Measuring coupling constants

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

We measure coupling constants in Hz, since if we measured them in fractions of ppm, they would not be constant, but would vary with the magnetic field strength of the spectrometer used. This would obviously be most inconvenient Note that 1 ppm = 250 Hz on a 250 MHz spectrometer and 400 Hz on a 400 MHz spectrometer, etc. 

Comparisons of the measured coupling constants to the geometry of the H-bond are hampered by the limited availability of very high-resolution diffraction data. Especially, no crystallographic data are available for most of the nucleic acids for which H-bond cou-... 

Most characteristic for the overall conformation of the cinchonan carbamates in solution is the dihedral angle of the torsion H8-C8-C9-H9, which can be inferred from the vicinal coupling constant of the NMR proton signal of H9. The experimentally measured coupling constant /hsh9 represents actually the average over the populations P(i) of the different conformers in solution according to [100]... 

Configurations of ethene compounds can be determined easily and securely by measuring coupling constants. The simplest case is the differentiation of diastereomeric 1,2-disubstituted ethenes, because vicinal H, H coupling constants are always larger in the E- than in the Z-isomer, and the ranges do not overlap. 

Most of the carbon-carbon coupling constants published recently have been measured using 2D 13C-detected INADEQUATE spectra.39-44 ID INADEQUATE experiments were used less frequently43 45 due to potential overlap problems. The measured coupling constants are usually used to resolve stereochemical problems and also compared with theoretical values. 

The radical ion of phenoxazine has a uniformly spaced four-line spectra with the line intensities in the ratio 1 2 2 1. The measured coupling constant was 9.83 gauss, the line width 8.19 gauss, and the g value 2.0049. This hyperfine structure can be interpreted as a basic three-line splitting by the nitrogen-14 nucleus (nuclear spin 1) and a doublet splitting with nearly identical spacing due to the attached acidic proton (nuclear spin ). 

The experimentally measured coupling constant J(HaHy)exp. = 5.5 Hz is reproduced by calculations for the endo-si y substituted model structure 44 using a finite perturbation level (FPT) 26) (Perdew/IGLO-III) approach J(HaHy)caic. = 5.9 Hz, whereas only 1.2 Hz is calculated for the HaHy-coupling constant in the exo-si y substituted model structure 45. Finally the bonding orbital of the bridging C-C-bond between Cq and Cy in these type of bicyclobutonium ions can be visualized by calculations of the natural bond orbitals (NBO s) (Figure 13). 

Zonfirming the conformations experimentally means measuring coupling constants in the f NMR so we need to look at the vital protons (marked H on the diagrams below) and der whether they can be seen in the spectrum. Fortunately, the interesting protons are all next nctional groups so they can be seen. We probably can t see the axial proton at the ring junction. e first example but trans-decalins must have axial protons there, so that is not so important. 

G. W. Vuister, M. Tessari, Y. Karimi-Nejad and B. Whitehead, Pulse Sequences for Measuring Coupling Constants , p. 195... 

Sensitivity and resolution improvements offered by the TROSY method have been utilised in two modified sequences for measuring coupling constants in proteins. Permi et proposed a 3D oc/p-HN(CO)CA-J TROSY sequence for evaluation of J(N,H ) couplings related to the torsion angle v]/. The H ... 

Our laboratory (21) has analyzed the variation in the P chemical shifts and H3 -p coupling constants in terms of fractional populations of the two thermodynamically stable Bi and Bn states. We have determined that the P chemical shift and Jh3 -p coupling constant of a phosphate in a purely Bi conformational state is estimated to be ca. -4.6 ppm and 1.3 Hz respectively. Similarly the P chemical shift and Jh3 -p coupling constants of a phosphate in a purely Bu conformational state should be ca. -3.0 ppm and 10 Hz respectively. The dispersion in the P chemical shifts of oligonucleotides is attributable to different ratios of populations of the Bi and Bu states for each phosphate in the sequence. The phosphate makes rapid jumps between these two states (30). Thus the measured coupling constant (and P chemical shift) provides a measure of the populations of these two conformational states. 

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