Room-Temperature FTIR Spectra of the Cyclic S4 (H2O)4 Cluster in Crystalline Li2(H2O)4(B12F12): Observation of B and E ν(OH) Bands and Coupling of Strong O-H···O and Weak O-H···F Vibrations.

Abstract

Room-temperature-attenuated total reflection-Fourier transform infrared (FTIR) spectra of the structurally characterized crystalline lithium salt Li2(H2O)4(B12F12), which contains a cyclic S4 symmetry (H2O)4 cluster, and several partially or completely deuterated isotopologs, suggest the following conclusions: (i) B and E normal modes gave rise to two distinguishable ν(OHbound) bands, separated by 42 cm-1, two distinguishable ν(OHfree) bands, separated by 13 cm-1, and two distinguishable δ(HOH) bands, separated by 19 cm-1, in the FTIR spectrum of Li2(H2O)4(B12F12); (ii) B and E normal modes gave rise to two distinguishable ν(ODbound) bands separated by 20 cm-1 in the FTIR spectrum of Li2(D2O)4(B12F12); (iii) coupling between ν(OHbound) and ν(OHfree) normal modes or between ν(ODbound) and ν(ODfree) normal modes is weak, but finite, and resulted in shifts of 4-10 cm-1 for the respective bands in the spectra of Li2(H2O)4(B12F12) vs Li2(HOD)(D2O)3(B12F12) or in the spectra of Li2(D2O)4(B12F12) vs Li2(HOD)(H2O)3(B12F12); and (iv) a δ(DOD) band for an S4 (D2O)4 cluster, at ca. 1202 cm-1, was observed for the first time. In addition, FTIR spectra of the lithium salt containing cyclic [(HOD)(H2O)3] or [(HOD)(D2O)3] clusters are the first examples in which bands that are unambiguously assigned to both HO-D···O and DO-H···O hydrogen bonds in the same sample have been observed for a water tetramer.