De. Woon et Th. Dunning, 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), Journal of the American Chemical Society, 117(3), 1995, pp. 1090-1097
Ab initio cluster calculations are reported in which the diatomic alka
li 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 basi
s sets: cc-pVDZ(Li, Na) and aug-cc-pVDZ (F, Cl, O, H). Cyclic structur
es, 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 st
ate between the two equivalent cyclic structures. The incremental bind
ing energies for the successive addition of water molecules tend to be
comparable within each species and very similar between different spe
cies 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 wi
th 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 err
or were also investigated. Other topics include the trends in the solv
ation dependence of the ionic asymptotes as well as an alternative aci
d-base channel for MX(H2O).