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.