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Theoretical investigation of the structure and stability of AlCO and Al(CO)2 and their cations
Volume 136, number 3,4
CHEMICAL PHYSICS LETTERS
8 May 1987
THEORETICAL INVESTIGATION OF THE STRUCTURE AND STARIJJTY OF AK0 AND Al( CO) 2 AND THEIR CATIONS V. BALAJI *, K.K. SUNIL and K.D. JORDAN Department of Chemistry, University ofpittsburgh, Pittsburgh, PA 15260, USA Received 4 January 1987
Ab initio calculations are performed to determine the geometries and binding energies of AlCO and Al(CO)2 and their cations. The calculations reveal that the bonding between Al and the CO groups is due almost entirely to electron correlation, the second CO binds to Al much more strongly than the first, and the C-Al-C angle in both AI( and its cation is 73-74”.
1. Introduction Although a large number of papers dealing with the interaction of small molecules with metal atoms, clusters and surfaces have appeared in recent years, our understanding of the nature of such interactions is still quite rudimentary. A large fraction of these studies have focused on the interactions involving CO. Yet, relatively little is known about the bonding characteristics of the MC0 and M(C0)2 species, particularly when M is a non-transition metal atom. One experimental technique which has proven particularly valuable for providing data on such systems is matrix isolation spectroscopy [ I]. In this method, the metal atoms and CO are co-condensed in a rare gas matrix and the resulting complexes studied spectroscopically. Recently, both IR [ 21 and ESR [ 31 studies have been performed on the products of cocondensation of Al and CO. These studies showed that Al( CO), is the major product of the co-condensation. No evidence for the formation of AlCO was found. From the IR intensities of the symmetric and asymmetric CO stretching modes and assuming linear Al-C-O bonds the C-Al-C angle was predicted tobeabout 110” [2,3]. In this paper we present the results of ab initio calculations on AlCO, Al (CO) 2 and their cations. Both Hartree-Fock (HF) self-consistent-field and many’ Present address: Department of Chemistry, University of Texas, Austin, TX 78712, USA.
0 009-26 14/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
body perturbation theory methods were employed. Most of the calculations were performed in an “allelectron” format. However to explore the importance of higher-order electron correlation effects, calculations were also performed in which an effective potential [ 41 was introduced on the Al atom to model the Is, 2s, and 2p electrons. With the use of the effective potential a considerably smaller basis set can be used for Al. All calculations were performed using the GAUSSIAN 82 program [ 5 1.
2. Computational details The all-electron calculations were performed using two different basis sets, one employing the 6-31G* basis set [ 61 of Pople and co-workers on all atoms and the other the 6-31 lG* basis set [ 71 on C and 0 and a 6-3 1G [ 2d] basis set on Al. The second, larger basis set, designed to minimize the basis set superposition errors in the neutral complexes, employs a triple-zeta description of the valence shells of C and 0 and replaces the single d function in the 6-31G* basis set for Al with two d functions with exponents 0.64 and 0.16. Basis set superposition errors result from the fact that when incomplete basis sets are employed an atom can lower its energy by utilizing basis functions centered on the other atoms. For the neutral species the superposition errors with the 63 1G* basis sets were on the order of 4-6 kcaYmo1, 309
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whereas they were less than 1 kcal/mol with the larger basis set. Hence, although results for both basis sets will be tabulated, unless otherwise indicated, the discussion will focus on the results with the more flexible basis set. The cations have much longer Al-C bonds than the neutral species, making superposition errors with the 6-3 lG* basis sets relatively unimportant. Thus, for the cations the geometries obtained with the 6-3 1G* basis sets were used for the single point calculations with the larger basis set. The spin-unrestricted and the spin-restricted HF methods were employed for the open- and the closedshell species, respectively. The perturbation theory calculations utilized the orbitals and orbital energies obtained from the appropriate HF calculations. Only the valence orbitals were correlated. The Hartree -Fock approximation did not give binding for the neutral species + and gave only relatively weak binding for the cations. All geometry optimizations were performed in the second-order perturbation theory procedure (the MP2 approximation of Pople and coworkers), which gives appreciable binding for both the neutral and cationic species. The MP2 geometry optimizations were performed via the Fletcher -Powell algorithm. For Al (CO) 2 and Al( CO) : , the geometries were optimized in planar CZysymmetry.
