Bent Allenes or Di-1,3-betainesAn Answer Given on the Magnetic Criterion

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent allene 1, the corresponding C-extended 1,3-butadiene derivative 2, and a number of related compounds 3–20 have been calculated using the gauge-independent atomic orbital perturbation method, employing the nucleus-independent chemical shift concept and visualized as isochemical shielding surfaces of various sizes and directions. Prior to that, both structures and 13C chemical shifts were calculated and compared with available experimental bond lengths and δ­(13C)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the δ­(13C)/ppm, and the TSNMRS values are employed to qualify and quantify the electronic structure of the studied compounds in terms of dative or classical electron-sharing bonds.