Abstract:
Ab initio, molecular orbital calculations of the structures, energetics, and vibrational spectra of six T2Obr-CO2 clusters and five T-[CO3]-T clusters (T = Al or Si, with and without Na+ present) have been completed using a 3-21G** basis set and the Gaussian 92 program to evaluate possible configurations of CO2 molecules and carbonate groups in melts and glasses on the NaAlO2-SiO2 join. Based on these calculations, we developed the following hypothesis for the solution mechanisms of CO2 molecules and carbonate groups in fully polymerized melts and glasses on this join. Molecular CO2 is weakly bound to bridging oxygen atoms in T2Obr-CO2 clusters. Calculated energetics indicate that molecular CO2 is more strongly bonded when associated with smaller angle T-O-T linkages, and thus may preferentially bond to linkages where at least one T = Al3+ . [CO3]2- is most likely present in T-[CO3]-T linkages. Si-[CO3]-Al linkages probably form in melts toward the silica-rich end of the NaAlO2-SiO2 join; Si-[NaCO3]-Al and/or Al-[CO3]-Al become more significant with increasing NaAlO2 content. The experimentally observed increase in [CO3]2-CO2 ratio accompanying higher NaAlO2 compositions can be understood in terms of the predicted increasingly negative ΔG° for T2Obr-CO2 -> T-[CO3]-T reactions when Si-[NaCO3]-Al and Al-[CO3]-Al rather than Si-[CO3]-Si and Si-[CO3]-Al are the reaction products. In addition, a reaction pathway with a low activation energy was calculated for forming T-[CO3]-T linkages from T2Obr-CO2 linkages. This model is consistent with available information on the vibrational and NMR spectra of C-bearing Na-aluminosilicate glasses, and on the relative proportions in these glasses of carbonate and molecular CO2 and their dependence on pressure, temperature, and composition.