Unusually sharp FTIR ν(OH) bands and very weak O H⋯F hydrogen bonds in M2(H2O)1,2B12F12 hydrates (M Na Cs)

Abstract We report attenuated total reflection FTIR spectra of microcrystalline samples of M 2 (H 2 O) n (B 12 F 12 ) (M Na, K, Rb, Cs) and Na 2 (H 2 O) 6 (B 12 Cl 12 ). The ν (OH) bands are unusually narrow compared to the spectra of other alkali metal salt hydrates, with most full widths at half-max less than 20 cm −1 and some less than 10 cm −1 . In addition, these bands are not as redshifted compared to the spectra of other alkali metal salt hydrates. These unusual effects are attributed to extremely weak O H⋯F and O H⋯Cl hydrogen bonding between the coordinated H 2 O molecules and the B 12 F 12 2− and B 12 Cl 12 2− superweak anions. The ν (OH) bands in these compounds are as narrow as the bands in the FTIR spectrum of monomeric H 2 O absorbed in poly(vinylidene fluoride), in which the only hydrogen bonding is O H⋯F C, or as the bands in the IR photodissociation spectrum of the gas-phase species [(Ar)Na(H 2 O) 2 ] + . These unusual spectra are compared to the literature spectrum of [NEt 4 ] 4 [Hg(CB 11 F 11 ) 2 ] 2 ∙H 2 O, which also exhibits sharp ν (OH) bands with relatively small redshifts from the ν (OH) values for H 2 O( g ).