NMR characterization of complex p-oligophenyl scaffolds by means of aliasing techniques to obtain resolution-enhanced two-dimensional spectra

The usefulness of computer-assisted aliasing to secure maximal resolution of signal clusters in H-1- and C-13-NMR spectra (which is essential for structure determination by HMBC 2D NMR spectroscopy) in minimal acquisition time is exemplified by the complete characterization of the two complementary p-octiphenyls 1 and 2 with complex substitution patterns. The need for digital resolution near 1 Hz/pt to dissect the extensive signal clusters in the NMR spectra of these refined oligomers excluded structure determination under routine conditions. High resolution was secured by exploiting the low signal density in the C-13 dimension of HMBC spectra by using computer-assisted aliasing to maximize signal density. Based on the observed shifts in DEPT and H-1-decoupled C-13-NMR spectra of 1 and 2, computer-assisted aliasing allowed to reduce the number of required time increments by a factor of 20 to 30 compared to full-width spectra with identical resolution. Without signal-to-noise constraints, this computer-assisted aliasing reduced the acquisition time for high-resolution NMR spectra needed for complete characterization of refined oligomers 1 and 2 by the same factor (e.g., from over a day to about an hour). With resolved signal clusters in fully aliased HSQC and HMBC spectra, unproblematic structure determination of 1 and 2 is demonstrated by unambiguous assignment of all C- and H-atoms. These findings demonstrate that computer-assisted aliasing of the underexploited C-13 dimension makes extensive molecular complexity accessible by conventional multidimensional heteronuclear NMR experiments without extraordinary efforts.