J . Phys. Chem. 1994,98, 2859-2863
2859
Theoretical Study of the AlCls-Catalyzed Decomposition of HClCO Paul G . Jasien Department of Chemistry, California State University San Marcos, San Marcos, California 92096 Received: November 10, 1993; In Final Form: January 3, 1994"
A b initio calculations which include the effect of electron correlation have been used to study the decomposition reaction of formyl chloride (HClCO) in the presence and absence of AlC13. The results predict a large decrease in activation energy in the presence of AlC13. The barrier for the uncatalyzed decomposition of HClCO to HC1 and C O is estimated to be 40 kcal/mol. In the presence of AlC13, the predicted barrier is 4 kcal/mol. The large decrease owes its origin to the elongated C-Cl bond in the AlC13-ClHCO precursor complex which is predicted to have a binding energy of 9 kcal/mol and to the different decomposition mechanism facilitated by the presence of the Lewis acid. The mechanism involves C1 atom exchange between the AlC13 catalyst and the HClCO. -The transition state for the catalyzed reaction involves the formation of a five-membered ring in which a nonbonded H- -C1 interaction guides the reaction. This structure differs dramatically from the highly distorted transition-state structure found for unimolecular HClCO decomposition.
It is well-known that many Lewis acids act as catalysts in organic reactions. Aluminum trichloride (AlC13) is one such Lewis acid which will readily form stable acid-base complexes with molecules that possess accessible electron pairs.' Upon complex formation, the electron distribution of the individual molecules may be dramatically changed, with the atom donating the electron pair becoming significantly more electronegative. It is this perturbation in the electron distribution which contributes to the utility of AlC13 as a catalyst.2 There are a number of chemical reactions in which trivalent A1 plays a catalytic role. Examples of these include the FriedelCrafts alkylation and acylation of aromatic rings,3 removal of tert-butyl groups from phenols: condensation of unsaturated alcohols with aldehyde^,^ and the well-known Ziegler-Natta polymerization reactions.6 There have been a limited number of ab initio computational studies of the Lewis acid chemistry of AlC13,and these have dealt specifically with the equilibrium structures of the acid-base pair. The bases in these studies have included H2C0,7.*HCl and HF,9 and C2H2, CzH4, HClCO, and H3CCL8 In an effort to understand the catalytic properties of A1C13,the present study will investigate a simple model for an AlC13-catalyzed reaction. The particular reaction under study is the decomposition of formyl chloride, HClCO. Formyl chloride was first identified in 1973 by its infrared spectrumlo and is known to undergo thermal decomposition to HCl and CO on the walls of reaction chambers.l1 The unimolecular decomposition of HClCO has been previously studied by Tyrell and Lewis-Bevan.l* Their results, from MP2/6-31G** calculations, indicated that the decomposition reaction had a barrier of 43.95 kcallmol. The present theoretical study will revisit the unimolecular decomposition reaction as a prelude to studying the AlC13-catalyzed decomposition. A general comparison of the characteristic properties of the catalyzed and uncatalyzed reactions will then be made. Computational Methods All calculations were performed on the Cray-YMP8164 computer at the San Diego Supercomputer Center or on an inhouse IBM RISC/6000 workstation. These calculations were performed with the Gaussian 92 program package13 and utilized a number of different basis sets and levels of electron correlation. e Abstract published
in Advance ACS Abstracts, February 15, 1994.
