Ab initio bonding and structures of chlorofluoroaluminate ions and

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478

J. Phys. Chem. 1994, 98, 478-485

Ab Initio Bonding and Structures of Chlorofluoroaluminate Ions and Their Sodium Salts Charles W. Bock' Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144, and American Research Institute, Suite 121 2, Upper Darby, Pennsylvania 19082

Mendel Tracbtman' Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144

Gilbert J. Mains' Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received: May 13, 1993'

The a b initio structures and energies of the isomers of NaAlCl,F&, (n = 0-4) have been calculated a t the HF/6-3 l G * level. Energies for their dissociation into ionic and neutral fragments are presented at the MP2/ 6-31G*//RHF/6-3 l G * and MP2/6-3 l+G*//RHF/6-3 lG* levels. Electron densities of AlC14-, AlClzFz-, AlFd-, and their sodium salts are reported and compared with electron densities for Na+Cl- and Na+F-. The agreement between the calculated and experimental equilibrium constants for the exchange reactions between the AICI,Fk,,- ( n = 0-4) structures is remarkable. The vibrational spectra of AlCl,F,- ( n = 0-4) anions are calculated and compared with experimental values.

Introduction The structure of molten mixtures of alkali metal halides and aluminum halides continues to be of current interest.lJ The primary driving force for research into these ionic liquids has been the potential application of these melts as solvents for high density batteries.' If the melts are acidic, Le., contain an excess of aluminum halide, the melting points are circa 100 OC for bromoaluminates and 200 OC for chloroaluminates and chlorofluor~aluminates,~ whereas if the melts are basic, Le., those containing an excess of alkali metal halide, the melting points are in excess of 500 OC. The early discovery that quaternary organic chlorides mixed with AIC4 are molten at room temperature4 inspired much research which was motivated by the prospect of room-temperature high-electron density batteries. Since these early melts did not contain excess alkali metal halides, small changes in their compositioncreated large changes in the ultimate electrochemistry, as these melts would change between acidic and basic during the discharge process. This problem was solved by Wilkes and co-workers,5whoshowed that the addition of NaCl to the chloroaluminate salts effectivelybuffered the melts on the basic side. Consequently, there is an enormous literature in this field, and the reader is directed to early reviews by Gale and Osteryoung6 and by Chum and Osteryoung.7 More recent advances are commonly found in the biennial International Symposia on Molten Salts published by the Electrochemical Society.8 The alkali metal chloroaluminate salts have been studied experimentally using IR emission?JO Raman spectroscopy,lI-*3 X-rayl'and recently by neutron diffraction.' Theoretical studies have been reported for LiAlF4 and NaAlF4 by Charkin et a1.15 and by Scholz and Curtiss.16 Nichols and CurtissI7 reported an ab initio molecular orbital study of AIX4- (X= F, Cl). Recently Gilbert et al.11 tentativelyidentified variouschlorofluoroaluminate ions, A1Cl,Fc,- ( n = 1 4 ) , using Raman spectroscopy. However, there have been no ab initio studies of these ions or of their sodium salts that may exist in a variety of bidentate and tridentate forms with different combinations of fluorine and chlorine atoms in the bridge. The purpose of this paper is to report the results of an .Abstract published in Advance ACS Absrracrs, December 15, 1993.

ab initio study of the structure, vibrational frequencies, and electron densities of the chlorofluoroaluminate ions. We also considerthe possible structures,relative stabilities,and dissociation energies of the associated sodium salts.

Computational Methods Ab initio calculations were performed using the GAUSSIAN 90 and GAUSSIAN 92 package of programs1*J9on the CRAY Y-MP/832 computer at the Pittsburgh Supercomputing Center on IRIS 4D/35 and V A X 8250 computerslocated in Philadelphia, on the CRAY-2, and the RS/6000 cluster at the National Center for Supercomputing Applications (NCSA) in ChampaignUrbana, and the IBM 3090 in Stillwater. Gradient optimizations were employed throughout using the 6-31G* basis set, resident in these packages. Frequency analyses at the RHF/6-31GS// RHF/6-3 1G*level were performedon all structures to determine whether they were stable states or transition states. Electron correlation was included by performing single-point calculations at the MP2/6-31G*//RHF/6-31G* and the MP2/6-31+G*/ /RHF/6-31G* levels for all structures and at higher levels with more complete basis sets in a few instances. Electron density maps were prepared using the CAChe Personal InnovatorZo program using a MacIntosh IIsi desktop computer equipped with 17 Mb of RAM, a 68882 coprocessor, and two 64k cache cards.

Results and Discussion The structures of the molecules studied and the charges found by Mulliken population analysis are given in Figures 1-6 and 14, with the specific geometrical parameters given in the captions to the figures. Electron density contours are given in Figures 7-1 3. Total molecular energies at the RHF/6-31G*//RHF/6-31G*, the MP2/6-31GZ//RHF/6-3 1G*, and the MP2/6-3 l+G*// RHF/6-3 lG*levels are listed in Table 1. In Table 2 energies are given for various isomerization and decomposition reactions. Scholz and CurtissI6showed the effect of diffuse functions on the energies of both chlorine- and fluorine-containing species is especially significant, which can be verified here by comparison of the last two columnsin these tables. Hence, energy comparisons will be made only at the MP2/6-31+G//RHF/6-31G* level herein.

QQ22-3654/94/2Q98-0418~Q4.5Q/Q Q 1994 American Chemical Society

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 479

ChlorofluoroaluminateIons

d03 AlCl ? -

AlCl,-

d

AlCl?;

AlCl I2 2

-.634

Al?

,-

Figure 1. AICIflh-: HF/6-31G*//HF/6-31G* values are given. ir bonds are shown as heavy black lines. (a) AQ-: bond AI-CI = 2.169. Bond angles (deg): CI-AI-CI = 109.5. (b) AIQF-: Bond lengths (A): AI-F = 1.659,AI-CI = 2.177. Bond angles (deg): F-AI-CI = 109.5,CI-AI-CI = 109.4. (c) AICI~FZ-:bond lengths (A): AI-F = 1.662, AI-CI = 2.188. Bond angles (deg): F-AI-CI = 109.2, CI-AI-CI = 109.0, F-AI-F = 110.8. (d) AICIF3-: Bond lengths (A): AI-F = 1.668, AI-CI = 2.206. Bond angles (deg): F - A I 4 = 108.6, F-AI-F = 110.4. (e) AIFd-: Bond lengths (A): AI-F = 1.677. Bond angles (deg): F-AI-F = 109.5.

