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Abstract

The proton NMR in single-crystal spheres of trans-diiodoethylene has been studied by multiple-pulse techniques at room temperature. In certain orientations the linewidth was found to be as low as 115 Hz despite the presence of the abundant 1211 Spins giving rise to strong dipole-dipole interactions with the protons. If these interactions were static, the resulting linewidths in the 1H multiple-pulse spectra would be of the order of 10 kHz. The narrow lines observed show that the quadrupolar relaxation of the 127I spins effectively reduces these dipolar interactions resulting in self-decoupling. The self-decoupling efficiency, however, shows a marked angular dependence, as evidenced by the angular variation of the proton linewidth, which has a maximum value of 410 Hz. To understand this, the dipolar coupling between a spin and a spin experiencing a strong quadrupole interaction, as is the case for 1271 in trans-diiodoethylene, is discussed in detail. It is shown that the angular dependence of the self-decoupling is not due to differences in the static 1H-127I dipolar coupling, but reflects the angular variation of the 127I relaxation rates. The data were analyzed to yield the proton shielding tensor σ in trans-diiodoethylene : σxx = −4.1, σyy = −5.9, σzz = −8.6 ppm relative to TMS. The least-shielded direction, the z axis, is perpendicular to the molecular plane, and the x and y axes both form an angle of 45° with the CC double bond.