Correlation spectroscopy coupling constant determination

Population transfer experiments may be selective or nonselective. Selective population transfer experiments have found only limited use for signal multiplicity assignments (SSrensen et al, 1974) or for determining signs of coupling constants (Chalmers et al., 1974 Pachler and Wessels, 1973), since this is better done by employing distortionless enhancement by polarization transfer (DEPT) or Correlated Spectroscopy (COSY) experiments. However, nonselective population transfer experiments, such as INEPT or DEPT (presented later) have found wide application. 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

In addition to these limited procedures a number of experimental methods (vibrational spectroscopy, dipole moment measurements, electron diffraction, NMR, etc.) have been employed to determine the relative stabilities of these complexes.11,23 Intense effort has been directed towards establishing some kind of correlation between NMR parameters and stability of the borane complexes. The chemical shifts alone rarely show good correlation. However, complexation shifts (the chemical shift difference between the free and complexed borane or ligand) and various spin-spin coupling constants correlate better with calorimetric data, especially for ligands or boranes belonging to structurally similar series (Table 2).10,24... 

The carbonyl frequency in the infrared spectrum provides a fairly characteristic method for differentiating between 1,4- and 1,5-lactones of aldonic acids. With few exceptions, the absorptions are in the range 1790-1765 and 1760 to 1725 cm-1, respectively.69 Configurational and conformational conclusions have been drawn from H and 13C NMR spectroscopy of aldonic acids and aldonolactones, using different correlation methods, enriched compounds, and shift reagents. For example, the solution conformation of aldono-1,4-lactones enriched with 13C at C-l have been determined on the basis of the coupling constants (homo and heteronuclear). In general, 0-2 is oriented quasi-equatorially due to stereoelectronic factors.36 Similar conclusions were made by Horton and Walaszek, who described the conformation of pentono- 1,4-lactones as an equilibrium between the 3E and forms.70 Conformations of D-hexono-1,4-lactones in solution have also been studied by NMR spectroscopy.70a The solution equilibrium of protected derivatives and their conformations have been described.71... 

NMR spectroscopy is a powerful analytical method for the determination of flavone structures. It has, however, some limitations as the sensitivity is rather low, and compounds have to be isolated. The assignments of the different proton and carbon signals in H and NMR can be based on chemical shifts (5) and coupling constants (J), and correlations observed in homo- and hetero-nuclear 2D NMR. NMR spectra of flavones have been extensively published previously (Markham and Chari, 1982 Agrawal, 1989 Markham and Geiger, 1993 Fossen and Andersen, 2006). 

Methylated trisilanes Me Si3Xg (X = H, F, Cl, Br, 1, Ph, OMe) can serve as model compounds for the study of correlations between structures and Si chemical shifts, Si Si coupling constants, bond strengths and bond lengths, as determined with vibrational spectroscopy and electron diffraction or for the study of rotational isomerism of Si-Si bonds. A report of these ongoing activities is given in this work. 

Broadband Hartmann-Hahn transfer can also be of assistance in alternative approaches to determine coupling constants that do not rely on E.COSY-type multiplets that are separated by large one-bond couplings. The homonuclear two-dimensional PICSY (pure in-phase correlation spectroscopy) experiment (Vincent et al., 1992, 1993), which is based on selective Hartmann-Hahn transfer using doubly selective irradiation, can... 

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