Origin of Shielding and Deshielding Effects in NMR Spectra of Organic Conjugated Polyynes

Nucleus
independent chemical shifts (NICS) for conjugated
polyynes H­(CC)nH are reported
(n = 2–10). There is marked shielding within
the carbon chain, gradually decreasing with radial distance and transitioning
to a weakly deshielding region at 2.5–3.0 Å, as well as
other trends. Interestingly, the magnitude of the shielding is only
very weakly dependent upon the chain length or distance from the termini. 1H and 13C NMR chemical shifts are also computed
and the trends analyzed.