Infrared and Ultrasonic Spectra of Sodium Thiocyanate and Lithium Thiocyanate in Tetrahydrofuran.

Infrared spectra of sodium thiocyanate solutions and of lithium thiocyanate solutions in tetrahydrofuran THF in the wavenumber range 2000-2200cm are reported. The digitized spectral envelope, which is due to the out of phase stretch of the SCN anion, is quantitatively described by the sum of three Gaussian-Lorentzian product functions. The three spectral bands are assigned respectively to the N-bonded pairs approx. 2057cm for NaNCS, approx. 2064cm for LiNCS, to the dimers or quadrupoles approx. 2043cm for NaNCS2, 2040cm for LiNCS2, and tentatively to triple ions 2074cm for sodium thiocyanate, approx. 2086cm for lithium thiocyanate. The maximum absorbances per unit lenght of cell are expressed as a function of concentrations by cubic polynomial functions. Ultrasonic spectra for sodium thiocyanate solutions in the THF in the frequency range 0.5-400 MHz are described by the sum of two Debye relaxation functions. For lithium thiocyanate solutions in the THF, the ultrasonic spectra are described by a single Debye relaxation function. The limitations of the infrared method in giving reliable formation stoichiometric constants in media of low permittivity are discussed. The advantage of combining the structural information from vibrational spectra to the kinetic results of the ultrasonic spectra for a molecular interpretation of the latter ones is reiterated.