Observation of potentially troublesome super(2)J sub(CC) correlations in 1,1-ADEQUATE spectra

• Published in: Magnetic resonance in chemistry 2016-04, Vol.54 (4), p.341-345
• Main Authors: Sauri, Josep, Liu, Yizhou, Williamson, RThomas, Martin, Gary E
• Format: Article
• Language: English
• Subjects: Carbonyls; Constants; Correlation; Equivalence; Gain; Joining; Mathematical analysis; Spectra
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.4385

Despite the tremendous usage of HMBC to establish long-range super(1)H- super(13)C and super(1)H- super(15)N heteronuclear correlations, an inherent drawback of the experiment is the indeterminate nature of the super(n)J sub(XH) correlations afforded by the experiment. A priori there is no reliable way of determining whether a given super(n)J sub(CH) correlation is, for example, via two-, three-, or sometimes even four-bonds. This limitation of the HMBC experiment spurred the development of the ADEQUATE family of NMR experiments that rely on, in the case of 1,1-ADEQUATE, an out-and-back transfer of magnetization via the super(1)J sub(CC) homonuclear coupling constant, which is significantly larger than super(n)J sub(CC) (where n=2-4) couplings in most cases. Hence, the 1,1-ADEQUATE experiment has generally been assumed to unequivocally provide the equivalent of super(2)J sub(CH) correlations. The recent development of the 1,1- and 1,n-HD-ADEQUATE experiments that can provide homodecoupling for certain super(1)J sub(CC) and super(n)J sub(CC) correlations has increased the sensitivity of the ADEQUATE experiments significantly and can allow acquisition of these data in a fraction of the time required for the original iterations of this pulse sequence. With these gains in sensitivity, however, there occasionally come unanticipated consequences. We have observed that the collapse of proton multiplets, in addition to providing better s/n for the desired super(1)J sub(CC) correlations can facilitate the observation of typically weaker super(2)J sub(CC) correlations across intervening carbonyl resonances in 1,1-HD-ADEQUATE spectra. Several examples are shown, with the results supported by the measurement of the super(2)J sub(CC) coupling constants in question using J-modulated-HD-ADEQUATE and DFT calculations. Certain large super(n)J sub(CC) couplings, in particular, the super(2)J sub(CC) coupling across a carbonyl, can lead to observable cross peaks in 1,1-ADEQUATE spectra, and to strong correlations due to the enhanced sensitivity afforded by 1,1-HD-ADEQUATE. Therefore caution should be used during data interpretation of carbonyl containing compounds.