JOURNAL OF MAGNETIC RESONANCE 55, 157-163 (1983) Low-Pass J Filters. Suppression of Neighbor Peaks in Heteronuclear Relayed Correlation Spectra H. KOGLER, 0. W. WRENSEN, G. BODENHAUSEN, AND R. R. ERNST Laboratorium fir Physihzlische Chemie, Eidgeniissische Technische Hochschule, 8092 Zurich, Switzerland Received June 20, 1983 ‘This communication describes a procedure for the selective removal of signals associated with coupling constants J exceeding a lower limit J,,,i,,. The procedure may be called “low-pass J filtering” since it leaves signals associated with small J couplings essentially unalfected. We explain the procedure in the following for the suppression of neighbor peaks in heteronuclear relayed correlation spectra. Other applications are also feasible but will not be treated in detail here. It is known that heteronuclear networks of coupled spins can be assigned by using a combination of homonuclear (1-3) and heteronuclear (6 7) correlation spectroscopy. Recently a new approach has been proposed that involves two consecutive coherence tra.nsfer steps, in order to “relay” information between nuclei that have a common coupling partner but are not necessarily coupled together (8-11). Applied to carbon- 13, relayed correlation spectroscopy makes it possible to explore the proton-proton coupling network even in crowded spectral regions by transferring coherence from a remote proton Ha to a carbon C via a neighbor proton HN, and to establish the sequence of carbon atoms in the molecular backbone. The salient feature of relayed heteronuclear correlation spectra is the appearance of “remote” cross-peaks with frequency coordinates wI = QuR and wz = Qc. In addi- tion to these signals, relayed spectra also contain “neighbor” peaks arising from direct transfer through one-bond couplings (q = QuN, 122 = Qc), thus replicating the in- formation contained in normal heteronuclear correlation spectra. Often, the proton- proton transfer period T,, must be kept short to avoid excessive losses of magnetization caused by relaxation and couplings to “passive” spins. As a side effect the amplitudes of the neighbor signals (II) may dominate and partially mask the weaker signals stemming from remote nuclei. In conventional relayed spectra, there is no simple wa.y of distinguishing signals from neighbor and remote protons, hence in most cases it :is necessary to compare a relayed spectrum with a normal correlation spectrum. This calls for a scheme to suppress neighbor signals in relayed spectra. We propose to incorporate a low-pass J titer in the normal relayed experiment to remove the magnetization of the direct neighbors HN without afFecting the magne- tization of the remote protons H R. We refer the reader to earlier papers (8-11) for 157 0022-2364183 $3.00 Copyright 0 1983 by Academic Press. Inc. All rights of reproduction in any form resewed.