Structural characterization of phosphorus-based networks and clusters: 31P MAS NMR spectroscopy and magnetic shielding calculations on Hittorf's phosphorus

The (31)P MAS NMR spectrum of Hittorf's phosphorus has been measured and assigned to the 21 crystallographically distinct phosphorus atoms based on two-dimensional dipolar correlation spectroscopies. Application of such 2D techniques to phosphorus-based networks is particularly challenging owing to the wide chemical shift dispersions, rapid irreversible decay of transverse magnetization, and extremely slow spin-lattice relaxation in these systems. Nevertheless, a complete assignment was possible by using the combination of correlated spectroscopy (COSY) and radiofrequency-driven dipolar recoupling (RFDR). The assignment is supported further by DFT-based ab initio chemical shift calculations using a cluster-model approach, which gives good agreement between experimental and calculated chemical shift values. The (31)P chemical shifts appear to be strongly correlated with the average P-P bond lengths within the P(P(1/3))(3) coordination environments, whereas no clear dependence on average P-P-P bond angles can be detected. Calculations of localized Kohn-Sham orbitals reveal that this bond-length dependence is reflected in energy variations in the corresponding localized p-p-σ orbitals influencing the paramagnetic deshielding contribution in Ramsey's equation. In contrast, the contributions of the lone pairs to shielding differences are small and/or do not vary in a systematic manner for the different crystallographically distinct phosphorus sites. The combined spectroscopic and quantum chemical approach applied here and the increased theoretical understanding of (31)P chemical shifts will facilitate the structural elucidation of other phosphorus-based clusters and networks.