Selective polarization transfer in homonuclear coupled spin systems. A possible one-dimensional multipulse technique for deducing CC connectivities

Abstract

In previous papers (1) we have considered the application of selective polarization transfer in heteronuclear coupled spin systems as a technique for the generation of rapid and accurate 13C,lH chemical-shift correlations without using a second Fourier transformation. In this paper we consider the homonuclear coupled case, specifically 13C-13C, and demonstrate that a homonuclear multipulse polarization-transfer procedure can be developed. Although the technique has associated with it a number of instrumental difficulties, the method does present a possible procedure whereby C-C connectivities can be deduced without using 2D NMR. Previous attempts (2) at developing such a method have used the DANTE hardpulse train for selective excitation; however, this technique allowed only a limited amount of polarization transfer. Our technique utilizes a soft “C pulse whose characteristics can be altered to produce the maximum amount of polarization transfer and, in principle, whose shape (tailored pulse) can be altered to increase its selectivity. As is well known, to induce polarization transfer in heteronuclear coupled spin systems, the initial step is the creation of a selective population inversion on one part of a multiplet. This can be done in either a selective manner using a soft pulse (3) or a nonselective manner employing a J-ordered state (4). To induce polarization transfer between homonuclear coupled spins CA and C, the two magnetization vectors associated with the spin acting as the polarization transfer source, CA, must be selectively aligned antiparallel along one transverse axis (the +x axes, for example) prior to the application of any hard pulse ((7r/2)[C, ~1 say) used to observe the complete NMR spectrum. If such an arrangement can be generated, the observed signal arising from C, will appear as a 0:2 multiplet being composed of two parts, the normal 1: 1 doublet arising from the C, population difference and a 1 :1 polarization-transfer signal arising from the CA population difference. We can satisfy the above physical requirements in the following novel way. Suppose we apply a soft carbon pulse, rf field strength Bi along the x axis at the CA resonance frequency. The two signals of interest arising from the J coupling to C’, will be *J/2 Hz off resonance from the soft pulse. We are interested in how the intensity and relative phases of these signals vary following soft-pulse excitation as the characteristics of the pulse are changed. If we define a dimensionless parameter 4 = ~?TAYIYBI, where Av is the amount off resonance of any signal of interest (as