Linewidth half-height

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

FIGURE 9.1 Linewidth versus standard deviation. A gaussian distribution of unit amplitude is plotted on an x-axis scale in units of the standard deviation (or sigma). At 3.15x0 the unit intensity has dropped to 0.1%. The linewidth in simulations is usually expressed as the halfwidth at half height (HWHH), which is equal to circa 1.17x0, or as twice this value that is, the full width at half height (FWHH). 

Bell et al. (2002) investigated the relationship between water mobility as measured by oxygen-17 NMR (transverse relaxation rate obtained from linewidth at half-height) and chemical stability in glassy and rubbery polyvinylpyrrolidone (PVP) systems. Reported results suggest that water mobility in PVP model systems was not related to Tg. The study did not find a link between water mobility and reaction kinetics data (half-lives) for degradation of aspartame, loss of thiamin and glycine, and stability of invertase. 

Table 5.4 A comparison of specific surface area ratio calculated from quadrupole splitting (Aq), spin-lattice relaxation rate (Rf, half-height linewidth (Avj/2) and isotherm data for an unbleached linerboard pulp beaten to various degrees.

It is again apparent from these results that the resonant line is inhomogeneous since the symmetry, linewidth at half-height, and peak maxima change with the position and strength of the irradiation. ... 

The Linewidtli option in the Analysis pull-down menu allows you to measure the linewidths at half height. The ability to recognize different linewidths in your spectrum is important because it may indicate additional molecular processes going on in solution. Broadening of some of the resonances may be indicative of additional non-resolved couplings, dynamic processes or different types of relaxation mechanisms selectively affecting a particular observed nucleus. This option will also be used to estimate linewidths for use as input data for WIN-DAISY as described in detail in Modem Spectral Analysis, volume 3 of this series. 

Load the proton spectrum of peracetylated glucose D NMRDATA GLUCOSE 1D H GH 002999,1R and expand a methyl resonance. From the Analysis pull-down menu choose the Linewidth option. With the cursor in Maximum Cursor mode, select a resonance line by clicking the left mouse button. With the cursor set on the top of the peak, click the right mouse button. A horizontal line indicates the intensity at half height and the corresponding line width (in Hz) will appear below the title bar. 

Determine the mean linewidth at half height Au,2, in the spectrum obtained... 

NMR spectrum is obtained by Fourier transformation as the distribution of frequency components that are present in an FID or an echo signal sampled over a period in time. Therefore, a rapidly decaying signal (short T2) has a broad function over frequency because of a wide spread of Larmor frequencies and vice-versa. The linewidth in Hz at half height, Avj/2, is related to T2 relaxation time by the equation,... 

The NMR spectral shape is a Lorentzian curve, and the linewidth, Avi/2, at half-height of the absorption peak, gives a measure of the spin relaxation rates through Avi/2 = 1 /ttT2. 

The frequency function is a Lorentzian with linewidth at half height of (jt72)-i(= Ri/n). The same, of course, holds in the continuous wave experiment. R2 is a measure of the uncertainty of the energy levels, which gives the linewidth in every spectroscopy. The uncertainty principle, according to which the uncertainty in energy of a level is inversely proportional to the lifetime, tells us that T2 is a measure of the lifetime of the energy levels. 

Figure 15.1a shows a typical 2H NMR spectrum obtained in a deuterated PDMS network at room temperature, in the absence of an external constraint [23]. The half-height linewidth is very small (about 60 Hz) compared to the static interaction Vq. This means... 

Fig. 12. Computer simulations of the X part of an AA X spin system where J(AX) = 25 Hz, J(A X) = 3 Hz and where J(AA ) is varied from 500 to 0.0 Hz. Linewidths at half-height for these simulations are 0.8 Hz [after Nelson et al. (55)].

Half-height linewidth in Hz. This notation is used throughout this review. 

In NMR spectroscopy, absorption parts A(phase-sensitive NMR spectrum. The linewidth Ao)i/2 at half height of A a>) is determined by Ti,... 

Alternatively, the magnitude spectrum A(uj) - -iZ>(ft)) can be displayed. Here, however, the linewidth at half height is broader by a factor of 3, so that the spectral resolution is... 

The magic-echo frequency-encoding method of Fig. 8.8.1 has been demonstrated on a phantom of adamantane and hexamethylbenzene [Mat 1 ]. The spatial resolution achieved was better than (100 p,m) . For both materials the H linewidth at half-height is about 15 kHz without homonuclear decoupling. 

---