JOURNAL OF MAGNETIC RESONANCE 60, 161-163 (1984) Pattern Recognition in Two-Dimensional NMR Spectra BEAT U. MEIER, GEOFFFCEY BODENHAUSEN, AND R. R. ERNST Laboratorium ftir Physikalische Chemie, Eidgeniissische Technische Hochschule, 8092 Zurich, Switzerland Received May 30, 1984 The fully automated determination of molecular structure by spectroscopic means is an old dream of analytical chemistry (Z-10). Many procedures proposed so far are based on comparison with extensive libraries of spectra or computer simulations rather than on direct analysis. The structure of NMR spectra suggests the feasibility of a deductive analysis, since the multiplet patterns allow one to trace out the connectivities of nuclear spins. Unfortunately, the assignment of multiplet patterns in conventional one-dimensional (1D) spectra is bjy no means unique, since a multiplet cannot be distinguished a priori from an accidental juxtaposition of chemical shifts. Pattern recognition (II, 12) in LD spectra can therefore be misleading. On the other hand, two-dimensional NMR spectra are much less ambiguous. Their information content is sufficiently high to avoid pitfalls in pattern recognition procedures. Of the many methods proposed, 2D correlation spectroscopy or “COSY” (Z3), appears particularly convenient for analyzing coupling networks. A cross-peak multiplet centered at (wi , w2) = (&, fix) indicates a resolved coupling between two nuclei with chemical shifts Q,., and 52 x. In a two-spin AX system, the cross-peak amplitude is distributed over four signals with alternating signs at (a, f rJAx, !I, + n;lAx), and four additional signals appear at mirror image positions with respect to the main diagonal. Diagonal multiplets are disregarded as they tend to suffer from severe overlap. The 16 coordinates of the eight cross-peaks of the AX system contain redundancy since they only carry information on two chemical shifts and one coupling constant. In AMX systems, each cross-peak multiplet is further split by couplings to the “passive” nucleus (13, Z4), and the total number of cross-peaks increases from 8 to 96 for a mixing pulse with a rotation angle /3 = 7r/2. For N inequivalent spins with Z = l/2 with nonvanishing couplings, N(N - 1)22’N-” cross- peaks, each with two coordinates, are available for the determination of iV(N + l)/ 2 parameters. By using a small flip angle (e.g., /3 = s/4), coherence transfer occurs predominantly between connected transitions, and the amplitudes of the correspond- ing cross-peaks are favored. The redundancy of the dominant signals with respect to the number of unknown parameters remains high, since N(N - 1)2N dominant cross-peaks appear, each with two coordinates. Pattern recognition is appropriate under these circumstances. Our preliminary experience indicates that the sign alternation of cross-peak multiplet patterns in phase-sensitive 2D spectra is the most useful feature for 161 0022-2364184 $3.00 Copyright 0 1984 by Academic F’ra, Inc. All rights of repmJuction in any form reserved.