3. Results and discussion The optimized geometries, total energies, and dissociation energies (0,) determined from the all-electron MP2 calculations are summarized in table 1, as are the Hartree-Fock dipole moments (obtained at the MP2 geometries). The dissociation energies are withrespecttoAl+nCOandAl++nCO(n=1,2)for the neutrals and cations, respectively. Full fourth-order many-body perturbation theory, MP4( SDTQ), calculations were performed for AlCO, AlCO+, and Al( CO), at the MP2 optimized geometries and using an effective potential and a 2s2pld
+ The lack of binding of AlCO at the Hartree-Fock level of theory has been noted by other researchers. See for example, ref.
[sl. 310
8 May 1987
basis set +t on Al. (For AlCO and AlCO+ all-electron MP4( SDTQ) calculations were also performed.) The results of these calculations, are summarized in table 2. The MP2 geometries and energy differences obtained from the all-electron and effective-potential calculations are in fairly good agreement. Comparison of the results obtained at the different orders of perturbation theory shows that the MP2 approximation considerably overestimates the contribution of double excitations to the De.However, single and especially triple excitations make sizable contributations to the De,with the net result that the MP2 and MP4 ( SDTQ) binding energies are comparable. The MP4( SDTQ) calculations give Deof 3.1 and 15.9 kcal/mol for AlCO and Al( CO)*, respectively. For AlCO the coupled cluster doubles (CCD) [ lo] and CCD + ST (CCD) [ 111 approximations were also considered. The latter, which adds to the CCD energies certain contributions of the single and triple excitations to all orders, is expected to be more accurate than the MP4(SDTQ) approximation. The CCD + ST( CCD) procedure gives a D, of AlCO about 0.6 kcal/mol smaller than the MP4(SDTQ) value. The optimized structures of AK0 and AlCOt are found to be linear. Since AlCO has a 211ground state, it is subject to the Renner-Teller effect and a bent structure might be expected. The present calculations show this not to be the case for the purely electronic problem. The A1(C0)2 and Al(CO)2+ molecules have 2BI and ‘Al ground states, respeo tively. Both of these species are found to have a C-Al-C angle between 73 and 74”) a point to which we will return later. Also surprising is the finding that the binding energy of the two CO groups in Al( CO)Z is about six times greater than the energy for binding the single CO in AlCO. The energy for binding the second CO to Al+ is nearly the same as for the first. ++The sp basis set employed on AI in conjunction with the effective potential is that developed by Topiol and Pople [ 91 and designated as the LP-31G basis set in the GAUSSIAN 82 program. In our calculations this basis set was augmented with two d functions with exponents of 0.78 and 0.195, similar to those utilized in the all-electron calculations. Trial calculations revealed that these functions could be contracted together (with contraction coefftcients of 0.3578 and 0.7596, respectively) with little error being introduced into the quantities of interest.
8 May 1987
CHEMICAL PHYSICS LETTERS
Volume 136, number 3,4
Table 1 Theoretical properties of AlCO, Al(CO)2 and their cations”) Property
R(AIC)
AlCO+
Alto
(4
NCG) (A) LY(CAlC) (deg) NGCAI) (deg) &~2 (au)
De (kcabmol) p(D)
2.1284 (2.1329) 1.1566 (1.1676)
2.8056 1.1456
- 354.9704 ( - 354.9095) 4.04 (4.76) 0.41 (0.15)
-354.7661 (-354.7098) 8.60 (9.67)
AI(C
AI(
2.0708 (2.0889) 1.1525 (1.1646) 74.04 (69.6) 166.45 (165.9) -468.0660 (-467.9461) 17.15 (17.30) 0.78 (0.83)
2.7795 1.1461 73.27 175.64 -467.8539 (-467.7411) 16.76 (18.87)
‘) The first set of entries under each molecule are obtained with 6.3 11G* [ C,O] 16-31G*( 2d) [ Al] basis set, while the results reported in parentheses are obtained with the 6-31G* basis set on all atoms. For the cations both sets of results were obtained at the 6.31G* optimized geometries. Table 2 Dissociation energies (kcal/mol) of AlCO, AlCO+, and Al( CO)r”’ Method
AlCO
AlCO+
AI
MP2 MP3
4.04 (3.57) 2.41 (2.15) 1.35 (1.27) 1.85 (1.78) 3.25 (3.14) (0.66) (2.56)
8.60 6.02 6.43 7.11 8.64
17.15 (17.40) (11.26) (8.54) (10.34) (15.89)
MWDQ) MP4( SDQ) MP4( SDTQ) CCD CCD+ST(CCD)
(9.88) (7.99) (8.14) (8.53) (9.63)
‘) The results given in parentheses were obtained from the calculations employing the effective potential on Al. The other results are from all-electron calculations using the 63llG*[C,O]/6-3lG(2d)[Al] basis set. All energies were determined using MP2 optimized geometries.
