Broadband Homonuclear Decoupling in Heteronuclear Shift Correlation NMR Spectroscopy

Abstract

Recently, Garbow, Weitekamp, and Pines introduced a new approach to homo-nuclear broadband decoupling (I). Independently, a closely related experiment has been proposed for semiselective heteronuclear J spectroscopy (2). The two types of experiments have the common feature that all protons which are not directly attached to a 13C nucleus experience an effect as if a nonselective 180 ” pulse were applied, while protons bonded to a r3C nucleus are not affected. This type of behavior can be used conveniently in many of the heteronuclear types of two-dimensional exper-iments (3-9). Here, its use for sensitivity and resolution enhancement in heteronuclear spectroscopy will be demonstrated. It is mentioned here that other approaches to homonuclear broadband decoupling in heteronuclear shift correlation spectroscopy have been proposed by Frenkiel (JO), but those experiments rely on a different principle (II), and are of a less general nature. The experimental scheme is sketched in Fig. 1. The theory of the conventional heteronuclear decoupled shift correlation experiment (3) has been described elsewhere (3-6) and will be repeated here. The difference between the conventional heteronuclear decoupled shift correlation experiment and the scheme of Fig. 1 is that the single 180 o 13C pulse at the center of the evolution period has now been replaced by a 90”,(‘H)-1/(2J)-180°,(‘H), 180”,(‘3C)-l/(2J)-900-,(‘H) sequence, where J is the magnitude of the heteronuclear coupling constant in hertz. It has been shown (I, 2) that this sequence has the effect of a single 180 ” pulse for those protons that are not attached to a 13C nucleus, while other protons (those attached to 13C) are essentially t,/2 v2.J 112.l t,12 A, ’ A2 ’ 12 FIG. 1. Pulse scheme of the homonuclear broadband decoupled shift correlation experiment. The delays A, and A2 are set to the same values as in the conventional shift correlation experiment (3, 6). The phases 4 and # are cycled according to Table 1