J. Phys. Chem. 1992, 96, 8309-8317 TABLE III: h m of DMABN d TMABN
exptl DMABN
TMABN
O P
(D) method" and ref 27.7 P, this work
solvent ACN THF n-BuOH dioxane ACN THF n-BuOH n-hexane cyclohexane dioxane
19 21 16.5
22.5 14.5 21 I10 12.9 14.5
T, ref 17 T, ref 17 E, ref 18 P, this work T, ref 17 T, ref 17 T, ref 17 E, ref 18 E, ref 18
= pressure shift; T = thermochromic shift; E = electrooptical.
previously determined by the measurement of thermochromic shift" and the electrooptical measurement'* in different solvents. Irrespective of the different experimental methods, the TICT state of TMABN possesses a higher dipole moment than that of DMABN. pTICT of TMABN tends to have a larger dependence on solvent. In such a case solvatochromic observation is not relevant to the basis for the calculation of p .
Concluding Remarks It should be emphasized that the pressure shift is a fundamental and useful experimental observation which must be considered for the study of the influence of the medium on the energy states of molecules. The pressure shift is large enough to determine the dipole moment of the TICT state with considerable accuracy. While in the present work the thermochromic shift is not sufficient for the analysis, the M T Ivalues ~ obtained from the pressure effect resulted in a good agreement with previous values determined by different methods.I7J8 Further, it is found that the TICT state
8309
of TMABN has a lower ~ T I C T value than that of DMABN and shows a comparatively large variation with solvents. In such a case the solvatochromic method is not relevant to use.
Acknowledgment. This research was supported in part by a Grant-in-Aid for Scientific Research No. 04640442 from the Ministry of Education, Science, and Culture. egist try NO. DMABN, 1197-19-9;TMABN, 60082-00-0; EIN, 89937-22-4; EICEE, 78357-15-0.
References and Notes (1)Grabowski, Z.R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, J.; Baumann, W. Noun J. Chim. 1979,3,449. (2)Rettig, W. Angew. Chem., Int. Ed. Engl. 1986,25, 971. Lippert, E.; Rettig, W.; Bonacic-Koutecky,V.; Heisel, F.; Miehe, J. A. Adv. Chem. Phys. 1987,68, 1. (3) Suppan, P. J . Photochem. Phorobiol., A 1990,50, 293. (4) Hagan, T.; Pilloud, D.; Suppan, P. Chem. Phys. Lett. 1987,139,499. Ghoneim, N.; Rohner, Y . ;Suppan, P. Faraday Discuss. Chem. Soc. 1988,86, 295. ( 5 ) Baumann, W. Z . Naturforsch. 1981, 36a, 868. (6) Hara, K; Suzuki, H.; Rettig, W. Chem. Phys. Lett. 1988, 145,269. (7) Wermuth, G.; Rettig, W.; Lippert, E. Der. Bunsenges. Phys. Chem. 1981,85,64. Rotkiewicz, K.;Grabowski, Z. R.; Krowczynski, A,; KUhnle, W. J . Lumin. 1976,12 & 13,877. (8) Hara, K Arase, T.; Osugi, J. J . Am. Chem. Soc. 1984,106, 1968. (9)Hara, K,Morishima, I. Rev. Sci. Instrum. 1988,59, 2397. (10) Wermuth, G.;Rettig, W. J. Phys. Chem. 1984,88,2729. (1 1) Hara. K, Obara, K. Unpublished data. W. In Excited States; Lim, E. C., Ed.; Academic Press: New
(f4) Robertson .W. W.. J . C h e k Phys. 1960,33, 362. (15) Robertson, W. W.; King, Jr., A. D. J . Chem. Phys. 1961,34,151 1. (16) Srinivasan, K.R.; Kay, R. L. J. Solution Chem. 1977,6,357. (17) Suppan, P.;Vauthey, E., private communication. (18) Baumann, W. Proc. Meet. Photoinduced Electron Transfer Relat. Phenom., Kyoto 1985,28.
