NMR in the quadrupole regime: Measurement of quadrupole and chemical shift parameters in single-crystal paradibromobenzene

The exact theory for the frequency of transition between the two lowest levels of a spin $\text{I =}\text{3}\text{2}$ nucleus experiencing a large asymmetric electric field gradient, an applied magnetic field, and an anistropic chemical interaction was presented in an earlier paper. Using the assumption that the quadrupolar and chemical shift tensors have the same principal axis system, the Hamiltonian was solved exactly — analytically for the applied field aligned along each of the three axes of the quadrupolar principal axis system, and numerically for arbitrary orientations.This theory is reviewed here and applied to our room-temperature experiments in single-crystal paradibromobenzene. The self-consistent least-squares fit to the field-dependencies and simultaneously the angular dependence (rotational pattern) of the resonance frequency was performed using the literature value for the pure quadrupole frequency $\text{ν}{}_{\mathrm{Q}}\text{(1 +}\text{η}{}^2\text{3}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 223·8}\text{MHz}$. The fit yielded values for the quadrupolar asymmetry η = 0·0461 ± 0·0004 and the chemical shift components σ_x = ?0·001 ± 0·001, σ_v = σ_z = 0·000 ± 0·001. Our value for η is in good agreement with values determined by other methods; it and our shift values are consistent with the information obtained by this method using a powdered specimen.The process of using the NMR signal itself to align the specimen yielded sufficient information for an unambiguous determination of the Euler angles of orientation of the crystal in its mounting within ± 0.6°.     

Intrinsic ferromagnetism in insulating cobalt doped anatase TiO2

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase Co:TiO(2) films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the +2 state substituting for Ti. Measurements at 300 K yield a M(S) of 1.1 mu(B)/Co atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of Co:TiO(2) leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.     

1H detected 1H,15N correlation spectroscopy in rotating solids

We describe new correlation experiments suitable for determining long-range 1H-1H distances in 2H,15N-labeled peptides and proteins. The approach uses perdeuteration together with back substitution of exchangeable protons during sample preparation as a means of attenuating the strong 1H-1H dipolar couplings that broaden 1H magic angle spinning (MAS) spectra of solids. In the approach described here, we retain 100% of the 1H sensitivity by labeling and detecting all exchangeable sites. This is in contrast to homonuclear multiple pulse decoupling sequences that are applied during detection and that compromise sensitivity because of the requirement of sampling between pulses. As a result 1H detection provides a gain in sensitivity of >5 compared to the 15N detected version of the experiment (at a MAS frequency of 13.5kHz). The pulse schemes make use of the favorable dispersion of the amide 15Ns resonances in the protein backbone. The experiments are demonstrated on a sample of the uniformly 2H,15N-labeled dipeptide N-Ac-Val-Leu-OH and are analogous to the solution-state suite of HSQC-NOESY experiments. In this compound the 1H amide linewidths at 750MHz vary from approximately 0.67 ppm at omega(r)/2pi approximately 5kHz to approximately 0.20 ppm at omega(r)/2pi approximately 30kHz, indicating that useful resolution is available in the 1H spectrum via this approach. Since the experiments circumvent the problem of dipolar truncation in the 1H-1H spin system, they should make it possible to measure long-range distances in a uniformly labeled environment. Thus, we expect the experiments to be useful in constraining the global fold of a protein.     

Rotational resonance NMR: separation of dipolar coupling and zero quantum relaxation

The solid state NMR technique of rotational resonance (R2) has been used extensively to measure distances approaching 5-6 A between 13C nuclei in a variety of compounds including amyloidogenic peptides and membrane proteins. The accuracy of the distance information extracted from the time-dependent spin dynamics at R2 is often limited by the accuracy with which the relevant zero-quantum lineshape parameters are estimated. Here we demonstrate that measurement of spinning frequency dependent magnetization exchange dynamics provides data from which both distance and zero-quantum relaxation parameters can be extracted independently. In addition to providing more accurate distance information, this technique allows examination of the zero-quantum lineshape, which can indicate the presence of correlated relaxation or chemical shift distributions between dipolar-coupled sites. With this approach we have separated the contribution of dipolar couplings and zero quantum relaxation to R2 exchange curves. Thus, we have significantly improved the accuracy of the measurement of the intramolecular, internuclear distances between a pair of 13C's in two model compounds (N-acetyl-D,L-valine and glycylglycine.HCl) that lie in the distance range 4.6-4.7 A.     

Why CH3CH3+* formation competes with H* loss from CCCO C3H6O+* isomers

How formation of CH3CH3+* competes with H* loss from C3H6O+* isomers with the CCCO framework has been a puzzle of gas phase ion chemistry because the first reaction has a substantially higher threshold and a supposedly tighter transition state. These together should make CH3CH3+* formation much the slower of the two reactions at all internal energies. However, the rates of the two reactions become comparable at about 20 kJ x mol(-1) above the threshold for CH3CH3+* formation. It was recently shown that losses of atomic fragments increase in rate much more slowly with increasing internal energy than do the rates of competing dissociations to two polyatomic fragments. This occurs because fewer frequencies are substantially lowered in transition states for the former type of reaction than for the latter. The resulting lower transition state sums of states cause the rates of dissociations producing atoms as fragments to increase much more slowly than competing processes with increasing energy. Here we show that this is why CH3CH3+* formation competes with H* loss from CH3CH2CHO+*. These results further establish that the dependence on energy of the rate of a simple unimolecular dissociation is usually directly related to the number of rotational degrees of freedom in the products, a newly recognized factor in determining the dependence of unimolecular reaction rates on internal energy.     

Phase-shifting shearing interferometer

A single-element phase-shifting interferometer has been developed based on the lateral shearing interferometer. This new interferometer requires no precise alignment, and the phase is continuously varied by changes in the voltage across a commercially available liquid-crystal phase retarder.     

1H-1H MAS correlation spectroscopy and distance measurements in a deuterated peptide

In this Communication, we demonstrate the use of deuteration together with back substitution of exchangeable protons as a means of attenuating the strong 1H-1H couplings that broaden 1H magic angle spinning (MAS) spectra of solids. The approach facilitates 15N-1H correlation experiments as well as experiments for the measurement of 1H-1H distances. The distance measurement relies on the excellent resolution in the 1H MAS spectrum and homonuclear double quantum recoupling techniques. The 1H-1H dipolar recoupling can be analyzed in an analytical fashion by fitting the data to a 2- or 3-spin system. The experiments are performed on a sample of the dipeptide N-Ac-Val-Leu-OH, which was synthesized from uniformly [2H, 15N] labeled materials and back-exchanged in H2O.