Isotropic spectra analysis

Measurement of rotational correlation times. Measurement in isotropic and nematic phases. FT analysis of spectrum. 

The last result was obtained independently in [27,269], In the logarithmic scale of Fig. 6.3 the dependence (6.25) is linear in both cases, but its slope in the isotropic case is opposite to that in the anisotropic case. This difference makes it possible to perform self-consistent verification of the theories. Unfortunately, independent information on xj is rather rare. It can be obtained from NMR investigations, or from analysis of the wings of the spectrum (6.20). Since both tasks are rather complex,... 

One possibility for this was demonstrated in Chapter 3. If impact theory is still valid in a moderately dense fluid where non-model stochastic perturbation theory has been already found applicable, then evidently the continuation of the theory to liquid densities is justified. This simplest opportunity of unified description of nitrogen isotropic Q-branch from rarefied gas to liquid is validated due to the small enough frequency scale of rotation-vibration interaction. The frequency scales corresponding to IR and anisotropic Raman spectra are much larger. So the common applicability region for perturbation and impact theories hardly exists. The analysis of numerous experimental data proves that in simple (non-associated) systems there are three different scenarios of linear rotator spectral transformation. The IR spectrum in rarefied gas is a P-R doublet with either resolved or unresolved rotational structure. In the process of condensation the following may happen. 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

The analysis of an isotropic ESR spectrum is relatively straightforward if a systematic approach is used, but it can be a frustrating experience for a... 

Here, A is the nearly isotropic nuclear coupling constant, I is the nuclear spin (Iun = I), and m is the particular nuclear spin state. It may be observed that the zero field splitting term D has a second-order effect which must be considered at magnetic fields near 3,000 G (X-band). In addition to this complication nuclear transitions for which Am = 1 and 2 must also be considered. The analysis by Barry and Lay (171) of the Mn2+ spectrum in a CsX zeolite is shown in Fig. 35. From such spectra these authors have proposed that manganese is found in five different sites, depending upon the type of zeolite, the primary cation, and the extent of dehydration. 

Upon y-irradiation of 1 in a CF3CCI3 matrix at 77 K [78], a radical cation was formed, the ESR spectrum of which consisted of nine broad hyperfine components spaced by ca. 0.75 mT (g = 2.0029 0.002), and the corresponding proton END OR spectrum exhibited two essentially isotropic signals at 25.83 and 24.58 MHz. The detailed analysis of the ESR and END OR spectra disclosed that the initially formed radical cation 1+ had transformed into the tetramethyleneethane radical cation 94+ (Scheme 17). In CFCI3 and CF2CICFCI2 matrices 1+ persists up to 100 K [79]. On going from 1 to l+, the set of eight equivalent protons splits... 

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Fig, 16. 50.1-MHz 13C MAS spectra of benzaldehyde-a-13C and benzene reacting on zeolite HY. The spectrum acquired at 120 K after the sample was heated at 448 K clearly shows an isotropic chemical shift at 207 ppm, consistent with the chemical shift of the trityl cation. Furthermore, the Herzfeld-Berger analysis of the sideband intensities reveals an axially symmetric tensor, thus providing unambiguous evidence for the trityl cation 16. 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

These workers (Adrian et al., 1962) also studied the spin resonance spectrum of DCO radicals and obtained remarkably narrow lines and shoulders which gave sufficient detail that the anisotropic hyperfine tensor could be deduced. This result then enabled them to extract the data tabulated from the spectrum of HCO. In particular, it is pointed out that as the g- and hyperfine-anisotropies have different principal axes, there has to be an extra term (Ayz) where the hyperfine tensor is expressed in terms of the axes of the gr-tensor. A careful analysis of all the data led these authors to the conclusion, based entirely upon experiment, that the large isotropic hyperfine coupling must be positive. 

Support for the above conclusions was also obtained from an analysis of the Cd NMR and solid state MAS spectra of the five, six, and seven coordinate cadmium texaphyrin complexes 116, 156, and 157, respectively [70]. From the solid-state MAS results, a single tensor was observed for the five-coordinate complex 116 with a corresponding isotropic chemical shift of 194 ppm. The MAS spectrum of a complex prepared in the presence of pyridine, however, revealed two tensors, presumed to be due to a mixture of six and seven coordinate species. Based upon the isotropic chemical shifts and the symmetry of the tensors the six coordinate species was assigned to the isotropic shift at 188 ppm and the seven coordinate species assigned to the isotropic shift at 221 ppm. The MAS spectrum of the benzimidazole complex of cadmium texaphyrin consists of only a single... 

Bis(4-inethylamino-3-penten-2-onato)nickel(II) forms diamagnetic green needlelike crystals which melt at 125°. It is fairly soluble in most weakly polar organic solvents, and especially soluble in chloroform. The proton magnetic resonance spectrum shows small isotropic contact shifts, the analysis of which indicates a 95 5 ratio of planar to tetrahedral stereoisomers at 25° in chloroform solution. In solution the complex is decomposed by small amounts of water. 

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