Compensation for spin-spin coupling effects during adiabatic pulses

It is widely recognized that adiabatic fast passage is the most effective method for inverting nuclear spins over a wide range of chemical shifts in situations where radiofrequency power is limited. One possible drawback of an adiabatic sweep is that different groups of spins are inverted at slightly different times, and this can perturb the symmetry of refocusing experiments by appreciable amounts (milliseconds), introducing a serious phase dispersion. We demonstrate how a pair of adiabatic pulses can be used to compensate such refocusing errors, Usually, but not always, this requires the use of two adiabatic pulses with opposite directions of frequency sweep, One important application is the ''isotope filter,'' a scheme for suppressing all proton responses except those from directly bound (CH)-C-13 groups; a pair of opposed adiabatic pulses allows compensation of refocusing errors over a range of different spin inversion times, In an extension of this technique, a range of (CH)-C-13 coupling constants can also be accommodated by exploiting the rough linearity between the C-13 chemical shift and the magnitude of (1)J(CH). Heteronuclear two-dimensional experiments that involve inverting C-13 spins during the evolution period can also benefit from a compensating pair of adiabatic pulses, in this case with the same sense of frequency sweep. Finally, a pair of opposed adiabatic pulses is used in a new scheme (''ECHO-WURST'') for suppressing cycling sidebands in broadband heteronuclear decoupling; we show a 500-MHz proton spectrum decoupled from C-13, where the residual cycling sidebands are below 0.06%. (C) 1997 Academic Press.