NMR effective radius of hydrogen in XIV group hydrides evaluated by NMR spectroscopy

Abstract

In the [ABr_nI_m] (A = C, Si, Ge, Sn; n + m = 4) compounds, with the heavier halides bonded to the central IV group elements, the experimental ¹³C, ²⁹Si, ⁷³Ge and ¹¹⁹Sn NMR chemical shifts of the central atoms were found to be strictly linearly proportional to the bonded halides ionic radii overall sum ∑(r_h). Based on this, calibration lines relating the chemical shifts to ∑(r_h) could be built for the considered subgroup of [ABr_nI_m] compounds. Using such calibration lines we could calculate the equivalent NMR radius, ^NMRr_H–A, attributable to each of the bonded hydrogens in [AH₄] species, according to the overall NMR shielding produced on the central A atom. Interestingly, the calculated ^NMRr_H–A value resulted to be almost constant in all [AH₄] examined systems (A = ¹³C, ²⁹Si, ⁷³Ge, ¹¹⁹Sn) with an average ^NMR_H–A value equal to 194.6 ± 1.6 pm. Based on this approach, we could calculate the ²⁰⁷Pb NMR chemical shift of the unstable [PbH₄] complex using the value of 192.7 pm calculated for ^NMRr_H–Sn in the stable closest hydride [SnH₄]. The obtained unprecedented NMR value is in accord with the ²⁰⁷Pb NMR chemical shift estimation, independently calculated for [PbH₄] from the [SnH₄] data, using the Pb/Sn chemical shift correlation defined in the Mitchell equation.