Nuclear Magnetic Shielding of Monoboranes: Calculation and Assessment of B NMR Chemical Shifts in Planar BX and in Tetrahedral [BX] Systems.

B NMR chemical shifts of tricoordinated BX and tetracoordinated BX compounds (X = H, CH, F, Cl, Br, I, OH, SH, NH, and CH═CH) were computed, and the shielding tensors were explored not only within the nonrelativistic GIAO approach but also by application of both relativistic ZORA computations including spin-orbit coupling as well as scalar nonrelativistic ZORA computations (BP86 level of density functional theory). The contributions of the spin-orbit coupling to the overall shieldings are decisive for X = Br and I in both series. No relationship was found between the 2p orbital occupancies or 1/ΔE (difference between LUMO and suitably occupied MO that can be coupled with LUMO) with the shielding tensors (or their principal values) in the BX series. However, a multidimensional statistical approach known as factor analysis (frequently used in chemometrics) revealed that three factors account for 92% of the cumulative proportion of total variance. The main components of the first factor are occupancies in the 2p and 2p orbitals and 1/ΔE; the second factor is mainly the occupancy in the 2p orbital and the inductive substituent parameters by Taft. Finally, the third factor consists exclusively (98.4%) of the electrostatic potential (V), which is directly related to the so-called π-hole magnitudes.