NMR spectral properties of the tetramantanes - nanometer-sized diamondoids

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their (1)H and (13)C NMR spectra because it translates into repeating structural features, such as diamond-cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond-lattice structures result in long-range (4)JHH, W-coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven (4)JHH). Isobutyl-cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their (13)C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W-couplings between these methines and methylenes clarify spin-network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid (13) C NMR chemical shifts are largely determined by α and β effects, however γ-shielding effects are important in [123]tetramantane. (1)H NMR chemical shifts generally correlate with numbers of 1,3-diaxial H-H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C-3,21. These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size.