:!;&

a

a

421

tridentat. N.AlCL

b

-.u9

m 461

bidentat. N~AIC~,

Figure X NaAIC4: HF/6-31G*//HF/6-31G* values are given. Electron pair bonds are shown as heavy black lines. (a) Tridentate: Bond lengths (A): Na-CI = 2.813, Na-AI = 3.007, AI-CI = 2.193. Bond angles (deg): CI-AI-Clb = 116.7, Clb-AI-Clb = 101.3. (b) Bidentate: Bond lengths (A): Na-Clb = 2.631, Na-AI = 3.433, AI-Clb = 2.230, A I 4 1 = 2.1 17. Bond angles (deg): CI-AI-CI = 117.0, CI-AI-CIb = 109.6, CIb-AI-clb = 100.1.

N a A Q zunI M a - . The HF/6-3 lG* optimized structure for AlCb-, provided in Figure la, exhibits tetrahedral symmetry similar to that reported by Curtiss and Nich0ls.1~The electron density at the HF/6-31G* level in a plane passing through the two chlorine nuclei and the A1 nuclei is given in Figure 7. Note that the minimum electron density between the chlorine and aluminum atoms is just larger than 0.06 e-/A3. The MP2/63 1+G* energy required to dissociate AlC14- into AlC13 and C1is 5 1.57 kcal/mol; see Table 2. Horn and AhlrichsZ1suggest that

there is covalent character to the A141 bond in agreement with the magnitude of the bond energy found. Two stable isomers were found for NaAlCl4, one in which the Na occupies the center of the three chlorine atom face, the tridentate form shown in Figure 2a, and a second where the Na occupies the edge of the tetrahedron, the bidentate form shown in Figure 2b. The minimum electron density between the sodium atom and the chlorine atoms (not shown, vide infra NaAlC12F2) is much less than for NaCl, suggesting that the sodium bonding is primarily ionic and the lowest energy structure is determined by the combined attractive Coulombic forces between the Na+ ion and the negatively charged C1 atoms and the repulsive Coulombic force between the Na+ ion and the positively charged aluminum atom. Since the sodium4hlorine attraction dominates the sodium-aluminum repulsion in the tridentate form, it is lowest in energy. No evidence for a monodentate conformerof NaAlCl4 was found, either computationally or in the literature. The isomerization energy, given in Table 2, shows that the tridentate structure is more stable than the bidentate structure by 1.83 kcal/mol at the MP2/6-3 1+G* level. The X-ray study of Koura et al.14 found interatomic peaks at 2.80 and 4.20 A. The former was attributed to NaCl and the latter to AI-Na. They predicted an AI-Na distance of 4.93 A for the monodentate, 3.42 A for the bidentate, and 2.65 A for the tridentate hard-sphere structures. Our calculationsfind 3.43 A for the bidentate and 3.00 A for the tridentate structure, suggesting that the hard-sphere analysis underestimates the AI-Na distance in the tridentate structure. We find the NaCl bond distance to be 2.48 A at the HF/6-31G* level compared with 2.40Aobtained at theHF/6-3 lG(2d) level2 and 2.36 A obtained experimentally.23 Thus, the assignment of the peak in the region of 2.80 A to NaCl alone may not be correct. We have calculated the transition state separating the bidentate and tridentate structures to be only 0.84 kcal/mol (0.49 kcal/ mol) above the bidentate structure at the HF/6-31G* (MP2/ 6-31G*) level, suggesting that interchange between the structures is probably rapid at the temperatures used experimentally. The dissociationof tridentate NaA1Cl4 into AIC13and NaCl requires only 54.24 kcal/mol at the MP2/6-31+G* level compared with 106.99 kcal/mol to dissociate into Na+ and AlC14- at the same level; see Table 2. These energiesare consistent with the bonding proposed for these structures. NaAlCl3F and A Q F - . The symmetry of AlC13F- is CjO,as shown in Figure 1b. The minimum electron density between the A1 and F atoms (similar to that shown in Figure 9a for AlC12F2-) is -0.12 e-/A3 compared to -0.06 e-/A3 between the A1 and C1 atoms, suggestingthat the AI-F bond is significantly stronger than the A I 4 bond. However, this is not reflected in the energy required todissociateAlCl3F- into AlCl3 and F-, 89.76 kcal/mol, compared to 77.49 kcal/mol required to dissociate AlC13F- into AlC12F and C1-. Thus a factor of 2 in the minimum densities between atoms results in only a 12.26 kcal/mol difference in the dissociation energies for AI-F and A141 bonds. This factor of 2 in electrondensitiesis characteristicof all the AlC1,Fk- studied (vide infra). These dissociation energies are consistent with the relative Lewis acidities and basicities reported p r e v i o u ~ l y . ~ ~ ~ ~ ~ The sodium salt structures, NaAlCl3F, are given in Figure 3. As expected, we find tridentate structures both with and without a fluorine atom on the face (Figure 3b,a, respectively). Similarly we find bidentate structures with and without a fluorine atom on the edge (Figure 3d,c, respectively). The minimum electron density between the sodium ion and the fluorine ion is greater than 0.04 e-/A3, similar to that in NaF, shown in Figure 11, suggesting the bonding between the sodium and the fluorotrichloroaluminate ion is predominately ionic. It is interesting to note that the most stable tridentate species is that shown in Figure 3a and not Figure 3b. Although the replacement of a chlorine atom with an F atom on the tridentate face enhancesthe Coulombic attraction, it also brings the sodium ion closer to the

Bock et al.

480 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

a

-.428

c l p tridontato i&A1Cl3?

C 5 tridontato

d

+. 738

-.576 +1.070 -.M2

?Cl bidontat. NaA1C13?

U C 1 1 bidontat. i&AlClJI

-.467

Figure 3. NaAlCl3F Hf/6-3 lG*//HF/6-3 1G*valuesare given. Electron pair bonds are shown as heavy black lines. (a) 3C1-tridentate: Bond lengths (A): Na-Clb = 2.8 15, Na-A1 = 2.994, Al-Clb = 2.191 AI-F = 1.632 A. Bond angles (deg): Clb-AI-Clb = 101.7, C1b-AI-F = 116.4 (b) 2C1-Tridentate: Bond lengths (A): Na-Clb = 2.898, Na-F = 2.177, Na-AI = 2.861, Al-Clb = 2.1 87, Al-CI = 2.099, AI-Fb = 1.701 A. Bond angles (deg): Clb-Al-Clb = 103.3, Clb-AI-Fb = 117.5. (c) 2CLbidentate: Bond lengths (A): Na-Clb = 2.626, Na-AI = 3.412, AI-Clb = 2.229, AI-CI = 2.1 15, AI-F = 1.641. Bond angles (deg): CI-AI-F = 117.1, F-AI-Clb = 108.6, Clb-Al-Clb = 110.4. (d) FCI-bidentate: Bond lengths (A): = 2.665, Na-F = 2.104, Na-AI = 3.135, AI-Clb = 2.230, AI-F = 1.713, AI-CI = 2.120. Bond angles (deg): CI-AI-CI = 117.3, F-Al-Clb = 95.7, CI-AI-Clb = 110.8.

a

fi

t451

(y+

b

490

h U

+.743 +1.497

-.567

Ittridontat.

