THE PRONOUNCED EFFECT OF MICROSOLVATION ON DIATOMIC ALKALI HALIDES - AB INITIO MODELING OF MX(H2O)(N) (M=LI, NA X=F, CL N=1-3)

摘要

Ab initio cluster calculations are reported in which the diatomic alkali halide (MX) species LiF, LiCl, NaF, and NaCl are microsolvated with up to three water molecules. Second-order Moller-Plesset perturbation theory (MP2) was used in conjunction with correlation consistent basis sets: cc-pVDZ(Li, Na) and aug-cc-pVDZ (F, Cl, O, H). Cyclic structures, with C-1, C-2, and C-3 symmetries respectively, were found for the addition of one, two, and three water molecules. Harmonic frequencies were determined for the one and two water clusters in order to verify that these are true minima. For the one water cluster, a local minima of C-2v symmetry was also characterized, as well as the transition state between the two equivalent cyclic structures. The incremental binding energies for the successive addition of water molecules tend to be comparable within each species and very similar between different species as well. The incremental binding energies (in kcal/mol) for each species were: LiF(H2O) [-20.3], LiF(H2O)(2) [-19.2], LiF(H2O)(3) [-16.8]; LiCl(H2O) [-20.2], LiCl(H2O)(2) [-19.0], LiCl(H2O)(3) [-16.9]; NaF(H2O) [-21.9], NaF(H2O)(2) [-23.1]; and NaCl(H2O) [-17.5], NaCl(H2O)(2) [-16.8]. The metal-halogen bond was found to lengthen appreciably with increasing degree of solvation. In the LiCl(H2O)(n) series, the gas-phase LiCl equilibrium bond length of 2.088 Angstrom (MP2) increased to 2.440 Angstrom upon addition of three water molecules, an increase of 0.352 Angstrom. Zero-point energy contributions, cluster frequency shifts, partial charges, and the effect of basis set superposition error were also investigated. Other topics include the trends in the solvation dependence of the ionic asymptotes as well as an alternative acid-base channel for MX(H2O). [References: 27]