Methods and procedures for gel-phase HR MAS spectroscopy

Methods and procedures for gel-phase HR MAS spectroscopy Solution NMR spectra were acquired on a 300 MHz Bruker AC-300P FT spectrometer at 303 K. HR MAS NMR spectra were acquired on a Bruker DRX400 spectrometer at room temperature or below using a Bruker HR MAS microprobe. Rotors containing a suspension of the beads in CDCl 3 were spun at 4 kHz. One-dimensional HR MAS spectra were obtained with 64 scans. CPMG pulse sequence contained 32 or 2000 π-pulses with a repetition time of 30 ms. Chemical shifts (δ) are reported in parts per million relative to residual solvent. It should be noted that some caveats are necessary in the use of this technique. A significant disadvantages of using HR MAS NMR spectroscopy to analyse solid-supported supramolecular systems, is the lack of accurate quantitation methods. This is due to two main factors; firstly, that the HR MAS NMR spectrum cannot be integrated as with standard 1 H NMR, due to the application of the CPMG pulse sequences used. While the CPMG pulse loops successfully filter out any broad resonances due to the bead core, this is indiscriminate and the intensity of signals arising from the tethered components can also be affected, depending on their relaxation properties. Signals broadened by relaxation or exchange processes are particularly affected, sometimes to such an extent that they are not observable in the filtered spectrum. Even relative integrations of individual protons within a single molecule, which are affected by the CPMG pulse sequences, can be anomalous. For example in the tethered pyridyl diimide tethered thread 5, the four pyridine protons are filtered to varying degrees, and therefore do not have the expected 1:1:1:1 integration. Thus integration cannot be relied upon as an accurate indication of the relative loading of different components, and even strategies that might be used which incorporate an 'internal standard' marker proton resonance are not feasible. A more reliable internal comparison of relative integrations can be obtained from the unfiltered spectrum, but this is only feasible for those signals which are not obscured by the large and broad residual peaks arising from the structural components of the beads. The second factor that renders any quantitation difficult, is that the loading of the tethered component on the bead is difficult to ascertain. The average percentage of functional sites on the beads is given by the manufacturer (around 0.46% mmol OH/g for ArgoGel beads), however, …