499

NMlCl?, C 4 ? trldontato i&Alc12?a

Cl bidantat. NhCl21

c +.742

*

-

-.481

?Cl bidontat. R.A1c12?2

c

+1.411

-.436

?2bidontato N~AICIQ

Figure 5. NaAICIF3: HF/6-31G*//HF/6-31G*

5 bidontat. NaAICla?a

Figure 4. NaAIC12F2: HF/6-31G*//HF/6-31G* values are given. Electron pair bonds are shown as hca black lines. (a) 2ClF-tridentate: Bond lengths (A): Na-Clb = 2.902,1, Na-F = 2.171, Na-AI = 2.851, Al-Clb = 2.186, AI-Fb = 2.171, AI-F = 1.632. Bond angles (deg): Clb-Al-Clb = 103.6, Clb-AI-F = 117.8, Fb-AI-F = 117.1. (b) 2C1-bidentate: Bond lengths (A): Na-Clb = 2.621, Na-AI = 3.396, = 2.230, AI-F = 1.640. Bond angles (deg): Clb-AI-Clb 101.O, C1b-AI-F = 109.2, F-AI-F = 117.7. (c) FC1-bidentate: Bond lengths = 2.232, (A): Na-Clb = 2.659, Na-Fb = 2.099, Na-Alz 3.121, AI-Fb = 1.7 14,AI-CI = 2.120, AI-F = 1.642. Bond angles (deg): F-AIc1 = 117.1, Fb-AI-Clb = 96.0, CI-Al-Clb = 111.1. (d) 2F-bidentate: Bond lengths (A): Na-Fb = 2.1 35, Na-A1 = 2.908, AI-Cl = 2.123. Bond angles (deg): Cl-AI-Cl = 117.7, Fb-AI-Cl = 110.8, Fb-Al-Fb = 93.4.

Al atom which raiscsthe Coulombicrepulsion. There is significant asymmetry in the structure shown in Figure 3b as the sodium ion moves toward the smaller fluorine atom. Observe that the tridentate structure, Figure 3b, is actually higher in energy than the bidentate structure, Figure 3c, with a double C1 edge at the MP2/6-3 1G*level. The tridentateisomers are within 3.52 kcal/ mol of each other at the MP2/6-31+G* level. The dissociation of NaAIC13F (see Figure 3a) intoNa+andAlC13F-requires107.33 kcal/mol, compared to 54.57 kcal/mol to dissociate into AlCl2F and NaCl and 75.59 kcal/mol to dissociate into AlC13and NaF, all at the MP2/6-3 1+G* level. Again, thesedissociation energies are consistent with the relative Lewis acidities and basicities reported pre~iously.2~JJ

values are given. Electron pair bonds are shown as heavy black lines. (a) 3F-tridentate: Bond lengths (A): Na-Fb = 2.286, Ne-A1 = 2.593, AI-Fb = 1.697, AI-CI=2.101. Bondangles(deg): Fb-AI-Fb-97.4,Fb-AI-Cl.L 119.8. (b) FCI-bidentate: Bond lengths (A): Na-Clb = 2.651, Na-Fb = 2.093, Na-AI = 3.109, Al-Clb = 2.237, AI-Fb = 1.715, AI-F = 1.643. Bond angles (de&: F-AI-CIb = 110.3, Fb-AI-Clb = 96.2, F-AI-CF = 117.3. (c) 2F-bidentate: Bond lengths (A): Na-Fb = 2.128, Na-AI = 2.896, AI-Cl = 2.123, AI-F = 1.644. Bond angles (deg): Cl-AI-F = 117.2, Fb-AI-CI = 110.5, Fb-AI-Fb = 93.6.

Figare6. NaAIF4: HF/6-31G*//HF/6-31G*valuesaregiven. Electron pair bonds are shown as heavy black lines. (a) Tridentate: Bond lengths (A): Na-F = 1.699, Na-AI = 2.583, AI-F = 1.634. Bond angles (de ): F-Al-Fb = 119.7, Fb-Al-Fb = 97.5. (b) Bidentate: Bond lengths Na-Fb = 2.120, Na-A1 = 2.887, = 1.722, AI-F = 1.646. Bond angles (deg): F-AI-F = 116.9, F-AI-Fb = 111.0, Fb-Al-Fb = 93.6.

(I):

Since it is known that diffuse functions are important in describing anions and that the A141 bonds are highly polar covalentbonds, there was somequestion in our minds as to whether the 6-3 1G*basis set was adequatelydtscribing thesecompounds. Therefore, we reoptimized the geometries of the two tridentate structures at the RHF/6-31+GS//RHF/6-3l+G* level. We

ChlorofluoroaluminateIons

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 481

NaF ~ r o ~ cnn d t ycontoursin 4 3 ThroughPlane with Na F A t o m

-

Figure 11. Electron density for NaF at the HF/6-31G* level. Contours . at constant e-/A3 at the values shown.

- -

u ~ a m ~nc n d t Contarla y in */A3 Through Plane with CI A I CI Atom

e

Figure 7. Electron density for AICI4-at the HF/6-31G* level. Contours at constant e-/A3 at the values shown through the CI-AI-CI plane.

0.08

0.00

NaCl ”E m t y contours in 4 3 Through Planewith Na CI Atom,

~ b t ~cnd&Fktarrs m in e - 1 ~ 3 Through Plane with F Ai F Atom

- -

-

Figure 8. Electron density for NaCl at the HF/6-31G* level. Contours at constant e-/A3 at the values shown.

Figure 12. Electron density for AlF4-at the HF/6-31G* level. Contours at constant e-/A3 at the values shown.

NnAIF4 ~ r o ~cnrity n contours in e-& Through Plane with Na F A I Atom

- - -F

Figure 13. Electron density for NaAlF~atthe HF/6-3 lG* level. Contours at constant e-/A3 at the values shown.

C

0.

.08

- -

Thmuih Plane with CI AI F A t m

Figure 9. Electron densit for AlCl2F2- at the HF/6-31G* level. (a) Contours at constant e-/K3 at the values shown through the F-AI-F plane. (b) Contours at constant e-/A3 at the values shown through the Cl-AI-F plane. (c) Contours at constant e-/A3 at the values shown through the CI-AI41 plane.

