RELAXATION DYNAMICS, VIBRATIONAL-SPECTRA, AND SOLVENT DONOR NUMBER EFFECTS FOR NA+ INTERACTING WITH ACYCLIC POLYETHERS

Ultrasonic relaxation spectra of Na+ salts and tetraglyme (TeG) at molar ratio R = [TeG]/[NaX] = 1 in the solvent acetonitrile (epsilon-25 = 36.0, donor number DN = 14) can be interpreted in terms of two Debye relaxation processes. Existence of three molecular configurations participating in the two relaxation processes is supported by infrared spectra in the 800-900 cm-1 wavenumber region. The tetraglyme spectral envelope splits into three Gaussian-Lorentzian bands. However, for Na+ + poly(ethylene oxides) (PEO) of average molar mass MBAR = 400, 1000, and 2000 and R = [(CH2CH2O)5]/[NaX] = 1 (with X = ClO4- or BPh4-) the slower relaxation of the two observed processes shifts to lower frequencies than those observed for the Na+ + TeG systems in acetonitrile. Dimethylformamide (DMF), which is almost isodielectric with acetonitrile, but with a much larger DN, when used as the solvent for Na+ + TeG at R = 1, yields an ultrasonic relaxation spectrum interpretable by a single relaxation process, which is comparable to the faster relaxation of the same sample system in acetonitrile. The same is true for Na+ + PEO with average molar mass ranging from 400 to 2000 and with R = 1 at 25-degrees-C. However, the reverse rate constant for the formation of the Na+PEO contact pair in DMF is smaller than the corresponding rate constant for the dissociation of Na+TeG. The infrared spectra in the 800-900-cm-1 region of Na+ + TeG in DMF show no evidence of an alteration of the polyether band, suggesting that no significant inclusive species (Na+TeG), with Na+ embedded in the TeG, is formed in DMF. Ultrasonic relaxation spectra of Na+ interacting with cyclic 15C5 in DMF at 25-degrees-C are also reported. The ultrasonic relaxation spectral envelope can be interpreted by the sum of two Debye relaxation processes at variance with the case of Na+ + TeG in DMF. The existence of an inclusive complex (Na+15C5) in DMF is suggested, despite the large solvent donor number. This reflects a greater affinity of the macrocycle 15C5 than of the acyclic TeG for the Na+ cation. Thus the solvent and the ligand configuration (and at times the anion) compete for the first coordination sphere positions of a given cation.