Fundamental aspects of parahydrogen enhanced low-field nuclear magnetic resonance.

We report new phenomena in low-field 1H nuclear magnetic resonance (NMR) spectroscopy using parahydrogen induced polarization (PHIP), enabling determination of chemical shift differences, δν, and the scalar coupling constant J. NMR experiments performed with thermal polarization in millitesla magnetic fields do not allow the determination of scalar coupling constants for homonuclear coupled spins in the inverse weak coupling regime (δν<J). We show here that low-field PHIP experiments in the inverse weak coupling regime enable the precise determination of δν and J. Furthermore we experimentally prove that observed splittings are related to δν in a nonlinear way. Naturally abundant 13C and 29Si isotopes lead to heteronuclear J-coupled 1H-multiplet lines with amplitudes significantly enhanced compared to the amplitudes for thermally prepolarized spins. PHIP-enhanced NMR in the millitesla regime allows us to measure characteristic NMR parameters in a single scan using samples containing rare spins in natural abundance.