Q2F tridcntrtc NnAlClZF2 ~ k a r o nDensity CUIIWN in e/h Through h e with Ne CI - A I F Atom

-

-

Cl2F tridmlntc NnAICi2F2 m r o n m t y ~ o nin et-& ~ Through Plane with Na F A I F Atom

- - -

~

Figure 10. Electron density for 2ClF tridentate NaAlC12F2 at the HF/ 6-31G* level. (a) Contoursatconsatnte-/A3at thevaluesshown through the Na-Cl-AI-F plane. (b) Contours at constant e-/A3 at the values shown through the Na-F-AI-F plane.

then computed the energy difference at the MP4SDTQ/63 1+G*//RHF/6-3 1+G* level and found the chlorine tridentate isomer to be more stable by 2.41 kcal/mol compared to the 3.52 kcal/mol noted above. Such diffuse functions do not appear to radically alter the energy difference between these two isomers, although there are significant differences between isomers containing more fluorine atoms (vide infra). Since the energies of the various conformersof NaAlC13F in Figures 3a-d differ by

Figure 14. Na(AlC4)2- HF/6-31G*//HF/6-31G* values are given. Electron pair bonds are shown as heavy black lines. (a) Double bidentate: Bond lengths (A): Na-Clb = 2.785, Na-AI = 3.583, AI-Clb = 2.198 A, AI-CIt = 2.137. Bond angles (deg): CIt-Al-CIt = 113.1, Clb-AI-CIb = 101.3, Clbl-Na-Clbl = 75.7, Clbl-Na-Clb2 = 128.6, clbAI-CIt = 110.3. Dihedral angle: C&-Al-Clb-Na = 117.2. (b) Double tridentate: Bond lengths (A): Na-Clb = 3.059, Na-AI = 2.236, AI-Clb = 2.175, A1-Q = 2.128 A. Bond angles (deg): CIt-AI-Clb = 114.5, CIb-AI-ch, = 104.0, Na-Al-Clb = 65.5, AI-Na-A1 = 180.

no more than 3.52 kcal/mol at the MP2/6-31+G level//RHF/ 6-3 lG* level, and in view of the relatively small barrier connecting the bidentate and tridentate forms of NaAlCld, it seems likely that the isomers of NaAlCl3F are fluxional at the experimental temperatures employed. NaAIC12F2and AICl2F2-. The symmetry of AlC12F2- is Cb as depicted in Figure IC, and structural details are provided in the caption. The electron density through the planes passing through the chlorine, aluminum, and fluorine nuclei are given in Figure 9a-q respectively. Note that the minimum density between A1

482 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

Bock et al.

TABLE 1: Energies for ChlorofluoroaluminateIons, Their Sodium Salts, and Related Compounds (au) species AICl4AICl3FAllClzFZAIClFsAlF4NaAlCl4 NaAlCl4 NaAlC13Fc NaAlC13Fc NaAIClaF NaAlCllF NaAlClzFZ NaAlClzFZ NaAlClZFz Na AlClzFz NaAIClF3 NaAlCIF3 NaAlClF3 NaAlF4 NaAlF4 bidentate Cl~AlCl~NaCl~AlC1~tridentate Cl~A1C1~NaCl~AlCl~AlC13 AlClZF AlCIFZ AlF3 NaCl NaF Na+ CIF-

figure

HF/6-3 lG*

ZPEO

MP2/6-31G*

MP2/6-31+G*

la lb

14a

-2 080.230 472 -1 720.187 641 -1 360.141 874 -1 000.092 835 -640.039 897 -2 242.052 759 -2 242.051 678 -1 882.013 809 -1 822.019 916 -1 882.013 308 -1 882.021 698 -1 521.980 082 -1 521.973 037 -1 521.982 308 -1 521.987 201 -1 161.943 628 -1 161.940 926 -1 161.946 626 -801.901 848 -801.903 826 -4 322.341 927

3.85 4.57 5.29 6.00 6.71 4.52 4.51 5.23 5.29 5.23 5.34 6.01 5.96 6.08 6.15 6.82 6.82 6.90 7.54 7.65 8.74

-2 080.796 214 -1 720.800 863 -1 360.802 191 -1 OOO.800 167 -640.794 592 -2 242.631 069 -2 242.627 794 -1 882.639 156 -1 882.636 437 -1 882.636 736 -1 882.634 001 -1 522.644 200 -1 522.643 246 -1 522.642 608 -1 522.640 89 -1 162.649 875 -1 162.648 902 -1 162.648 77 -802.656 354 -802.654 327 -4 323.491 263

-2 080.810 783 -1 720.825 505 -1 360.839 172 -1 000.851 982 -640.864 135 -2 242.641 643 -2 242.638 712 -1 882.656 892 -1882.651 39 -1 882.654 984 -1 882.647 869 -1 522.666 566 -1 522.670 898 -1 522.663 852 -1 522.658 857 -1 162.677 199 -1 162.679 925 -1 162.675 155 -802.693 107 -802.691 77 4323.515 239

14b

-4 322.331 842

8.40

-4 323.483 68

-4 323.507 778

nsb

-1 620.576 087 -1 260.535 351 -900.493 476 -540.450 452 -621.399 618 -261.301 704 -161.659 288 -459.525 997 -99.350 482

3.16 3.85 4.54 5.25 0.51 0.85 0

-1 621.007 982 -1 261.014 224 -901.018 974 -541.022 398 -621.534 366 -261.490 906 -161.659 288 459.652 104 -99.526 607

-1 621.015 034 -1 26 1.029 726 -901 -044 522 -541.059 09 -621.538 817 -261.519 451 -161.659 288 -459.671 145 -99.623 847