Experimental and Theoretical Study of C,H,OAI+ Complexes in the Gas Phase J. Tortajada,*s+A. Total,+J. P. Morizur; M. AlcamI,t 0. M6,t and M. YBiiez*qf Laboratoire de Chimie Organique Structurale, UniversitO Pierre e! Marie Curie, CNRS URA 455, 4 Place Jussieu 75252, Paris Cedex 05, France, and Departamento de Quimica C-XIV, Universidad Autdnoma de Madrid, Cantoblanco, 28049-Madrid, Spain (Received: January 29, 1992)
SCF calculations with the 3-21G, 6-31G*, and 6-31+G(d,p) basis sets have been performed to investigate the structure and energetics of C2H40Al+complexes. Electron correlation effects on their relative stabilities were taken into account by using the Merller-Plesset perturbation theory. At the same time, the structures of these complexes, prepared by gas-phase electrophilic addition, have been investigated by means of tandem mass spectrometry with collision-induced dissociation. Our results show that the most stable C2H40Al+cations correspond to oxygen association to acetaldehyde, vinyl alcohol, and oxirane, respectively. Other isomers corresponding either to the insertion of Al+ into the bonds of these neutrals, as [CH3-Al-COH]+, or to the interaction of ethylene, acetylene and ketene with OAl+, OH2A1+,and A1H2+,respectively were also found to be very stable. Cationization of oxirane and acetaldehyde by Al+ association parallels gas-phase protonation, while this is not the case for vinyl alcohol. Al+ association produces significant C-0 bond activation of the three neutrals. This effect is particularly important for oxirane, which may undergo a C-O breaking process yielding either ethylene + OAl+ or the acetaldehyde-Al+ complex (which correspondsto the global minimum). Al+ exhibits some peculiarities with respect to other closed-shell monocationsas Na+, since it is able to bond to C or H yielding dicoordinated species (which are not stable when Al+ is replaced by Na+).
Introduction Investigation of the properties and reactions of metal ions and metal-containing compounds represents one of the most active research areas in contemporary gas-phase ion chemistry. This rapidly increased activity has been stimulated on one hand by the need for basic physical data for organometallic species and on the other hand by the rich reactivity that metal ions have shown with 'Universitb Pierre et Marie Curie. Univenidad Autdnoma de Madrid.
*
0022-3654f 9212096-8309$03.0010
different organic compounds.' In this respect, the advent of pulsed-laser techniques to generate gas-phase metal species together with ion cyclotron resonand (as well as other experimental setup as flowing afterglow,13high-pressure mass spectrometry?-" and pulsed molecular beams5) has yielded valuable information on the dynamics of metal ion binding with neutral ~ubstrates.~*~~' These experimental investigations, in particular for protonated species, have often been complemented by ab initio calculations on the structures of the different molecular ions,8-20 offering a careful analysis of the potential energy surface9J9associated to 0 1992 American Chemical Society
8310 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
Tortajada et al.
+.q
a given ionic system. However, studies of this sort are ~ c a r c e r ~ ~ - ~AE ~ (kcal.mol"1 for metal ion complexes, as they have been basically devoted to estimating the corresponding metal ion binding energies. Among the different metal ions, Al+ presents a 2-fold interest: (a) it is a closed-shell monocation with a highly polarizable 3s2 core that is responsible from some of its p e c u l i a r i t i e ~with ~~,~~ respect to other closed-shell monocations as Li+ or Na+; (b) it is of considerable biological interest since aluminum seems to be involved in a number of disease^.^' Some studies concerning the relative binding energies of Al+ to different organic molecules have been p ~ b l i s h e d , 2 ~ *and ~ ~ aJ ' basicity scale relative to the Al+ acid was established2*and compared to those of Li+ and H+. Complexes of saturated and unsaturated 0,N, and F bases with Al+ have been theoretically investigated,2'~~~ and the calculated absolute AI+ affinity of ethanol has been taken as reference to establish a basicity scale.M Recently the stabilities of aluminum hydrocarbon complexes, as CH2Al+ and CH3Al+,whose existence has been experimentally demon~ t r a t e d , by ~ neutralization reionization mass spectrometry (NRMS) experiments, have been reported also.38 As part of our investigation of bond activation of organic +lo4 19.7 molecules by metallic monocations, we report in this paper a study of the [C2H40Al]+potential energy surface. This particular *' molecular ion has been chosen because it can be produced by AI+ association with three different neutrals: oxirane, acetaldehyde, and vinyl alcohol, which are stable in the gas phase (although the latter is not stable in solution). On the other hand, there are many possible isomers arising from the attachment of AI+ either to oxygen or to carbon atoms or from the insertion of this metal ion into the different bonds of the neutrals. We have investigated 19 different [C2H40AI]+isomers in all, as summarized in Figure -20 1. Some of them, such as forms 17 and 18, involve other neutral species (the former corresponds to the interaction between ketene and AIH2+and the latter to that of Al+with methane and carbon monoxide), Others are involved in the isomerization processes between different stable ions. In this respect preliminary exFigure 1. Relative stabilities (in kcal/mol) of C2H40Al+complexes. perimental results are also presented. In particular the structures Values obtained at the MP3/6-31G1//3-21G level after ZPE corrections. of the [C2H40Al]+ions prepared by gas-phase electrophilic addition reactions have been investigated by means of tandem mass complexes included in this study. All calculations have been spectrometry with collision-induced d i s s ~ c i a t i o n(CID). ~~ performed by using the MONSTERGAUSS~and GAUSSIAN 8s4' and 9 0 series ~ ~ of programs. Computational Details AI+ binding energies were evaluated by subtracting from the energy of each complex the energy of the neutral (in its most stable The geometries of the complexes included in this study and of conformation) plus that of the Al+ cation. The binding energies the neutral bases were fully optimized at the SCF level of theory defined in this way are affected by the so-called basis set suemploying suitable gradient techniques. These optimizations were perposition error (BSSE),which has been estimated using the carried out using a 3-21G basis set.