T-1, EXCHANGE AND ANOMALOUS BROADENING IN METAL DIHYDROGEN COMPLEXES

A substantial difference is found between the observed linewidth of th e H-2 resonance in the H-2 complex [Ir(PPh3)(2)(H-2)(H) (7,8-benzoquin oline)] [BF4] (1) and the expected linewidth based on the true T-1 of the signal deduced from the initial part of the magnetization-time plo t. This procedure was adopted because exchange processes cause the mag netization to show biexponential recovery, only the first part of whic h corresponds to the true T-1 of the site. If this effect is ignored, erroneous T-1 data are obtained. These observations are made at temper atures above the T-1 minimum, where T-1 should be equal to T-2.We have called the currently unexplained contribution to the linewidth anomal ous broadening. The origin of this anomalous contribution to the linew idths of H-2 complexes remains to be elucidated. In addition, the line width increases sharply at low temperature for the H-2 but not for the HD complex and the broadening is larger at 500 MHz than at 300 MHz. A strong candidate for the origin of this effect is slowing of H-2 rota tion with quantum coupling effects making the J(H,H') very large.