Applications of shaped pulses in NMR spectroscopy.

摘要

Traditional methods of performing NMR spectroscopy involve the application of rectangular pulses which, when constructed into complex pulse sequences, can probe many details of molecular structure and reactivity. Rectangular pulse sequences are, however, vulnerable to many imperfections which can distort and ultimately misrepresent true chemical composition.;Another class of shaped pulses has been developed for use in solvent suppression pulse sequences. The "narrow reject" and self-refocused pulse shapes were used in simple one-dimensional and two-dimensional (COSY) experiments and were shown to yield excellent suppression over a region of the spectrum.;Pulse shaping has also been extended to three level systems where a shaped pulse was designed for broadband excitation in a two pulse quadrupole echo and inversion pulse sequences. This pulse shape was shown, through simulations and experiments, to triple the bandwidth available from a rectangular pulse sequence and eliminate the distortions seen in using composite pulses in these same applications. Inversion sequences were also simulated, but no experiments have been performed.;The difficulties with each of these experiments are detailed as are the modifications that are necessary to make a commercial NMR spectrometer capable of giving undistorted shaped pulses. The pulse shaping hardware and software are presented as are copies of the simulation programs used for three level systems.;In order to correct some of the shortcomings of rectangular pulse sequences, we have introduced shaped pulses, which can replace rectangular pulses or pulse sequences, in many experiments. We have also introduced a perturbation expansion which allows us to better understand the effects of pulse shaping in two level systems. Shaped pulses, as we have shown experimentally, use much less peak power and have greater frequency selectivity than rectangular pulses with similar bandwidths. These "Hermite" pulses also retain phase coherence which was demonstrated experimentally by constructing a composite inversion sequence using shaped pulses to eliminate the off-resonance oscillations and increase the on-resonance inversion. We have also demonstrated a self-refocused selective pulse shape which eliminates phase-roll which is present when pulses of long duration are used. Using this pulse, super-selective experiments are now possible, after which, the peaks in the excited region remain phased.