The 74’ C-Al-C angle in Al( CO), at first appears to be inconsistent with the 110” value predicted by Kasai and Jones [ 31 and Chennier et al. [ 21. However, it should be noted that their analyses actually give the angle between the two CO groups. If the AlCO bonds are bent, this angle will differ from the C-Al-C angle. Our calculations show that the Al-C-ObondsofAl(CO)zandAl(CO): areindeed bent, deviating 13.55” from linear for the neutral species and by 4.36 ’ for the cation. In both cases, the bending is “outward”, increasing the separation between the oxygen atoms. For Al( CO), the energy lowering associated with the AlCO bending is about 2.3 kcal/mole. We note that the metal-C-O bonds of many transition metal carbonyl complexes are known to be bent [ 121. The MP2 calculations give an angle of 10 1.1 o between the two CO groups of Al( CO)z, in
good agreement with the predictions of Kasai and Jones and of Chennier et al. The CO bond lengths of the neutral complexes are found to be over 0.01 A longer than that of the isolated CO molecule at the same level of theory. The lengthening of the CO bonds is consistent with the presence of Al(p) -X0( n*) back donation as well as Al(p,d) cCO( n,x ) donation. The SCF dipole moments of AlCO and Al(CO)2 are 0.41 and 0.78 D, respectively. These should be compared with the 0.42 D dipole moment obtained for CO with bond length of the CO group of AlCO and the 0.5 3 D dipole moment for two CO groups arranged as in Al( CO)z. These results indicate that at the SCF level of theory there is little net charge transfer between the Al atom and the CO groups. There are at least two factors which could contribute to the strong binding and small CAlC angle in Al(C First, since the distance between the two carbon atoms is only 2.5 A, there could be significant CC bonding and a sizable O-C-GO component to the wavefunction. The possibility of O-C=C-0 bonding in transition metal L,M (CO) z complexes has been discussed by Hoffmann et al. [ 131. Secondly, the small angle in Al ( CO ) z could simply result from the fact that the overlap between the orbitals of Al and those of the CO groups is much more favorable at small angles than at large angles. Some insight into the relative importance of these two factors and of the role of the highest occupied molecular orbital (HOMO) in the bonding in the neutral complexes can be gained by comparing the structures of the neutral and cationic complexes. The 311
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CHEMICAL PHYSICS LETTERS
a-type HOMO of AlCO and the 3b, HOMO of Al( CO), contain small, but important, contributions from the CO x orbitals, as evidenced by the 0.7 A increase in the AlC distances accompanying ionization. In spite of the much longer AlC bond lengths in the cations, the binding in AlCO+ is over twice that in AlCO and that in Al( CO); is only about 0.5 kcal/mol less than in Al( CO)Z. Apparently, the HOMO of Al( CO)2 is largely responsible for the anomalously strong binding of the second CO group in this species. The long CC distance (x3.3 A) in Al(C0): makes it unlikely that significant CC bonding could be occurring in this species. Perhaps the most surprising result of these calculations is that the CAlC angle is nearly the same in Al (CO) $ as in Al( CO) 2. These results suggest that CO+Al donation may be an important factor in determining the CAlC angles in both the neutral and cationic complexes. (Examination of the SCF MOs reveals that both CO( n:) +Al and CO( o ) *Al donation are important. ) The CAlC angle of Al( CO), is essentially unchanged when d functions are omitted from the basis set, ruling out CO+Al(d) donation as being responsible for the small angle. We caution that it will be necessary to examine the charge redistributions which occur in the correlated wavefunctions in order to establish unambiguously the factors responsible for the small CAlC angles in Al( CO) Zand its cation. It might be expected that the large binding energies of the cationic species are due to the importance of charge-induced-dipole interactions. However, while such interactions are usually well described in the Hartree-Fock (HF) approximation, this is not the case for AlCO+ and Al( CO);, for which the HF method yields binding energies two to three times smaller than the MP2 values.
