Decoupling molecular motion

Briefly, it happens that for the case of an anisotropic chemical shift the lowest order average Hamiltonian and all hi er order corrections to it vanish identically. Thus, the linewldths in the spectra, such as Fig. 1, should be limited by Inhomogeneous broadening not removed by the sample rotation and by homogeneous broadening, arising from incomplete decoupling, molecular motion, etc. 

C.-F. Hoelger, E. Rossler, B. Wehrle, F. Aguilar-Parrilla and H.-H. Limbach, Self-decoupling of 15N-14N dipole-quadrupole couplings in 15N CPMAS NMR spectra and molecular motions in crystalline hydrazine sulfate, p-(diethylamino)benzaldehyde, and its solid solution in polycarbonate.. Phys. Chem., 1995, 99,14271-14276. 

In contrast to n-paraffins, which exhibit no or only a slight C NMR CSA, aromatics or hydrocarbons with double or triple bonds show a much larger anisotropy. Therefore, benzene (57) and 2-butyne 14) were chosen as suitable probe molecules to study molecular motions by C NMR lineshape analysis. Theoretical lineshapes for different motional states of benzene and 2-butyne molecules are depicted in Figs. 3 and 4. The proton-decoupled C NMR spectra were recorded by means of the homemade NMR spectrometer UDRIS (University of Leipzig) and a BRUKER MSL 400 (Central Institute of Physical Chemistry, Berlin) at frequencies of 22.6 and 100.6 MHz (9,14,57). 

A key feature of the DSMC technique in comparison to continuum methods is its relatively high computational expense. To allow the decoupling between molecular motion and intermolecular collisions to occur in a physically accurate way, the time-step used in the DSMC technique must be smaller than the mean time between collisions. Similarly, the size of the cells employed in the DSMC computational grid must be of the order of the local mean free path everywhere in the flow domain. These physical restrictions on the size of the numerical parameters results in the time steps and cell sizes employed in DSMC calculations being usually significantly smaller than those employed in continuum computations. For this reason, significant work has been performed in the optimization of the DSMC technique for different types of computer hardware. Examples of specific implementations are described in Refs. 23-26. 

Rotary resonance recoupling of heteronuclear C- H dipolar interactions in NMR spectra (recorded under conditions of H decoupling at frequency vi and MAS at frequency Vr) has been studied for three examples of molecular solids (adamantane, ferrocene and hexamethylbenzene) in which substantial molecular motion is known to occur. It has been shown that when rotary resonance conditions are satisfied (i.e. l Vr = , for n = 1 or 2), the recoupling can lead to motionally averaged Pake-like powder patterns from which information on C- H intemuclear distances and/or molecular motion can be derived. 

The transverse magnetization of the REDOR experiment decays as a function of the decoupling field [23, 24]. Dipolar decoupling may be strongly interfered with by molecular motion, if any, when the motional frequency is of the same order of magnitude as the decoupling field and, hence, the transverse relaxation times (Ta s) are significantly shortened. In... 

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