Nuclear Magnetic Resonance Gas-Phase Studies of Spin-Spin Couplings in Molecules

This paper overviews gas phase experiments with respect to one fundamental part of nuclear magnetic resonance (NMR) spectra. Indirect spin-spin coupling is an important parameter of NMR spectra and is observed as the splitting of spectral signals. A molecule containing two different magnetic nuclei (e.g., hydrogen HD, HT, or DT) exhibits this interaction in an external magnetic field measured as the spin-spin coupling parameter, nJ(NN′). Modern quantum chemical methods allow the precise calculation of spin-spin coupling, but it is never easy because nJ(NN′) is modified by temperature and intermolecular interactions. Accurate calculations can be performed only for small isolated molecules. NMR spectroscopy can deliver measurements of spin-spin couplings for isolated molecules if nJ(NN′) parameters are observed in the gas phase as a function of density. The extrapolation of such measurements to the zero-density limit permits nJ0(NN′) determination free from intermolecular interactions. The latter technique can also be applied to liquid vapors in molecules like acetonitrile or water. Spin-spin couplings across one chemical bond (1J0(NN′)) are the largest and most important for theoretical modeling. The present review reports numerous 1J0(NN′) parameters recently measured by multinuclear NMR spectra of gaseous samples.