0022-3654/94/2098-2859$04.50/0
The primary basis sets used were the CEP-31G*(*) basis sets of Stevens et al.14 The CEP basis sets utilize compact effective potentials (CEPs) to replace the core electronsin heavy atoms,14J5 making calculations with heavy atoms more computationally tractable. Calculations with CEPs have been shown to give comparable results to similar all electron c a l ~ u l a t i o n s . ~Other J~1~ basis sets that wereused in this investigation included the standard 6-31G*(*) basis sets. Since the results from the CEP-31G*(*) basissets werecomparable to thosefrom6-31G*(*),onlyselected 6-31G** basis set results will be presented. Calculations were performed both at the HartreeFock self-consistent field (HF) level and at the correlated level. Correlated calculations utilized Maeller-Plesset perturbation theoryl8to second (MP2) and fourth order (MP4(SDTQ)). Geometry optimizations were carried out at the H F and MP2 levels for a variety of basis sets. Energy evaluations not done at the same level as the geometry optimization are labeled using the standard X/Y//X'/Y' n0tati0n.l~ All geometry minimizations achieved convergence in the structures to 0.001 A in bond length and 0.1' in bond angle. For the HClCO system, all geometries were found to be of C, symmetry. The AlC13- -ClHCO minimum energy geometry was previously found to be of C1 symmetry: with a small rotational barrier to a C, symmetry structure. The current calculations indicate that the equilibrium symmetry is highly dependent on the basis set and that the difference in energies between the two structures is -0.1 kcal/mol or less at the H F level for a number of basis sets tested. Therefore, only results for the C, symmetry complex will be reported here. In the case of the transition state for the AlC13-catalyzed decomposition reaction, the saddle point was found to possess a true C, symmetry structure. The nature of a particular stationary point on the potential energy surface (PES) was determined by the number of imaginary eigenvalues in the Hessian matrix. All transition-statestructures possessed one imaginary eigenvalue, while all minima had none. The only exception was for the AlC13--ClHCO C, symmetry complex, which as discussed before possesses a very low barrier to AlC13 rotation and therefore had a very small imaginary eigenvalue(15i cm-1) correspondingto a deformation that breaks symmetry. The vibrational frequencies used to estimate zero point energy (ZPE) corrections were calculated via 2-point finite differencing of the analytically calculated first derivatives at the CEP-31G**/MP2 level. 0 1994 American Chemical Society
2860 The Journal of Physical Chemistry. Vol. 98. No. 11. 1994
Jasien
TABLE 1: Calculated Relative Energies for the HCICO Decomposition Reaction' level AE (kcal/mol) A . P (kcal/mol) HF/CEP31G'* MP2/CEP3IGW*
MP2/CEP3lGW'/HF/CEP3lG**
-12.6 -3.5 -2.9 -5.4
MP4(SDTQ)/CEP3 IG"//HF/ CEP3 IG" MP4(SDTQ)/CEP3 IG"//MP2/ 4.2 42.8 CEP-31G" HF/6-31G** -13.0 43.3 MP2/6-3 IG**//HF/6-3 IG" -1.9 46.5 MP4(SDTQ)/6-3lG"//HF/ -3.6 46.0 6-31G" MP2/6-3 IG**b -1.6 43.9 AE refers to the dissociation energy for HCICO HCI CO, and AiP corresponds to the energy barrier starting from HCICO. Results are no101 corrected for ZPE. Reference 12. This work included the core electrons in the MP2 calculation.
-
TABLE 2 complex
co ~~
HCI HClCO (TS) HCICO
Zero Point Energies. ZPE (kcal/mol) complex 2.97. 4.44 9.08 12.12
Alrl, AICI, AICIA!IHCO AICI3--CIHCO (TS) AICI,--CIH
+
ZPE (kcal/mol) 2 I1 3.11 15.73 13.58 8.63
Calculated from MPZ/CEP-31G** frequencies. Results and Discussion HCICO. Energetics. Given in Table 1 are the calculated energies for the composition reaction of HCClO from this work as well as the results of Tyrell and Lewis-Bevan.12 The first column in Table 1 represents the calculated dissociation energies for the reaction HCICO
-
HCI
+ CO
It can he seen that the calculated energetics from the all-electron calculations (Le., 6-31G8*) arecomparahle to thoseutilizing the CEP and associated basis sets. For all basis sets, the calculated AE's were quite sensitive to the inclusion of the electron correlation. The results listed in the second column of Table 1 are the predicted energy barriers for the unimolecular decomposition reaction. These results indicate that the H F predicted barrier heights are within a few kcaljmol of those from correlated calculations and that the effect of higher order correlation (MP4(SDTQ) vs MP2) is small. Unlike the case of the HCICO minimum, reoptimization of the transition-state structure at the correlated level has a larger effect on the energies and lowered the predicted harrier height by -3 kcal/mol (cf. MP2/CEP31G" and MP2/CEP-31G*'//HF/CEP31G**). The results of Tyrell and Lewis-Bevan are in reasonable agreement with the MP2/CEP-31G'* results