Proton Spin—Spin Coupling Constants from the NMR Spectra of Paramagnetic Molecules

Abstract

``Chemical shifts'' for protons in the paramagnetic Ni II aminotroponeimineates are up to 250 times greater than for shifts of analogous protons in the diamagnetic chelates or ligands. These increased shifts are attributed to isotropic hyperfine contact interactions between protons and pπ spin densities transferred from Ni II to aminotroponeimine ligand as the result of pπ—dπ bonding. Widths of the nuclear resonances for these chelates in solution are sufficiently narrow that in many cases spin—spin (I·I) couplings can be resolved. The large separations between the different resonances lead to simple first-order spectra from which the spin—spin coupling constants can be obtained directly. Proton coupling constants for a number of simple conjugated groups obtained in this way are reported and compared with values for analogous diamagnetic compounds. Close agreement is found. Proton coupling constants have also been obtained for the first time for a number of more complex groups which give rise to prohibitively complicated spectra in diamagnetic compounds. For a number of olefinic systems the coupling constants for cis protons are found to correlate with the C=C bond length.