NMR Crystallography of Campho[2,3-c]pyrazole (Z `=6): Combining High-Resolution H-1-C-13 Solid-State MAS NMR Spectroscopy and GIPAW Chemical-Shift Calculations
H-1-C-13 two-dimensional magic-angle spinning (MAS) solid-state NMR correlation spectra, recorded with the MAS-J-HMQC experiment, are presented for campho[2,3-c]pyrazole. For each C-13 moiety, there are six resonances associated with the six distinct molecules in the asymmetric unit cell (Z' = 6). The one-bond C-H correlations observed in the 2D H-1-C-13 MAS-J-HMQC spectra allow the experimental determination of the H-1 and C-13 chemical shifts associated with the separate CH, CH2, and CH3 groups. H-1 and C-13 chemical shifts calculated by using the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach are presented. Calculations for the whole unit cell (12 x 29 = 348 atoms, with geometry optimization of all atoms) allow the assignment of the experimental H-1 and C-13 chemical shifts to the six distinct molecules, The calculated chemical shifts for the full crystal structure are compared with those for isolated molecules as extracted from the geometry-optimized crystal structure. In this way, the effect of intermolecular interactions on the observed chemical shifts is quantified. In particular, the calculations are sufficiently precise to differentiate the small (<1 ppm) differences between the H-1 chemical shifts of the six resonances associated with each distinct CH or CH2 moiety.