Nuclear Magnetic Resonance in Transition-Metal Diborides

Nuclear magnetic-resonance studies of the groups IV, V, and VI transition-metal diborides show that the boron nuclei have a small quadrupole coupling constant, and negative frequency shift of the order of 10—4. The V51 resonance in VB2 provides a measure of the quadrupole coupling constant for V51 as 2.35 Mc/sec and a frequency shift of —3.37×10—3. The frequency shifts are enhanced at low temperatures. These data are explained within the framework of a suggested metal—boron donor—acceptor bonds by proposing s—d exchange polarization and diamagnetic ``ring current'' contributions to the field at the nucleus.