& The geometries of the three counterpoise method of Boys and Bernardi.49 neutrals (a, b and c) and their more stable Al+-complexes 1,2, The characteristics of the Al+-basc interactions were analyzed and 3 as well as complex 18 were reoptimized at the 6-31G* level4I by means of the Laplacian of the electronic density. As has been to check whether the inclusion of polarization functions in the basis shown by Bader and worker^,*^^ Vzp identifies regions of space set would affect, in a significant way, the description of these wherein the electronic charge of a given system is locally consystems and in particular their relative stabilities. For species 18, centrated or depleted. In the first situation Vzp < 0, whereas in which is a minimumof the potential energy surface with very weak the latter V2p > 0. In general, negative values of V2p are typical vibrational modes, the optimization at both levels (3-21G and of covalent bond, where charge is concentrated in the interatomic 6-31G*) was carried out by computing the analytical second region leading to a lowering of the energy associated with the derivatives at every point of the optimization. As shall be shown predominance in this region of the potential energy density. In below, 6-3 1G*//6-3 1G* relative stabilities differ very little from contrast, positive values of Vzp are associated with interactions those obtained with the same basis set at the 3-21G optimized between closed-shell systems, as in typical ionic bonds, hydrogen geometries (6-31GS//3-21G). The harmonic vibrational frebonds, or van der Waals molecules, where the electronic charge quencies were determined at both levels by analytical secondis depleted in the interatomic region, leading to a predominance derivative techniques and used both to characterize stationary of the kinetic energy density. Therefore, an analysis of the topoints of the potential surface and to evaluate zero-point energies pological properties of V2p(r) will yield direct informationon the (DEwhich were scaled by the empirical factor 0.8Y2). Electron nature of the interactions between the base and the Al+ ion. We correlation effects have been taken into account by evaluating the have also located the relevant bond critical points (bcp), i.e., points energies at second- and third-order Merller-Plesset theorf, (MP2 and MP3), keeping the core electrons frozen. Since it has been where the electronic charge density, p, has one positive curvature (A,) and two negative curvatures (A1,A2),because the values of claimed" that at this level the contributions from the diffuse functions may be significant, post-SCF calculationsfor the three p and V2p at this point permit us to characterize quantitatively the bonding between the base and the attaching ion. For instance, neutrals and complexes 1,2,3, and 18 were carried out employing the ratio between the two negative curvatures along axes perthe 6-3 1+G(d,p) basis set45at the 6-3 1G* optimized geometries pendicular to the bond yields information on the ~ - ? character r (MP2/6-31+G(d,p)//6-31G*). Since the results obtained do not of the bond. For single and triple bonds, where the electronic differ significantly from those found at the MP3/6-31G*//3-21G density around the bond axis presents cylindrical symmetry, both level, this was the computational level adopted for the remaining
-I +iH'cHO
1
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8311
Gas-Phase C2H40Al+Complexes TABLE I: Totd Emriges (au) and ZPE (kcnl mol-') 3-21G//3-21G -392.436 160 -392.480 483 -392.456 546 -392.418 839 -392.378 414 -392.384 158 -392.358 991 -392.329099 -392.348118 -392.361 653 -392.3 15 261 -392.396 408 -392.379 587 -392.280 41 5 -392.418 109 -392.423 493 -392.409 130 -392.441 905 -392.364 229 -392.349010 -392.403 551 -392.385 208 -240.348 308 ~~
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 9
b C
Al'
ZPE 35.3 34.2 34.6 31.9 34.8 32.3 33.6 33.6 31.5 32.9 31.4 33.6 33.4 31.9 34.7 32.2 28.4 30.1 30.9 34.3 33.4 33.4
6-31G*//3-21G -394.514023 -394.624 196 -394.581 308 -394.540 159 -394.521 185 -394.532 961 -394.506 013 -394.415 103 -394.487 121 -394.491 186 -394.470212 -394.546 161 -394.533 198 -394.433 846 -394.561 491 -394.566 199 -394.554 051 -394.593 509 -394.496 689 -394.514685 -394.565 100 -394.536 196 -241.652 164
MP2/ 6-31G*//3-21G -395.043 381 -395.082098 -395.041 499 -395.012989 -395.018 252 -394.982 653 -394.964 052 -394.910932 -394.949 334 -394.936 150 -394.982 158 -395.003 550 -395.006 0 16 -394.928 322 -395.033 199 -395.032 508 -395.010 41 3 -395.042402 -394.945 543 -394.980 105 -395.024 595 -394.993 393 -241.678131
curvatures are identical. However, this symmetry is lost when the bond presents a doublebond character, since the charge density is preferentially distributed in a particular plane containing the bond axis. This is quantitatively measured by the elipticity c = XI/X2 - 1, which, according to the previous arguments, should be zero for single and triple bonds and different from zero for double bonds or single bonds with a given T character. The nonbonded maxima in the valence shell charge concentrations of a base may also provide information" about its relative base strength. These nonbonded maxima, which are associated with a lone pair of electrons, correspond to maxima in IVzpI. Therefore, in an attempt to provide information on the intrinsic basicity of the neutrals included in this study, we shall also analyze the critical points of lV2pI for them and for the most stable Al+ complexes.
