Geminal coupling constants

Figure 13 20 there are two other vinylic protons Assuming that the coupling constant between the two geminal protons in ArCH=CH2 is 2 Hz and the vicinal coupling constants are 12 Hz (cis) and 16 Hz (trans) describe the splitting pattern for each of these other two vinylic hydrogens... 

Useful compilations of NMRspectra are , . The H and NMR chemical shifts and the geminal and vicinal proton-proton coupling constants for oxirane and other heterocycles are given in a very readily compared manner,... 

Table 2.7. Structural features and geminal (two-bond) CH coupling constants Jqh (Hz)...

Rather large HH coupling constants in the aliphatics range (72.5 and 15.0 Hz) indicate geminal methyl protons in rings. In order to establish clearly the relevant AB systems, it makes sense first to interpret the CH COSY diagram (Table 52.1). From this, the compound contains two methylene groups, A and B. 

J, V, V, V nuclear spin-spin coupling constant (in Hz) through two, three, four and five bonds (geminal, vicinal, longer-range couplings)... 

In view of the possible ambiguities which can attend conformational assignments based on vicinal coupling constants, it is fortunate that both long-range and geminal couplings each exhibit stereospecific dependences. 

The fact that the absolute magnitudes of these two sets of coupling constants are different is also expected on the basis of the Pople, Bothner-By treatment (48). It would clearly be of some considerable interest if a general study were to be made of the stereospecificity of this geminal dependence since this may lead to another method for determining carbohydrate conformations. 

The H and 13CNMR spectra of various cyclopentazepines have been recorded (Tables 1 and 2), as has the HNMR spectrum of 7V,7V-dimethylcyclopent[e]azepin-l-amine.68 A detailed analysis of geminal and long-range 13C-H coupling constants for cyclopent[c]azepine is also available.87 The HNMR spectra of 9//-pyrrolo[l,2-a]azepin-9-one (8b) and its fully delocalized cation have been recorded in various solvents.7... 

Geminal coupling constants are presumed to be negative, even if not so stated.1411 "Measured in D20 al 400 MHz. 

Geminal coupling constants are assumed to be negative even if not so stated.147 Measured al 250 MHz in CDC13. 

Note the impact of the geminal chlorine or bromine substituent to diminish the cis and tram F—H coupling constants in these systems. 

Figure 4.10 provides the 19F NMR spectrum of 1,1-difluorobutene. The chemical shifts for its Z- and E-fluorines are -92.8 and -90.8ppm, respectively, with the geminal 2JFF coupling constant being 50 Hz, and the trans 3/HF coupling constant being 25.5 Hz. The cis coupling was too small to be seen in the fluorine spectrum, but was determined to be 2.7 Hz from the proton spectrum shown in Fig. 4.11. The magnitudes of these vicinal F—H coupling constants are considerably diminished as compared to those of monofluoroalkenes.

The fluorine substituent at the 2-position of a trifluorovinyl group is much more highly shielded than the other two fluorines, and its presence gives rise to an enhanced split of the diastereotopic fluorines at the 1-position and enhanced coupling constants, both geminal and vicinal. 

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>. 

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