Vicinal coupling constants bond orders

The 1H NMR spectrum of the four central protons of 48h was analyzed as an AA BB spin system. The coupling constants between II7/II7 and the deuterium nuclei on C6/C6 (ca 2 Hz) were taken into account as first-order perturbations. In all cases coupling constants over four (—0.58 to —0.87 Hz) and five (+0.32 to +0.69 Hz) bonds were also considered in performing the spectral analysis. Chemical shifts and vicinal coupling constants for 48a-f are reported in Table 14. 

Similarly, the vicinal coupling constants, 3-/H( >h( +1)> also take alternating values related to the 7r-bond order by a linear relationship86. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

The combination of ring size and bond order effects explain the notable decrease in the magnitude of vicinal coupling constants in heterocyclic molecules, especially those with a five-membered ring (Figure 12.75). 

For the discussion of conformation of poly(i -oxypropylene) molecules, we have to consider stereochemistry with respect to C-C and C-0 bonds along a polymer chain, 40-CH(CH3)-CH2)n In order to get Information on the rotational Isomers around the C-C bond, a deuterated poly(/ -oxypropylene) was prepared starting from 2Ms-deuterated methyloxirane monomer (7),W. The vicinal coupling constants, Jac, between the methj ne and the methine protons were found to be 5.3, 4.9 and 5.2 (Hz) In cyclohexane, chloroform and benzene, respectively. By assuming the standard values J o =2.6 (Hz) and Ji8f=9.3 (Hz) reported In a variety of 1,2-d1oxygen-subst1tuted propane derivatives. It was possible to estimate the population of the three rotational Isomers In cyclohexane, chloroform and benzene, respectively. 

For the triarylmethane dye (130) and isomeric structures a correlation has been found between H chemical shifts and tt charges and vicinal coupling constants and 7r-bond orders a pronounced charge delocalization is indicated. 

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