AGeneral Method for Extracting Individual Coupling Constants from Crowded H NMR Spectra

Couplings between protons, whether scalar or dipolar, provide a wealth of structural information. Unfortunately, the high number of H-H couplings gives rise to complex multiplets and severe overlap in crowded spectra, greatly complicating their measurement. Many different methods exist for disentangling couplings, but none approaches optimum resolution. Here, we present a general new 2D Jresolved method, PSYCHEDELIC, in which all homonuclear couplings are suppressed in F2, and only the couplings to chosen spins appear, as simple doublets, in F1. This approaches the theoretical limit for resolving H-H couplings, with close to natural linewidths and with only chemical shifts in F2. With the same high sensitivity and spectral purity as the parent PSYCHE pure shift experiment, PSYCHEDELIC offers a robust method for chemists seeking to exploit couplings for structural, conformational, or stereochemical analyses. Homonuclear coupling is a double-edged sword. On the one hand, it is a well-established source of valuable information on molecular structure, reflecting torsion or bond angles in the case of scalar couplings, and internuclear vectors in the case of residual dipolar couplings (RDCs). On the other hand, when, as is almost always the case, multiple couplings are present, the complexity of the resulting multiplets and the narrow range of H chemical shifts conspire to cause spectral overlap. This complicates spectral analysis and often prevents measurement of individual H-H couplings. In recent years, much effort has been invested in generating broadband homonuclear decoupled, or “pure shift”, spectra, collapsing multiplets to singlets and yielding a limiting resolution close to the natural linewidth. Unfortunately, the gain in resolution comes at the cost of losing direct access to valuable coupling information. Here, we present a method that retains pure shift resolution and high sensitivity, while displaying all couplings to a selected proton or protons as simple doublets in a second dimension. This delivers the maximum information content with minimum complication, making H-H coupling measurements straightforward even in challenging molecules. At first sight, the classic 2DJ spectroscopy (J-resolved 2D) experiment ought to provide a good tool for coupling measurements, but it suffers from two severe limitations. First, the presence of a large number of couplings can make analysis of F1 traces difficult, and limits accuracy. Second, and more seriously, signals are phase modulated and therefore give rise to phasetwist lineshapes. The classic experiment therefore uses absolute value display, which requires brutal time-domain weighting functions to be used if lineshapes with acceptable resolution are to be obtained. Unfortunately, this is costly in sensitivity and, crucially, distorts multiplet structure, making the determination of couplings unreliable. The problems of phasetwist lineshapes can be avoided by the use of recent phase-sensitive 2DJ experiments, although these come at a cost in sensitivity, broadband character, or simplicity of data processing. The first problem, however, remains. It is helpful to consider what the characteristics of an ideal 2DJ method for determining coupling constants would be. First, the experiment would be phase-sensitive, giving absorption mode 2D lineshapes. Second, and importantly, only a subset of couplings of interest would be active, allowing these to be measured without interference from other splittings. Third, it would be broadband, working over the full range of chemical shifts and allowing simultaneous measurement of as many individual couplings as possible. Fourth, and finally, the method would be generally applicable, that is, sensitive, simple to set up, and tolerant of the breakdown of the weak coupling approximation. The new experiment, dubbed PSYCHEDELIC (Pure Shift Yielded by CHirp Excitation to DELiver Individual Couplings), meets all these requirements. A demonstration is shown in Figure 1 for the steroid 17b-estradiol. Steroids typically pose challenging cases for the determination of HH coupling constants because of the high incidence of couplings between protons in a narrow chemical shift range. In Figure 1B,C, only the couplings involving, respectively, proton H9 and proton H14 were selected, using a selective 1808 pulse, and appear as simple doublets along F1 at the chemical shifts of their coupling partners, while all other signals remain as singlets in F1. Because the experiment has the form of a traditional 2DJ spectrum, the doublets are dispersed at ¢458 to the principal axes, so applying a conventional 458 tilt (more strictly, a shear) of the spectrum fully [*] Dr. D. Sinnaeve, Dr. M. Foroozandeh, Dr. M. Nilsson, Prof. Dr. G. A. Morris School of Chemistry, University of Manchester Oxford Road, Manchester M13 9PL (UK) E-mail: [email protected] Homepage: http://nmr.chemistry.manchester.ac.uk