JOURNAL OF MAGNETIC RESONANCE 65,531-534 (1985) The Appearance of Forbidden Cross Peaks in Two-Dimensional Nuclear Magnetic Resonance Spectra Due to Multiexponential T2 Relaxation NORBERTMOLLER,*-~ GEOFFREY BODENHAUSEN,~~$. KURT W~THRICH,* ANDRICHARD R. ERNST? Vnstitut ftir Molekularbiologie und Biophysik, Eidgeniissische Technische Hochschule. CH-8093 Zurich, Switzerland, and fLaboratorium fu’r Physikalische Chemie, Eidgeniissische Technische Hochschule, CH-8092 Zurich, Switzerland Received August 22, 1985 The interpretation of complex nuclear magnetic resonance spectra can be facilitated by using two-dimensional techniques, such as correlation spectroscopy (COSY) and multiple-quantum NMR. These methods exploit the phenomenon of coherence trans- fer, which is governed by coherence-transfer selection rules (1, 2). These rules have been derived on the assumption that transverse relaxation can be neglected. They predict, for example, that it should be impossible in systems with magnetically equiv- alent spins (AX*, AX3) to transfer single-quantum coherence of the X spins into multiple-quantum coherence involving more than one X spin. In pquantum filtered correlation spectroscopy (PQF-COSY) (2-5) this would imply that in systems of the type AX2 and AX3 no cross peaks should be observed for p > 2. Since the presence or absence of such signals is often taken as evidence for the existence of specific mo- lecular fragments, violations of these selection rules might lead to errors in resonance assignments. It turns out that, in contrast to the expectations based on these selection rules, single-quantum coherence of the X spins in AX, groups can in practice be transferred into multiple-quantum coherence involving several X spins. This phenomenon is due to unequal transverse relaxation of degenerate X-spin single-quantum coherences in the course of the evolution period. Consider the three degenerate transitions (1, 2) (2, 3), and (3,4) between the symmetric eigenstates of the AX3 system (I = 1 coupled to a group spin F = $, see Fig. 1). According to the “naive” selection rules mentioned above, the failure of coherence transfer from these transitions into the four-quantum coherence u18 can be understood by noting that the transfer from the single-quantum coherences cr12 and c34 is precisely canceled by the transfer from ~23, since the latter’s transfer coefficient has opposite sign. However, this cancellation will be complete only if the three degenerate single-quantum coherences (r12, ~23, and us4 decay with the same T2 behavior. We wish to point out that this condition does not generally prevail. $ Current address: Institut de chimie organique, Universitk de Lausanne, rue de la Barre 2, CH-1005 Lausanne, Switzerland. 531 0022-2364185 $3.00 Copyright 0 1985 by Academic Press Inc. All rights of reproductmn in any form resewed.