NMR Crystallography of Campho[2,3-c]pyrazole (Z′ = 6): Combining High-Resolution 1H-13C Solid-State MAS NMR Spectroscopy and GIPAW Chemical-Shift Calculations
journal contributionposted on 30.09.2010, 00:00 by Amy L. Webber, Lyndon Emsley, Rosa M. Claramunt, Steven P. Brown
1H-13C 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 13C 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 1H-13C MAS-J-HMQC spectra allow the experimental determination of the 1H and 13C chemical shifts associated with the separate CH, CH2, and CH3 groups. 1H and 13C 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 × 29 = 348 atoms, with geometry optimization of all atoms) allow the assignment of the experimental 1H and 13C 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 1H chemical shifts of the six resonances associated with each distinct CH or CH2 moiety.