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The difference in the local environment of CO 3  2- , NO 3  - , SO 4  2- , and ClO 4  -  in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO 3  2-  a shift toward lower wavenumbers was observed. A band around 3000-3200 cm -1  has been attributed to the bridging mode H 2 O-CO 3  2- . The IR spectrum of CO 3  -  hydrotalcite clearly shows the split ν 3  band around 1365 and 1400 cm -1  together with weak ν 2  and ν 4  modes around 870 and 667 cm -1 . The ν 1  mode is activated and observed as a weak band around 1012 cm -1 . The Raman spectrum shows a strong ν 1  band at 1053 cm -1  plus weak ν 3  and ν 4  modes around 1403 and 695 cm -1 . The symmetry of the carbonate anions is lowered from D 3h  to C 2s  resulting in activation of the IR inactive ν 1  mode around 1050-1060 cm -1 . In addition, the ν 3  shows a splitting of 30-60 cm -1 . Although NO 0  hydrotalcite has incorporated some CO 3  2-  the IR shows a strong ν 3  mode at 1360 cm -1  with a weak band at 827 cm -1 , and the ν 4  band is observed at 667 cm -1 , although it is largely obscured by the hydrotalcite lattice modes. The Raman spectrum shows a strong ν 1  mode at 1044 cm -1  with a weaker ν 4  band at 712 cm -1 . The ν 3  mode at 1355 cm -1  is obscured by a broad band due to the presence of CO 3  2- . The symmetry of NO 3  -  did not change when incorporated in hydrotalcite. The IR spectrum of SO 4- hydrotalcite shows a strong ν 3  at 1126, ν 4  at 614 and a weak v 1  mode at 981 cm -1 . The Raman spectrum is characterized by a strong ν 1  mode at 982 cm -1  plus medium ν 2  and ν 4  bands at 453 and 611 cm -1 ; ν 3  cannot be identified as a separate band, although a broad band can be seen around 1134 cm -1 . The site symmetry of SO 4  2-  is lowered from T d  to C 2ν . The distortion of ClO 4  -  in the interlayer of hydrotalcite is reflected in the IR spectrum with both ν 3  and ν 4  bands split around 1096 and 1145 cm -1  and 626 and 635 cm -1 , respectively. A weak ν 1  band is observed at 935 cm -1 . The Raman spectrum shows a strong ν 1  mode at 936 cm -1  plus ν 2  and ν 4  bands at 461 and 626 cm -1 , respectively. A ν 3  mode cannot be clearly recognized, but a broad band is visible around 1110 cm -1 . These data indicative a lowering of symmetry from T d  to C s .

Infrared and Raman study of interlayer anions CO 3 2– , NO 3 – , SO 4 2– and ClO 4 – in Mg/Al-hydrotalcite