Line shape studies

If a molecule contains one or more unpaired electrons it is usually possible to detect an electron spin resonance signal and at a very low concentration of unpaired electrons, commonly 1018 spins with modem instruments. Several pieces of information can be obtained in this way. The number of unpaired spins can be found, the symmetry of the molecule in the region of the unpaired electron can be determined, and, if the unpaired electron is delocalized over nuclei with nuclear spins, then the extent of delocalization can be determined. Perhaps more importantly for our purpose, the rotational time of molecules can be determined from line shape studies. 

Spectral line shape studies by Frommhold and Biondi60 (using Fabry-Perot interferometry) indicated measurable broadening of several lines coming from Ne2 and Ar2 recombination. The authors showed that dissociative recombination,... 

Because density fluctuations produce a long-lived perturbation, whereas collisional dynamics are fast, the Raman echo is a definitive experiment for testing this prediction. An excellent system for this test is the sym-methyl stretch of acetonitrile (3). There have been many Raman line shape studies of this mode, which concluded that the IBC theory of collisional dynamics alone can account for the linewidth (84,90,91,133,134). On the other hand, calculations by George and Harris using Schweizer and Chandler s theory predicted that a substantial fraction of the linewidth is due to density fluctuations (135). 

Line shape studies of 7Rb and deuteron NMR lines have thrown more light on the frustration in the system. Blinc et al.5 have shown clearly the measurement of second moment can be related to the Edwards-Anderson spin-glass order parameter in the glass phase. 

Molecular motions not only affect the NMR line shape, but also determine the spin-lattice relaxation time T. Measurement of the T relaxation provides information about fast motions with frequencies near the NMR fiequency of deuterons, i.e., 46 MHz [25]. Moreover, Ti relaxation experiments are very useful for detecting motional heterogeneity in polymers [25]. Motional effects in both relaxation and line shape studies are completely dominated by reorientations of C-D bonds. Therefore, in motionally heterogeneous polymers, different T] relaxation times can be related to chain units with different mobility as reflected in different line shapes. 

Schlick S, Narayana PA, Kevan L. (1976) ESR line shape studies of trapped electrons in y-irradiated enriched lOM NaOH alkaline ice glass Model for the geometrical structure of the trapped electron. J Chem Phys 64 3153-3160. 

Roy, A. K., Jones, A. A., and Inglefield, P. T. (1986) Phenylene ring dynamics in solid polycarbonate an extensive probe by carbon-13 solid state NMR line-shape studies at... 

The instrumental resolution of the spectrometers is limited by the combined frequency fluctuation from each CO2 laser (about 15 kilohertz). This, of course, is less than any Doppler limited linewidth and, therefore, does not limit our resolution except for possible sub-Doppler work. This high resolution provides an excellent way of studying pressure shifts and line shape studies of spectral lines. The measurement of OH concentrations in our atmosphere as a function of altitude using absorption and emission measurements requires an accurate knowledge of its linewidth in the atmosphere. 

In the early reports, line-shape studies predominated however, many of the more recent reports use 2-D exchange spectroscopy. 

Binkley, J.S. (1984) A Carbon Auger Line Shape Study of... 

CARBON AUGER LINE-SHAPE STUDY OF NITROAROMATIC EXPLOSIVES... 

LaSb 1.4 temperature dependence of line shape studied single crystal Davidov et al. (1975a)... 

Changes in relaxation rates also arise from changes in the electric field gradient in the vicinity of the nucleus. Such variations can be induced by intermolecular aggregation, solvation, or binding of added species (e.g., metal ions to basic binding sites). Phase changes (isotropic liquid crystal > plastic crystal) can also be probed via relaxation and line-shape studies (8). 

As far as sub-Tg dynamics (often called p-relaxation) and its relation to ductility is concerned, bisphenol-A polycarbonate (PC) is one of the most studied, yet still controversial systems. For a recent review of the field, see Reference 48. From NMR line shapes, large-angle phenylene rotations described by 180° flips augmented by additional oscillations were identified at room temperature and above. Such 180° flips were subsequently confirmed by NMR line shape studies and 2D... 

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