CALIBRATION OF STUD-BAND ADIABATIC DECOUPLING IN A SINGLE TRANSIENT( PARAMETERS TO ACHIEVE OPTIMALLY EFFICIENT BROAD)

To provide the most efficient conditions for spin decoupling with leas t RF power, master calibration curves are provided for the maximum cen terband amplitude, and the minimum amplitude for the largest cycling s ideband, resulting from STUD+ adiabatic decoupling applied during a si ngle free induction decay. The principal curve is defined as a functio n of the four most critical experimental input parameters: the maximum amplitude of the RF field, RFmax the length of the sech/tanh pulse, T -p, the extent of the frequency sweep, bwdth, and the coupling constan t, J(o). Less critical parameters, the effective (or actual) decoupled bandwidth, bw(eff,) and the sech/ tanh truncation factor, beta, which become more important as bwdth is decreased, are calibrated in separa te curves. The relative importance of nine additional factors in deter mining optimal decoupling performance in a single transient are consid ered. Specific parameters for efficient adiabatic decoupling can be de termined via a set of four equations which will be most useful for C-1 3 decoupling, covering the range of one-bond (CH)-C-13-H-1 coupling co nstants from 125 to 225 Hz, and decoupled bandwidths of 7 to 100 kHz, with a bandwidth of 100 kHz being the requirement for a 2 GHz spectrom eter. The four equations are derived from a recent vector model of adi abatic decoupling, and experiment, supported by computer simulations. The vector model predicts an inverse linear relation between the cente rband and maximum sideband amplitudes, and it predicts a simple parabo lic relationship between maximun sideband amplitude and the product J( o)T(p). The ratio bwdth/(RFmax)(2) can be viewed as a characteristic t ime scale, tau(c), affecting sideband levels, with tau(c) approximate to T-p giving the most efficient STUD+ decoupling, as suggested by the adiabatic condition. Functional relationships between bwdth and less critical parameters, bw(eff) and beta, for efficient decoupling can be derived from Bloche-quation calculations of the inversion profile for a single sech/tanh pulse. Residual splitting of the centerband, norma lly associated with incomplete or inefficient decoupling, is not seen in sech/tanh decoupling and therefore cannot be used as a measure of a diabatic decoupling efficiency. The calibrated experimental performanc e levels achieved in this study are within 20% of theoretical performa nce levels derived previously for ideal sech/tanh decoupling at high p ower, indicating a small scope for further improvement at practical RF power levels. The optimization procedures employed here will be gener ally applicable to any good combination of adiabatic inversion pulse a nd phase cycle. (C) 1998 Academic Press.