CALCULATION OF THE 13C NMR SHIELDINGS OF THE CO2 COMPLEXES OF ALUMINOSILICATES

Abstract

13C NMR shieldings have been calculated using the random-phase-approximation, localized-orbital local-origins version of ab initio coupled Hartree-Fock perturbation theory for CO2 and and for several complexes formed by the reaction of CO2 with molecular models for aluminosilicate glasses, H3TOT'H-n3, T,T' = Si,Al. Two isomeric forms of the CO2-aluminosilicate complexes have been considered: (1) ''CO2-like'' complexes, in which the CO2 group is bound through carbon to a bridging oxygen and (2) ''CO3-like'' complexes, in which two oxygens of a central CO3 group form bridging bonds to the two TH3 groups. The CO2-like isomer of CO2-H3SiOSiH3 is quite weakly bonded and its13 C isotropic NMR shielding is almost identical to that in free CO2. As Si is progressively replaced by Al in the --H terminated aluminosilicate model, the CO2-like isomers show increasing distortion from the free CO2 geometry and their 13C NMR shieldings decrease uniformly. The calculated 13C shielding value for H3AlO(CO2)AlH-23 is only about 6 ppm larger than that calculated for point charge stabilized CO-23. However, for a geometry of H3SiO(CO2)AlH-13, in which the bridging oxygen to C bond length has been artificially increased to that found in the --OH terminated cluster (OH)3SiO(CO2)Al(OH)-13, the calculated13 C shielding is almost identical to that for free CO2. The CO3-like isomers of the CO2-aluminosilicate complexes show carbonate like geometries and 13C NMR shieldings about 4-9 ppm larger than those of carbonate for all T,T' pairs. For the Si,Si tetrahedral atom pair the CO2-like isomer is more stable energetically, while for the Si,Al and Al,Al cases the CO3-like isomer is more stable. Addition of Na+ ions to the CO-23 or H3AlO(CO2)AlH-23 complexes reduces the 13C NMR shieldings by about 10 ppm. Complexation with either Na+ or CO2 also reduces the 29Si NMR shieldings of the aluminosilicate models, while the changes in 27Al shielding with Na+ or CO2 complexation are much smaller. Complexation with CO2 greatly increases the electric field gradient at the bridging oxygen of H3AlOAlH-23, raising it to a value similar to that found for Si--O--Si linkages. Comparison of these results with the experimental 13C NMR spectra support the formation of CO2-like complexes at SiOSi bridges in albite glasses and CO3-like complexes at SiOAl and AlOAl bridges in albite and nepheline glasses. Changes in the calculated shieldings as Na+ ions are added to the complexes suggest that some of the observed complexes may be similar in their CO2-aluminosilicate interactions, but different with respect to the positions of the charge-compensating Na+ ions.