4. Conclusions
I3May 1987
energy of 2.6 kcal/mole for AlCO. We estimate that basis set superposition error could be responsible for as much as 0.8 kcal/mol of the predicted binding and that the actual binding energy of AK0 could be as small as 1.8 k&/mole. Thus, although AlCO is likely to be a bound species, its detection in co-deposition/ matrix isolation experiments is likely to be realizable only when relatively low concentrations of CO are utilized (else formation of Al ( CO) 2 will result ) . Both AlCO+ and Al( CO): are predicted to be strongly bound with the binding of the second CO to Al+ is somewhat weaker than that of the first CO. This is a common situation for ion-“solvent” complexes. The CO bond lengths in the cationic species are significantly shorter than those in the neutral complexes and even somewhat ( %0.006 A) shorter than that of a free CO molecule. Electron correlation effects are found to be responsible for most of the stability of the cationic species and all of the stability of the neutral complexes. Al( CO), and its cation are found to have small CAlC angles. Finally, we note that at the MP2 level of theory the adiabatic IPs of AlCO and Al ( CO) Z are predicted to be 5.6 and 5.8 eV, respectively. The corresponding vertical IPs are 6.3 and 6.6 eV. At the same level of theory the IP of Al is 5.9 eV.
Acknowledgement This research was carried out with the support of the National Science Foundation. The calculations were performed on the Department of Chemistry’s H 1000 minicomputer, funded by grants from NSF and Harris Corporation, and on the Cray XMP-48 at the Pittsburgh Supercomputer Center.
References [ 1] H. Huber, E.P. Kundig, M. Moskovits and G.A. Ozin, J.
In summary, the present study shows that both the AlCO and Al( species are bound, with the binding energy associated with the second CO being much larger than with the first. The stability of AlCO is of interest since this species has not yet been observed. Our most sophisticated calculations using the CCD+ ST( CCD) approximation yield a binding 312
Am. Chem. Sot. 97 (1975) 2097; A.J. Hinchliffe, J.S. Ogden and D.D. Oswald, J. Chem. Sot. Chem. Commun. (1972) 338. [ 21 J.H.B. Chennier, C.A. Hampson, J.A. Howard and B. Mile, J. Chem. Sot. Chem. Commun. (1986) 730. [3] P.H. Kasai andP.M. Jones, J. Am. Chem.Soc. 106 (1984) 8018. [4] AK Rappe, T.A. Smedly and W.A. Goddard, J. Phys. Chem. 85 (1981) 1662.
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CHEMICAL PHYSICS LETTERS
[ 51 J.S. Binkley, M.J. Frisch, K. Raghavachari, D.J. DeFrees, H.B. Schlegel, R. Whiteside, E. Ruder, R. Seeger, J.A. Pople, J.H. Yates and K.K. Sunil, GAUSSIAN 82, Harris VOS version, Carnegie-Mellon University, Pittsburgh (1985). [6] M.M. Francl, W.J. Pietro, W.J. Hehre, M.S. Gordon, D.J. DeFrees and J.A. Pople, J. Chem. Phys. 77 (1982) 3654. [ 7 ] R. Krishnan, J.S. Binkley, R. !3eegerand J.A. Pople, J. Chem. Phys. 72 (1980) 650. [ 81 P.S. Bagus, C.J. Nelin and C.W. Bauschlicher Jr., Phys. Rev. B28 (1983) 5423.
8 May 1987
[ 91 S. Topiol and J.A. Pople, unpublished.
[lo] J. Ciiek and J. Paldus, Physica Scripta 21 (1980) 251; R.J. Bartlett, Ann. Rev. Phys. Chem. 32 (1981) 359.
[ 1l] K. Raghavachari, J. Chem. Phys. 82 (1985) 4607; Y.S. Lee, S.A. Kucharski and R.J. Bartlett, J. Chem. Phys. 81 (1984) 5906. [ 121 S.F.A. Kettle, Inorg. Chem. 4 (1965) 1661; E.W. Abel and F.G.A. Stone, Quart. Rev. Chem. Sot. 23 (1969) 325. [ 131 R. Hoffmann, C.N. Wilkes, S.J. Lippard, J.L. Templeton andD.C. Brower, J. Am.Chem. Sot. 105 (1983) 146.
