J. Phys. Chem. 1995,99, 12786-12789
12786
Ab Initio Studies of AIH3-H2O, AlF3-H20, and AlCl3-H20 Complexes David W. Ball Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 15 Received: April 17, 1995; In Final Form: June 22, 1 9 9 9
HF/6-3 lG(d,p) and MP2/6-3 lG(d,p) calculations were performed to determine minimum-energy structures for the molecular complexes AlH3-H2O, A l F r H 2 0 , and AlC13-H20. The structures are qualitatively similar to that of BF3-H20, with the H20 molecule leaning toward the Lewis acid in such a way as to eclipse two of the hydrogedfluorine/chlorine atoms in the AlH3/AlF3/AlCI3 moiety. Predicted MP2 binding energies were 90.2 kl/mol for AlH3-H20, 139.0 kJ/mol for AlF3-H20, and 126.5 kJ/mol for AlC13-H20, which were all substantially higher than the predicted binding energy of BF3-H20. Basis set superposition error corrections lowered these binding energies by ca. 10 kJ/mol; however, BSSE was a proportionately smaller component of the binding energy when compared to other A1S complexes.
Introduction Several of the aluminum trihalides are useful chemical species. Aluminum trifluoride, AlF3, is important in the commercial production of metallic aluminum and some optical glasses. Aluminum trichloride, AlC13, is used as a FriedelCrafts catalyst for alkylation and acylation of aromatic compounds.’ Lately, the Lewis acid behavior of aluminum hydride and aluminum halides has been receiving attention, not only for energetic and mechanistic reasons but also as a model of an aluminum surface. Lewis acid-base complexes with trivalent aluminum compounds as the Lewis acid have been the focus of some theoretical studies. An early report by Groper et aL2 presented results of self-consistent field (SCF) calculations on AlH3-H20. They found that the H20 plane was tilted from being in a “straight-up” orientation by a -27” angle and calculated a noncorrelation binding energy of 17 kcaYmole (7 1 kJ/mol). This appears to be the only available study on this molecular complex. More recently, Scholz reported3 on the equilibrium geometries and binding energies in AlXj-HX’ complexes (X = F, C1; X’ = F, Cl). Wilson et aL4 performed MP4(SDTQ)/6-3lG(d)//HF/6-31G(d) calculations on AlH3-HX (X = F, C1) and AlF3-HF complexes as well as a series of mixed hydride-halide complexes. Burkhardt et aL5 showed that the aluminum fluoride complex with two HF molecules formed a six-membered ring at the energy minimum. In most of these studies, it was found that a basis set superpositionerror correction lowered the predicted binding energy significantly, in some cases by more than half. Bates and Dwyefl used an AlH3 fragment in their model of CO adsorption in zeolites; their conclusion was that a “Lewis site model” for adsorption of CO was energetically favorable and more consistent with experimental work. Papatheodorou, Curtiss, and Maroni’ have reported on the amine complexes NH3-AlX3, where X = F, C1, and Br. There are more references available on complexes of AlC13, presumably because of its utility as a catalyst. Connolly and Dudis8 studied AlC13 complexes with NH3 (reporting a binding energy of 42 kcal/mole) and H2S (binding energy of 14 kcaY mole). Jasien9 reported on a series of AlC13 complexes, including those with formaldehyde, chloroformaldehyde (0bonded and C1-bonded), ethylene, chloromethane, and acetylene. Both of these reports reference quite a few previous studies on aluminum chloride complexes. @Abstractpublished in Advance ACS Absfrucrs, August 1, 1995.
