Takehiko Terao

13C–1H dipolar-assisted rotational resonance in magic-angle spinning NMR

Abstract A new 13 C – 13 C recoupling mechanism which occurs under magic-angle spinning (MAS) is presented. The mechanism can basically be attributed to rotational resonance (R 2 ), but the conventional R 2 condition is modified by a recoupled 13 C – 1 H dipolar interaction. The 13 C – 1 H recoupling is attained by 1 H rf irradiation fulfilling a rotary-resonance condition. The present method does not have the drawbacks associated with rf irradiation on 13 C and is applicable for band-selective recoupling between carbonyl/aromatic carbons and aliphatic carbons. The 13 C – 13 C recoupling mechanism under 13 C – 1 H recoupling is theoretically explained and is experimentally demonstrated using N -acetyl[1,2- 13 C] dl -valine and uniformly 13 C, 15 N-labeled glycylisoleucine.

13C-1H dipolar-driven 13C-13C recoupling without 13C rf irradiation in nuclear magnetic resonance of rotating solids

Two recently proposed 13C–13C recoupling methods under magic angle spinning (MAS), resonant interference recoupling (RIR), and 13C–1H dipolar-assisted rotational resonance (DARR), are examined on a common theoretical foundation using the average Hamiltonian theory. In both methods, a rf field is applied on not 13C but 1H to recouple the 13C–1H dipolar interactions, and spectral overlap necessary to conserve energy for 13C–13C polarization transfer is achieved by the 13C–1H dipolar line broadening. While DARR employs time-independent 13C–1H interactions recoupled by suitable rf irradiation to 1H spins, RIR uses time-dependent 13C–1H interactions modulated appropriately by 1H rf irradiation. There are two distinct cases where 13C–1H line broadening realizes 13C–13C spectral overlap. For a pair of a carbonyl or aromatic carbon and an aliphatic carbon, spectral overlap can be achieved between one of the spinning sidebands of the former 13C resonance and the 13C–1H dipolar powder pattern of the latter. On the ...

An NMR relaxation study on the proton transfer in the hydrogen bonded carboxylic acid dimers

We have studied the proton spin‐lattice relaxation times (T1) of a series of benzoic acid (BAC) derivatives and decanoic acid (DAC) over a wide range of temperature and analyzed the results in terms of the double proton switching along the hydrogen bonds. The proton T1 in the high temperature region are analyzed using the classical jump model and the barrier heights for the proton transfer are determined. The thermodynamic parameters for the equilibria between the two configurations in the solid state are also determined by the FT–IR measurements. It is shown that the energetics and dynamics of the proton transfer in DAC and the para‐ and meta‐substituted BAC are all similar, but they are very different in the ortho‐substituted ones. It is suggested that the low temperature behavior of the proton T1 of the dimers of carboxylic acid is due to the tunneling and the asymmetry of the potential brings in a small activation energy.

A study on the proton transfer in the benzoic acid dimer by 13C high-resolution solid-state NMR and proton T1 measurements

Abstract By a solid-state 13 C and proton NMR study of the hydrogen bonded dimer of benzoic acid, the rate of proton transfer, the height of the potential barrier for the transfer and the relative population of two different configurations were determined.

Switching-angle sample-spinning NMR spectroscopy for obtaining powder-pattern-resolved 2D spectra: Measurements of 13C chemical-shift anisotropies in powdered 3,4-dimethoxybenzaldehyde

Abstract A general method is proposed which allows us to obtain individual powder patterns of chemically distinct nuclei. As an example, the measurement of 13 C chemical-shift anisotropies in powdered 3,4-dimethoxybenzaldehyde is reported.

Excitation and Detection of Coherence between Forbidden Levels in Three-Level Spin System by Multi-Step Processes

The coherent superposition of the states between which magnetic dipole transition is forbidden has been excited and detected by means of a multi-step application of resonant rf pulses in a three-level NMR system. The experiment was carried out on 27 Al nuclei in Al 2 O 3 where the quantization axes of the quadrupole and the magnetic coupling were parallel. Free decay and echo signals associated with the coherence were observed, and transfer of coherence between nonresonant pairs of levels was studied in detail.

Relayed anisotropy correlation NMR: Determination of dihedral angles in solids

Abstract A two-dimensional solid-state NMR method is proposed for determining the mutual orientation of the two interaction tensors in each of which a different chemical group participates, from which information on the dihedral angle can be extracted. The two-dimensional powder pattern was observed for 1,2- 13 C-labeled DL-alanine as a demonstration, correlating the 13 C 1 chemical shift anisotropy and the 13 C 2  1 H dipolar coupling via polarization transfer. The OC 1 C 2 H dihedral angle as well as the orientation of the 13 C 1 chemical shift tensor were determined from the spectrum.

Dipolar SASS NMR spectroscopy: Separation of heteronuclear dipolar powder patterns in rotating solids

Two‐dimensional sample‐spinning NMR techniques for separately obtaining heteronuclear dipolar powder patterns of chemically distinct nuclei are proposed, which involve switching the spinning axis on and off the magic angle. Procedures for the computation of dipolar spectra obtained by this method are described, and various features of such spectra are discussed using simulated spectra. Practical applications of this method are demonstrated with 13C spectra of calcium formate, β‐quinol‐methanol clathrate, and urea‐trans‐4‐octene clathrate, providing information on the absolute sign and magnitude of the indirect dipolar couplings, 13C–1H distances, and dynamics of trapped molecules in clathrates.

Rotational resonance in the tilted rotating frame

Abstract We propose a new solid-state NMR technique under magic-angle spinning to recouple the dipolar interaction for a particular pair of homonuclear spins. In this method, a weak radio-frequency field is applied during the mixing time of the exchange NMR, and its frequency and intensity are chosen to realize rotational resonance for a particular pair of spins in the tilted rotating frame. The selective recoupling is demonstrated for the dipolar interaction between the CH and CH 3 carbons in triply 13 C-enriched l -alanine, whose chemical shift difference is too small to apply the conventional rotational resonance method.

13C1H dipolar recoupling dynamics in 13C multiple-pulse solid-state NMR

Abstract The recoupling of 13 C 1 H dipolar interactions by applying 13 C pulses under a 1 H decoupling rf field is discussed. It is theoretically and experimentally shown that 13 C spectral line-broadening due to such recoupling in a 13 C π-pulse train can be minimized by setting the rf field intensity for 1 H decoupling to around three times that for 13 C π pulses, or stronger.