313
CHEMICAL PHYSICS LETTERS
8 May 1987
THEORETICAL INVESTIGATION OF THE STRUCTURE AND STARIJJTY OF AK0 AND Al( CO) 2 AND THEIR CATIONS V. BALAJI *, K.K. SUNIL and K.D. JORDAN Department of Chemistry, University ofpittsburgh, Pittsburgh, PA 15260, USA Received 4 January 1987
Ab initio calculations are performed to determine the geometries and binding energies of AlCO and Al(CO)2 and their cations. The calculations reveal that the bonding between Al and the CO groups is due almost entirely to electron correlation, the second CO binds to Al much more strongly than the first, and the C-Al-C angle in both AI( and its cation is 73-74”.
1. Introduction Although a large number of papers dealing with the interaction of small molecules with metal atoms, clusters and surfaces have appeared in recent years, our understanding of the nature of such interactions is still quite rudimentary. A large fraction of these studies have focused on the interactions involving CO. Yet, relatively little is known about the bonding characteristics of the MC0 and M(C0)2 species, particularly when M is a non-transition metal atom. One experimental technique which has proven particularly valuable for providing data on such systems is matrix isolation spectroscopy [ I]. In this method, the metal atoms and CO are co-condensed in a rare gas matrix and the resulting complexes studied spectroscopically. Recently, both IR [ 21 and ESR [ 31 studies have been performed on the products of cocondensation of Al and CO. These studies showed that Al( CO), is the major product of the co-condensation. No evidence for the formation of AlCO was found. From the IR intensities of the symmetric and asymmetric CO stretching modes and assuming linear Al-C-O bonds the C-Al-C angle was predicted tobeabout 110” [2,3]. In this paper we present the results of ab initio calculations on AlCO, Al (CO) 2 and their cations. Both Hartree-Fock (HF) self-consistent-field and many’ Present address: Department of Chemistry, University of Texas, Austin, TX 78712, USA.
0 009-26 14/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
body perturbation theory methods were employed. Most of the calculations were performed in an “allelectron” format. However to explore the importance of higher-order electron correlation effects, calculations were also performed in which an effective potential [ 41 was introduced on the Al atom to model the Is, 2s, and 2p electrons. With the use of the effective potential a considerably smaller basis set can be used for Al. All calculations were performed using the GAUSSIAN 82 program [ 5 1.
2. Computational details The all-electron calculations were performed using two different basis sets, one employing the 6-31G* basis set [ 61 of Pople and co-workers on all atoms and the other the 6-31 lG* basis set [ 71 on C and 0 and a 6-3 1G [ 2d] basis set on Al. The second, larger basis set, designed to minimize the basis set superposition errors in the neutral complexes, employs a triple-zeta description of the valence shells of C and 0 and replaces the single d function in the 6-31G* basis set for Al with two d functions with exponents 0.64 and 0.16. Basis set superposition errors result from the fact that when incomplete basis sets are employed an atom can lower its energy by utilizing basis functions centered on the other atoms. For the neutral species the superposition errors with the 63 1G* basis sets were on the order of 4-6 kcaYmo1, 309
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whereas they were less than 1 kcal/mol with the larger basis set. Hence, although results for both basis sets will be tabulated, unless otherwise indicated, the discussion will focus on the results with the more flexible basis set. The cations have much longer Al-C bonds than the neutral species, making superposition errors with the 6-3 lG* basis sets relatively unimportant. Thus, for the cations the geometries obtained with the 6-3 1G* basis sets were used for the single point calculations with the larger basis set. The spin-unrestricted and the spin-restricted HF methods were employed for the open- and the closedshell species, respectively. The perturbation theory calculations utilized the orbitals and orbital energies obtained from the appropriate HF calculations. Only the valence orbitals were correlated. The Hartree -Fock approximation did not give binding for the neutral species + and gave only relatively weak binding for the cations. All geometry optimizations were performed in the second-order perturbation theory procedure (the MP2 approximation of Pople and coworkers), which gives appreciable binding for both the neutral and cationic species. The MP2 geometry optimizations were performed via the Fletcher -Powell algorithm. For Al (CO) 2 and Al( CO) : , the geometries were optimized in planar CZysymmetry.
