Spin-selective multiple quantum excitation: Relative signs of the couplings and ambiguous situations

Homonuclear higher quantum NMR spectra of heteronuclear spin systems result in fewer transitions aiding the analyses. In such experiments the spin states of the heteronuclei do not get disturbed in both single and multiple quantum dimensions resulting in the separation of active homonuclear and passive heteronuclear couplings in two dimensions. The cross sections of the single-quantum dimension get displaced according to the strengths of the passive couplings. The directions of the displacement of these cross sections provide relative signs among the passive couplings. The present study demonstrates the situations when the displacement vectors, though provide the relative signs, could be ambiguous. The dynamics of the spin systems in homo- and heteronuclear multiple quantum studies have been discussed using polarization operator approach. The experimental results on 13C- and 15N-labeled isotopomers of acetonitrile, in both isotropic and thermotropic liquid crystalline phases, are reported.