1 3 C NMR shieldings have been calculated using the random-phase-approximation, localized-orbital local-origins version of ab initio coupled Hartree-Fock perturbation theory for CO 2 and and for several complexes formed by the reaction of CO 2 with molecular models for aluminosilicate glasses, H 3 TOT H - n 3 , T,T = Si,Al. Two isomeric forms of the CO 2 -aluminosilicate complexes have been considered: (1) CO 2 -like complexes, in which the CO 2 group is bound through carbon to a bridging oxygen and (2) CO 3 -like complexes, in which two oxygens of a central CO 3 group form bridging bonds to the two TH 3 groups. The CO 2 -like isomer of CO 2 -H 3 SiOSiH 3 is quite weakly bonded and its 1 3 C isotropic NMR shielding is almost identical to that in free CO 2 . As Si is progressively replaced by Al in the --H terminated aluminosilicate model, the CO 2 -like isomers show increasing distortion from the free CO 2 geometry and their 1 3 C NMR shieldings decrease uniformly. The calculated 1 3 C shielding value for H 3 AlO(CO 2 )AlH - 2 3 is only about 6 ppm larger than that calculated for point charge stabilized CO - 2 3 . However, for a geometry of H 3 SiO(CO 2 )AlH - 1 3 , in which the bridging oxygen to C bond length has been artificially increased to that found in the --OH terminated cluster (OH) 3 SiO(CO 2 )Al(OH) - 1 3 , the calculated 1 3 C shielding is almost identical to that for free CO 2 . The CO 3 -like isomers of the CO 2 -aluminosilicate complexes show carbonate like geometries and 1 3 C NMR shieldings about 4-9 ppm larger than those of carbonate for all T,T pairs. For the Si,Si tetrahedral atom pair the CO 2 -like isomer is more stable energetically, while for the Si,Al and Al,Al cases the CO 3 -like isomer is more stable. Addition of Na + ions to the CO - 2 3 or H 3 AlO(CO 2 )AlH - 2 3 complexes reduces the 1 3 C NMR shieldings by about 10 ppm. Complexation with either Na + or CO 2 also reduces the 2 9 Si NMR shieldings of the aluminosilicate models, while the changes in 2 7 Al shielding with Na + or CO 2 complexation are much smaller. Complexation with CO 2 greatly increases the electric field gradient at the bridging oxygen of H 3 AlOAlH - 2 3 , raising it to a value similar to that found for Si--O--Si linkages. Comparison of these results with the experimental 1 3 C NMR spectra support the formation of CO 2 -like complexes at SiOSi bridges in albite glasses and CO 3 -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 CO 2 -aluminosilicate interactions, but different with respect to the positions of the charge-compensating Na + ions.