3. Results and discussion The optimized geometries, total energies, and dissociation energies (0,) determined from the all-electron MP2 calculations are summarized in table 1, as are the Hartree-Fock dipole moments (obtained at the MP2 geometries). The dissociation energies are withrespecttoAl+nCOandAl++nCO(n=1,2)for the neutrals and cations, respectively. Full fourth-order many-body perturbation theory, MP4( SDTQ), calculations were performed for AlCO, AlCO+, and Al( CO), at the MP2 optimized geometries and using an effective potential and a 2s2pld
+ The lack of binding of AlCO at the Hartree-Fock level of theory has been noted by other researchers. See for example, ref.
[sl. 310
8 May 1987
basis set +t on Al. (For AlCO and AlCO+ all-electron MP4( SDTQ) calculations were also performed.) The results of these calculations, are summarized in table 2. The MP2 geometries and energy differences obtained from the all-electron and effective-potential calculations are in fairly good agreement. Comparison of the results obtained at the different orders of perturbation theory shows that the MP2 approximation considerably overestimates the contribution of double excitations to the De.However, single and especially triple excitations make sizable contributations to the De,with the net result that the MP2 and MP4 ( SDTQ) binding energies are comparable. The MP4( SDTQ) calculations give Deof 3.1 and 15.9 kcal/mol for AlCO and Al( CO)*, respectively. For AlCO the coupled cluster doubles (CCD) [ lo] and CCD + ST (CCD) [ 111 approximations were also considered. The latter, which adds to the CCD energies certain contributions of the single and triple excitations to all orders, is expected to be more accurate than the MP4(SDTQ) approximation. The CCD + ST( CCD) procedure gives a D, of AlCO about 0.6 kcal/mol smaller than the MP4(SDTQ) value. The optimized structures of AK0 and AlCOt are found to be linear. Since AlCO has a 211ground state, it is subject to the Renner-Teller effect and a bent structure might be expected. The present calculations show this not to be the case for the purely electronic problem. The A1(C0)2 and Al(CO)2+ molecules have 2BI and ‘Al ground states, respeo tively. Both of these species are found to have a C-Al-C angle between 73 and 74”) a point to which we will return later. Also surprising is the finding that the binding energy of the two CO groups in Al( CO)Z is about six times greater than the energy for binding the single CO in AlCO. The energy for binding the second CO to Al+ is nearly the same as for the first. ++The sp basis set employed on AI in conjunction with the effective potential is that developed by Topiol and Pople [ 91 and designated as the LP-31G basis set in the GAUSSIAN 82 program. In our calculations this basis set was augmented with two d functions with exponents of 0.78 and 0.195, similar to those utilized in the all-electron calculations. Trial calculations revealed that these functions could be contracted together (with contraction coefftcients of 0.3578 and 0.7596, respectively) with little error being introduced into the quantities of interest.
8 May 1987
CHEMICAL PHYSICS LETTERS
Volume 136, number 3,4
Table 1 Theoretical properties of AlCO, Al(CO)2 and their cations”) Property
R(AIC)
AlCO+
Alto
(4
NCG) (A) LY(CAlC) (deg) NGCAI) (deg) &~2 (au)
De (kcabmol) p(D)
2.1284 (2.1329) 1.1566 (1.1676)
2.8056 1.1456
- 354.9704 ( - 354.9095) 4.04 (4.76) 0.41 (0.15)
-354.7661 (-354.7098) 8.60 (9.67)
AI(C
AI(
2.0708 (2.0889) 1.1525 (1.1646) 74.04 (69.6) 166.45 (165.9) -468.0660 (-467.9461) 17.15 (17.30) 0.78 (0.83)
2.7795 1.1461 73.27 175.64 -467.8539 (-467.7411) 16.76 (18.87)
‘) The first set of entries under each molecule are obtained with 6.3 11G* [ C,O] 16-31G*( 2d) [ Al] basis set, while the results reported in parentheses are obtained with the 6-31G* basis set on all atoms. For the cations both sets of results were obtained at the 6.31G* optimized geometries. Table 2 Dissociation energies (kcal/mol) of AlCO, AlCO+, and Al( CO)r”’ Method
AlCO
AlCO+
AI
MP2 MP3
4.04 (3.57) 2.41 (2.15) 1.35 (1.27) 1.85 (1.78) 3.25 (3.14) (0.66) (2.56)
8.60 6.02 6.43 7.11 8.64
17.15 (17.40) (11.26) (8.54) (10.34) (15.89)
MWDQ) MP4( SDQ) MP4( SDTQ) CCD CCD+ST(CCD)
(9.88) (7.99) (8.14) (8.53) (9.63)
‘) The results given in parentheses were obtained from the calculations employing the effective potential on Al. The other results are from all-electron calculations using the 63llG*[C,O]/6-3lG(2d)[Al] basis set. All energies were determined using MP2 optimized geometries.