Experimental Section The mass spectrometric experiments were carried out on a VG-ZAB 2F mass s p e c t r ~ m e t e r . ~ ~ The aluminum-organic compound adduct ions were generated in the combined EI-CI ion source under chemical ionization conditions (source temperature 200 OC, 140-eV ionization energy emission, current 1 mA, and source housing pressure lo4 Pa) by addition of Al+ cation with suitable neutral precursors. Al+ was produced by electron impact from AlC13, which was introduced into the ion source via a direct inlet probc. The organic compound was introduced in the ion source via a heated inlet system at 50 OC. The metal ion adduct complexes formed with the organic substrate are mass selected (using an acceleration voltage of 8 kV and a mass resolution of 2500 in the 10% valley definition) with the magnetic analyzer. Metastable and collision-induced dissociations occurring in the second field-free region (FFR) between the magnetic analyzer and the electric analyzer are monitored by scanning of electric analyzer. The metastable ion reactions were studied by mass-analyzed ion kinetic energy (MIKE) t e c h n i q ~ e s .Collision-induced ~~ dissociations (CID) were performed by admitting argon (as a target gas) into the second drift region collision cell to reduce the parent ion signal to 70%. Commercially available oxirane, acetaldehyde, and acetaldehyde-d, were used without purification.
Results and Discussion The calculated total energies of acetaldehyde, oxirane, and vinyl alcohol and the 19 [C2H40AI]+molecular cations obtained at various theoretical levels are listed in Table I. The optimized
MP3/ 6-3lGS//3-21G -395.065112 -395.106236 -395.013 941 -395.035 219 -395.032 392 -395.009 666 -394.989 116 -394.984 294 -394.914911 -394.968 548 -394.913 158 -395.031 581 -395.022 851 -394.925 093 -395.051 542 -395.050257 -395.028 952 -395.065 195 -394.969 061 -395.004215 -395.041 199 -395.020809 -241.681 133
MP2/ 6-3lG*//6-31GS ZPE 6-31G'G(d,p)//6-31G* -394.518 564 35.5 -395.083 116 -394.626331 34.1 -395.121 236 -394.583 415 34.4 -395.091 526
-394.591 609
30.4
-395.080 902
-394.520 120 -394.568 129 -394.538 153 -241.652164
34.8 33.5 33.8
-395.023 532 -395.065 936 -395.043 919 -241.619342
geometries are available from the authors upon request. As mentioned above the values in Table I show that, in general, there is a fairly good agreement between the relative stabilities obtained at the 6-31G*//6-31GS and at the 6-31G*//3-21G level. Similarly, there are not significant discrepancies between the MP2/6-3 1+G(d,p)//6-3 lG* relative stabilities and the MP3/ 6-31G*//3-21G ones. Hence, for the sake of consistency, the relative stabilities presented in Figure 1 for all systems included in this study were obtained at the latter level, after ZPE corrections and taking isomer 1 as reference. These values do not include the BSSE corrections. In this respect two things should be noticed: (a) This error is practically identical (=2.0 kcal/mol) for oxirane, acetaldehyde, and vinyl alcohol and therefore does not affect their relative Al+ binding energies. (b) For some species, such as 16, where Al+ is bonded to an oxygen atom and to a hydrogen simultaneously, it is not possible to determine this error quantitativelyby the counterpoise method, since the neutral system obtained by removing the Al+ cation would be a quite unstable species. Nevertheless, in view of the relative values obtained for this error when this problem is not present, we can conclude that its effect should not change the relative stability trends found for the different cations investigated. Stable OxygewAV Adducts 1-3. One of the aims of our study was to investigate the possible bond activation mechanisms involved in the oxirane Al+, acetaldehyde + Al+, and vinyl alcohol Al+ reactions. We shall focus first on the oxygen adducts (1,2, and 3, respectively) which turn out to be the most stable species. The calculated stability difference between acetaldehyde and oxirane (27.9 kcal/mol) is in very good agreement with the experimental value" (27.0 kcal/mol) and very similar to that between l