Chemical shift tensors of directly bonded phosphorus nuclei in unsaturatedfour-membered rings. Solid-state P-31 NMR and theoretical study of trans- and cis-substituted diphosphetes

The chemical shift (CS) tensors of two four-membered heterocyclic diphosphe tes, trans- and cis-1,2-dihydro-1-methyl-2-phenyl-3,3-bis(tert-butyl)-1,2-d iphosphete, have been investigated by solid-state P-31 NMR spectroscopy. Th e dipolar chemical shift method has been employed to determine the magnitud es of the principal components of the P-31 CS tensors in these compounds, a s well as their orientations relative to the (31)p-(31)p dipolar vector. Th e spans of the phenyl- and methyl-substituted phosphorus CS tensors of the trans isomer are 248 and 280 ppm, respectively, significantly larger than t he corresponding values for the cis isomer, 139 and 173 ppm. The orientatio ns of the CS tensors in the molecular framework are proposed on the basis o f ab initio calculations using the gauge-independent atomic orbitals method . The combined experimental-theoretical results suggest that, for both the trans and cis isomers, the least shielded principal components, delta(11), are oriented in the plane defined by the four membered ring. For the trans isomer, the most shielded principal components, delta(33), are approximatel y perpendicular to this plane; however, for the cis isomer, it is the delta (22) components that are perpendicular to the plane of the four-membered ri ng. The spectra of magic-angle spinning samples of the trans isomer exhibit asymmetric, spinning-frequency dependent line shapes, while those of the c is isomer are invariant to spinning frequency. These observations are consi stent with the CS tensors derived from the P-31 NMR spectra of stationary s amples.