IC

Id le 2a

2b 3a

3b 3c 3d 4a

4b

4c 4d 5a 5b 5c 6a

6b

ns ns ns ns ns ns ns ns

0 0

kcal/mol at HF/6-31GS. ns = not shown. Additional energies in footnote 26, and F in AIClzFz- is larger than 0.08 e-A3 in both Figure 9a,b. The actual minimum is about 0.10 e-/A3, lower by about 0.02 e-/A3 than in AlClpF-, where only one fluorineatom is competing for the electron density. Similarly, parts b and c of Figure 9 show that the presence of two fluorine atoms reduces the minimum electron density in the Al-Cl bond slightly from that found in AlC14- to just over 0.06 e-/A3. Thus, one anticipates that the Al-CI bond will be weaker than the A1-F bond, an expectation borne out by the energy required to dissociateAlCl2F2-into AlClFz and C1- (76.75 kcal/mol) and into AlC12F and F- (1 15.03 kcal/ mol) at the MP2/6-31+G* level. The structures of the NaAlClZFz salts are shown in Figure 4 and theenergiesare listed inTable 1. Theonly tridentatestructure found had two chlorine atoms on the face, Figures 4a, and is not the most stable of the NaAlClzFz structures. The lowest energy structure is given in Figure 4b. Note here the important effect of a diffuse basis set, MP2/6-31+GS, which causes the relative energies of Figure 4a,b to invert when compared with the corresponding energies of MP2/6-3 1G*. Initialgeometrieswhich placed two fluorine atoms on the tridentate face rearranged to give the two fluorine bidentate structure, Figure 4d, instead of the more stable fluorine edge, Figure 4c, presumably because of the starting gcometry. The order of stability of the structures, is consistent with the electrostatic nature of the interaction, i.e., thedichlorineedge (Figure 4b) has the longest sodium-aluminum distance and is the most stable of the isomers, and the difluorine edge (Figure 4d) has the shortest and is the least stable of the bidentate structures, with the chlorinefluorine edge intermediate (Figure4c). The tridentatestructure, Figure4a, brings thesodium ion closer to the aluminumion but the repulsion is counterbalanced by the gain in Coulombic energy by interacting with the fluorine atom. The electron densities through the Na-Cl-AI-F and the Na-F-AI-F planes are given in Figure lOa,b, respectively. In figure loa, one again observes the huge difference. in minimum electron density in the AI-Cl bond, just over 0.06 c-/A3, and in the AI-F bond, 0.12 e-/A3. The presence of a sodium ion, Na+,

has a very small effect on the electron density in the adjacent Al-Cl bond in Figure 10a (compared with Figure 9b) and the adjacent AI-F bond in Figure 10b (compared with Figure 9a). On the other hand, the presence of the Na+ ion raises the density in the AI-F bond to the right of the A1 atoms in Figure 10a,b suggesting that ion polarizationis significant where both the Na+ ion and the positively charged aluminum are acting in concert. The minimum electron density between the Na+ ion and the chlorine atom in Figure 10a is significantly lower than in NaCl, reflecting again the electrovalent character of this interaction. Similarly, the electron density between the Na+ ion and the fluorine atom in Figure 10b is similar to that in NaF, Figure 1 1. (The electron densities of NaCl in Figure 8 and NaF in Figure 11, respectively, are similar to those found by Curtiss et ale2’See also Jordan28 for additional discussion of these densities.) The energy required to dissociate the tridentate form of NaAlC12Fz (Figure 4a) into Na+ and AlChFz-, into NaCl and AlClFz, and into NaF and AlClZF are 104.77, 51.25, and 72.35 kcal/mol, respectively, all at the MP2/6-31+G* level. These dissociation energies are consistent with breaking an ionic bond and with breaking A141 and A1-F polar covalent bonds. Note that less than 7.8 kcal/mol separate the NaAlC12F2 isomers at the MP2/ 6-3 1+G* level, suggesting that this salt is probably also fluxional at the high temperatures employed in the experimental studies. NaAlCW3 and AlCIF3--. The structure of AlClF3- with C3, symmetry is given in Figure Id and the energy is listed in Table 1. We anticipate that the presence of another fluorine atom will lower the electron density in the Al-Cl bond slightly below that in Figure 9b,c, since each addition of fluorine has lowered the minimum electron density between the AI and C1 atoms in the preceding cases, The energy required to dissociate AlClF3- into C1- and AlF3 is 75.65 kcal/mol compared with 113.76 kcal/mol for the dissociation into F- and AlClF2, both at the MP2/631+G* level. The isomeric structures of the sodium salts, NaAlClF3, are given in Figure 5, and the energies are listed in Table 1. The

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 483

Chlorofluoroaluminate Ions

TABLE 2 Energiea for hmerizntion and Decomposition R ~ c ~ ~(kcrrl/mol)' oM

---

isomerization reactions bidentate NaAIC4 tridentate NaAlCl4 Clz bidentate NaAlCllF Cl3 tridentate NaAlCl3F FCI bidentate NaAlClsF cl3 tridentate NaAlC4F C12 bidentate NaAlClaF ClF2 tridentate NaAlCl3F FCl bidentate NaAlClsF ClF2 tridentate NaAlCLF ClFz tridentate NaAlCllF + Cl3 tridentate NaAlCloF Clz bidentate NaAlC12F2 FClz tridentate NaAlClzFz FCl bidentate NaAIC12Fz FClz tridentate NaAlCbF2 Fz bidentate NaAlClzF2 + FC12 tridentate NaAIClzF2 FCl bidentate NaAlClFa F3 tridentate NaAlClF3 Fz bidentate NaAlClF3 F3 tridentate NaAlClF3 Fz bidentate NaAlFd F3 tridentate NaAlF4

HF/6-3 1G* -0.67 -0.31 4.84 -4.09 1.07 3.77 -4.37 1.40 4.33 -1 -70 1.80 1.13

+

--

+

- ++ -- ++ -- ++ --- +++ -- +++ --- +++ --

decomposition reactions into ion products Cl3 tridentate NaAlCl4 Na+ AIC4C12 bidentate NaAlC4 Na+ AlC4C13 tridentate NaAlClsF Na+ AlCllFClF2 tridentate NaAICl3F Na+ AlCl3FC12 bidentate NaAlCllF Na+ AlClnF FCl bidentate NaAlCl3F Na+ AlClsFFClz tridentate NaAICIzF2 Na+ AlCl2F2C12 bidentate NaAlC12F2 Na+ AlClzFzFCl bidentate NaAlC12F2 Na+ AlCl2F2F2 bidentate NaAlClzFz Na+ AlC12FzF3 tridentate NaAlCIF3 Na+ AlClF3FCl bidentate NaAlClF3 Na+ AlCIF3Fz bidentate NaAlClF3 Na+ AlClF3F3 tridentate NaAlF4 Na+ AlF4Fz bidentate NaAlF4 Na+ AIF4bidentate Cl~AIClzNaC1~AlCl2-Cl3 tridentate NaAlC4 + AlF4tridentate Cl2AlCl2NaCl2AlClz- Cl3 tridentate NaAlCl4 + AlF4-

HF/6-3 1G* 101.61 100.95 104.78 107.83 103.74 108.90 111.55 107.18 112.88 115.88 119.35 117.66 121.15 126.34 127.47 36.34 30.38 79.87 162.42 78.53 159.23 76.06 154.71 72.28 148.49

