JOURNAL OF MAGNETIC RESONANCE 61,22-27 (1985) Applications of 95Mo NMR Spectroscopy. XI. 95M~, 97M~, and 170 Relaxation Times, Quadrupole Coupling Constants, and the Molybdenum Relaxation Mechanism in MOM ROBERT T. C. BROWNLEE,* MAXWELL J. O’CONNOR, B. PHILIP SHEHAN,* AND ANTHONY G. WEDD Department of Chemistry, La Trobe University, Bundoora, Victoria, 3083, Australia Received March 20, 1984; revised June 25, 1984 Spin-lattice relaxation times (T,‘s) of 95Mo, 97Mo, “0, and 13C for Mo(CO), in CDC& are reported. The rotational correlation time of the molecule is obtained from the chemical-shift anisotropy relaxation of “C. Quadrupole coupling constants are calculated for 95Mo, 97Mo, and “0. The results indicate that 95Mo and 97Mo relaxation is entirely quadrupolar and is caused by rotational reorientation of a permanent quadrupole coupling constant rather than from solvent molecule collisions and vibrationally induced momentary electric field gradients. 0 1985 Academic PXS. IIIC. Over the last few years, a growing number of reports on 95Mo NMR have appearedin the literature, and a chemical-shift scale covering 7000 ppm has been established (1). Apart from two early studies of aqueous molybdate (2, 3), much less attention has been paid to the measurement of 95Mo relaxation times. As part of a general study of the factors governing 95Mo relaxation and its possible utility as a probe of molecular structure and molecular dynamics, we have carried out a detailed investigation of relaxation processes in MOM. The 95Mo spin-lattice relaxation time (TI) for this compound in CDC13 at 40°C is 7 s. This is an exceptionally long relaxation time for a quadrupolar nucleus and is a consequence of the octahedral symmetry of the complex. The question therefore arisesas to what extent the quadrupolar mechanism contributes to relaxation and how such a mechanism may operate. Spin-lattice relaxation rates due to various mechanismsj are additive (4): T;’ = 2 T,‘. [II For spin > i nuclei, such as 95Mo(I = $), the quadrupolar relaxation rate is generally severalorders of magnitude larger than that due to any other mechanism. Relaxation may then be regardedas being totally quadrupolar and is describedby (4) * To whom correspondenceshould be addressed. 0022-2364185$3.00 22 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved PI