Benchmark fragment-based H, C, N and O chemical shift predictions in molecular crystals

The performance of fragment-based ab initio 1 H, 13 C, 15 N and 17 O chemical shift predictions is assessed against experimental NMR chemical shift data in four benchmark sets of molecular crystals. Employing a variety of commonly used density functionals (PBE0, B3LYP, TPSSh, OPBE, PBE, TPSS), we explore the relative performance of cluster, two-body fragment, and combined cluster/fragment models. The hybrid density functionals (PBE0, B3LYP and TPSSh) generally out-perform their generalized gradient approximation (GGA)-based counterparts. 1 H, 13 C, 15 N, and 17 O isotropic chemical shifts can be predicted with root-mean-square errors of 0.3, 1.5, 4.2, and 9.8 ppm, respectively, using a computationally inexpensive electrostatically embedded two-body PBE0 fragment model. Oxygen chemical shieldings prove particularly sensitive to local many-body effects, and using a combined cluster/fragment model instead of the simple two-body fragment model decreases the root-mean-square errors to 7.6 ppm. These fragment-based model errors compare favorably with GIPAW PBE ones of 0.4, 2.2, 5.4, and 7.2 ppm for the same 1 H, 13 C, 15 N, and 17 O test sets. Using these benchmark calculations, a set of recommended linear regression parameters for mapping between calculated chemical shieldings and observed chemical shifts are provided and their robustness assessed using statistical cross-validation. We demonstrate the utility of these approaches and the reported scaling parameters on applications to 9- tert -butyl anthracene, several histidine co-crystals, benzoic acid and the C-nitrosoarene SnCl 2 (CH 3 ) 2 (NODMA) 2 . The performance of fragment-based ab initio 1 H, 13 C, 15 N and 17 O chemical shift predictions is assessed against experimental NMR chemical shift data in four benchmark sets of molecular crystals.