Static linewidth

For rigid Si-F bonds, the 19F CSA is characterized by a span, Q = Sn - S33, of about 80 ppm [150]. This parameter describes the total static linewidth of 19F NMR of rigid Si-F bonds in zeolites. Line narrowing (reduced span values) is observed, when the fluoride ion carries out motional jumps between two or more Si sites. 

The superposition of the different magnetic interactions complicates the interpretation and calculation of the resulting normalized line shape function,/(v). It is therefore advantageous to make use of the so-called second moment M2 as a measure of the linewidth of the solid-state NMR signals. The full-width at halfmaximum of an NMR signal in frequency units, also called the static linewidth, is... 

MAS <5P, peak position from saturated aqueous CaCf solution, t At/, MAS linewidth, except J which is the static linewidth. 

Figure 11 Temperature dependence of the H static linewidth of H4Ru4(CO)i2...

The 2 polymorphs of CaCOs, aragonite and calcite, can readily be distinguished on the basis of their " Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can he extracted by spectral simulation. The narrower Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum... 

MAS linewidth, except t - which represents the static linewidth. 

The criterion for such motional narrowing is Zl- r 1, where A is the static linewidth of the relevant species and ris its motional correlation time. "" We reasonably substitute the... 

The reduction of the second moment by motions occurring on the static linewidth time-scale can be used to measure a corresponding correlation time. The integration of the spectral density gives the following interpolation formula between the static M2o and motionally averaged M2 oo second moments expressed in angular units ((rad/s)2) ... 

The motional narrowing of a resonance occurs when the frequency of motion exceeds the static linewidth and an averaging of the dipole... 

The geometric term in Equation 4.5 vanishes when =54°44 the magic angle. The dipolar interaction (also referred to as dipolar coupling) should average to zero if the sample is spun at this angle with respect to the static field. However, to completely eliminate the dipolar interactions the speed of rotation must be several times the static linewidth, typically several lOOkHz which is not currently possible. Most commercial MAS systems are capable of rotating 4mm sample rotors at a speed of 10-15 kHz or so. 

The different cases are given in Table 5.2. The static linewidth, i.e., the linewidth one would observe without motion, is denoted by wo, and D is the characteristic hopping frequency. When the interchain couplings are such that the cutoff frequency is larger than the linewidth, A [Pg.146]

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