Complementary to the discussed experiments, the authors applied density functional calculations (DFT) for the inclusion complexes in vacuum. In agreement with the gas phase experiments, the complex [ 17 2 + PF 6( exo) + S ( endo)] − was energetically favoured compared to [ 17 2 + PF 6( endo) + S ( exo)] − (with S = acetone). Calculated electrostatic potential surfaces were used to illustrate the cause for the observed behaviour, showing that the polarised and electron-rich interior is barely suitable for anion binding ( Fig. 12B). However, the electron-poor exterior (isobutyl chains) was revealed to have a better affinity towards anions. Y. Cohen, L. Avram, T. Evan-Salem, S. Slovak, N. Shemesh and L. Frish, Analytical Methods in Supramolecular Chemistry, Wiley, 2nd edn, 2012, pp. 197–285 Search PubMed. L. Paudel, R. W. Adams, P. Kiraly, J. A. Aguilar, M. Foroozandeh, M. J. Cliff, M. Nilsson, P. Sandor, J. P. Waltho and G. A. Morris, Angew. Chem., Int. Ed., 2013, 52, 11616 CrossRef CAS PubMed .

L. Sellies, I. Reile, R. Aspers, M. C. Feiters, F. Rutjes and M. Tessari, Chem. Commun., 2019, 55, 7235 RSC . To investigate its structure further in solution, SAXS was used for 11 and the [4]-rotaxane 12 ( Fig. 7B), which had already been fully characterised with X-ray crystallography. A pair distance distribution function (PDDF) was obtained from each SAXS data set ( Fig. 7C). Such X-ray scattering data provides a probability for the distance between electrons in the investigated molecule. 69 M. Foroozandeh, R. W. Adams, N. J. Meharry, D. Jeannerat, M. Nilsson and G. A. Morris, Angew. Chem., Int. Ed., 2014, 53, 6990 CrossRef CAS PubMed . A. P. France, L. G. Migas, E. Sinclair, B. Bellina and P. E. Barran, Anal. Chem., 2020, 92, 4340–4348 CrossRef CAS PubMed. A. P. Deshmukh, D. Koppel, C. Chuang, D. M. Cadena, J. Cao and J. R. Caram, J. Phys. Chem. C, 2019, 123, 18702–18710 CrossRef CAS.M. R. Chierotti and R. Gobetto, Supramolecular Chemistry: From Molecules to Nanomaterials, John Wiley & Sons, Ltd, 2012 Search PubMed. G. Dal Poggetto, L. Castanar, M. Foroozandeh, P. Kiraly, R. W. Adams, G. A. Morris and M. Nilsson, Anal. Chem., 2018, 90, 13695 CrossRef CAS PubMed . G. Bertho, L. Lordello, X. Chen, C. Lucas-Torres, I. E. Oumezziane, C. Caradeuc, M. Baudin, S. Nuan-Aliman, C. Thieblemont, V. Baud and N. Giraud, J. Proteome Res., 2022, 21, 1041 CrossRef CAS PubMed . S. J. Duncan, R. Lewis, M. A. Bernstein and P. Sandor, Magn. Reson. Chem., 2007, 45, 283 CrossRef CAS PubMed .

Another interesting analysis, arguably also related to the structural characterisation of 18 and 19 in their aggregated forms, is the determination of the average number of monomers in a polymer (polymerisation degree). The authors showed that this is possible with fluorescence spectroscopy, because they were able to assign one peak/shoulder (≈620 nm) to the monomers and a second band (≈700 nm) to the assemblies. Due to broad overlapping signals, reliable deconvolution was not possible for the assembly of 18. However, the authors obtained average polymerisation degrees between 10 and 140 for 19 in solution, with the value depending on the concentration and the temperature of the system. These studies shed light on thermodynamic properties of polymeric 19, but further details are beyond the scope of this review. G. Dal Poggetto, L. Castanar, R. W. Adams, G. A. Morris and M. Nilsson, J. Am. Chem. Soc., 2019, 141, 5766 CrossRef CAS PubMed . A. Bornet, R. Melzi, A. J. Perez Linde, P. Hautle, B. van den Brandt, S. Jannin and G. Bodenhausen, J. Phys. Chem. Lett., 2013, 4, 111 CrossRef CAS PubMed . D. Li, I. Keresztes, R. Hopson and P. G. Williard, Acc. Chem. Res., 2009, 42, 270 CrossRef CAS PubMed .N. Eshuis, B. J. A. van Weerdenburg, M. C. Feiters, F. P. J. T. Rutjes, S. S. Wijmenga and M. Tessari, Angew. Chem., Int. Ed., 2015, 54, 1481 CrossRef CAS PubMed .

