Nuclear electric quadrupole relaxation

Nuclei with spin quantum number 1 greater than or equal to 1 have an asymmetric charge distribution and thus possess an electric quadrupole moment eQ. The interaction of the electric quadrupole moment with the electric field gradient of the bonding electrons provides the dominant relaxation mechanism of the quadrupolar nuclei, which are approximately three-quarters of the naturally occurring magnetic nuclei. This review summarizes the basic principles of nuclear electric quadrupolar relaxation. Particular emphasis is given to the effects of the magnitude of the quadrupole moment, the spin quantum number, the linewidth factor, the electric field gradient (which originates from charge density variations of the bonding electrons near the nucleus), and the effective correlation time for molecular tumbling. Relaxation outside the extreme narrowing condition for integer and half-integer spins is examined and experimental examples are provided. Modulation of the electric field gradient due to time-dependent intermolecular interactions and chemical exchange are discussed briefly, together with the effects of relaxation mechanisms other than quadrupolar. (C) 1996 John Wiley & Sons, Inc.