from this study. The value of -3.6 kcal/mol for the AE from this work is close to the value of -1.6 kcal/mol from ref 12. In addition, their predicted value of 43.95 kcal/mol for the barrier compares favorably with the 46.0 kcal/mol calculated here. Given inTable2are thecalculated ZPEs forvariousstationary points studied in this work. Inclusion of ZPEcorrectionscoupled with relative energetics determined from MP4(SDTQ)/CEP31GW*//MP2/CEP31G** calculations gives estimates for the energy barrier and AE of 39.8 kcal/mol and -10.9 kcal/mol, respectively, for the decomposition reaction. The results for HCICO presented here are in no way meant to be definitive for the decomposition energetics of this system, hut they do provide a basis for comparison with the AICI,catalyzed decomposition reaction. Larger basis set calculations and the inclusion of higher levels of electron correlation will certainlyrevealchangesin thecalculated energetics. For instance,
0
Q
45.3 43.6 46.3 46.0
2.335
1.781
Figure 1. MP2/CEP-31G** structures for HCICO uilibrium and for the transition-state configuration. Bond lengths in?, bond angles in
degrees.
the use of a multiconfiguration (MC) reference wave function to describe thebondbreakingmay benecessarytoprovidethehighest quality results as has been discussed in previous work for the case of HzCO decomposition.20 I n that work, the use of MC wave functions lowered the barrier by -5% compared to a CISD calculations with a single reference configuration. Structures. Given in Figure 1 are the MPZ/CEP-31G" predicted geometries for the minimum energy and saddle point structures of HCICO. Table 3 presents a more complete listing of structural parameters for a number of basis sets and levels of electron correlation. This tablealso contains the results ofTyrell and Lewis-Bevan, as well as the experimentally determined structures of HCICO, HCI, and CO for comparison. The results from this work are in reasonable agreement with thosefrom ref 12, although not asclose to theexperimentalvalues as that work. The results from theMPZ/CEP-3 IG*'calculations seem to consistently predict slightly longer absolute bond lengths than theresultsofTyrellandLewis-Bevin. However, the relative shiftsinbondlengthsandanglesfor thesecalculationsareinvery good agreement. AICb- -CIHCO. Previously published calculations have dealt with some of the aspects of the binding in the AIC13--CIHCO complex.8 Thesecalculations utilized a CEP-3IG. basis set and demonstrated the importance of electron correlation in the prediction of the binding energy for CI donor complexes with AICI,. This work predicted a binding energy of 8.9 kcal/mol at the MP2/CEP-3IG'//HF/CEP-3lG8level. Given in Figure 2 is a representation of the minimum energy structure for the AIC1,- -CIHCO complex. Before proceeding to a detailed discussion of the energetics of this reaction, we note the 2.49-A distance for the nonbonded H- -CI pair from HClCO and A1C13. This distance is less than the sum of the van der Waals radii for H and CI (1.2 1.8 = 3.0 A).z4 The interaction between these atoms will play a crucial role in the reaction pathway. Energetics. Given in Table 4 are the calculated energetics for some stationary points identified on the PES of AICI, + HCICO. Consistent with the results of ref 8, the CEP-3 IG** calculations indicatea large increasein thebondingenergyforAIC1,--CIHCO upon theinclusion ofelectroncorrelation. Structural optimization at the MP2 level with this basis set leads to dramatic changes in the geometry (vide infra) and a minor 1-2 kcal/mol change in the predicted energetics. Results from the MP2/CEP-31G'* calculations predict that the AICI3- -CIHCO complex lies 9.5 kcal/mol below the AlC13 HCICO dissociationlimit,andhigher order effects change this by only 0.2 kcal/mol. Despite this relatively strong interaction, there are two other points on the PES which lie even lower in energy. The first point consistsofan AICl3- -CIHcomplex (Figure 3a) and a free CO which is predicted to lie 13.1 kcal/mol (MP4(SDTQ)) below A1C13 HCICO. The AIC13--CIH complex has been studied previously by Wilson et aL9 Their predicted result (without ZPE correction) from MP4(SDTQ)/
+
+
+
The Journal of Physical Chemistry. Vol. 98, No. 11,1994 2861
AICI,-Catalyzed Decomposition of HCICO
TABLE 3
Structural Parameters for HCICO Deeomposition' coordinate HF/CEP31G**
reactant
C-CI
1.765
MP2/6-3IG**'
expt
1.781
1.7644 1.092 1.1987
1.765ff 1.0897 1.1820
109.9073 123.8169 2.3601 1.115 I.1521
110.44 123.07
1.102
C-H
transition state
MP2/CEP31GW* ~~~~~
MI C-H
2.451 1.111
1.215 2.398 110.3 123.8 2.335 1.127 1.170 1.847 ~
~~
50.2144 123.4866
51.0
products a
H-CI
Go
1.271 1.127
122.4 1.277 1.169
1.274606 1.1281'
Bond lengths in A. bond angles in degrees. Reference 12. Reference 21. Reference 22. e Reference 23.