The 74’ C-Al-C angle in Al( CO), at first appears to be inconsistent with the 110” value predicted by Kasai and Jones [ 31 and Chennier et al. [ 21. However, it should be noted that their analyses actually give the angle between the two CO groups. If the AlCO bonds are bent, this angle will differ from the C-Al-C angle. Our calculations show that the Al-C-ObondsofAl(CO)zandAl(CO): areindeed bent, deviating 13.55” from linear for the neutral species and by 4.36 ’ for the cation. In both cases, the bending is “outward”, increasing the separation between the oxygen atoms. For Al( CO), the energy lowering associated with the AlCO bending is about 2.3 kcal/mole. We note that the metal-C-O bonds of many transition metal carbonyl complexes are known to be bent [ 121. The MP2 calculations give an angle of 10 1.1 o between the two CO groups of Al( CO)z, in
good agreement with the predictions of Kasai and Jones and of Chennier et al. The CO bond lengths of the neutral complexes are found to be over 0.01 A longer than that of the isolated CO molecule at the same level of theory. The lengthening of the CO bonds is consistent with the presence of Al(p) -X0( n*) back donation as well as Al(p,d) cCO( n,x ) donation. The SCF dipole moments of AlCO and Al(CO)2 are 0.41 and 0.78 D, respectively. These should be compared with the 0.42 D dipole moment obtained for CO with bond length of the CO group of AlCO and the 0.5 3 D dipole moment for two CO groups arranged as in Al( CO)z. These results indicate that at the SCF level of theory there is little net charge transfer between the Al atom and the CO groups. There are at least two factors which could contribute to the strong binding and small CAlC angle in Al(C First, since the distance between the two carbon atoms is only 2.5 A, there could be significant CC bonding and a sizable O-C-GO component to the wavefunction. The possibility of O-C=C-0 bonding in transition metal L,M (CO) z complexes has been discussed by Hoffmann et al. [ 131. Secondly, the small angle in Al ( CO ) z could simply result from the fact that the overlap between the orbitals of Al and those of the CO groups is much more favorable at small angles than at large angles. Some insight into the relative importance of these two factors and of the role of the highest occupied molecular orbital (HOMO) in the bonding in the neutral complexes can be gained by comparing the structures of the neutral and cationic complexes. The 311
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a-type HOMO of AlCO and the 3b, HOMO of Al( CO), contain small, but important, contributions from the CO x orbitals, as evidenced by the 0.7 A increase in the AlC distances accompanying ionization. In spite of the much longer AlC bond lengths in the cations, the binding in AlCO+ is over twice that in AlCO and that in Al( CO); is only about 0.5 kcal/mol less than in Al( CO)Z. Apparently, the HOMO of Al( CO)2 is largely responsible for the anomalously strong binding of the second CO group in this species. The long CC distance (x3.3 A) in Al(C0): makes it unlikely that significant CC bonding could be occurring in this species. Perhaps the most surprising result of these calculations is that the CAlC angle is nearly the same in Al (CO) $ as in Al( CO) 2. These results suggest that CO+Al donation may be an important factor in determining the CAlC angles in both the neutral and cationic complexes. (Examination of the SCF MOs reveals that both CO( n:) +Al and CO( o ) *Al donation are important. ) The CAlC angle of Al( CO), is essentially unchanged when d functions are omitted from the basis set, ruling out CO+Al(d) donation as being responsible for the small angle. We caution that it will be necessary to examine the charge redistributions which occur in the correlated wavefunctions in order to establish unambiguously the factors responsible for the small CAlC angles in Al( CO) Zand its cation. It might be expected that the large binding energies of the cationic species are due to the importance of charge-induced-dipole interactions. However, while such interactions are usually well described in the Hartree-Fock (HF) approximation, this is not the case for AlCO+ and Al( CO);, for which the HF method yields binding energies two to three times smaller than the MP2 values.
