Proton transfer dynamics in the hydrogen bond. Inelastic neutron scattering, infrared and Raman spectra of Na 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SO 4) 2

Abstract

Na 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SO 4) 2 crystals are composed of (SO 4HSO 4) −3 dimers linked by rather strong hydrogen bonds ( R O…O=2.43 Å for Na 3H(SO 4) 2, R O…O=2.48 Å for Rb 3H(SO 4) 2 and R O…O=2.49 Å for K 3H(SO 4) 2). Crystallographic data of the salts at room temperature indicate either asymmetric (Na 3H(SO 4) 2) or symmetric (K 3H(SO 4) 2 and Rb 3H(SO 4) 2) hydrogen bonds. Inelastic neutron scattering (INS), infrared and Raman spectra of crystal powders at 20 K are reported for these three compounds. The OH bending modes, which give large INS intensities, appear only weakly in the infrared. The two bending modes are degenerate in Na 3H(SO 4) 2 which has the shortest hydrogen bond but are well separated in K 3H(SO 4) 2 and Rb 3H(SO 4) 2. The OH stretching band profiles in INS are also quite different from those in the infrared. Strong INS bands at 57 and 44 cm −1 for K 3H(SO 4) 2 and Rb 3H(SO 4) 2, respectively, are assigned to 0→1 transitions in quasi-symmetric double-minimum potentials for the OH stretching coordinates. For K 3H(SO 4) 2 the frequency is unaffected by temperature between 2 and 100 K. Potential functions are calculated and the dynamics of the proton transfer are discussed. Infrared spectra are thus dominated by OH stretching transitions in asymmetric double-minimum potentials with low barriers, with relative intensities indicating a large electrical anharmonicity.