Chirality in Gold Nanoclusters Probed by NMR Spectroscopy

We report the analysis of chirality in atomically precise gold nanoclusters by nuclear magnetic resonance (NMR) spectroscopic probing of the surface ligands. The Au(38)(SR)(24) and Au(25)(SR)(18) (where, R = CH(2)CH(2)Ph) are used as representative models for chiral and nonchiral nanoclusters, respectively. Interestingly, different (1)H signals for the two germinal protons in each CH(2) of the ligands on the chiral Au(38)(SR)(24) nanocluster were observed, so-called diastereotopicity. For α-CH(2) (closest to the chiral metal core), a chemical shift difference of up to ~0.8 ppm was observed. As for the nonchiral Au(25)(SCH(2)CH(2)Ph)(18)(-)TOA(+) nanocluster, no diastereotopicity was detected (i.e., no chemical shift difference for the two protons in the CH(2)), confirming the Au(25) core being nonchiral. These two typical examples demonstrate that NMR spectroscopy can be a useful tool for investigating chirality in Au nanoclusters. Since the diastereotopicity induced on the methylene protons by chiral nanoclusters is independent of the enantiomeric composition of the chiral particles, NMR can probe the chirality of the nanoclusters even in the case of a racemic mixture, while circular dichroism spectroscopy is not useful for racemic mixtures.