E. Martineau, J. N. Dumez and P. Giraudeau, Magn. Reson. Chem., 2020, 58, 390 CrossRef CAS PubMed . P. N. Reardon, C. L. Marean-Reardon, M. A. Bukovec, B. E. Coggins and N. G. Isern, Anal. Chem., 2016, 88, 2825 CrossRef CAS PubMed .The power of NMR spectroscopy also results from the vast space of possible experiments that can be carried out, with a given instrument and a given sample, through the use of different pulse sequences. This makes it possible, through the controlled manipulations of nuclear spins, to access chemical information that would otherwise remain invisible or inaccessible. The process of developing NMR pulse sequences has sometimes been referred to as “spin choreography” or “spin alchemy”. 3 C. J. Bruns and J. F. Stoddart, The Nature of the Mechanical Bond: From Molecules to Machines, Wiley, 2016 Search PubMed.

J. Ferrando-Soria, A. Fernandez, D. Asthana, S. Nawaz, I. J. Vitorica-Yrezabal, G. F. S. Whitehead, C. A. Muryn, F. Tuna, G. A. Timco, N. D. Burton and R. E. P. Winpenny, Nat. Commun., 2019, 10, 1–7 CrossRef CAS PubMed. Key learning points (1) The reader will repeat and/or learn basic concepts for a range of structural characterisation methods and get new insights into evaluating the obtained data critically. Fig. 2 500 MHz (a) 1D 1H spectrum, (b) 1H pure shift spectrum recorded using the unmodified Zangger–Sterk pulse sequence, (c) vertical expansion of the spectrum (b and d) 1H pure shift spectrum acquired with the pulse sequence modified to remove sidebands, for a mixture containing rosuvastatin (1) and 2.8% of its precursor BEM (2). (e) 1D 1H spectrum of BEM. Spectra c and d were acquired in approximately 9 h with an sw1 of 39.0625 Hz and 16 chunks using an RSNOB spatially selective spin inversion element with a peak RF amplitude of 47 Hz. Reproduced from ref. 25 with permission from the Royal Society of Chemistry. An efficient alternative to broadband homonuclear decoupling is the use of band-selective decoupling. 5,6 In this approach, the active spins are those within a selected frequency region of the spectrum, and the passive spins are all those outside that region. The main advantage of band-selective decoupling methods is that they have virtually no sensitivity penalty, in contrast to other pure-shift methods. Also, they are more straightforward to implement in a single scan. When the properties of the mixture of interest allow it, they are a method of choice to resolve the signals of structurally similar compounds. Band-selective pure-shift NMR was for example used, in 2D spectra, to distinguish diastereoisomers with near-identical 1H and 13C spectra, 28 or enantiomers in chiral solvating agents. 29Fig. 5 13C NMR spectra (quaternary region) of mature green tomato fruit pericarp extracts. (a) Conventional (non-hyperpolarised) spectrum of 20 mg extract (prepared from 20 mg lyophilized grounded tissue) dissolved in 700 μL D 2O, recorded with 1024 scans (11 h 45 min) at 700 MHz with a cryogenic probe. (b) Spectrum of an identical extract recorded with D-DNP combined with CP. The extract was first dissolved in a 200 μL mixture of H 2O/D 2O/glycerol-d 8 (2 : 3 : 5) doped with 25 mM TEMPOL, then polarized for 30 min at 1.2 K and 6.7 T, and finally dissolved with 5 mL of hot D 2O and transferred in about 10 seconds to a 500 MHz spectrometer equipped with a cryogenic probe. (c) Same as (b), but the tomato extract was replaced by a control sample prepared under strictly identical conditions without any biological material. The hyperpolarized spectra result from the sum of six consecutive acquisitions using 30° pulses spaced by 7.7 s. *Indicates the signal of a 13C-labeled pyruvate impurity from the D-DNP setting. Cit: citrate; GABA: γ-aminobutyrate; Gln: glutamine; Mal: malate. Reproduced from ref. 102 with permission from the Royal Society of Chemistry. M. V. Salvia, F. Ramadori, S. Springhetti, M. Diez-Castellnou, B. Perrone, F. Rastrelli and F. Mancin, J. Am. Chem. Soc., 2015, 137, 886 CrossRef CAS PubMed .