Polarization Transfer NMR Spectroscopy for Silicon-29: The INEPT and DEPT Techniques

Abstract

Publisher Summary Silicon-29 nuclear magnetic resonance (NMR) is a highly valuable form of spectroscopy for organosilicon compounds; it is only now becoming a routine technique. Silicon nuclei that are coupled to protons, the NMR spectrum can be greatly improved by the transfer of polarization from 1 H to 29 Si. Polarization transfer techniques have been available for over a decade, they have not been widely used to obtain 29 Si-NMR spectra. J-cross polarization (JCP), which evolved from methods used for enhancing the solid-state spectra of rare spin nuclei, has been applied to 29 Si-NMR spectroscopy. Selective polarization transfer (SPT) is another NMR technique recently used to obtain enhanced 29 Si-NMR spectra. To obtain SPT spectra, a silicon satellite of a proton signal is selectively irradiated prior to a nonselective 29 Si pulse. Two new polarization transfer techniques have recently been reported: insensitive nuclei enhanced by polarization transfer (INEPT) and distortionless enhancement by polarization transfer (DEPT). There are differences between the INEPT and DEPT techniques; the fundamental polarization transfer mechanism is the same. Dodecamethylcyclohexasilane was chosen as a representative member of this important class of compounds upon which to test INEPT and DEPT methods. Siloxanes represent another large class of silicon compounds for which 29 Si-NMR data would be useful. Bendall, Pegg, and Doddrell showed that the results of multipulse sequences, such as INEPT and DEPT, can be analyzed using the Heisenberg vector approach. DEPT theory is best described mathematically. Two different analyses have been employed: the Schrodinger approach and the Heisenberg approach.