posted on 2023-12-08, 14:35authored byA. A. Rybakov, D. P. Vercauteren, A. V. Larin
The
relationship between the s-fraction of O orbitals,
electronic
density at the O lone pair, and Si–O–Si angles in the
total magnetic shielding σ(29Si) of zeolite materials
is typically approximated using the cos(⟨Si–O–Si⟩)/(cos(⟨Si–O–Si⟩)
– 1) function, with the ⟨Si–O–Si⟩
being the averaged Si–O–Si angle over the four O neighbors
of Si. In this work, an analogous sin(⟨Si–O–Si⟩)
dependence is introduced based on the approximation of the absolute
O dipoles Q1(O) versus the Si–O–Si angle in five zeolite structures (BEA,
FER, GOO, ZSM-12, and ZSM-57). The models, along with α-quartz,
were considered to evaluate the advantages of the effects on both
chemical shifts and chemical shift anisotropies, as measured for α-quartz
and ZSM-12. The decrease of the O dipole with the Si–O–Si
angle is usually interpreted as a variation of the electronic density
at the O lone pairs. The dependence of Q1(O) versus sin(Si–O–Si) demonstrated
a similar high level of accuracy for Q1(O) in the zeolites. The sin(⟨Si–O–Si⟩)
function for the magnetic isotropic δiso(29Si) chemical shifts was also demonstrated for some of the five structures.
The final conclusion regarding a better approximation of the angular
dependence of the σ(29Si) chemical shifts between
four candidate functions is linked to the distribution of Si–O
bond lengths in the zeolites.