Despite the amount of work on aluminum hydride and halide complexes, there seems little work on H20 complexes with aluminum hydride and halides, despite the ubiquity of H20 and the reactivity of such aluminum compounds with water. AlH3 reacts violently with water; aluminum chloride is “watersensitive”, reacting explosively with water and fuming in air. Only aluminum trifluoride is stable in the presence of water. Hydrated crystalline aluminum fluoride is known’O and has even been identified in volcanic incrustations,” but appears to not have been studied theoretically. Because of the ubiquitous nature of H20 and the apparent interest in aluminum compounds as Lewis acids, presented here is an ab initio study of the structures, vibrations, and binding energies for AlH3-Hz0, AlF3-H20, and AlC&-H20. Estimates of the basis set superposition errors show that such corrections are of similar orders of magnitude as for the HX complexes, but since the binding energies are higher for aquo complexes, BSSE corrections to the binding energies have a lesser overall impact. This study follows the report on the BF3-H2O complex, published earlier.I2 Computational Details Ab initio calculations were performed using the GAUSSIAN 92 p r ~ g r a m . ’The ~ calculations were performed in the Windows environment on a 66-MHz 486 PC having 16 Mbytes of RAM and -130 Mbytes of available disk storage space or on a Cray Y-MP supercomputer. The 6-31G(d,p) basis set was used for computational efficiency and also because the previous work on BF3-H20I2 showed that there were no major differences in the structure or binding energy when basis sets larger than this were used. Restricted Hartree-Fock and second-order MollerPlesset methods were used to determine minimum-energy geometries for the three aquo complexes and the individual molecules. D3h and C2“ symmetries were imposed on the AlX3 and H20 individual molecule optimizations, respectively, but for the optimizations of the complexes an initial CI symmetry was presumed. Vibrational frequencies were also determined using both methods to verify the existence of the true potential minimum and to determine zero-point energies.
Results and Discussion Because H2O and AlX3 have been the subject of much previous study (Wilson, Coolidge, and Mains? for example, performed a very complete A& analysis in their study of HF
0022-365419512099-12786$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 34, 1995 12787
AlH3-H20, AlF3-H2O, and AlCl3-HzO Complexes
Lx2 Irl
x3
XI
XI H)Lx3
XI
x3
x2
H2
x2 ( X I = X ~ = X= ~H, F, C1)
Figure 1. Schematics of the optimized geometries of AIX,-H20 complexes (X = H, F, C1). Three perspectives of the general structure of the complex are shown, all complexes optimized having similar geometries. Specific bond lengths and angles for X = H, F, and CI are given individually in Table 2. TABLE 1: Calculated Structural Parameters for the D3h Am3, AIF3, and AICI3 and CzVH20 AlH3 HF16-3 IG(d,p) MP2/6-3 IG(d,p) A1F3 HF/6-31G(d;p) MP2/6-3 IG(d,p) AlC13 HF/6-3 lG(d,p) MP2/6-3 lG(d,p) H20 HF/6-3 IG(d,p) MP2/6-3 lG(d,p)
r(AI-H) = 1.582 8, r(AI-H) = 1.577 8, r(AI-F) = 1.620 8, r(A1-F) = 1.645 8, r(A1-Cl) = 2.077 8, r(A1-Cl) = 2.072 8,
4,
r(0-H) = 0.9430 a = 105.97' r(0-H) = 0.9613 A, a = 103.79'
