It is widely recognized that adiabatic fast passage is the most tion applications there is no significant loss of signal through effective method for inverting nuclear spins over a wide range of relaxation on this timescale, but during an adiabatic sweep chemical shifts in situations where radiofrequency power is limthere is an appreciable variation in the instant at which a ited. One possible drawback of an adiabatic sweep is that different given group of spins is inverted, determined by their chemigroups of spins are inverted at slightly different times, and this cal shift.
can perturb the symmetry of refocusing experiments by apprecia-
In the majority of these experiments the frequency range ble amounts ( milliseconds) , introducing a serious phase disperof the adiabatic sweep is very large compared with the radiosion. We demonstrate how a pair of adiabatic pulses can be used frequency field intensity (expressed in frequency units), and to compensate such refocusing errors. Usually, but not always, to a good approximation we may neglect the relatively short this requires the use of two adiabatic pulses with opposite directime required to achieve a given degree of spin inversion.
tions of frequency sweep. One important application is the ''isotope filter,'' a scheme for suppressing all proton responses except We may regard inversion as occurring at the point where those from directly bound 13 CH groups; a pair of opposed adiathe adiabatic frequency sweep passes through the chemical batic pulses allows compensation of refocusing errors over a range shift of a given group of spins. However, there will be a of different spin inversion times. In an extension of this technique, significant time difference for spin inversion of groups of a range of 13 CH coupling constants can also be accommodated spins with different chemical shifts. by exploiting the rough linearity between the 13 C chemical shift Several authors (16-19) have studied the problem of and the magnitude of 1 J CH . Heteronuclear two-dimensional excompensating these timing variations when an adiabatic periments that involve inverting 13 C spins during the evolution pulse is used to refocus chemical shift effects in a homonuperiod can also benefit from a compensating pair of adiabatic clear spin-echo sequence. Here we address a different quespulses, in this case with the same sense of frequency sweep. Fition-how to compensate the evolution due to spin-spin nally, a pair of opposed adiabatic pulses is used in a new scheme coupling when adiabatic pulses are used in heteronuclear ( ''ECHO-WURST'') for suppressing cycling sidebands in broadband heteronuclear decoupling; we show a 500-MHz proton spec-spin-echo experiments. The present article proposes several trum decoupled from 13 C, where the residual cycling sidebands new schemes that balance the evolution of spin-spin couare below 0.06%. ᭧ 1997 Academic Press
pling caused by one adiabatic pulse against the opposing effect of a second adiabatic pulse, usually with the direction of frequency sweep reversed. We illustrate this use of ''compensating pairs'' of adiabatic pulses with reference to filtra-