NMR line shapes from AB spin systems in solids - The role of antisymmetric spin-spin coupling

作者：Harris Kristopher J; Bryce David L; Wasylishen Roderick E^*

来源：Canadian Journal of Chemistry, 2009, 87(10): 1338-1351.

DOI：10.1139/V09-089

• J coupling
• solid-state NMR spectroscopy
• indirect nuclear spin-spin coupling
• (119)Sn NMR
• symmetry
• NMR relaxation
• antisymmetric tensor

NMR parameters such as indirect nuclear spin-spin coupling (J), nuclear magnetic shielding (sigma), direct dipolar coupling (D), and electric field gradient (V) are properly described by second-rank tensors. Each may be decomposed into isotropic, symmetric, and antisymmetric components; the number of these three components which may be nonzero is a distinguishing attribute of each interaction tensor. The rank-1 antisymmetric portion of J (J(anti)) holds the distinction of remaining the only nonzero part of these fundamental NMR interaction tensors which has never been observed experimentally. Accordingly, effects from J(anti) are usually ignored, but it is important to consider when this is valid. An experimental strategy for observing J(anti) in powdered samples of tightly coupled homonuclear spin pairs, based on ideas originally presented by Andrew and Farnell (Mol. Phys. 1968, 15, 157), is described. The theory of Andrew and Farnell is extended to powder samples, and methods for analyzing NMR spectra from powdered samples are presented. It is found that, in certain rare cases, J(anti) has the potential to affect the NMR line shapes from AB spin systems, but that even in these systems, the most intense features of the spectra are not affected and may be analyzed independently of J(anti). Furthermore, J(anti) will only have an observable effect on the NMR spectra when its magnitude is comparable with that of J(iso) and with the difference in chemical shifts (in Hz) between the two sites. Finally, the first experimental attempts to measure J(anti) are reported, and experimental proof that no elements of J(anti)((119)Sn,(119)Sn) in hexa(p-tolyl)ditin are larger than 2900 Hz is given. The benefits of modern double-quantum filtering NMR pulse sequences in isolating effects from J(anti) are also illustrated.

• 出版日期2009-10