Thursday, April 06, 2006

Joel's ChemPhysChem Update

Quenching Echo Modulations in NMR Spectroscopy
Jens Dittmer adn Geoffrey Bodenhausen
ChemPhysChem (2006) 7(4),831.
In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr–Purcell–Meiboom–Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal p rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor 15N nuclei I=ND and S=NA in RNA Watson–Crick base pairs with homonuclear scalar couplings JIS=2hJ(ND, NA) across the hydrogen bonds.

Fingerprints of Damped Quantum Rotation Observed in Solid-State Proton NMR Spectra.
P. Gutsche, H. Schmitt, U. Haberlen, T. Ratajczyk, and S. Symanski.
ChemPhysChem (2006) 7(4),884.
1H NMR spectra of the methyl group in an oriented crystal sample of methylmalonic acidwith all three non-methyl protons replaced by deuterons are interpreted in terms of the damped quantum rotation (DQR) theory of NMR line shapes. The DQR approach offers a perfect theoretical reproduction of the observed spectra while the conventional Alexander-Binsch line-shape model shows evident defects in the present case. The temperature trends of the quantities characterizing the coherent and incoherent dynamics of the methyl group in the DQR approach (the effective tunnelling frequency andtwo coherence-damping rates) derived from the spectra are fairly reproduced using a model reported previously. The present findings provide further evidence of limitations to the validity of the common belief that molecular rate processes in condensed phases are necessarily classical.

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