more importantly the predicted energy harrier to HCICO decomposition is only 4.1 kcal/mol. Therefore, the presence of the Lewis acid AlCI3 has lowered the decomposition barrier for HClCO from 40 kcal/mol in the isolated case to 4 kcal/mol in the catalyzed reaction. This extremely large decrease in the energy harrier for the decomposition implies a potential increase in the reaction rate. Once again, the energetics as reported from these calculations are not meant to he definitive. The current calculations have not rigorously assessed the energy changes arising from larger basis setsor from basisset superpositionerrors (BSSE).2S,z6Preliminary Figure 2. MPZ/CEP-31G** structure for the C,symmetry minimum calculations indicate that the errors from BSSE may be as large energyconformationof AICb- -CIHCO. Bond lengths in A, bond angles as 1 kcal/mol at the H F level and 4 kcal/mol at the MP2 level in degrees. at the AICI,- -CIHCO equilibrium geometry. However, the predictedenergybarrierwill bemuchlesssensitivetotheseerron. 6-31G8//HF/6-31G* calculations gave a complexation energy As mentioned previously,theabsolute energetics may necessitate relative to AICI, + HCI of 6.58 kcal/mol. This result is in good the use of an MC reference wave function. However, given the agreement with the MP4(SDTQ)/CEP-?.lG*'//MP2/CEPinsensitivity of the energy barrier to the inclusion of electron 31G" result of 6.9 kcal/mol found here. correlation the effect of such a reference will probably be small. The second system that is energetically lower than the AICbTherefore, the large decrease in energy harrier in the presence -CIHCO complex is the trimer of AICI,, HCI, and CO pictured of AICI, should certainly hold up to future more rigorous in Figure 3b. This structure can be thought of as two Lewis calculations. acid-base complexes, the first of AICI, and HCI and the other Structure. Given in Figure 5 is the transition-state structure of HCI and CO. The relative energy of this complex is -I6 for the catalyzed decomposition reaction. Listed in Table 5 are kcal/mol below that of AICI, HCICO. The additional selected structural parameters for several ofthestationarypoints stabilization of this complex over AIC1,- -CIH + CO arises from on the AICI, HCICOPES. Theseresultsshowthat the predicted the weak interaction between the lone pair on the C of CO and intermolecular AI-CI distance in AICI,- -CIHCO decreased by the H from HCI. Although this complex is formally lower than 0.15 A on going from the H F to the MP2 level. This dramatic AICI,. -CIH CO on the PES, the weakness of the CIH- -CO shortening is consistent with that found in other weakly bound interaction means that it would only be bound at extremely low complexes." This same size decrease is also seen in AICI,-. temperatures. Therefore, for practical pu'poses, the AICb- CIH. Despite thesedramatic changes in structure, theuseofthe CIH CO asymptote will he considered the lowest energy H F optimized geometry to evaluate the MP2 energy was seen to dissociation products. leadtolessthana 2kcal/moldifferencein thepredictedenergetics. The most important of the energetics listed in Table 4 are for This insensitivity is related to the flatness of the PES in the the transition-state structure that separates the AICI,- -CIHCO direction of the intermolecular AI-CI stretching coordinate. complex from the possible dissociation products. The energy for Moreimportant than theabsolutebond lengthsandanglesare the saddle point relative to AICI, + HCICO ranges from +3.9 the relativeshiftsin