4. Conclusions
I3May 1987
energy of 2.6 kcal/mole for AlCO. We estimate that basis set superposition error could be responsible for as much as 0.8 kcal/mol of the predicted binding and that the actual binding energy of AK0 could be as small as 1.8 k&/mole. Thus, although AlCO is likely to be a bound species, its detection in co-deposition/ matrix isolation experiments is likely to be realizable only when relatively low concentrations of CO are utilized (else formation of Al ( CO) 2 will result ) . Both AlCO+ and Al( CO): are predicted to be strongly bound with the binding of the second CO to Al+ is somewhat weaker than that of the first CO. This is a common situation for ion-“solvent” complexes. The CO bond lengths in the cationic species are significantly shorter than those in the neutral complexes and even somewhat ( %0.006 A) shorter than that of a free CO molecule. Electron correlation effects are found to be responsible for most of the stability of the cationic species and all of the stability of the neutral complexes. Al( CO), and its cation are found to have small CAlC angles. Finally, we note that at the MP2 level of theory the adiabatic IPs of AlCO and Al ( CO) Z are predicted to be 5.6 and 5.8 eV, respectively. The corresponding vertical IPs are 6.3 and 6.6 eV. At the same level of theory the IP of Al is 5.9 eV.
Acknowledgement This research was carried out with the support of the National Science Foundation. The calculations were performed on the Department of Chemistry’s H 1000 minicomputer, funded by grants from NSF and Harris Corporation, and on the Cray XMP-48 at the Pittsburgh Supercomputer Center.
References [ 1] H. Huber, E.P. Kundig, M. Moskovits and G.A. Ozin, J.
In summary, the present study shows that both the AlCO and Al( species are bound, with the binding energy associated with the second CO being much larger than with the first. The stability of AlCO is of interest since this species has not yet been observed. Our most sophisticated calculations using the CCD+ ST( CCD) approximation yield a binding 312
Am. Chem. Sot. 97 (1975) 2097; A.J. Hinchliffe, J.S. Ogden and D.D. Oswald, J. Chem. Sot. Chem. Commun. (1972) 338. [ 21 J.H.B. Chennier, C.A. Hampson, J.A. Howard and B. Mile, J. Chem. Sot. Chem. Commun. (1986) 730. [3] P.H. Kasai andP.M. Jones, J. Am. Chem.Soc. 106 (1984) 8018. [4] AK Rappe, T.A. Smedly and W.A. Goddard, J. Phys. Chem. 85 (1981) 1662.
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[ 51 J.S. Binkley, M.J. Frisch, K. Raghavachari, D.J. DeFrees, H.B. Schlegel, R. Whiteside, E. Ruder, R. Seeger, J.A. Pople, J.H. Yates and K.K. Sunil, GAUSSIAN 82, Harris VOS version, Carnegie-Mellon University, Pittsburgh (1985). [6] M.M. Francl, W.J. Pietro, W.J. Hehre, M.S. Gordon, D.J. DeFrees and J.A. Pople, J. Chem. Phys. 77 (1982) 3654. [ 7 ] R. Krishnan, J.S. Binkley, R. !3eegerand J.A. Pople, J. Chem. Phys. 72 (1980) 650. [ 81 P.S. Bagus, C.J. Nelin and C.W. Bauschlicher Jr., Phys. Rev. B28 (1983) 5423.
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[ 91 S. Topiol and J.A. Pople, unpublished.
[lo] J. Ciiek and J. Paldus, Physica Scripta 21 (1980) 251; R.J. Bartlett, Ann. Rev. Phys. Chem. 32 (1981) 359.
[ 1l] K. Raghavachari, J. Chem. Phys. 82 (1985) 4607; Y.S. Lee, S.A. Kucharski and R.J. Bartlett, J. Chem. Phys. 81 (1984) 5906. [ 121 S.F.A. Kettle, Inorg. Chem. 4 (1965) 1661; E.W. Abel and F.G.A. Stone, Quart. Rev. Chem. Sot. 23 (1969) 325. [ 131 R. Hoffmann, C.N. Wilkes, S.J. Lippard, J.L. Templeton andD.C. Brower, J. Am.Chem. Sot. 105 (1983) 146.
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