Nuclear symmetry effect

Another effect found in the flat-surface study of Refs. 113, 114 and 117 is the nuclear symmetry effect which states that for initial rotation states satisfying j + m =... 

Chemists pay much less attention to the NMR relaxation rates than to the coupling constants and chemical shifts. From the point of view of the NMR spectroscopist, however, the relaxation characteristics are far more basic, and may mean the difference between the observation or not of a signal. For the quadrupolar nucleides such as 14N the relaxation characteristics are dominated by the quadrupole relaxation. This is shown by the absence of any nuclear Overhauser effect for the 14N ammonium ion despite its high symmetry, which ensures that the quadrupole relaxation is minimized. Relaxation properties are governed by motional characteristics normally represented by a correlation time, or several translational, overall rotational and internal rotational, and thus are very different for solids, liquids and solutions. 

Symmetry-induced kinetic isotope effects cause changes in the rate of a chemical process involving identical nuclei when the nuclear symmetry of the system is altered... 

Figure 20. Energy dependence of total cross section for He (2 5) + He calculated from potentials of Fig. 14 and Table 111. Oscillations at low energies attributable to nuclear-symmetry. Glory effect is amplified in curve I, in which difference between cross sections for identical and distinguishable particles is plotted on an expanded scale. 

SYMMETRY BREAKING, NON-ADIABATIC ELECTRON-PHONON COUPLING AND NUCLEAR KINETIC EFFECT ON SUPERCONDUCTIVITY OF MgB2... 

A.l.Maergoiz, E.E.Nikitin, and J.Troe, Adiabatic channel study of the capture of nitrogen and oxygen molecules by an ion Effect of nuclear symmetry and spin-spin interaction. Z. Phys D- Atoms, Molecules and Clusters, 36, 339 (1996)... 

This model has been used for the description of Arrhenius curves of H-transfers as described in more detail in Chapter 6. The next two chapters show applications of these symmetry effects. First the para-hydrogen induced polarization (PHIP) experiments are discussed. There the symmetry induced nuclear spin polarization creates very unconventional NMR lineshape patterns, which are of high diagnostic value for catalytic studies. Then in Section 21.4 symmetry effects on NMR line-shapes and relaxation data of intramolecular hydrogen exchange reactions are discussed and examples from iH-liquid state and H-solid state NMR are presented and compared to INS spectra. The last section gives an outlook on possible future developments in the field. 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

In contrast to the vibrational effect, the rotational effect on hydrogen dissociation on Cu is much less understood, until very recently. Most 3D quantum calculations have used the plane rotor model, which is not appropriate for studying rotational effects. The studies of Refs. 113, 114, and 117 using the spherical rotor treatment have obtained important results on the effect of rotational orientation and the nuclear symmetry. The rotational orientational effect is clearly shown in Fig. 15, where reaction probabilities for different initial rotational orientation states are plotted as a function of kinetic energy. Significant enhancement of reaction probability is seen for the state with j = m ( helicopter mode) while the m = o ( cartwheel mode) is least effective for dissociation. 

Walters KJ, Matsuo H et al. (1997) A simple method to distinguish intermonomer nuclear Overhauser effects in homodimeric proteins with C2 symmetry. J Am Chem Soc 119 5958-5959... 

Although A = 2 degeneracy might be expected, symmetry requirements dictate that each rotational level can be associated with only one A component aU of the rotational levels in Ag O2 have positive parity. Consequently the excited state mixing which, in a heteronuclear molecule, would lead to A-doublet splitting here produces only a small alternating shift in the energies of the rotational levels. Moreover there are no nuclear hyperfine effects in the predominant isotopomer. 

Fig. 1. Qualitative molecular state model based on topology, symmetry, effective nuclear potentials as well as electron distribution, emphasizing the structure <-> energy relationship as well as molecular dynamics as essential prerequisites for reactions (cf. text).

For isobaric nuclei a symmetry term must be added to the radius formula (see Angeli (1992) and further references therein). Nuclear shell effects were considered in the nuclear r.m. s. radii by Angeli (1991a). A systematics of nuclear charge radii are given in Nadjakov et al. (1994), Angeli (2004), and Fricke and Heilig (2004). 

To analyze the recorded spectra, the spectrometer needs to be calibrated. The three main calibration parameters are the velocity scale, the center point of the spectrum and the nonlinearity of the velocity/time profile of the oscillation compared to a standard reference. The calibration is performed using a spectrum recorded from an a-iron foil at room temperature using the well defined line positions of the sextet from a-iron, which occur at 5.312mms , 3.076mms , and 0.840mms The center of this a-iron spectrum at room temperature is taken as the reference point (0.0 nun s ) for isomer shift values of sample spectra. The typical Mossbauer spectrum of the 14.4 keV transition of Fe in natural iron (Fig. 4.10) represents a simple example of pure nuclear Zeeman effect. Because of the cubic symmetry of the iron lattice, there is no quadrupole shift of the nuclear energy levels. The relative intensities of the six magnetic dipole transitions are... 

Hydroxide-ion mobility has received much less attention. On the basis of symmetry arguments, viewing OH as water with a missing proton, one could envisage similar mechanisms for proton and hydroxide transport in water. Recent computational studies, however, do not support this notion and stress the formation of specific hydration complexes influenced by nuclear quantum effects. ... 

To determine whether the catalyst remains in this conformation in solution, heteronuclear NOESY experiments were set up. If one of the ligands flips, nOe (nuclear Overhauser effect) contacts between the side chain protons and the phthaloyl carbons should be visible. No cross peaks were detected in the case of 20, meaning that the all-up conformation is also predominant in solution, whereas cross peaks in the spectrum of 19 indicated that the C4 symmetry is not maintained in solution. Variable temperature NMR experiments with 20 confirmed this, as the aromatic carbon atoms give rise to two singlets at room temperature, which merge into one broad singlet at higher temperatures (comparable to the spectmm of 19 at room temperature). 

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