Oriented sample, lineshape

Fig. 2 Mechanically oriented bilayer samples as a membrane model for ssNMR. (a) Illustration of the hydrated lipid bilayers with MAPs embedded, the glass supports, and the insulating wrapping, (b) A real sample consists of 15 stacked glass slides, (c) Schematic solid-state 19F-NMR lineshapes from an oriented CF3-labelled peptide (red), and the corresponding powder lineshape from a non-oriented sample (grey), (d) Illustration of typical orientational defects in real samples - the sources of powder contribution in the spectra...

In organic radicals in solution, the y-factor anisotropy cannot be detected one needs oriented samples. In crystals of free radicals, this anisotropy is easily measured—for example, in crystals of sodium formate (Na+ HCOO-) the principal-axis components are gxx = 2.0032, gyy = 1.9975, and gzz = 2.0014. If there is some spin-orbit interaction in an organic molecule (e.g., if a compound contains S or Cl), then y-values as high as 2.0080 are encountered. In disordered powders with narrow EPR lineshapes, the y-factor anisotropy can produce considerable distortion in the overall signal, due to averaging of the y-tensor. 

Stark effect measured for the ground state absorption and fluorescence should be the same. When Am dominates changes in polarizability and hyperpolarizability for a transition, then the Stark effect for an isotropic sample has approximately the shape of the second derivative of the absorption or emission band this is observed for the absorption of P [10,11]. The origin of this second derivative lineshape is illustrated in Figs. 3A and B for the simplest case of an oriented sample with equal populations parallel and anti-parallel to the field. It is evident upon inspection of Fig. 2, that there is little, if any, evidence for such a second derivative component in the AF spectrum of the RC. Rather, the AF spectrum is very well described by an enhancement relative to the zero-field fluorescence spectrum (zeroth derivative) and a slight shift to lower energy (approximately 60 cm ). When we first observed this effect [6], we simply analyzed the AF spectrum in terms of a sum of the expected second derivative component and a zeroth derivative component due to the net reduction in the rate of electron transfer which competes with fluorescence. The best fit indicated little if any contribution from the second derivative, i.e., App - 0, a result which made little physical sense and provided no explanation for the slight red-shift. 

Spectra frequently rely upon spectral simulations in which Avq, the linewidth, the lineshape function, and, in the case of overlapping powder spectra, the relative spectral contribution must be fitted. Clearly, this is difficult since only the approximate value of Avq is easily determined. Recently M. Bloom and co-workers (16) have developed a means of calculating an oriented-sample spectrum from the powder spectrum. Since the orientational frequency distribution of a powder spectrum is scaled by (3cos 0-l)/2 as in eq. 6 (for n=0), and if one assumes that the linewidth scales in the same way, one half of the powder spectrum can be defined in terms of one component (one orientation) of the spectrum according to eq. 19, in which F9q(x) is the... 

Fig. 2 Schematic representation of the 13C NMR signal of a single crystal containing the functional group AB, oriented (A) perpendicular to the applied field, and (B) parallel to the applied field. The lineshape in (C) represents the NMR signal of a polycrystalline sample with a random distribution of orientations yielding the chemical shift anisotropy pattern displayed. (From Ref. 15.)...

One way to obtain highly resolved, selective, solid state information plus uniquely defined information on the orientation-dependencies of spin interactions is to perform NMR experiments on oriented single crystals. However, for various reasons, such single crystal NMR experiments appear impractical in standard applications to organometallic chemistry. The vast majority of solid state NMR experiments on organometallic compounds are— and probably will be— performed on polycrystalline powder samples. In principle, also for polycrystalline powders, all relevant information on spin interactions is contained in the shape of the powder patterns obtained imder static conditions. The problem then is the extraction of well-defined single parameters from such lineshapes resulting from either a multitude of resonances and/or the simultaneous presence of multiple spin interactions. In practice, it turns out that only very rarely is this possible. 

If the peptide is not oriented in the membrane but forms large immobilized aggregates, this is easily seen in the NMR spectrum. In this case, all different orientations of the peptide are present in the sample and a very broad lineshape is seen, as for a peptide powder. When this is the case, no orientation of the peptide can be determined. Looking at the NMR spectral lineshape is a useful method for investigating the aggregation behavior of peptides interacting with membranes, which may be intimately related to the function or malfunction of the peptide. 

A polymer sample usually consists of amorphous or polycrystalline regions that are randomly oriented with respect to the magnetic field. The resulting H NMR spectrum then has a characteristic lineshape. shown at the top of Fig. 6.2.1. 

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