and 2 (26.5 kcal/mol). This means that both isomers exhibit practically the same gas-phase basicity versus Al+. A similar behavi06~has been observed upon protonation, even though oxirane and acetaldehyde have quite different electronic structures and, more importantly, in both systems the oxygen atom exhibits quite different bonding patterns.s8 Since, as indicated above, it is reasonable to assume that the 0 lone pairs should be involved in the stabilization of the 0-Al' complexes through charge transfer and/or polarization effects, we have investigated the characteristics of the oxygen lone pairs of both neutrals in order to try to understand this behavior. We have recently showd4 that when the attaching ion is H+, this charge-transfer ability can be quantitatively measured by the values of the maxima of IV2pI corresponding to lonepair nonbonded charge concentrations. For oxirane, acetaldehyde, and vinyl alcohol these values are summarized in Table I1 and show, coherently with the previous rea-
+
+
8312 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
Tortajada et al. of the complex and the neutral molecule permits easy detection of bond activation effects. The Laplacian of the charge density of oxirane-Al+ complex evaluated at the C-O and C-C bcp (see Table 111) shows that, upon Al+ association, the C-O bonds of the three-membered ring became depopulated (as shown by a much less negative Laplacian) while the Laplacian at the C-C bonding region becomes more negative, showing an increase of the charge density within this area. The following explanation is proposed: when the oxiranc-Al+ is formed, there is strong polarization of the oxygen lone pairs of the base toward the Al+ and hence a decrease of the charge density at the oxygen atom (see Table 11). Since the oxygen is highly electronegative, it recovers part of this charge by depopulating the C-O bonds. The concomitant polarization of the C atom valence shell produces also a slight increase of the charge density into the C-C bonding region. This interpretation is consistent with the shifts observed in the corresponding harmonic vibrational frequencies. The vibrational mode involving C-O stretching displacements is shifted by 107.2 cm-'to lower frequency values, while that involving C-C stretching displacementsincreases by 45.2 an-'.Therefore, one may conclude that Al' association produces a noticeable bond activation effect of the C-O bonds of oxirane, which became weaker. Is this activation enough to produce a C-O bond breaking? The answer to this question would require a dynamical treatment of the process, which is beyond the scope of this paper; but it must be noticed that the intermediate 6, which corresponds to this process, lies 32.2 kcal/mol above 1 and that the ethylene4Al+ complex 11 is a h 54.2 kcal/mol above 1. Taking into account the decrease of the vibrational frequency mentioned above and the fact that oxirane presents quite a high ring strain, such a process would perhaps be possible if a selective accumulation of energy takes place on these bonds yielding ethylene + OAP. Similar charge polarizations take place for acetaldehyde and vinyl alcohol, although the charge depletion of the C-O bond of the former is less important in relative terms (see Table 111). As a consequence we conclude that C and 0 atoms of acetaldehyde remain still tightly bound after Al+ association and that the C-C bond becomes slightly reinforced. The calculated harmonic frequency shifts (Au = -171 cm-' for the C-O stretching, Au = +27 cm-' for the C-C stretching) are in good agreement with these results. Values in Table I11 show that the C-O activation
TABLE II: Maxima of IV*ppI of Nonbonded Charge Concentrations Associated with Oxygen Lone Pairs (p in e/au3 and V 2 p in e/aus) and AI+ B d h g Eoergies" (E(A1') in kcal/mol) P V2P E(A1') -7.371 36.7 oxirane 1.019 -6.308 oxirane-Al' 0.973 acetaldehyde 1.005 -7.021 34.1 acetaldehyde-Al' 0.962 -6.1 12 vinyl alcohol 0.979 -6.386 29.3 vinyl alcohol-AIt 0.960 -6.038 ,
'Values obtained at the MP2/631+G(d,p)//6-31-GS level after
ZPE and BSSE corrections.
