19F and 29Si nuclear magnetic shielding and spin–spin coupling constants in silicon tetrafluoride and hexafluorodisiloxane in the gaseous state

Abstract

In this contribution the gas-phase measurements of 19F and 29Si chemical shifts and scalar spin–spin couplings in SiF4 and (SiF3)2O have been reported. The density dependence of silicon and fluorine nuclei chemical shifts is strictly linear for both compounds. An extrapolation of gaseous data to the zero-density limit permitted the measurements of 19F and 29Si absolute nuclear magnetic shielding of isolated molecules at 300K. They were referenced against general accepted standards and are as follows: σ0(SiF4)=362.87ppm, σ0(SiF4)=488.15ppm, σ0((SiF3)2O)=356.50ppm, and σ0((SiF3)2O)=487.95ppm. The direct measurements of shielding constants on the basis of NMR frequencies were also performed. Indeed, we have successfully demonstrated the equivalence of both shielding values refinements. Careful investigation of 19F spectra shows isotope effects connected with the presence of three naturally occurring silicon isotopes: 28Si (92.19%), 29Si (4.70%) and 30Si (3.09%). The isotope effect measured on 19F nuclei does not exceed 0.01ppm. The 1J(Si,F) indirect spin–spin coupling constants in both molecules were extracted from an experimental spectra of gaseous samples. The intermolecular effects in the gaseous state as well as in coming from gas to organic solutions were also observed. The new experimental results were used for verification of the theoretical evaluations presented earlier.