NMR Characterization of Complex p‐Oligophenyl Scaffolds by Means of Aliasing Techniques to Obtain Resolution‐Enhanced Two‐Dimensional Spectra

Abstract

Dedicated to Professor Ulrich Burger on the occasion of his 65th birthdayThe usefulness of computer-assisted aliasing to secure maximal resolution of signal clusters in 1 H-and 13 C-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 13 C dimension of HMBC spectra by using computer-assisted aliasing to maximize signal density. Based on the observed shifts in DEPT and 1 H-decoupled 13 C-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 highresolution 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 Hatoms. These findings demonstrate that computer-assisted aliasing of the underexploited 13 C dimension makes extensive molecular complexity accessible by conventional multidimensional heteronuclear NMR experiments without extraordinary efforts.

Interface Engineering of Synthetic Pores: Towards Hypersensitive Biosensors

“…[41] However, the recording of high-resolution 2D HSQC and HMBC NMR spectra allowed unambiguous Figure 1. Self-assembly from monomer 4 and the theoretical active structure of pore 1 with hydrophilic anchors together with anchor-free control pores 2 and 3. b Sheets are shown as gray arrows in the theoretical pore structure, and in the chemical structure, the external amino acid residues are indicated within an empty circle, whereas the internal residues are indicated within a filled circle (the residues are indicated with single-letter abbreviations, see Scheme 2 for full structures).…”

New and old NMR experiments for the resonance assignment of complex oligosaccharides—application to a cyclodextrin derivative

“…It has already found many applications in physicochemical studies [15,16] and for the structure elucidation of complex molecules. [17,18] The major problem with folding is identifying of the number of 'folds', also named 'aliasing order', a signal has undergone. This problem has recently received an elegant solution with the recording of two complementary experiments, [19] as illustrated by the HSQC-TOCSY spectrum of Compound 1.…”

Spectral Aliasing: A Super Zoom for 2D-NMR Spectra. Principles and Applications

“…[4] For HMBC experiments one may consider using pulse sequences tailored for long t 1 -evolution times. [4,5] But, in both cases, the experiments are quite convenient and can be easily added to the set of standard NMR experiments. An example of comparison of a full and aliased spectra is shown in Fig.…”

“…The factor of reduction depends on the number of carbons and the severity of the clustering of signals but is typically between 20 and 100. [5,9] One of the typical applications consists in recording a finely tailored ADEQUATE experiment when the carbon backbones of natural products have to be confirmed. [10] …”