Phenomena in J-coupled nuclear magnetic resonance spectroscopy in low magnetic fields

摘要

We present the theory and experimental results of phenomena associated to J-coupled nuclear magnetic resonance (NMR) spectroscopy at low magnetic fields (< 10(-4) T). So far it was believed that in low field the chemical shift and with it the homonuclear J-coupling information is lost. This contribution shows that the network of all homo- and heteronuclear J-coupling constants can be measured in low magnetic fields, thus revealing the whole molecular structure even in the absence of any chemical shift information. The chemical group of the form YXN (Y=rare spin 1/2, X=observed spin 1/2, N=number of spins X) can be identified by the number of lines in the heteronuclear coupled X spectrum if the strong J-coupling condition is valid. If two molecular groups, such as YXN and AX(M-N) (A=group without nuclear spin, M=total number of coupled spinsX), are bound together then all homo- and heteronuclear J-coupling constants appear in the X-NMR spectrum as pairs of multiplets. A vector model is presented which explains the relation between the molecular structure and the number of observed lines in a multiplet pair. The linewidths of the different NMR lines inside one multiplet are measured to be substantially different and depend on the total spin state of the molecule. If M is an odd number and M-1 spins X of the molecule are coupled into (M-1)/2 singlets, then intramolecular dipole-dipole relaxation as well as J-coupling mediated relaxation processes are suppressed and very narrow lines are observed.