Information from coupling constants

Data from NOE experiments nicely supplement information from coupling constants in the determination of three-dimensional stereochemistry loo. Reduction of this bicyclic ketone with a bulky hydride reducing agent gives one diastereoisomer of the alcohol, but which Irradiation of the proton next to the OH group leads to an NOE to tlie green proton. 

Information from coupling constants 4.8.1 General principles... 

Nowadays, the NMR specialist finds an arsenal of assignment techniques at his disposal for structural determination. Long before they became available this responsibility lay mostly in the hands of chemists who could treat the information gleaned from coupling constants. Older NMR texts dedicated a large part of their content to the descriptions of spin-spin coupling patterns (order of spectra, two and higher order systems) as exemplified by Reference 5, which contains relevant J values. This content has been updated and placed on a very sound theoretical basis that is described in recent reviews. ... 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

It has been possible recently to perform DFT calculations [44] for J couplings as well and obtain rather reliable values. This has opened the possibility to infer from couplings constants structural information without the need for calibration. A rather new and important example of this breakthrough is the measurement and structural interpretation of scalar couplings across H-bridges in biomolecules. It turns out that couplings across... 

A second approach is the semiquantitative use of chemical shifts, mostly from a-carbons, for restricting the range of backbone torsion angles in proteins. Statistical data show that deviations of these shifts from their values in unfolded polypeptides correlate with the presence of regular secondary structures. Thus, the resonances of a-carbons in a-helices are typically shifted down-field by about 3 ppm. those in P-sheets up-field by about 1.5 ppm. Measurements of a-carbon chemical shifts may therefore be converted to constraints on the backbone torsion angles 4> and of selected residues, and these constraints complement the information obtained from coupling constants. 

Information from an n.m.r. spectrum is classified into the chemical shift, <5 (the relative shift from a standard [Me Si for H, CC13F for which is rendered independent of the field), and the coupling constants, J, which are determined directly from the spectra. 

Specinfo, from Chemical Concepts, is a factual database information system for spectroscopic data with more than 660000 digital spectra of 150000 associated structures [24], The database covers nuclear magnetic resonance spectra ( H-, C-, N-, O-, F-, P-NMR), infrared spectra (IR), and mass spectra (MS). In addition, experimental conditions (instrument, solvent, temperature), coupling constants, relaxation time, and bibliographic data are included. The data is cross-linked to CAS Registry, Beilstein, and NUMERIGUIDE. 

After a suitable number of pulses have been acquired, the time-domain spectmm is transformed into a frequency-domain spectmm and phased so that the base line of the spectmm is fiat and the peaks all fall on the same (positive) side of the base line. Information obtainable from this spectmm includes chemical shifts, coupling constants, and integrated peak areas. 

The direct coupling constants (A/) of pyridine, obtained from a spectrum of the molecule in a nematic liquid crystal solvent, are listed in Batterham s monograph (B-73NMR, p. lo). These data provide information about the geometry of the molecule, as the couplings are proportional to the inverse cube of the distance (r,/) between the nuclei. 

Aside from C-H coupling constants (often obtained from satellites in NMR spectra) not very much information is available on the NMR spectra of small heterocycles. A recent review (B-79MI50101) devotes only seven pages to the NMR spectra of all three- and four-membered heterocyclics. 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Hyperfine coupling constants provide a direct experimental measure of the distribution of unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for electronic wave functions [1,2], Conversely, given an accurate theoretical model, one can obtain considerable information on the equilibrium stmcture of a free radical from the computed hyperfine coupling constants and from their dependenee on temperature. In this scenario, proper account of vibrational modulation effects is not less important than the use of a high quality electronic wave function. 

Chemical information from hafnium Mossbauer spectroscopy can primarily be deduced from the quadrupole-splitting parameter. In Table 7.4, we have listed the quadrupole coupling constants eQV for some hafnium compounds. Schafer et al. 

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). 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

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