Line narrowing of I = 12 spins coupled to quadrupolar nuclei in liquids: effects of weak decoupling fields

In this paper an exact description of the observed transverse magnetization of spin 12 nuclei, coupled to quadrupolar spins which are subjected to RF irradiation, is presented. It is shown that for on-resonance CW decoupling at weak to intermediate irradiation levels, the transverse decay of the spin 12 magnetization is modulated with a period of 1/nu(2), where nu(2) is the amplitude of the decoupling irradiation. When the spin 12 signal is created as a spin echo, and the quadrupolar resonance continuously irradiated during the echo evolution, the echo amplitudes experience much stronger modulation with period 2/nu(2). In previous treatments of such spin systems, the regime of weak decoupling power was usually neglected, and approximate analytical expressions seeking to define the "adequate" or "minimal" decoupling power, necessary to achieve the collapse of the spin 12 multiplet into a single narrow line, were derived. It is demonstrated here, both by experiments and by simulations using a full Redfield formalism, that simple analytical predictions for the T(2) decay of the spin 12 magnetization are still possible, even when the scalar relaxation is not in the "fast exchange" limit and the transverse decay is considerably modulated due to insufficient decoupling power. In this case, the expected single exponential decay rate is obtained for the nonmodulated component of the signal. The theoretical solution for spin I = 12 coupled to S = 3 is derived, and results for the proton decay in (10)B-enriched sodium borocaptate in aqueous solution are presented. The effects of irradiation by several composite pulse decoupling sequences are also considered. Copyright 1999 Academic Press.