Friday, August 24, 2007

Joel's Journal Update

13C-13C Chemical Shift Correlation in Rotating Solids without 1H Decoupling During Mixing.
C. Herbst, K. Riedel, J. Leppert, O. Ohlenschlager, M. Gorlach,R. Ramachandran.
Adiabatic RF pulse schemes using inversion pulses with low power factor are difficult to implement but are shown to be feasible and advantageous for generating 13C-13C MAS solid-state NMR chemical shift correlation spectra via longitudinal magnetisation exchange without 1H decoupling during mixing.

Quenching and Recoupling of Echo Modulations in NMR Spectroscopy.
K. Gopalakrishnan, N. Aeby, G. Bodenhausen
Trains of spin echoes are normally modulated by homonuclear scalar couplings. It has long been known that echo modulations are quenched when the pulse-repetition rates are much larger than the offsets of the coupling partners, because the spin systems behave as if they consisted of magnetically equivalent spins when the offsets are suppressed. This type of quenching of the echo modulations can occur when the radio-frequency (RF) pulses are ideal, that is, when they are perfectly homogeneous, properly calibrated to induce rotations through an angle, p, and have an RF amplitude, w1=-gB1, that is strong compared to the largest offset, WS=w0S-wRF, with respect to the carrier frequency. Recently, it was discovered that echo modulations can also be quenched when the RF pulses are nonideal, that is, when they are too weak to bring about an ideal rotation of the magnetization of the coupling partners, so that the effective fields associated with the RF pulses are tilted in the rotating frame. This phenomenon typically occurs when the pulse-repetition rates are much slower than the offset of the coupling partner. Under such conditions, it turns out, however, that for certain offsets, when the phase, fS (which arises from a free precession of the magnetization of the coupling partner, S, in the pulse interval, 2t, and the pulse length, tp), approaches a multiple of 2p, the echo modulations are restored. However, the frequencies of these echo modulations are not simply determined by the homonuclear scalar coupling, JIS. The Fourier transforms of the echo trains (the so-called J spectra) reveal surprising multiplet patterns, and the amplitudes of the echo modulations depend on the offsets of the coupling partners. Herein, we present a unified theory, based on an average-Hamiltonian approach, to describe these effects for two-spin systems. Experimental evidence of echo modulations in a system of two spins is presented. Experiments with three and more spins, backed up by extensive numerical simulations, will be presented elsewhere.

NMR Studies of Double Proton Transfer in Hydrogen bonded cyclic N,N'-diarylformamidine Dimers: Conformational Control, Kinetic HH/HD/DD Isotope Effects and Tunneling.
J.M Lopez, F. Mannle, I. Wawer, G. Buntkowsky, and H.H. Limbach.
Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline 15N,15N-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of 15N NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal 15N and 2H relaxation measurements. 15N CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline 15N,15N-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and 15N,15N-di-(4-fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell–Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.

Selectivity of guest-host interactions in self-assembled hydrogen-bonded nanostructures observed by NMR.
G. Brunklaus, A. Koch, D. Sebastiani, H.W. Spiess
We studied the incorporation of various small guest molecules into calix[4]hydroquinone nanotubes and nanoclusters using solid-state proton NMR spectroscopy in combination with quantum chemical calculations. While the molecules exhibit different types of hydrogen bonding and van der Waals interactions, they show different affinities to the nanotube host structures. As the guest molecules are located inside the complexes, they experience a shift in the NMR resonance line caused by screening effects from the aromatic electrons of the host superstructure. The abilities to fill the otherwise empty space within the tubes can hence be measured indirectly by the displacement of the NMR lines relative to the free molecules. In this way, we can probe which guest molecules are recognized by the calix[4]hydroquinones as suitable for filling their nanoporous superstructures. Selective guest–host interactions have been successfully achieved for the three component mixture of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes.

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