Complex Geometries. The minimum energy geometries for AlH3-Hz0, AlF3-Hz0, and AlCl3-H20 were all similar and showed a nominal plane of symmetry. Figure 1 shows a diagram of the general structure of all three complexes, while the individual values for all of the bond distances and bond angles are listed in Table 2. The H20 and AlH3 are slightly farther apart than are the H20 and either AlF3 or AlC13, and in the latter two cases the calculated A1-0 distance is almost the same. There is very little variation in the calculated 0-H bond lengths, either among the complexes or within a particular complex. Comparison of the A1-X and 0-H bond distances from Tables 1 and 2 shows that AI-X bonds are affected more by the complexation than are the 0-H bonds. The A1-H and AI-F bonds are calculated to increase in length by ca. 0.02 A upon complexation, while the Al-Cl bond lengthens by ca. 0.04 A. However, the 0-H bonds lengthen by only about a tenth as much, by ca. 0.002 A. Upon complexation, the H-0-H bond angle (given by a(H10H2) in Table 2) increases by 4-5'. For all complexes, the MP2/6-31G(d,p) method calculates an H-0-H bond angle about 2.2" less than the corresponding Hartree-Fock calculation. The complexed A1x3 now deviates from planar local (This is indicated by the -AK2X3) entry in Table 2.) The dihedral angles for the AlX3 moiety are about the same for all complexes. An apical angle of -1 16-1 18" in the complex AlX3 indicates that the deviation from planar is slight. Perhaps the biggest surprise in these optimized structures is the relative orientation of the H20 with respect to the AlX3. In an early studyI4 of the BFs-HzO complex, it was reported that the H20 molecule was pointing directly away from the BF3 molecule; these calculation used the relatively small 4-3 1G basis. In our later studyI2 of BF3-H20, we noted a distinct tilt of the H20 toward the BF3 when larger basis sets having polarization functions were used. The optimum geometry of BF3-H20 included an almost perfect eclipsing of two of the fluorine atoms on the BF3 by the hydrogen atoms on the H20. We suggested that intramolecular hydrogen bonding was a stabilizing factor. As our generalized Figure 1 implies, in all cases presented here a very similar relative geometry is predicted: the water molecule tilts toward the A 1 x 3 in such a way as to almost perfectly eclipse two of the three X atoms attached to the A1 atom. Electrostatic interaction between the A1 atom and the 0 atom is not the only interaction between the two molecular components (with AVO atomic charges calculated as +0.63/-0.66, 1.59F0.67, and
and HC1 complexes), optimum geometries, vibrational energies, etc., for these species will not be discussed at length here. In the interest of comparison, however, the calculated structural parameters of the flat, triangular AlX3 molecules and bent H20 are listed in Table 1. TABLE 2: Structural Parameters for Optimized AIHJ-H~O,AIF3-Hz0, and AIC13-H20" AIH3-H20 (X = H)
dAl-0) r(0-Hi) r(O-H2) r(A1-Xl) r(AI-X2) r(AI-X3) a(H 1OH2) a(A1OH1) a(AlOH2) a(OAlX1) a(OA1X2) a(OAlX3) d(H2-OAIHl) d(X1-AlOHl) d(X2-AIOH 1) d(X3-AIOH 1) &AI-HOH) b(Xl-AlX2X3)
HF163 1G(d,p) 2.05 1 0.9469 0.9460 1.599 1.599 1.591 109.15 116.13 116.29 96.07 96.21 102.78 133.88 -4.653 -123.69 115.87 133.80 151.72
MP2/63 IG(d,p) 2.062 0.9662 0.9657 1.594 1.594 1.584 106.99 111.81 1 1 1.70 95.62 95.51 102.99 122.60 -1.011 - 119.57 119.72 122.67 152.57
AIF3-HlO (X = F) HF/63 1G(d,p) 1.953 0.9490 0.9487 1.644 1.644 1.636 109.92 114.86 114.67 96.00 95.87 106.00 131.06 -6.143 -125.22 114.25 131.21 148.90
MP2/63 lG(d,p) 1.980 0.9683 0.9687 1.668 1.667 1.657 107.70 108.36 108.92 94.68 95.03 106.71 117.69 0.6550 - 117.63 121.42 117.33 150.11
" See Figure 1 for atom labels 6 = dihedral angle. All r in angstroms; all a and d in degrees.
