JOURNAL OF MAGNETIC RESONANCE 62, 346-349 (1985) Some Aspects of Coherence Transfer by Isotropic Mixing N. CHANDRAKUMAR* AND S. SUBRAMANIAN~ *Chemical Laboratory, Central Leather Research Institute, Adayaru, Madras 600020, Tamil Nadu, India, and tRegiona1 Sophisticated Instrumentation Centre, Indian Institute of Technology, Madras 600036, India Received October 15, 1984; revised December 26, 1984 Coherence transfer (CT) between spins is mediated by a suitable interaction that couples them; for example, weak scalar coupling between nuclei is basic to CT in the INEPT and DEPT (1) experiments. Evolution under strong coupling, on the other hand, leads to certain characteristic differences in CT processes compared to the weak coupling situation. For heteronuclear spin systems in liquids, spin locking with Hartmann-Hahn- matched rf fields introduces strong-coupling effects, leading to net coherence transfer, which is the J-cross polarization process (2-4). This is to be contrasted with the INEPT situation, which leads basically to no net CT. More recently, Braunschweiler and Ernst (5) have demonstrated net CT in homonuclear spin systems as well, introducing strong coupling by a rapid train of pulses to achieve spin locking (5). All the components of the scalar coupling Hamiltonian are operative in this situation, which has therefore been termed “isotropic mixing.” This process has been analyzed for an AX system, and has been employed to achieve total J correlation in the homonuclear TOCSY experiment (5). Isotropic strong coupling in the average Hamiltonian sense has also been proposed by Weitekamp et al. (6) as a means of net heteronuclear CT in solids, by employing identical pulse sequences on both spin channels, an experiment termed SHRIMP (6). In an attempt to understand net CT phenomena under scalar coupling, we have undertaken a study of AIWXN systems evolving under collective modes. We define a collective mode as an evolution of coupled spin systems, wherein one or both components of the total transverse spin angular momentum is a constant of the motion. This confers on the system the unique property that each partner in the coupling evolves with identical frequency/frequencies. In contrast, single spin modes, which do not conserve either component of the total transverse spin angular momentum, can lead to different evolution frequencies of the partners in coupling. With this general definition, the Hartmann-Hahn mixing in liquids (2-d), in which the spin-locked component of the total spin angular momentum is conserved, is obviously one means of establishing collective modes. During isotropic mixing on the other hand, both components of the total transverse spin angular momentum are conserved. Our studies are aimed at unravelling the salient features of isotropic mixing as a collective modes process that is distinctly different from the Hartmann- Hahn experiment in liquids. 0022-2364185 $3.00 Copyright @ 1985 by Academic Press. Inc. All rights of reproduction in any form reserved. 346