thesequantitieson going from free monomers kcal/mol (HF/CEP-31G**) to -3.6 kcal/mol (MPZJCEPto complex4 to the saddle point configuration. These changes 31G**). Once again, the effect of electron correlation on the will help to explain the large calculated decrease in activation relative energies is seen to be significant for this AE. energy for the Lewis acid catalyzed reaction. The predicted MP4(SDTQ)/CEP-31G** barrier to dissociThe complexation of the AlCl, molecule facilitates the ation with respect to AICI3- -CIHCO is 6.3 kcal/mol. This value isquiteclosetothe6.8 kcal/molpredictedfromHF/CEP-31G8* decomposition reaction in a number of ways. The first way is by an initial elongation of the C-CI bond to the HCICO monomer calculationsand isrelatively insensitiveto theinclusionofelectron uponcomplexation. At the MPZ level, thiselongation is predicted correlation. Therefore, even though the absolute energetics for toheO.ll A ( 1 . 8 9 2 ~1.781 .&)andcomeswithoutanyenergetic complexation are grossly underestimated at the H F level, the expense. In fact, as has been previously noted, this distortion is activation harrier is reasonably well predicted. This is consistent accompanied by a net stabilization of -9 kcal/mol. Obviously, with the results from the uncatalyzed decomposition reaction since the C-CI bond must he broken in this reaction, this energy discussed previously. free elongation is quite important in lowering the barrier. Given in Figure 4 are the best estimates of the ZPE corrected The second manner in which the complexation of AlCh energetics for the AlCl3 + HCICO PES. The binding energy for facilitatesdecomposition is by allowing a less stericallyconstrained the A1C13- -CIHCO complex is found to be 8.8 kcal/mol, hut
+
+
+
+
3862 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
TABLE 4
Jasien
Calculated Relative Energies for AICl3-Containing Species.
complex AICI, + HCICO AICI3--CIHCO [AICIi--CIHC0lTS AICb-HCI + CO AlCll+ HCI + CO AICli--CIH--CO AETS
HFI CEP3 1G** 0.0 -2.9 3.9 -15.9 -12.6 -18.1 6.8
MPZ/ CEP31G" 0.0 -9.5 -3.6 -11.0 -3.5 -15.9 5.9
MPZ/CEP31G'*// HF/CEP310** 0.0 -8.6 -1.8
MP4(SDTQ)/CEP3IG**// MPZ/CEP31G** 0.0 -9.3
MP2/631G*' 0.0 -9.3
-3.0
-2.9
-13.1 -5.2
6.8
6.3
-1.9
'Energies are relative to AlCh + HCICO and are not corrected for ZPE. S represents the barrier height for HCICO decomposition relative
to AICII-CIHCO.
, Figure 5. MPZ/CEP-310** structure for the C, symmetry transition state of AICII- -CIHCO. Band lengths in A. bond anglcs in degrees.
TABLE 5: Selected Structural Parameters for AICI, Comnlexes! parametera AICb--CIHCO AICb--CIHCOt AlCli + HCICO
Figure 3. MP2/CEP-3IGw* structures for the C, symmetry minimum energyconformationof(a)AICI,. -CIHand (b) AICII- -CIH- -CO. Bond lengths in A, bond angles in degrees. AlCl rHClCO
+r\
\
CXI, AI-CI, AI-CI' AI-CI" C-O C-H H--CY LCCLAI LCIX6O LChCH LCI,AICI'
1.892 (1.830) 2.481 (2.636) 2.130 (2.118) 2.104 (2.097) 1.197 (1.165) 1.099(1.087j 2.486 (2.730) 107.8 (111.1) 119.9i119.7j 107.5 (109.7) 98.2 (96.5)
2.420 (2.350) 2.239 (2.290) 2.241 (2.210) 2.112 (2.114) 1.163 (l.II9) i.wo(i.ii5j 1.776 (2.063) 98.5 (103.0) 114.9(11j.i) 85.8 (88.1) 98.9 (98.1)
.~~~
1.781 (1.765) 2.086 (2.083) 2.086 (2.083) 1.215 (1.178) i.iozii.09ii 123.8(123.1) 110.3 ( I 10.8)
Bond lengths in A, bond angles in degrees. Results are from MPZ/ CEP-310'. and HFJCEP-310.. (parentheses) calculations. 'The subscript x on CI indicates it is the donor atom from the HCICO, while the'and"supcrscripts on the Cl's indicate the in-plane and out-af-plane CI atoms on the AICI,.