+
AlC13-H20 (X = C1) HF/63 lG(d,p) 1.957 0.9493 0.9486 2.1 19 2.119 2. IO7 110.65 119.40 119.28 98.33 98.25 104.51 144.37 -11.17 - 130.75 109.06 144.42 145.60
MP2/63 1G(d,p) 1.985 0.9688 0.9684 2.112 2.112 2.095 108.37 113.81 113.93 97.57 97.48 104.62 127.85 3.129 -1 17.42 122.05 127.79 146.77
12788 J. Phys. Chem., Vol. 99, No. 34, 1995
Ball
TABLE 3: Unscaled MP2/6-31G(d,p) Vibrational Frequencies (in cm-') for AlHS-HzO, AlF3-HZ0, and AlC13-HZO AIH3H2O 84 376 383 406 549 735 809 825 829 1664 1931 1938 1974 3850 3976
AIF3AIC13approx assgmt H20 approx assgmt H20
TABLE 4: HF/6-31G(d,p) Energies and Energy Differences (Total Energy Is in hartrees; All Others in kJ/mol)
approx assgmt
H2Owag 112 H2O/AIF3 wag 110 H2O wag 154 H20/AIF3 tilt AI-0 str 123 H2O/AIC13 wag HzO/AIH3wag 224 H20wag 167 H20/AIC3 wag H20/AIH3 tilt 255 H20/AIF3 tilt 176 H20 rock H2O/AIH3 tilt 275 H20/AIF3 tilt 176 H20/AIC13 tilt HzOrock 292 AIF3def 200 AIC13 0.0.p. bend AIH2 bend 464 AI-0 str 387 AIC13def 0.0.p.AIH bend 557 H20 rock 452 AI-0 str AIH3rock 679 H20rock 478 H20rock HOHbend 696 AlF3rock 564 H20/AIC13 def asym AIH str 942 sym AIF str 622 AlCl str 953 asym AIF str asym AIH str 724 H20 twist AIHstr 1656 HOHbend 1653 HOHbend sym OH str 3822 sym OH str 3810 sym OH str asym OH str 3944 asym OH str 3933 asym OH str
E AlH3 H20 AIH3-H20
-243.618 99 -76.023 62 -319.675 96
AlF3 H20 AIF3-H20 AICI3 HzO AIC13-H20
A,!?
Eihermal
AEthemal
binding energy (=-A&I)
-87.58
58.95 68.34 138.87 $11.58
76.00
-540.450 45 -76.023 62 -616.528 85 -143.82
33.27 68.34 111.94 $10.33
133.49
- 1620.576 09 -76.023 62 -1696.649 31 -130.25
26.94 68.34 105.28 f10.00
120.25
TABLE 5: MP2/6-31G(d,p) Energies and Energy Differences (Total Energy Is in hartrees; All Others in kJ/mol) binding
+0.89/-0.66 respectively for the AlH3-H20, AlF3-H20, and AlC13-H20 complexes). There appears to be significant electrostatic interaction between the hydrogen atoms on the water and the X atoms on the AX3, as evidenced by the WX atomic charges calculated as being +0.40/-0.26, +0.41/-0.58, and +0.42/-0.37 for the AlH3-Hz0, AlF3-H20, and AlC13H20 complexes, respectively. The eclipsing orientation thus allows for maximum electrostatic stability in the complexes. There are some trends in the specific orientations of the H20 and the AlX3. The Hartree-Fock calculations predict in all cases that the H20 molecule is directed somewhat more directly away from the AlX3 molecules in the complexes. This is indicated by the G(A1-HOH) entry in Table 2. For AlH3H20, the plane of the water molecule is calculated as making a 133.8' angle with the A1-0 bond by the Hartree-Fock method, but an angle of 122.67' is predicted in the Moller-Plesset calculation. This is a larger tilt than predicted by Gropen et a1.* The relative angles of orientation calculated for AlF3-H20 and AlC13-H20 show even larger differences. Also, the H-0-H bond angle is calculated as slightly higher for the Hartree-Fock method over the Mflller-Plesset method. The net effect of these two differences is that, when viewing the structure of the complex along the A1-0 bond, the HFoptimized geometries show the water molecule being slightly wider than the X-AI-X segment that the molecule is eclipsing. However, for the MP2-optimized geometries, the tilt of the water molecule and the H-0-H angle are such that the eclipsing of two X atoms by the hydrogen atoms in water is almost perfect. Vibrational Energies. Table 3 lists the unscaled vibrational frequencies for the three complexes and their approximate assignments. Although the complexes have a nominal plane of symmetry, all 3N - 6 = 15 vibrational frequencies are expected to be IR-active. With very few exceptions, the approximate assignments for the vibrations were inconsistent throughout the three complexes. Also with very few exceptions, the approximate assignments are very approximate, because especially for AlF3-H20 and AlCl3-H20 the normal vibrations were very complicated motions of the complexes as a whole. Because of this, any detailed comparison between the vibrational motions is moot. Only the approximate HOH bending motion and symmetric and asymmetric OH stretches were clearly identifiable and, in fact, varied little between the complexes. For AlH3-H20, most of the vibrational motions were clearly identifiable. Unlike BF3-H20, where matrix isolation experiment^'^.'^ were available for comparison, there are no known references for experimental vibrational spectra of these three aquo complexes. It is hoped that in the near future matrix isolation or
energy E
A!?
