Extracting coupling constants

The isotropic and anisotropic hyperfine coupling terms in a arise from interactions between electron and nuclear spins, and provide information about the nature of the orbital containing the unpaired electron and the extent to which it overlaps with orbitals on adjacent atoms. The anisotropic term can cause similar difficulties to the g tensor anisotropy in analysing spectra of polycrystalline powders extracting coupling constants from spectra of transition metal ions or radicals in zeolites can be difficult or impossible without computer simulation. 

With relatively simple spectra, it is usually possible to extract the individual coupling constants by inspection, and to pair them by size in order to discover what atoms they coimect. However, the spectra of larger molecules present more of a challenge. The multiplets may overlap or be obscured by the presence of several unequal but similarly sized couplings. Also, if any chiral centres are present, then the two hydrogens in a... 

A fluxional amido-salt (24), in which the y-nitrogen atom acts as an internal nucleophile, has been identified by variable-temperature n.m.r. spectroscopy. At — 63 °C two methyl signals are observed, one a singlet, one a doublet (J = 11 Hz) whereas at + 60 °C there is only one signal, a doublet with J = 5.5 Hz (the average of the low-temperature coupling constants). The solvent extraction of organophosphorus compounds has also been studied by and H n.m.r, ... 

A great deal of information on the electronic structure and geometry of radicals in solution can be extracted from their ESR spectra, as it is well established that the values of hyperfine coupling constants (hfcc), arising from the spin density of the s-orbitals, markedly increase with increasing of the SOMO s-character. The pyramidalization of the radicals is manifested in higher values of their hfccs (o-radicals), whereas smaller values of the hfccs are indicative of the more planar radicals (tt-radicals). 

Here we have irradiated the OCH2 group in the proton spectrum the result is a doublet of quartets with two coupling constants CJch = 127.7 Hz, 3JPOcc 5.5 Hz). We can thus extract 2JCCH from the multiplets in Fig. 18b its value is 2.7 Hz. 

Fitting of these curves yields the sum of the dipolar coupling plus the /-coupling. Referencing to a spectrum without alignment allows extraction of the dipolar coupling constant. 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

CCR is easily measured by heteronuclear NMR experiments of isotopically labeled molecules. The information extracted from these experiments will significantly improve the resolution of NMR structures, especially bound conformations of weakly bound ligands, since /-couplings cannot be used in this case. The reason is the fact that the nonbound conformation significantly contributes to the averaged values of the coupling constant. 

In numerous complex aldol adducts it is not always possible to extract the relevant vicinal proton coupling constants. Heathcock and co-workers have recently noted that for the -hydroxy ketones and esters that exist in the preferred hydrogen-bonded conformations A and A, NMR spectroscopy may be conveniently employed to assign stereochemistry (13). 

A systematic development of relativistic molecular Hamiltonians and various non-relativistic approximations are presented. Our starting point is the Dirac one-fermion Hamiltonian in the presence of an external electromagnetic field. The problems associated with generalizing Dirac s one-fermion theory smoothly to more than one fermion are discussed. The description of many-fermion systems within the framework of quantum electrodynamics (QED) will lead to Hamiltonians which do not suffer from the problems associated with the direct extension of Dirac s one-fermion theory to many-fermion system. An exhaustive discussion of the recent QED developments in the relevant area is not presented, except for cursory remarks for completeness. The non-relativistic form (NRF) of the many-electron relativistic Hamiltonian is developed as the working Hamiltonian. It is used to extract operators for the observables, which represent the response of a molecule to an external electromagnetic radiation field. In this study, our focus is mainly on the operators which eventually were used to calculate the nuclear magnetic resonance (NMR) chemical shifts and indirect nuclear spin-spin coupling constants. 

It is fortunate that most applications devolve into one of two camps small molecules or proteins. In the former case, the size of these molecules has stayed fairly constant and the inexorable rise in magnetic fields has meant that the number of incidences of second-order spectra has decreased (although complications will always exist with virtual couplings). It is therefore pertinent to examine methods, which are not only designed to extract couplings from first-order spectra, but are also amenable to automation. 

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