AlCI~-CIHCO'
-10.9
.16.7
Figure 4. ZPE-corrected relative energies for the AICI,
+ HClCO PES.
Energiesare based upon MP~/I~EP-~IG*'//MP~/CEP-~IG** calculations. transition state by permitting the ClCH angle of HCICO to bc 86' in the catalyzed reaction as compared to 51' in the uncatalyzed (MP2/CEP-31G**). Although the final d m m position products for both the catalyzed and uncatalyzed decomposition of HClCO are HCI and CO, the origin of the CI atom in the HCI is different in these processes, as may be seen by examining Figure 5. The catalyzed reaction occurs through a CI exchange mechanism in which the H- - -CI distance of importance is that between the H atom from HCICO and a CI atom from AICI,. This particular H--CI distance is only 1.776 A in the transition-state structure for the catalyzed reaction compared to 1.847 A in the uncatalyzed. What occurs at the transition state can best be illustrated with reference to Figure 6. This figure shows the in-plane atoms and their relative motions as given by the eigenvector associated with
Fi8nre6. Relativeatomicmotionofin-planeatomsatthe AICb- -CIHCO
transition state. the imaginaryeigenvalueat the transition state. In thecatalyzcd reaction, the C?, atom is transferred from HCICO to the AI simultaneously with the CI' transfer from the AI to the H. All this occurs while the C-Cl, and C-H bonds of the HCICO are lengthening. At the transition state in the catalyzed reaction, the AI-CI'distanccisessentially thesameas theAl-Clxdistancc (2.241 vs 2.239 A), while the AI-CI" distances are considerably shorter at 2.112 A.
AlCl3-Catalyzed Decomposition of HClCO
2.75
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2863
{
2 Qo
5
m
2.25-
0
II
3
9
0
1.75-
" I -4
I
. :*: - 2
0
2
R(CCI) R(ACClx) R(Al-cl') H-CI
i
Reaction Coordinate (a.u.1
Flgm 7. Bond length changes from an IRC calculation from the AICl3-CIHCO transition state calculated at the HF/CEP-31G** level. Negative Q proceeds toward the AICI3- -CIHCO complex, while positive Q goes toward the dissociation products.
Given in Figure 7 are results from intrinsic reaction coordinate (IRC)*b%alculations at the HF/CEP-31GS* level. This figure more quantitatively demonstrates the atomic motions that are discussedaboveand give further evidencethat the transition state does indeed connect the reactant and products described. That this reaction occurs as it does may be related to the weak interaction between the C1 atom of AlC13 and the H of HCICO. A weak interaction of this type has been noted previously in AlC13 complexes.* Although not formally a H-bonding interaction, this interaction seems to be strong enough to guide the current decomposition reaction. The presence of such an interaction brings to mind a generalquestion: Could such a situation influence other AlC13-catalyzed reactions? Further investigation of this question is currently under study. The existence of the C1-exchange mechanism above could be experimentally investigated through spectroscopic means. This exchange could be probed by generation of a mixture of AlC13 and HClCO in which one of the reactants had been prepared with isotopically enriched C1. Observation of the relative intensities of H W l and H3'Cl vibrational absorption peaks that would appear can provide information as to whether or not the AlCl3-catalyzed mechanism presented here plays any role in HClCO decomposition, and if it does, what the branching ratio for the reaction is.
