as) followed Kirkwood's model for the continuum solvation of a dipole, indicating that water also enhanced catalysis by altering the active-site's electronic environment and increasing its polarity to better stabilize the transition state. These results reveal that favorable dynamic and electrostatic effects both contribute to accelerated catalysis by solubilized subtilisin Carlsberg upon hydration in organic solvents.

Water dynamics and salt-activation of enzymes in organic media: mechanistic implications revealed by NMR spectroscopy

Deuterium spin relaxation was used to examine the motion of enzyme-bound water on subtilisin Carlsberg co-lyophilized with inorganic salts for activation in different organic solvents. Spectral editing was used to ensure that the relaxation times were associated with relatively mobile deuterons, which were contributed almost entirely by D(2)O rather than hydrogen-deuteron exchange on the protein. The results indicate that the timescale of motion for residual water molecules on the biocatalyst, (tau(c))(D(2)O), in hexane decreased from 65 ns (salt-free) to 0.58 ns (98% CsF) as (k(cat)/K(M))(app) of the biocatalyst preparation increased from 0.092 s(-1) x M(-1) (salt-free) to 1,140 s(-1) x M(-1) (98% CsF). A similar effect was apparent in acetone; the timescale decreased from 24 ns (salt-free) to 2.87 ns (98% KF), with a corresponding increase in (k(cat)/K(M))(app) of 0.140 s(-1) x M(-1) (salt-free) to 12.8 s(-1) x M(-1) (98% KF). Although a global correlation between water mobility and enzyme activity was not evident, linear correlations between ln[(k(cat)/K(M))(app)] and (tau(c))(D(2)O) were obtained for salt-activated enzyme preparations in both hexane and acetone. Furthermore, a direct correlation was evident between (k(cat)/K(M))(app) and the total amount of mobile water per mass of enzyme. These results suggest that increases in enzyme-bound water mobility mediated by the presence of salt act as a molecular lubricant and enhance enzyme flexibility in a manner functionally similar to temperature. Greater flexibility may permit a larger degree of local transition-state mobility, reflected by a more positive entropy of activation, for the salt-activated enzyme compared with the salt-free enzyme. This increased mobility may contribute to the dramatic increases in biocatalyst activity.

An effective stochastic excitation strategy for finding elusive NMR signals from solids

The versatility of using a stochastic pulse sequence to elucidate peaks with a wide range of shifts, peak widths, and T(1) relaxation times is demonstrated. A stochastic sequence is combined with high speed magic angle spinning (MAS) to obtain the broad and largely shifted peak associated with (31)P in LiNiPO(4). A stochastic sequence is also used to obtain a spectrum of 85% H(3)PO(4), which has a much longer T(1) value. The signal-to-noise was comparable for spectra of 85% H(3)PO(4) obtained with either a stochastic sequence or an optimized Ernst angle experiment. Experimental parameters for the stochastic experiment are set depending only on the ringdown of the probe and not on any inherent qualities of the sample. A stochastic sequence, therefore, combined with MAS provides a useful strategy for finding peaks with unknown T(1) relaxation constants, peak widths, and shifts.