Vibrational study and spectra-structure correlations in ammonium saccharinate: comparison with the alkali saccharinates

The FT IR spectra, at temperatures from liquid-nitrogen boiling (LNT) up to room temperature (RT), as well as the RT Raman solid-state spectra of prot onated and deuterated ammonium saccharinate and of a series of alkali (Na, K, Rb, Cs) saccharinates are studied. The spectral assignments are aided wi th ab initio calculations on the free saccharinato anion at the HF/6-31 + G(d,p) level. Attention is paid to the ND, CO and SO, stretching regions. Correlation splitting is believed to be responsible for the presence of a n u(CO) doublet. The averaged nu(CO) frequency in (purely ionic) ammonium sac charinate is found to be the lowest in the so far studied saccharinates, al ong with the assumptions that the nu(CO) frequency (or the corresponding av eraged value) can have predictive value for the type of the metal-to-saccha rinato ligand/ion bonding. The appreciably higher contribution of the domin ating internal coordinate in the corresponding normal vibration in case of nu(as)(SO2) than in nu(s)(SO2) makes it suitable for spectra-structure corr elations. Contrary to RT, even though no phase transitions were observed in the studied temperature range, some polycentered character is prescribed t o the hydrogen bonds in which the ammonium ions of effective symmetry C-s p articipate at LNT. Certain structural predictions about the saccharinates o f K, Rb and Cs are made. (C) 2000 Elsevier Science B.V. All rights reserved .