Push–pull allenes-conjugation, (anti)aromaticity and quantification of the push–pull character

Structures, 1H/13C chemical shifts, and π electron distribution/conjugation of an experimentally available and theoretically completed set of push–pull allenes Acc2Cdouble bond; length as m-dashCdouble bond; length as m-dashCDon2 (Acc=F, CHO, CF3, Ctriple bond; length of mdashN; Don=t-Bu, OMe, OEt, SMe, SEt, NCH2R) have been computed at the DFT level of theory. Both orthogonal linear and orthogonal bent structures have been obtained. In the latter case the push–pull character could be quantified by the quotient method. The 13C chemical shift of the central allene carbon atom C-2 and chemical shift differences Δδ(C-1, C-2) and Δδ(C-2, C-3) of allene carbon atoms proved to be a quantitative alternative. TSNMRS of ring-closed push–pull allenes have been computed in addition and were employed to identify polar, carbene-like and carbone-like canonical structures of these molecules.