Gary Dabbagh

Measurement of nuclear magnetic dipole—dipole couplings in magic angle spinning NMR

Abstract We describe a method for measuring nuclear magnetic dipole—dipole couplings in NMR spectra of solids undergoing rapid magic angle spinning (MAS). We show in theory, simulations, and experiments that the couplings, which are averaged out by MAS alone, can be recovered by applying simple resonant radiofrequency pulse sequences in synchrony with the sample rotation. Experimental 13 C dipolar powder pattern spectra of polycrystalline ( 13 CH 3 ) 2 C(OH)SO 3 Na obtained in a two-dimensional experiment based on this method are presented. The method provides a means of determining internuclear distances in polycrystalline and noncrystalline solids while retaining the high resolution and sensitivity afforded by MAS.

Determination of chemical-shift-anisotropy Lineshapes in a two-dimensional magic-angle-spinning NMR experiment

Abstract We describe a technique for measuring chemical-shift-anisotropy (CSA) lineshapes in NMR spectra of polycrystalline or amorphous solids. By combining magic-angle spinning (MAS) with a radiofrequency pulse sequence synchronized with the sample rotation in one time period of a two-dimensional experiment, we obtain two-dimensional spectra in which the CSA lineshapes appear along one axis and the normal MAS spectrum appears along the other axis. The CSA lines are thereby resolved as long as the inequivalent nuclei have resolved isotropic chemical shifts in the MAS spectrum. Our technique differs from previous, related techniques in that we employ pulse sequences designed so that the CSA lineshapes in the two-dimensional spectrum are precisely the same as those obtained from one-dimensional spectra of nonspinning samples in the absence of spectral overlap; the analysis of the spectra is thus simplified substantially. We describe the theory and experimental implementation of the technique in detail, and present resolved 13C CSA lineshapes for methyl-α- d -glucopyranoside. We analyze the effects of pulse imperfections on the observed lineshapes and show how such effects can be minimized.

The preparation of lithium organocuprates from various Cu(I) salts

Abstract Various Cu(I) salts have been tested as precursors for organocuprates in side-by-side comparisons under controlled conditions. CuCN and CuBr·SMe2 appear to be superior to CuI, CuBr, and CuCl. CuSCN and CuOTf are also good precursors in some circumstances.

Factors governing the thermal stability of organocopper reagents. Two new classes of heterocuprates with greatly improved thermal stability

New Heterocuprates based upon phosphido-and amido-ligands have exceptional thermal stability, which can be traced to steric, electronic, and co-ordination effects.

Conjugate addition of lithium isopropyl dithioisobutyrate to 4-acetoxycyclopent-2-enone. Assignment of stereochemistry in flexible 5-membered rings

Lithium isopropyl dithioisobutyrate is alkylated on the S atom by 4-bromocylopent-2-enone but on the C atom by cyclopent-2-enone and 4-acetoxycyclopent-2-enone via conjugate addition; the stereochemistry of the 3,4-disubstituted cyclopenanone from the latter reaction is established by the determination of the long-range coupling constants, 4JHH.

Solid State Dynamics of C60 And C70 Revealed by Carbon-13 NMR

Carbon-13 NMR spectra of powder samples of pure C 60 and of a mixture of C 60 and C 70 provide information about molecular motions in the solid state. At room temperature, C 60 molecules rotate rapidly and isotropically. The transition from a rapidly rotating to a stationary (on the time scale of the experiment) system occurs over a temperature range from 120 K to 60 K, suggesting a distribution of activation energies. C 70 molecules also rotate rapidly at room temperature, but more anisotropically.

Titanium enolates from epoxides and bis(pentamethylcyclopentadienyl)dimethyltitanium(IV)

Typical epoxides such as cyclohexene oxide and cis- and trans-but-2-ene oxide are converted to titanium enolates by heating at 80 °C with the title compound.