soning, that oxirane and acetaldehydtmxygen lone pairs are quite similar, the charge density at the former being slightly greater than at the latter, in good agreement with the fact that oxirane is only slightly more basic than acetaldehyde. Similarly, our results indicate that the intrinsic basicity of vinyl alcohol with respect to Al' (see Table 11) is much less than that of oxirane and acetaldehyde since complex 3 is only 5.9 kcal/mol more stable than 1, while neutral vinyl alcohol is 11.3 kcal/mol more stable than oxirane. Again, this energetics is in good agreement with the topological description of the corresponding oxygen lone pairs, but in clear contrast with the behavior upon protonation. Although the experimental gas-phase proton affinity of vinyl alcohol is not known, the theoretically estimated value59 is greater than that of oxirane. This is so because vinyl alcohol protonates on carbon rather than on oxygen.s9 In fact, oxygen protonation would yield a complex analogous to 3 while carbon protonation would yield the same cation (similar to 2) as protonated acetaldehyde. Such a situation does not occur when the reference acid is Al+. Al+ association at the carbon of vinyl alcohol yields a complex, 15, which is about 31.O kcal/mol less stable than the corresponding acetaldehydc-Al+complex 2. This constitutes another case where not only the intrinsic basicity but the nature of the basic center depends on the reference acid. It must be also mentioned that our calculated value for the acetaldehyde-Al' binding energy is in fairly good agreement with previously reported ~alues.~~~~~ As has been shown previously,6' comparison between the topological characteristics of the Laplacian of the charge density
TABLE 111: Bond Characteristics of Oxirane (a), Acetaldehyde (b), Vinyl Alcohol (e), and the Different AI+ Complexes (p in e/ad, V 2 p in e/aus) C-O c-c &AI C-AI RCP comwund
P
v2P
P
a
0.232 0.4 12 0.27 1 0.196 0.368 0.201
-0,345 0.535 -0.146 -0.008 0.331 0.1 16
9 10
0.212 0.294 0.376 0.144 0.231 0.370 0.21 1
-0.442 -0.393 0.097 -0,325 -0,325 0.507 -0.023
0.263 0.272 0.369 0.277 0.283 0.373 0.416 0.232 0.304
11 12 13
0.318 0.409
0.409 0.620
14 15 16
0.311 0.231
17
b C
1 2 3 4 5 6 7
8
v2P -0.652 -0.777 -1.334 -0.765 -0.883 -1.361 -1.290 -0.558 -1.053
0.297 0.342 0.278
-0.899 -1.106 -0.825
-0.063 0.383
0.286 0.343 0.372
-0.903 -1.186 -1.326
0.404
0.508
0.349
-0.156
I8
0.460
1.200
19
0.247
0.437
"Ring critical point. bAl-H bond. CH-C=C
bond. dAl-C=C
P
0.057 0.052 0.049 0.02W 0.104 0.076
V2P
0.436 0.417 0.303 0.056c 0.975 0.775
0.059 0.043 0.129
0.550 0.238 1.306
0.126
1.218
0.116 O.09Ob 0.056 0.087b 0.022 0.0056 0.109
1.328 0.227b 0.541 0.243b 0.038 0.0126 1.173
bond.
P
VZP
0.070
0.153
0.054
0.169
0.070 0.076 0.027 0.048d 0.042 0.0896 0.082b 0.069 0.033
0.181 0.312 0.033 0.043d 0.085 O.31Ob 0.2206 0.281 0.016
0.090
0.286
0.203
VZP 0.259
0.187
0.323
0.044
0.174
0.037
0.155
0.024
0.050
P
The Journal of Physical Chemistry, Vol. 96, NO. 21, 1992 8313
Gas-Phase C2H40Al+Complexes TABLE I V Topological Features of the *AIt compound
1 2 3 4
S 6 9 10 11 14 16 17 18 19
Pc
+AIt 0.057 0.052 0.049 0.052 0.104 0.076 0.059 0.043 0.129 0.126 0.116 0.090' 0.056 0.087' 0.022 0.005a 0.109
V2P, Bonds 0.436 0.417 0.303 0.349 0.975 0.775 0.550 0.238 1.306 1.218 1.326 0.320' 0.541 0.243' 0.038 0.012' 1.173
and C-AIt Bonds
R
(4
1.869 1.882 1.966 1.934 1.702 1.735 1.823 2.025 1.603 1.603 1.603 1.550' 1.832 1S63" 2.399 3.193' 1.638
C-AI+ Bonds
S I 8 9
10 12
13 1s 19
0.070 0.054 0.070 0.076 0.069 0.027 0.042 0.089 0.082 0.033 0.090
0.153 0.169 0.181 0.312 0.218 0.033 0.085 0.310 0.220 0.015 0.286
2.030 2.199 2.050 1.961 2.063 2.538 2.340 1.95 1 2.040 2.475 1.95 1
"AI-H bond.
