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.