E-BURP

The last three pulses listed in table 1, namely the two E-BURPs and the are non-universal pulses designed to transfer Zeeman polarization Iz) into detectable transverse magnetization I or ly), but not to achieve the reverse operation. The conversion of or ly into Iz can be achieved by using the time-reversal of the considered pulse shape. An alternative... 

Fig. 1. Computer simulations of four selective excitation pulses. (Top) Pulse shapes. From left to right 90° rectangular pulse, 270° Gaussian truncated at 2.5%, Quaternion cascade Q, and E-BURP-1. The vertical axis shows the relative rf amplitudes, whereas the horizontal axis shows the time. (Middle) Trajectories of Cartesian operators in the rotating frame...

Fig. 5. Experimental calibration of a 15 ms E-BURP-1 around the optimum amplitude value on a DMX300. (Top) Excerpt of the one-dimensional spectrum of BPTI. (Bottom) Same region centered around 7 ppm excited by a 15 ms E-BURP-1 with amplitudes increasing from left to right by 1 dB steps. The optimum is located at the intermediate value (determined by smaller amplitude steps, typically of 0.1 dB).

Better profiles may sometimes be needed, in which case an E-BURP-1 is recommended, particularly for band-selective experiments. The calibration is relatively easy and very narrow transition regions are very useful (fig. 1). If a universal pulse is needed, the E-BURP-1 may either be replaced by a pulse, or if only a conversion of or ly into Iz is required, by a time-reversed E-BURP-1. 

FIGURE 9.6 Illustration of the E-BURP selective excitation pulse, (a) Depiction of the time excitation function. (b) Frequency domain excitation profiles for absorption (solid line) and dispersion... 

As noted above, one implicit impediment to the utilization of SIMBA is the requirement of an accurate chemical shift for the resonance to be pulsed selectively. The problem is exacerbated rather than ameliorated when Gaussian 270° pulses are employed. The need for an accurate chemical shift is circumvented by utilizing the recently reported E-BURP-2 pulse of Geen and Freeman (1991) for the final 90° pulse of the SIMBA experiment. Since the E-BURP-2 pulse gives uniform phase and excitation across a relatively wide bandwidth, in contrast to the Gaussian 270° pulse which provides uniform phase and excitation across only a relative narrow... 

An experiment that actually preceded SIMBA, but which probably has somewhat lower utility, was the selective ID HMQC-TOCSY experiment described by the authors (Crouch et al. 1990c). The selective experiment differs from the HMQC-TOCSY pulse sequence only in the replacement of the final C 90° pulse by a selective pulse. Initially, we utilized a square pulse in this application but, obviously, the improvement in the SIMBA experiment by using a Gaussian 270° or an E-BURP-2 pulse would be realized in this experiment as well. To the best of our knowledge, there have been no applications of this experiment to alkaloids reported in the literature. 

From equation (8) three modes of adiabatic spin inversion using RF pulses may be defined as (a) amplitude modulated pulses, e.g. I-BURP [13], G3[16], I-SNOB [17], (b) frequency modulated pulses, e.g. chirp [18,20], tangential sweep [20,21] and (c) both amplitude and frequency modulated pulses, e.g. the hyperbolic secant [22] or WURST (Wide band Uniform Rate Smooth Truncation) [23] pulse. 

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