+

+

-

~ 1 ~ 4~ - 1 + CI~ 1

~

AlCl3P-AlCl3 + FAlClaF- AlCl2F + C1AlClzF2- AlClzF + FAlCl2F2- AlClF2 + C1AlClF3- AlClF2 + FAIClF3--AIF, + C1AlF4- AlF3+ P +

+

+

+

-------

decomposition reactions into neutral products Cl3 tridentate NaAIC4 NaCl + AlCl3 CI2 bidentate NaAlC4 NaCl + AlCl3 Cl3 tridentate NaAlClrF NaCl + AlClzF Cl3 tridentate NaAlCl3F NaF + AlC13 ClFz tridentate NaAlClsF NaCl + AlCl2F CIF2 tridentate NaAlCl3F NaF + AlCl3 C12 bidentate NaAIClsF NaCl + AlClzF CIz bidentate NaAlClsF NaF + AICl3 ClF bidentate NaAlCl3F NaCl + AlClzF ClF bidentate NaAlChF NaF + AlCh Cl2F tridentate NaAlClZFz NaCl + AlClFz Cl2F tridentate NaAIClZFz NaF + AlCIzF C12 bidentate NaAlClzFz NaCl + AIClF2 Clz bidentate NaAlCIzF2 NaF + AlClzF ClF bidentateNaAIClzF2 NaCl + AICIF2 CIF bidentate NaAlClzF2 * NaF + AlClzF Fa bidentate NaAlCIzF2 NaCl + AIClFz F2 bidentate NaAlCbF2 NaF + AlCl2F F3 tridentate NaAlCIF3 NaCl + AlF3 F3 tridentate NaAlClF3 NaF + AlClF2 ClF bidentate NaAlClF3 NaCl + AlF3 ClF bidentate NaAlClF, 4 NaF + AlClF2 F2 bidentate NaAlClF3 NaCl + AlF3 Fz bidentate NaAlClF3 NaF + AlClF2 F3 tridentate NaAlF4 NaF + AlF3 Fz bidentate NaAlF4 NaF + AlF3 Including zero-point energy at HF/6-3 1G* level. +

----+

+

+

HF/6-31G* 47.50 46.83 48.60 84.13 52.38 87.90 48.29 83.82 53.44 88.97 53.63 88.44 49.26 84.07 54.95 89.77 57.95 92.77 57.65 91.72 55.95 90.03 59.45 93.52 92.49 93.62

tridentate all-fluorine face is not the lowest energy structure, instead the bidentate with bridging C1 and F atoms is lowest. Initial tridentategeometries with a chlorineon the face, rearranged to give the difluoro bidentate structure, Figure Sc, instead of the slightly more stable structure, Figure Sb. The dissociation of the structure in Figure Sa into Na+ and AlClF3- requires 103.30

MP2/6-31G* -2.05 -1.52 -3.34 0.25 -1.58 -1.77

MP2/6-3 1+G* -1.83 -1.20 -2.26 2.32 -4.57 -3.51 2.77 -1.77 -4.98 1.71 -1.36 4.95

-0.55

-1.07 -2.22 4.61 -0.77 -1.38 MP2/6-3 1G* 109.50 107.45 112.39 109.90 110.15 108.32 113.94 113.39 112.87 111.72 118.67 118.06 117.90 126.22 124.84 39.65 35.25 84.73 165.68 83.70 162.57 81.52 158.30 78.11 152.65

MP2/6-3 1+G* 106.99 105.16 107.33 103.82 106.14 101.56 104.77 107.54 103.00 99.79 103.30 105.01 101.94 105.65 104.70 38.92 34.59 5 1.57 89.76 77.49 115.03 76.75 113.76 75.65 112.24

MP2/6-31G* 54.82 52.78 55.96 86.80 54.19 85.03 54.44 85.28 52.62 83.45 56.06 85.96 55.51 85.41 54.99 84.89 53.84 83.74 57.37 86.42 56.76 85.81 56.59 85.64 88.33 93.62

MP2/6-31+GS 54.24 52.41 54.57 75.59 5 1.06 72.08 53.37 74.39 48.80 69.82 5 1.27 72.35 54.03 75.12 49.49 70.58 46.29 67.38 48.70 69.62 50.41 71.33 47.33 68.26 70.45 69.50

kcal/mol at the MP2/6-31+G* level, consistent with an ionic bond dissociation in the gas phase. Similarly, the dissociation of the structure in Figure Sa into NaCl AlF3 and into NaF + AlC1F2 requires 48.70 and 69.62 kcal/mol, respectively, at the MP2/6-31+G* level, consistent with breaking polar covalent AlC1 and polar covalent AI-F bonds. Note that the energy

