ON FREQUENCY-SHIFTS IN OH STRETCHING VIBRATIONS OF HYDRATED CATIONS
1992 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 96, no 12, 8995-9004 p.Article in journal (Refereed) Published
Ab initio calculations of force constants up to fourth order and anharmonic frequencies for the uncoupled O-H stretching vibration in hydrates of the ions Li+, Na+, Mg2+, and Al3+ have been performed at the Hartree-Fock level for M(n+) (H2O)m and M(n+) (H2O)m...(H2O)2 complexes, with m = 5-7. The dependence of force constants and frequency shifts on ion type, cation-water distances, second hydration shell, hydrogen bonding and coordination number is discussed. In the optimized Li+(H2O)6 system, the OH frequency is downshifted 20 cm-1 compared to the free water molecule, whereas, in the optimized Li+ (H2O)6...(H2O)2 complex, the downshift is 120 cm-1. The corresponding numbers for the Al3+ hexaaqua complexes are - 250 and - 1215 cm-1. Frequency shifts with respect to bulk water for the cation-coordinated water molecules have been estimated from the difference between the frequencies of M(n+) (H2O)6...(H2O)2 and a (H2O)2...H2O...(H2O)2 complex. The values are + 85 cm-1 for Li+ and Na+, - 150 cm-1 for Mg2+, and - 1000 cm-1 for Al3+, to be compared with the infrared frequency shifts of + 50 for Li+ and Na+, - 155 for Mg2+ and - 500 cm-1 for Al3+ (experimental aqueous solution values from the literature). The activation energies for water exchange between the hydration shells in the hexaaqua complexes have been calculated for dissociative and associative exchange processes and suggest a dissociative mechanism to be dominant.
Place, publisher, year, edition, pages
1992. Vol. 96, no 12, 8995-9004 p.
IdentifiersURN: urn:nbn:se:uu:diva-228778DOI: 10.1063/1.462257ISI: A1992HZ09800037OAI: oai:DiVA.org:uu-228778DiVA: diva2:734863