&hen"
AEthermal
(=-A&oi)
-243.690 34 -76.219 79 -319.948 89 -101.75
58.46 64.91 134.92 $11.55
90,20
-541.023 61 -76.219 79 -617.300 27 -149.33
32.56 64.91 107.81 $10.34
138,99
26.94 64.91 101.87
126.50
-1621.00801 -76.219 79 -1697.279 78 -136.48
+9.98
gas phase spectra of these molecular complexes will be measured for comparison with the predicted vibrational frequencies. Energies. Tables 4 and 5 show the HF/6-3IG(d,p) and MP2/ 6-3 lG(d,p) calculated energies for the isolated molecules and the molecular complexes. The final column lists the energy of interaction, the binding energy, between the water molecules and the AlX3 molecule. That binding energy has been corrected for thermal energy differences and is defined as the negative of the change in energy for the complexation process. There is not much of a difference in the binding energies predicted for the HF calculations versus the MP2 calculations except for the AlH3-H20 complex, where a difference of 14 kJ/mol is seen. For the other two complexes, the difference between HF binding energies and MP2 binding energies is a relatively small fraction of the total binding energy. All of these predicted binding energies are substantially higher than most of the predicted binding energies for AlX3 complexes previously studied. For example, Wilson et aL4 calculate for the strongest bound AlH3 complex, AlH3-HF, a binding energy of 55.3 Wmol (reported as 13.22 kcaymole in the original reference). These calculations indicate that the AlH3-H20 complex is bound by 76-90 kJ/mol, slightly more than Gropen's 17 kcaymole (71 kJ/mol) result. The strongest AlF3 complex as deterqined by W i l ~ o nAlS-HF, ,~ is predicted to be bound by 99.3 kI/mol (reported as 23.74 kcaymole), compared to the 133-138 kJ/mol calculated here for AlF3-H20. Finally, the strongest AlC13 complex reported, AlCL-HF, has a predicted4 binding energy of 81.1 kJ/mol (reported as 19.39 kcaymole), compared to the 120-126 kJ/mol for AlC13-Hz0. (All of these calculations from ref 3 use the 6-31G(d) basis set.) These relative energies are consistent with the results of Scholz3 also, who predicts a binding energy of 88.9 kJ/mol for AlS-HF, 66.2 kJ/mol for AlC13-HF, 28.3 kJ/mol for AISHC1, and 20.8 kJ/mol for AlC13-HCl. (These calculations were based on a double-5 polarized basis set.) Higher binding energies
AlH3-H20, AlF3-H20, and AlCl3-H2O Complexes TABLE 6: Comparison of Binding Energies (kJ/mol) with and without BSSE (All Calculations MP2/6-31G(d,p)) complex
without BSSE
with BSSE
A1H3-H20 A~FI-H~O AlC13-H20
90.20 138.99 126.50
81.71 129.00 120.41
have been calculated. Papatheodorou et aL7 calculated binding energies for AlF3-NH3 and AlCL-NH3 as 41.3 kcal/mole (173 kJ/mol) and 63.0 kcallmole (264 kJ/mol), respectively, using an STO-3G basis. These may be on the high side; our previous calculationsI2 on BF3-H20 binding energies versus basis set showed a consistent lowering of the binding energies for any basis set having polarization functions on any of the atoms, when compared to calculations where the basis set had no polarization functions. In all cases, an attempt was made to estimate the effect of basis set superposition error, as mentioned in earlier