Carbon coupling constants ring strain

Carbon-hydrogen spin-spin coupling constants are included in a review by Hansen and discussed in terms of ring strain, steric effects, electronegativity, lone pair effects and electric field effects. Additivity of these effects in a few systems is commented on. 

The C NMR spectrum of cyclobutadiene-iron tricarbonyl consists of a singlet at 209.0 ppm for the carbonyl carbons and a doublet centered at 61.0 ppm, Jc-H = 191 Hz (193). The large C-H coupling constant suggests hybridization at carbon between sp and sp which is consistent with the strained cyclobutadiene ring. In a plot of C versus chemical shifts the cyclobutadiene resonances fall on the same line as resonances for aromatic molecules, their ions and their metal n complexes. 

C-proton coupling constants ( 7) are very dependent on the hybridization of the carbon atom. Strained ring compounds have larger coupling constants because of increased s-character. Reich, Hans J., University of Wisconsin http //www.chem.wise.edu/areas/reich/nmr/06-cnn-05-ljch.htm 7ch. 7ch. Tch coupling constants... 

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