Accentuated selective population transfer difference spectroscopy: a new method for revealing hidden NMR resonances.
✍ Scribed by Niels H. Andersen; Hugh L. Eaton; Khe T. Nguyen
- Book ID
- 104233943
- Publisher
- Elsevier Science
- Year
- 1984
- Tongue
- French
- Weight
- 253 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Bock difference FID accumulatlon wlth an Inltlal selectlve decoupler pulse alternately placed at the hlgh and low frequency Ilnes of a multlplet provldes. upon transform, a dlfference spectrum that shows only those other mult/plets that are scalar coupled to the probed resonante wlth full retention of all spllttlngs for the multlplets so revealed. Eigh-field programmable multipulse FFNWR instruments are now routinely available for structure elucidation of moderately complex natural products (RW 3OD-600). If obtaining a ful1 complement of 2D spectra t-18 precluded due to either instrument time constraints or compound instability, the major obstacle in the spectral assignment procesa is locating (and visualizing the multiplicity of) individual resonantes in crowded spectral regions. Two techniquesdecoupling difference and ANOE spectra -have been advocated' for revealing hidden multiplets that display either scalar or dipolar coupling to resonantes in less crowded regions. Both of these methods suffer from significant subtraction error problems t that can be only partially corrected4. The correction routines preclude direct accumulation of a difference PID and require software that is not routinely available. In ANOE spectroscopy an additional complication appears. Imprecise placement of the on-resonante decoupler pulse (or the use of low power settings to obtain selectivity) results in differential saturation of lines within a multiplet and superimposes selective population transfer (SPT) effects upon the ANOE spectra. H-1Ofi JIOC% Jn I s J s/mn Control ASPT I,lOa,l Jma J ~ho traces imediately above show the lOj+B multiplet of PGp2a and prwide an illustrative example of how J values can be derived by this technique. (SPT effects are also seen for strongly coupled spin systems, but their quantitative interpretation is more complicated). Accentuated SPT differencing is Clearly a simple, and rapid, technique for unmasking the identity and multiplicity of hidden proton resonantes.
We anticipate that the method will find wide applicability in NWR assignment and structure elucidation.