+

484 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 TABLE 3

Energies for Reactions with NaF (kcal/molP

31G* level, consistent with the results presented above. The dissociation of the bidentate-bidentate structure, Figure 14a, and the tridentate-tridentate structures into NaA1Cl4and AIC14requires 38.92 and 34.59 kcal/mol, respectively, at the MP2/ 6-31+G* level suggesting that these species may be important in basic melts at high concentration. In addition to the dissociation reactions for each of the moieties described abovethere are other reactionsof interest. For example Gilbert et ala3in their explanation of the vibrational spectra of the chlorofluoroaluminate salfs assume the reaction

~~~~~

reaction NaF AlCl4-- NaCl AICl3FNaF + AIC13F- NaCl AIC12F2NaF AlCl2F2- NaCl AICIF3NaF AlCIFa- NaCl + AIF4-

+ + +

a

--

+ + +

HF/ MP2/ 6-31G* 6-31G* -34.19 -29.81 -32.34 -27.72 -30.30 -25.63 -27.85 -23.40

MP2/ 6-31+G* -21.01 -20.35 -19.82 -19.41

Including zero-point energy at HF/6-31GS level.

differences between all of the NaAlClF3 isomers is less than 3 kcal/mol, so these structures are most likely fluxional. NaAlF, and AIF4-. As expected, the symmetry of AIF4- is Td, and the structure is given in Figure l e with the energy listed in Table 1. The electron density map is given in Figure 11, where it can be seen that the minimum in AI-F bond electron density is reduced to approximately0.080 e-/A3, compared with the AI-F bonds in AlC12F2- (see Figure 9a,b) as a consequence of the other three fluorine substituents. The energy required to dissociate AlF,-intoAlFg and F-is 1 12.24 kcal/mol,consistent with breaking a polar covalent AI-F bond. The sodium salts, NaA1F4, are shown in Figures 6a,b, and the energies are listed in Table 1. These structures were reported recently by Curtiss and Scholz2 as part of a careful study of MAlF4 (M = H, Li, Na) which employed a spectrum of basis sets, themostsophisticatedofwhichwas6-31+G*.Our structural parameters at the RHF/6-31G* level agree with theirs for both the structures. They report that the structure in Figure 6a is more stable than that of Figure 6b by 1.24 kcal/mol at the MP2/ 6-31G*//RHF/6-31G* level, whereas we find 1.38 kcal/mol, where the difference is due to a 0.1 1 kcal/mol zero-point energy correction. Thus, these structures are also probably fluxional under experimental conditions. We calculate the energy to dissociate the structure in Figure 6a into NaF and AlF3 to be 70.45 kcal/mol in excellent agreement with the result of Curtiss and Scholz at the MP2/6-3l+G*//RHF/6-3lG* level. The experimental value is somewhat higher, 8 1.3 f 4 kcal/mol.29 Curtiss and Scholz attribute the difference between computation and experiment to be due to the use of a limited basis set and basis set superposition error. Optimizing NaF, AlF3, and NaAlF, at the MP2(FC)/6-31G(2d) level and using zero-point energy corrections from the RHF/6-3 1G*//RHF/6-31GS level raise the dissociation energy to 85.4 kcal/mol, in better agreement with experiment. It is interesting to note the profound effect of the presence of a sodium atom on the electron densities of the neighboring AlF,-; see Figure 13. Note that the minimum electron density in the adjacent AI-F bond is raised above -.08 e-/A3 and the minimum electron density in the bond to the right of the aluminum atom in Figure 13 is raised above 0.12 e-/A3 (contour not shown). Thus, the presence of a sodium ion polarizes the electron density in these chlorofluoroaluminate salts. The halogen atoms nearest the Na+ ion has the polarization mediated by the positively charged aluminum atom, but the Al-X bond across the aluminum atom from the Na+ ion experiences a much larger field and is polarized significantly. Other Reactions. In addition we have considered the ions, NaA12C18-, which may be of importance in concentrated melts. We find two stable isomers, which are shown in Figure 14, and the energies are given in Table 1. The bidentatebidentate structure has larger sodium ion to aluminum ion distance and is more stable by 4.33 kcal/mol at the MP2/6-31+G//RHF/6-

TABLE 4

Bock et al.

+

*

NaAlCl, + nNaF NaAlCl,F,, nNaCl (1) to be quantitative. We have calculated the thermodynamic properties for these reactions and they are provided in Table 3. They are all quite exothermic, confirming the assumption of Gilbert et al.' It is interesting to note that the reaction k m e s less exothermic as fluorine is added, suggesting that additional fluorine atoms have more difficulty in the competitionfor 7 backbonding. Gilbert et al.' also deduce equilibrium constants, K,, for the exchange reactions between the various fluorinated anions, i.e.

+ AlCl,F,-,-

(2) wheren= 1,2,3. Theseequilibriaat82OOChavebeencalculated in Table 4 using the MP2/6-31+G* energies (thermally corrected), and HF/6-3 1G* entropies and zero-point energies. The agreement between the theoretical and experimental equilibrium constants is remarkable, especially when one recalls that solvation effects were not taken into account in the ab initio calculations. Vibrational Analysis. Since the evidence presented above overwhelmingly favors fluxional behavior in the chlorofluoroaluminate melts, it should be possible toassignvibrationstovarious chlorofluoroaluminate ions as suggested by Gilbert et al.3 based on their observed changes in the Raman spectrumof theNaAIC1, melt as varying amounts of NaF are added. The computed frequencies are assigned and compared with experiment in Table 5 . Here we note that good agreement is obtained between the computed vibrations of AIC14- and that reported by Gilbert et al.30 The largest discrepancy, 11.4 cm-I, is for the T2 vibration at 51 1.4 cm-l which Mamantov et al.30 report to be 490 cm-1. Rytter and Oye3I report 498 cm-I in better agreement with our calculations. The substitution o f F for one of the C1 atoms causes the T2 vibrations to split. The agreement between the computed and the experimental assignments of Gilbert et al.3 for AlCl3Fis reasonable. We predict an AICl3F- Al vibration at 857.9 cm-I which is beyond the range of the cell windows employed. Substitution of F for two C1 atoms causes further splits in the vibrational levels due to the reduced symmetry. Here the agreement between our calculations and the experimental assignments is poor. Similarly, the agreement between our calculated vibrational frequencies and those assigned experimentally is also poor for AIFICI-. In contrast, our calculations agree reasonably well for AlF4- with those assigned by Gilbert et al.30 If the source of the disagreement were progressive ion association as one proceeds from left to right on Table 4, one would expect poorest agreement for the perfluoroaluminate ion, which is not observed. Our calculated frequencies agree well with those assigned from spectra taken in the absence of glass window,30 suggesting that there may be window problems in the experimental measurements in ref 3. 2AlCl,Fn-