Conclusion Ab initio calculationshave been wed to study the decomposition reaction of formyl chloride (HCICO) in the presence and absence of AIC13. The results predict a large decrease in the energy barrier in the presence of the catalyst. The barrier for the uncatalyzed decomposition of HClCO to HC1 and CO is estimated to be 40 kcal/mol. In the presence of AlCl3, the predicted barrier is 4 kcal/mol. The large decrease owes its origin to the elongated C-Cl bond in the AlCl~-ClHCOprecursor complex which is
predicted to have a binding energy of 9 kcal/mol and to the different decomposition mechanism facilitated by the presence of the Lewis acid. The mechanism involves C1 atom exchange between the AlCL catalyst and the HCICO. The transition state for the catalyzed reaction involves the formation of a 5-membered ring in which a nonbonded H- -C1interaction guides the reaction. This structure differs dramatically from the highly distorted transition state structure found for unimolecular HClCO decomposition.
Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. The author also thanks the San Diego Supercomputer Center for an allocation of Cray-YMP/I computer time used to perform some of the calculations. References and Notes (1) Cotton, F. A.; Wilkinaon, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 217 ff. (2) Carey, F. A.; Sundbcrg. R. J. Aduanced Organic Chemistry, Part A: Structure and Mechanisms, 3rd ed.; Plenum: New York, 1990; p 229. (3) Ibid. pp 570-576. (4) Lewis, N.; Morgan, I. Synth. Comm. 1988, 18, 1783. (5) Coppi, L.; Ricci, A.; Taddei, M. J. Org. Chem. 1988, 53, 91 1. (6) Reference 1, pp 1257 ff. (7) LePage, T. J.; Wiberg, K.B. J. Am. Chem. Soc. 1988,110, 6642. (8) Jasien, P. G. J. Phys. Chem. 1992, 96,9273. (9) Wilson, M.; Coolidge, M. B.; Mains, G. J. J. Phys. Chem. 1992,96, 4851. (10) Hisatsune, I. C.; Heiclden, J. Can. J. Spectrosc. 1973, 18, 77. (11) Niki, H.; Maker, P. D.; Savage, C. M.;Breitenbach, L. P. Int. J. Chem. Kinct. 1980,12,915. (12) Tyrell, J.; Lewis-Bevan, W. J. Phys. Chem. 1992, 96, 1691. (13) Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.;Raghavachari, K.; Binkley, J. S.; Gonzalcz, C.; hfrcts, D. J.; Fox, D. J.; Whiteside, R. A.; Sceger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, S.; Pople, J. A. Gaussian 92; Gaussian Inc.: Pittsburg, PA. (14) Stevens,W. J.; Basch, H.;Krauss, M. J. Chem.Phys. 1984,81,6026. (15) Krauss, M.; Stevens, W. J. Ann. Reu. Phys. Chem. 1984,35, 357. (16) Jasien, P. G. Chem. Phys. Lett. 1992, 188, 135. (17) Jasien, P. G.; Stevens, W. J. J. Chem. Phys. 1986,84, 3271. (18) Szabo, A.; Ostlund, N. S . Modern Quantum Chemistry, 1st ed.; MacMillan: New York, 1982. (19) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods: A Guide to UsingGaussian,1stcd.;Gaussian: Pittsburgh, 1993. (20) Dupuis, M.; Lester, W. A., Jr.; Lengsfield, B. H., 111; Liu, B. J. Chem. Phys. 1983, 79,6167. (21) Davis, R. W.; Gerry, M.C. J. Mol. Spectrosc. 1983, 97, 117. (22) Herzburg, G. MolecularSpectra andMolecularStructure I. Spectra of Diatomic Molecules; Van Nostrand Reinhold New York, 1950; p 534. (23) Ibid. p. 522. (24) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 57th cd.; CRC Press: Cleveland, 1976. (25) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (26) Scheiner, S. ReviewsinComputationalChemistry, Vol.2; Lipkowitz, K. B., Boyd, D. B. (ed.),VCH New York, 1991; p 172 ff. (27) Ibid. p. 184. (28) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (29) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90,2154. (30) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