follows the sequence vinyl alcohol > acetaldehyde > oxirane, although in the latter both C-O bonds are activated. The ability of the neutrals to be polarized is also mirrored in the characteristics of the O-AP bonds of species 1,2, and 3. As shown in Table IV the greatest charge concentration at the bond critical point corresponds to oxirane-AI+ 1, which presents the shortest O-AP bond, while the opposite is true for vinyl alcohol-AI+ 3. This is a striking feature common to all species considered, in the sense that *Al+ and C-Al+ bond lengths vary within a wide range of values (from 1.60 to 2.40 A the former and from 1.95 to 2.54 A the latter), indicating that the bonds in which the AI+ cation participates are quite sensitive to the molecular environment and reflect quite different electronic charge distributions. This interpretation is ratified by the corresponding topological analysis. As shown in Table IV, there is a clear relationship between the charge density or its Laplacian at the 0-Al+ or C-Al+ bond critical points and the bond length. It is significant that this charge density is twice as great for species 14 or 16, for instance, than for the oxirane-Al+ complex 1, which accordingly presents a 0-Al+ bond that is about 0.26 A longer. A similar analysis can be applied to forms 5, 13, and 19, where both O-Al+ and C-AI+ bonds present large charge densities at the corresponding bond critical points. As we shall discuss below, this is related to the availability of low-lying empty 3p orbitals in Al+ and to the polarization of the 3s2 electronic core. For the discussion which follows it is convenient to consider the remaining cations investigated as grouped in three sets, depending on the neutral from which they can be considered derived. A fourth set will contain those species which cannot be directly related to any of the three neutrals. [OxiraneAlJ+Association: 1 pad 510. As indicated above, the most stable species found upon AI+ association corresponds to adduct 1, which is 37.6 kcal/mol more stable than the isolated subsystems. The ring opening of 1 yields structure 6, which is an intermediate in the 1-2 isomerization by hydrogen transfer. It should be emphasized that form 6 is found to be a minimum of the potential energy surface at the 3-21G level but not at the 6-31G* level, similarly to what has been reported by Ford and Smith62
regarding a similar structure in the C2H50+series. Therefore, parallel to what has been proposed62for protonated oxirane and acetaldehyde,we conclude that species 6 is actually the transition state between forms 1 and 2. Insertion of Al+ on the C-O bond of oxirane yields a stable four-membered ring 5, which, as we have mentioned above, presents very strong 0-A1 and C-A1 linkages (see Table IV). The result is that form 5 is 11.8 kcal/mol more stable than 6, showing that Al+ prefers double coordination. It is not unlikely that while closed-shell monocations, such as Li+ or Na+, can bridge between two basic centers of a given molecule, the dicoordination found in 5 is an important peculiarity of Al+. In general, the net positive charge on A1 at the equilibrium conformation of the complex is smaller than 1. However, in form 5 (and also in forms 13 and 16) this net charge (evaluated at the 3-21G level) is greater than 1. This seems to indicate that the 3s2 subshell of Al+ may be easily polarized. Hence, it would be reasonable to expect that complexes as 5, 13, and 16 should be much less stable if the metal cation were Na+, since polarization of a 2p6 closed-shell should be more difficult. To confirm this expectation we have optimized, at the 3-21G level, the geometries of species 5, 13, and 16 replacing Al+ by Na+ (which we shall call 5', 13') and 16', respectively). Our SCF results show that while 5 and 16 are 17.2 and 31.0 kcal/mol more stable than oxirane Al+ (see Figure l), structures 5' and 16' are 46.2 and 186.3 kcal/mol less stable than oxirane Na+. Form 13' is not stable and evolves without activation to yield the corresponding G N a + adduct. Of course,one should expect an enhanced stability of Al+ complexes with respect to similar Na+ adducts, due to the smaller size of Al+ cation; however, this effect alone cannot account for the energy differences mentioned above and very likely, some other effects as the participation of the low-lying empty 3p orbitals of Al+ as well as the 3s polarization must be i n v ~ l v e d . ~Our ~.