publications. These earlier report^^,^ indicated that BSSE corrections lowered the predicted binding energies substantially, in some cases by a factor of 50%. Utilizing the counterpoise method, singlepoint energies of H20, AlH3, AlF3, and A1C13 in their equilibrium complex geometries were determined. The energy differences are compared in Table 6. In the cases of AlH3-H20 and AlF3-H20, the estimated corrections are almost 10 kJ/mol, which is on the order of the BSSE corrections estimated for HF and HC1 c o m p l e x e ~ . ~However, .~ the BSSE correction is a smaller portion of the entire binding energy for all three complexes. Conclusions The complexes AlH3-H20, AlFs-HzO, and AlC13-H20 were studied using HF and M E methods. Optimum geometries showed that the H20 made an angle with the A1-0 bond such that the hydrogens almost perfectly eclipsed two of the X substituents on the Al; electrostatic interactions between the water H atoms and the X atoms on the aluminum moiety are implicated. Vibrational frequencies for all complexes were determined. However, there is no known experimental data for comparison. Binding energies for the complexes ranging from
J. Phys. Chem., Vol. 99, No. 34, 1995 12789 90 kJ/mol for AlH3-H20 to 139 kJ/mol for AlF3-H20, with the AlCl3-H2O having an intermediate binding energy of 126.5 kJ/mol, were predicted. BSSE estimations showed that such corrections have the same absolute value as such corrections for halide complexes, but are much less a factor for the aquo complexes because the absolute binding energies were much greater. The energy stabilizations upon complexing are large enough that all three complexes should be readily observable in the gas phase or the matrix phase. Acknowledgment. Computational time on the Cray Y-MP8 at the Ohio Supercomputing Center was obtained through Grant PFS183-3. Assistance from Dr. Michael J. Zehe at NASA Lewis Research Lab, Cleveland, OH, is appreciated. Support was also provided through the Ohio Board of Regents. References and Notes (1) Greenwood, N. N.; Eamshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1984. (2) Gropen, 0.;Johansen, R.; Haaland, A,; Stokeland, 0.J. Organomet. Chem. 1975, 92, 147. (3) Scholz, G. J. Mol. Struct. (THEOCHEM) 1994, 309, 227-34. (4) Wilson, M.; Coolidge, M. B.; Mains, G. J. J. Phys. Chem. 1992, 96, 4851-9. (5) Burkhardt, A.; Wedig, U.; Scholz, G.; Menz, D. Chem. Phys. Lett. 1991, 182, 556-60. (6) Bates, S.; Dwyer, J. J. Phys. Chem. 1993, 97, 5897-900. (7) Papatheodorou, G.N.; CUrtiSS, L. A,; Maroni, V. A. J. Chem. Phys. 1983, 78, 3303-15. (8) Connolly, J. W.; Dudis, D. S. Macromolecules 1994, 27, 1423-7. (9) Jasien, P. G.J. Phys. Chem. 1992, 96, 9273-8. (10) Grobelny, M. J. Fluorine Chem. 1977, 9, 187-207. Ibid. 1977, 10, 63-73. (11) Rosenberg, P. E. Am. Mineral. 1988, 73, 855-60. (12) Ball, D. W. J. Mol. Struct. (THEOCHEM) 1995, 331, 223-8. (13) 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.; Gonzales, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92, revisions B and D.2; Gaussian, Inc.: Pittsburgh, PA, 1992. (14) Evans, D. G.; Yeo, G. A.; Ford, T. A. Faraday Discuss. Chem. SOC. 1988, 86, 55. (15) Hunt, R. L.; Auk, B. S. Spectrosc. Int. J. 1982, 1 , 45.
JP95 1092V
