Line broadening interactions

Is the spectrometer/probe capable of variable temperature operation Dynamic samples need to be measured at low temperatures to freeze out the dynamic processes. Qnadrupolar nnclei are often best observed at high temperatures the increased molecnlar tumbling rates help to average ont the qnadrupolar line-broadening interaction. 

Quadrupole nutation NMR is a technique, introduced by Samoson and Lippmaa (27), where in a two-dimensional way the effect of the quadrupole interaction is separated from other line broadening interactions, simply by allowing the magnetization to evolve during an incremented periode tj, under the influence of a strong radio frequency field. 

Obviously, the magic angle spinning techniques described briefly above rely on coherent averaging out of resonance line-broadening interactions. An alternative approach is to cause solids to assume the incoherent motion of molecules in the liquid phase. Essentially, two such methods exist. The first is the so-called ultra-fine particle NMR (UFPNMR) method which was proposed originally by Yesinowski [21 ] and developed further by Kimura [22,23]. The second is the sonically induced narrowing of the NMR spectra of solids (SINNMR) that was demonstrated recently by Homer et al. [24,25]. 

The dominating line broadening interactions of spin 7=1/2 nuclei are described by the total Hamiltonian ... 

A further characteristic line broadening interaction occurs for nuclei with nuclear spinsf>l/2 in addition to the interactions discussed above [seeEq.(l)]. These nuclei exhibit an electric quadrupole moment that interacts with the electric field gradient at the site of the nucleus. The NMR spectriun of quadrupole nuclei with half-integer nuclear spins (7=3/2,5/2,...) consists of the -hl/2 -1/2 central transition and the so-called satellite transitions ( l/2 3/2, 3/2 quadrupole interaction it is practically impossible to excite the whole spectriun non-selectively, i.e., the central transition is often selectively observed. 

The influence of the line broadening interactions described above (see Sect. 2.1) can be suppressed at least partly by the appHcation of special multiple-pulse sequences [8,13-15]. A typical example is the so-called WAHUHA (Waugh, Huber, Haeberlen) sequence [8] which consists of four "/2-pulses with alternating phases -[-r- nl2)x-2T- nl2) -T-(nl2)y-2r-(nl2).y-]-. This pulse cycle which exhibits a cycle time of t =6ris appHed repeatedly to the resonating spins. Sam-pHng the FID at multiples of leads to a suppression of the influence of the homonuclear magnetic dipole-dipole interaction. It has, however, to be stated... 

Fig. 3. Line shape function of the central transition of a quadrupole nucleus with a half-integer nuclear spin, calculated for /Jq=0 in the limiting case of rapid magic angle spinning assuming that the electric quadrupole interaction is the dominating line broadening interaction...

Of great interest to physical chemists and chemical physicists are the broadening mechanisms of Raman lines in the condensed phase. Characterization of tliese mechanisms provides infomiation about the microscopic dynamical behaviour of material. The line broadening is due to the interaction between the Raman active chromophore and its environment. 

The cross-correlation effects between the DD and CSA interactions also influence the transverse relaxation and lead to the phenomenon known as differential line broadening in a doublet [40], cf Figure Bl.13.8. There is a recent experiment, designed for protein studies, that I wish to mention at tire end of this section. It has been proposed by Pervushin etal [4T], is called TROSY (transverse relaxation optimized spectroscopy) and... 

Even for a single radical tire spectral resolution can be enlianced for disordered solid samples if the inliomogeneous linewidth is dominated by iimesolved hyperfme interactions. Whereas the hyperfme line broadening is not field dependent, tire anisotropic g-matrix contribution scales linearly with the external field. Thus, if the magnetic field is large enough, i.e. when the condition... 

Although the rotation barrier is chiefly created by the high-frequency modes, it is necessary to consider coupling to low-frequency vibrations in order to account for subtler effects such as temperature shift and broadening of tunneling lines. The interaction with the vibrations q (with masses and frequencies m , tu ) has the form... 

It is much more difficult to observe the Mossbauer effect with the 130 keV transition than with the 99 keV transition because of the relatively high transition energy and the low transition probability of 130 keV transition, and thus the small cross section for resonance absorption. Therefore, most of the Mossbauer work with Pt, published so far, has been performed using the 99 keV transition. Unfortunately, its line width is about five times larger than that of the 130 keV transition, and hyperfine interactions in most cases are poorly resolved. However, isomer shifts in the order of one-tenth of the line width and magnetic dipole interaction, which manifests itself only in line broadening, may be extracted reliably from Pt (99 keV) spectra. 

We have referred to the various interactions which can cause line broadening in the solid state. One of these, which is normally not a problem in liquid state NMR, is due to the fact that the chemical shift itself is a tensor, i.e. in a coordinate system with orthogonal axes x, y and z its values along these axes can be very different. This anisotropy of the chemical shift is proportional to the magnetic field of the spectrometer (one reason why ultra-high field spectrometers are not so useful), and can lead in solid state spectra to the presence of a series of spinning sidebands, as shown in the spectra of solid polycrystalline powdered triphenylphosphine which follows (Fig. 49). In the absence of spinning, the linewidth of this sample would be around 75 ppm ... 

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