+ AlCl,-,,F,,-

Standard Energies and Entropies for Anion-Exchange Reactions (kcal/moP and cal/(mol K))

reaction 2AlCliF- = AIClzF2- + AlCl42AIChF2- = AICIFs- + AIC13F2AlCIF3- = AlFr AlCl2Fz-

+

AH.b

As0

AGO

K(ca1c)

0.6614 0.5271 0.3903

0.709 -1.754 -2.193

-0.0113 2444 2787

1.os

0.32 0.19

Includes zero-point energy at HF/6-31GS level. bCalculated at the MP2/6-31+Gt level. cCorrected to 820 OC.

K(expt) 0.59 0.35 0.19

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 485

Chlorofluoroaluminate Ions

TABLE 5: Vibrational Assignments for the ChlorofluoroaluminateIons (cm-l) AICl4-

AlClaF-

AlClzF2-

AI 141.3 b(Al-Cl2) [nflbJ A2 166.9 p,(AI-Clz) [nflb"

a1f4-

AlClF3-

E 178.4 p,(AIF3) [nflbSc E 218.8 &(FAIF) [210]'

BZ235.0 p,(AI-ClZ) [nflb' Bl 261.8 pw(Al-C1z) [260Ib BI273.3 6(Al-F2) [nflbVc

A1 287.7 S(AIF3) [279Ib E 296.6 6(AlF,) [nflbgc

Tz 332.7 &(FAIF) [322]" A1 421.0 v(A1-Cl) [404Ib BI 501.0 u(A1-CI) [nflb-c

AI 456.2 U(A1-Cl) [453]* A1 668.9 v(A1-F) [622]" A1 733.9 v(Al-F3) [nflbJ

A1 799.8 V(A1-F) [665Ib E 885.5 V(Al-Fa) [nab&

A1 857.9 v(AI-F) [nflbSc

Tz 863.6 vd(A1F) [7601°

Bz 898.7 v(A1-F) [ n f l b c 0

Experimental.% b E~perimental.~nf = not found.

Conclusions The chlorofluoroaluminate ions are tetrahedrally bonded as expected, and electron density calculations support the bonding to be polar covalent. The stability of the many isomers of the sodium salts is dictated primarily by electrostatic considerations. On the basis of electron densities the bonding between the sodium ion and the chlorofluoroaluminateions is essentiallyionic; however, there is evidence that the sodium ion polarizes the AI-X bond normal to the face in tridentate structures. None of the isomers found for a particular sodium salt differs in energy by more than a few kcal/mol, suggesting that these structures are fluxional at the melt temperatures studied experimentally. We confirm the conclusions of Gilbert et al.3 that the NaF-NaCl exchange reaction is quantitative, and we find the computed equilibrium constants for the chlorine-fluorine exchange reactions in remarkable agreement with those based on experimentalvibrational spectra. The computed vibrations of the chlorofluoroaluminate ions are in fair agreement with experiment for AlCl3F- and in poor agreement for AlClZF2- and AlCIF3-. Good agreement between the computed and experimental vibrational frequencies for AlC14- and AIF4- is obtained, leading us to suggest window problems in the experimental measurement. We present two structures for the ion complexes between AlC4- and NaAlCL and based on their high dissociation energies, suggest that these complex ions may be an important species in these melts.

Acknowledgment. M.T. acknowledges support from the Pittsburgh Supercomputer Center, Grant CHR55JP, for computer time on CRAY Y-MP/832. G.J.M. thanks the USAFOSR for an RIP Grant which supported this research and the National Center for SupercomputingApplicationsfor Grant CHE890003N and for computer time on the CRAY-2 and RS/6000 cluster. References and Notes (1) Blander, M.; Bierwagen, E.; Calkins, K. G.;Curtis, L. A.; Price, D. L.; Saboungi, M.-L. J . Chem. Phys. 1992, 97,2733. (2) Curtiss, ]L. A.; Scholz, G.J. Mol. Struct. (THEOCHEM) 1992,258, 251. (3) Gilbert, B.; Williams, S.D.; Mamantov, G. Inorg. Chem. 1988.27, 2359. (4) Hurley, F. H.; Wier, J. P. J . Electrochem. Soc. 1951, 98, 203. ( 5 ) Wilkce, J. S.;Melton, T. J.; Joyce, J.; Maloy, J. T.; Boon, J. A. J . Electrochem. Soc. 1990, 137, 3865. (6) Gale, R. J.; Osteryoung, R.A. In MoltenSalt Techniques; Lovering, D. G.. Gale, R. J., Us.; Plenum Prcss: New York, 1983; Vol. 1.

(7) Chum,H. L.;Osteryoung,R. A. InfonicLiquids;Inman,D., Lovering, D. G.,Eds.; Plenum Prcss: New York, 1981. (8) The most recent is 8th Int. Symposium on Molten Salts, 1992. (9) Hvistendahl, J.; Klaeboe, P.; Rytter, E. Oye, H. A. Inorg. Chem. 1984, 23, 706. (10) Mamantov, G.;Smyrl, N. R.; McCurry, L. E. J . Inorg. Nucl. Chem. 1978,40, 1489. (11) Gilbert, B.; Williams, S.D.; Mamantov, G. Inorg. Chem. 1988,27, 2359. (12) Wallart, F.; Lorriaux-Rubbens, A.; Mairisse, G.; Barbier, P.; Wignacourt, J. J . Raman Spectrosc. 1980, 9, 55. (13) Rytter, E.; Oye, H. A,; Cyvin, S.J.; Cyvin, B. N.; Klaeboe, P. J. Inorg. Nucl. Chem. 1973, 35, 1185. (14) Takahashi, S.;Muneta, T.; Koura, N. J . Chem.Soc., Faraday Trans. 2 1985.81. 1107. -(IS) Charkin, 0. P.; Zakzhevskii, V. G.;Boldyrev, A. I. Sou. J . Coord. Chem. 1982,8, 232. (16) Scholz,G.; Curtiss, L. A. J. Mol. Strucr. (THEOCHEM)1992,258, 251. (17) Nichols, R.; Curtiss, L. A. Proceedings of the Fifth International Symposium on Molten Salts, The Electrochemical Society,Pennington, New Jersey, 1986. (18) Gaussian 90; Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H.B.; Raghavachari, K.; Robb, M. A.; Binkley, J. S.;Gonzalez, C.; Defrees, Fox, D. J.; Whitcside, R. A,; Seeger, R.; Melius, C. F.; Baker, J.;Martin,R.L.;Kahn,L.R.;Stewart, J. J.P.;Topiol,S.;Pople, J. A.; Gaussian, Inc.: Pittsburgh, PA, 1990. ( 19) Gaussian 92, RevisionA,; Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.;Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.;Stewart, J. J. P.;Pople, J.A.;Gaussian, Inc.: Pittsburgh,PA, 1992. (20) CAChe, P.O. Box 500, OR 97077. (21) Horn, H.; Ahlrichs, R. J. Am. Chem. Soc. 1990, 112, 2121. (22) Dickey, R. P.; Maurice, D.; Cave, R. J. J . Chem. Phys. 1993, 98, 2182. (23) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IY: Constants of Dlastomic Molecules; Van Nostrand Reinhold New York, 1979. (24) Wilson, M.; Coolidge, M. B.; Mains, G.J. J. Phys. Chem. 1992,%, 4851. (25) Bock, C. W.; Trachtman, M.; Mains, G. J. J . Phys. Chem. 1993,97, 2546. (26) The HF, MP2, and MP4(SDQ) energies for the structures in Figure 3a,bare-1 882.032 542,-1 882.763 467,-1 882.695 388and-1 882.036 997, -1 882.755 516, -1 882.691 549 au, respectively. (27) Curtiss, L. A,; Kern, C. W.; Matcha, R. L. J. Chem. Phys. 1975,63, 1621. (28) Jordan, K. D. In Alkali Halide Vapors; Davidovits, P., McFadden, D. L., Eds.; Academic Prcss,New York, 1979. (29) Chase, M. F., Jr.; Davies, C. A.; Downey, J. R., Jr.; Trump, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Re$ Data, Suppl. I 1985, 14. (30) Gilbert, B.;Mamantov, G.;Begun, G.M. Inorg. Nucl. Chem. Lett. 1974, 10, 1123. (31) Rytter, E.; Oye, H. A. J. Inorg. Nucl. Chem. 1973, 35, 4311.