Study of coupled spin systems by flip-angle dependent one and two-dimensional NMR experiments

Abstract

Flip-angle effects of the RF pulses on systems in non-equilibrium states are studied in detail for three different systems oriented in liquid crystalline phase ZLI-1167. It is shown that in the non-linear regime the effect of the pulse on such systems is such that it picks up various transitions connected in a coupled network depending on the flip angle of the pulse. When the pulse is of a small angle then it acts as a linear pulse, picking up only the transitions directly connected to the perturbed transition. In order to obtain a complete connectivity information, a two-dimensional experiment is proposed which yields the connectivity of all the transitions. A computer program for reconstruction of the energy-level diagram, from the connectivity information, has been developed. This logic yields the complete energy-level diagram which in turn helps in obtaining all the parameters of the spin-Hamiltonian. This algorithm has been utilized for the analysis of the spectra of three oriented molecules namely, (i) Acetone (ii) Benzene and (iii) Cis, cis-mucononitrile. In all the cases it is possible to obtain all the parameters of the spin-Hamiltonian from the connectivity information. However, it may not be always possible to do so in the case of complex spin systems. In such cases, after constructing the energy-level diagram using the above algorithm, the parameters of the spin-Hamiltonian can be obtained by the iterative method outlined by Pfandler et al. (26). To conclude, it may be mentioned that strong coupling can give rise to cross peaks unconnected to either relaxation or zero-quantum coherences in the NOESY spectrum. These effects should be taken into account while attempting to use NOESY cross-peak intensities for distance estimation. Fig. III.9: 2D NOESY (90°-U-90°-?m-90°-t2) spectrum of acetone oriented in ZLI-1167, recorded with a negligible mixing time (?m ? 1 ?sec). Data points were collected with 8 scans per increment and zero-filled once in t1. 2D Fourier transformation was done. The transitions in the 1D spectrum belong to irreducible representations namely A1g and B1u. The cross-peaks in this spectrum are within each irreducible representation. Fig. III.10: Same 2D spectrum as in Fig. III.9, except that zero-quantum coherences are shifted during the interval ?m using ?m = ?1 + k?2, where ?1 = 3 ?sec and k = 2, yielding a maximum value of ?m = 125 msec. Fig. III.11: Cross-sections parallel to ?2 taken from Fig. III.10 at ?1 = d, h, k and i with the corresponding calculated cross-sections shown below each experimental cross-section. Transitions d and h belong to A1 domain, transition k belongs to B1 domain and transition i belongs to A1 domain.