Experimental and computational study of thaumasite structure

• The thaumasite structure was studied experimentally and using DFT method.
• We used DFT method for the refinement of the positions of hydrogen atoms.
• A detailed analysis of the hydrogen bonds was done.
• A complete assignment of all bands to particular types of vibrations was done.

The structure of thaumasite has been studied experimentally by means of a single crystal X-ray diffraction and FTIR methods, and theoretically using density functional theory (DFT) method. Very good agreement was achieved between calculated and experimental structural parameters. In addition, calculations offered the refinement of the positions of the hydrogen atoms. The detailed analysis of the hydrogen bonds existing in the thaumasite structure has been performed. Several types of hydrogen bonds have been classified. The water molecules coordinating Ca2 +cation act as proton donors in moderate OH⋯O hydrogen bonds formed with CO32 −and SO42 −anions. The multiple OH⋯O hydrogen bonds exist among water molecules themselves. Finally, relatively weak hydrogen bonds form water molecules with the OH groups from the coordination sphere of the Si(OH)62 − anion. Further, calculated vibrational spectrum allowed complete assignment of all vibrational modes which are not available from the experimental spectrum that has a complex structure with overlapped bands, especially below 1500 cm− 1.