Resonance signals overlapping

Owing to signal overlap, only partial assignment H (6.55-7.99 ppm) and 13C resonances were possible for anions 6-9. Of course, most of the compounds described in this chapter were well characterized mainly due to the NMR techniques. 

In our experience resonance assignment is speeded up so much in the case of 15N-la-beled peptides that the additional effort in the laboratory is quickly paid for. The HSQC spectra will also help to immediately recognize signal overlap. 

The experimental data closely resemble the simulation based on the all-trans PSB values. The discrepancy between the two solid-state NMR studies on rhodopsin arises in part from a difference, in signal-to-noise ratio and in part from possible problems associated with a fatty acid resonance which overlaps with the centerband in the previous study. The simulations illustrate the sensitivity of the sideband intensities to changes in the chemical shift tensor, as well as the quality of data necessary to accurately determine the shift tensor values. 

Next we deal with the nature of the correction Awc. The resonance signals in a spectrum may be composed of overlapping, closely spaced components. Usually this is caused by either weak spin-spin couplings (we consider here only those which are included explicitly in the assumed model of an exchanging spin system) or by almost degenerate transitions in the spin multiplets. (81) Such effects should be compensated for by the correction Awc which can be determined from measurement of an apparent line-width, wa, in a theoretical spectrum calculated for a reasonably assumed value of the natural line-width, wtheor The correction (wa — wtheor) is virtually independent of the value... 

In the N NMR spectrum of quinazoline in dibromomethane only one broad resonance signal (half height 950 + 50 Hz) was observed at 90 + 3 ppm (relative to internal nitromethane) due to overlapping. The N NMR chemical shifts depend almost linearly on the n-charge density at the nitrogen atom obtained from SCF-MO calculations of the Pariser-Parr-Pople type. NNQR parameters for quinazoline at 77 K showed reasonable agreement with ab initio MO calculations. ... 

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