186
Communications to the Editor
References and Notes (1) (2) (3) (4) (5)
the SCF potential7 suggests that our interaction energy terms for halide ions would be much better represented by having the attractive ion dipole terms based closer to the H atoms rather than at the H20 center of mass. For alkali ions the center of mass terms are similar to those implied in the SCF algebraic representation. In summary we can conclude that the original selection of repulsive parameter^^,^ should have been based on a slightly different form of the attraction between halide ions and H20. This more attractive potential expression would have resulted in larger repulsive energy parameters for the H atoms of H 2 0 thereby yielding a larger equilibrium separation in the calculations of Table I. The availability of SCF results allows a more accurate choice of the attractive part of the halide ion-H20 potential in the future. The use of the electrostatic m e t h ~ d l allows -~ an accurate representation of interaction effects in complicated clusters. As shown in ref 1,remarkable agreement with SCF results is obtained for the case of Li+. Indeed, more recent SCF results for clusters with Li' and Na+ that only contain pairwise additive terms8 do not agree as well with experiment as the electrostatic meth0d.l Large basis set SCF computations are necessary for these ions8to agree with experiment.
R. Field, E. Koros, and R. Noyes, J Am Chem Soc.,94,8649 (1972). H. Degn, Nature (London), 213, 589 (1967). L. Bornmann et al., 2. Naturforscb., 286, 824 (1973). G. Kasperek and T. Bruice, Inorg. Chem., 10, 382 (1971). S. Jacobs and I. Epstein, J. Am. Chem. Soc., 98, 1721 (1976).
201th Nosrticzius
Institute of Physics Department of Chemical Engineering Technical University of Budapest 152 1 Budapest, Hungary Received June 21, 1976
Hydration Structures For Halide (-) Ions
Sir: We report the calculated hydration energies of halide (-) ions for the symmetrical hydration structures containing 1-6 waters of hydration. The method is identical with the semiempirically based electrostatic method previously used for alkali (+) i0ns.l Empirically based repulsive parameters were previously selected from alkali halide diatomic spectra and ion-rare gas beam scattering
TABLE I: Energies a n d Distances for Symmetric H a l i d e Ion Hydrationa Hydration no. 1 Ion
FC1Br-
1-
-E 24.1 (23.3) 14.3 (13.1) 12.6 (12.6) 10.5 (10.2)
2
R 2.14 2.86 3.04 3.34
-E 44.8 (39.9) 27.6 (25.8) 24.4 (24.9) 20.5 (20.0)
3
R 2.22 2.88 3.08 3.36
-E 60.2 (53.6) 40.1 (37.5) 34.8 (36.4) 29.4 (29.4)
4
R
-E
2.32 2.92 3.12 3.38
72.7 (67.i) 49.1 (48.6) 43.9 (47.3) 37.4 (-
1
5
R
-E
2.40
81.0 (80.3) 57.1
2.96
(-
3.16
1
51.4
6 R
-E
R
2.48
86.0
2.62
(-
3.02
44.2
3.08
(- )
3.20
58.0 (-
(-)
3.42
1
63.9
3.46
(- )
1
50.2
3.24 3.50
(- )
a Energies are in k c a l / m o l and the equilibrium distance in angstroms is from the ion t o t h e H,O center of mass. T h e energy value enclosed in parentheses is experimental AH from r e f 4.
data.2, These parameters have been applied to halide (-1 ion hydration for the symmetrical structures including a linear dimer, a planar symmetrical trimer and tetrahedron, trigonal bipyramid, and an octahedron. The equations necessary to compute the negative ion hydration structures were included in the extensive description of the alkali (+) hydration calcu1ations.l The results are given in Table I with comparisons to the experimental AH values of Kebarle et al.4 The agreement of our values with experiment has substantial deviations as the size of the hydration sphere increases. This is particularly noticeable for the F-ion with 1-4 ligands. This disagreement shows a fundamental flaw in our previous construction of an empirical potential for negative ions and a single water molecule. As previously stated,' uniformly excellent experimental agreement of the alkali (+) hydration energies shows a reasonably correct radical dependence empirical potential for positive ions. The source of the halide ion difficulty is easily seen by comparison to recent SCF results for the equilibrium distance of F--Hz0.6,6An SCF calculation gives the F--H20 center of mass distance of 2.44 A6 compared to our 2.14 A. Additional comparison to the simple algebraic form of
The Journal of Physical Chemistry, VoL SI,No. 2, 1977
Acknowledgment. Sang Hyung Kim deserves thanks for his assistance in calculating the results for halide ion binding. The author has benefited by being awarded an Alfred P. Sloan Fellowship during 1974-1976.
References and Notes K. G. Spears and S. H. Kim, J. Pbys. Cbem., 80, 673 (1976). K. G. Spears, J. Cbem. Pbys., 57, 1842 (1972). K. G. Spears, J. Cbem. Pbys., 57, 1850 (1972). M. Arshadi, R. Yamdagni, and P. Kebarle, J. Pbys. Cbem.,74, 1475 (1970). G. H. F. Diercksen and W. P. Kraemer, Cbem. Phys. Lett., 5, 570 (1970). H. Kistenmacher, H. Popkie, and E. Clementi, J. Cbem. Phys., 58, 5627 (1973). H. Kistenmacher, H.Popkie, and E. Clementi, J. Cbem. Pbys., 59, 5842 (1973). H. Kistenmacher, H. Popkie, and E. Clementi, J. Cbem. Phys., 61, 799 (1974).
Department of Chemistry Northwestern University Evanston, Illinois 60201 Received August 23, 1976
Kenneth G. Spears