~ topological analysis clearly shows, however, that the Al+-C2H40 interaction is basically electrostatic, so we cannot strictly speak of a sp hybridization of the Al+. Insertion of Al+ on the C-C bond yields structure 8, which is not a minimum of the potential energy surface but a transition state. The imaginary frequency corresponds to a rotation of the A1-CH2 group, which would yield its conformational isomer 7. This is corroborated by the fact that the C-C bond distance is the same for both species. A topological analysis of this conformer shows that the AlCH2-0 bond should be considerably weaker than the H2C-0 bond (see Table 111), since the charge density at the former is almost 3 times smaller than that at the latter. This would imply that a fragmentation mechanism of this ion ought to yield formaldehyde + CH2Al+. In this respect, it must be also taken into account that 7 can also be obtained by a 0-A1 breaking process of 5. The relative stabilities of 5, 7, and 8 are consistent with the topological features of their charge distributions. According to our results, oxirane has a quite strong C-C bond, so that at a crude level of approximation it could be considered as the result of the interaction between ethylene and an oxygen atom. Therefore, breaking the C-C bond greatly destabilizes the system, and which accounts for the low stability of forms 7 and 8. On the contrary, in 5 the C-C bond remains practically unchanged as well as one of the C-O bonds, while the other C-O bond is replaced by a 0-A1-C linkage that, as indicated above, is particularly stable. Consistent with these arguments is the low stability of species 9, which is the result of a C-C bond breaking process of 5. Ion 10 which arises from a 1, 3 proton transfer in 5, is also unstable due to a weakening of both the 0-Al and the C-AI linkages (seeTable 111). The energy difference between 6 and 7 indicates that the Al+-oxygen affinity is markedly greater than Al+-carbon affinity. [Acetaldehyde-AI]+ Association: 2 and 12-14. As mentioned above oxygen-Al+ association of acetaldehyde leads to the most stable C2H40Al+complex reflecting the high stability of the corresponding neutral. We find no minima corresponding to C attachment because they evolve without activation barrier to yield the oxygen-Al+ derivative. On the other hand, insertion of Al+ into the C-C bond yields a stable cation (13) where, as in 5 and 16, A1 bears a (3-21G) net charge greater than unity. It must
+
+
8314 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 be noticed that a fragmentation of this species would be expected to yield AlCH3,'+ which has recently been reported.38 In 13 both C-Al+ bonds are relatively strong but not equivalent, the Al-CH3 linkage being slightly weaker than the Al-CHO one (see Table 111). Insertion of Al+ into the C-O bond yields a much less stable structure 14,since the breaking of the C-.O bond is energetically very expensive and is not counterbalanced when the weaker C-Al and 0-A1 bonds are created. We have also found that 12 is a minimum of the potential energy surface. It corresponds to the Al+ association to the carbene H3C-C-OH, which is believed to be formed in the pyrolysis of pyruvic acid63and which has been found to be a minimum of the C2H40potential energy surface.64 In this respect it should be noticed that the energy difference between H3C-COH and acetaldehyde (or vinyl alcohol) decreases appreciably upon Al+ association, which suggests that bonding of the metal cation to the divalent carbon atom considerably stabilizes the carbene. [Vinyl AlcohoCAl]+ Association: 3,15, and 16. Once again calculations show that oxygen association is preferred to carbon association, and hence 0-complexed vinyl alcohol 3 is -10 kcal/mol more stable than 15. Our topological analysis shows that interaction with carbon leads to a very small charge polarization, producing one of the longest C-AI+ found in the series. Concomitantly the C - C double-bond character of vinyl alcohol is retained in species IS,which presents a high ellipticity (0.36) comparable to that of the neutral alcohol (0.47). The insertion of Al+ into the OH bond yields a stable structure 16,which, as indicated above also corresponds to a case where AI presents a net (3-21G) charge greater than unity. In this respect it should be noticed that a 1,2 hydrogen transfer destabilizes considerably this molecular ion yielding species 6,while 1.3 hydrogen transfer would yield the most stable species 3. Species 4, 11, and 17-19. In our search of the C2H40Al+ potential energy surface, we have found five additional minima, which cannot be directly related to oxirane, acetaldehyde, or vinyl alcohol. The first three of these minima correspond to interaction between three different ions with acetylene, ethylene, and ketene, respectively. The other two can be considered as derivatives of 13 species 18 is the result of a 1,3 hydrogen transfer, while 19 is the consequence of the appropriate rotation of the COH fragment. The least stable of the five corresponds to a *-complex, 11, between ethylene and NO+ cation (a cation that is produced6s by the reaction Al+ + 02).Atomic A1 has been observed to yield stable r-bonded complexes with ethylene,&for which the predicted Al binding energy is around 11 kcal/mol. For the ethylene-AlO+ complex we calculate a binding energy greater than 50 kcal/mol, for attachment of the O-AP moiety to the double bond. Species 4 is also a stable (only 15.7 kcal/mol less stable than 1) *-bonded complex between acetylene and H20Al+cation. As shown in Table I11 structure, 4 has a bond critical point between one of the hydrogen atoms of the A10H2+moiety and the r-system of the C=C linkage. Structure 17 is the only one in which Al+ appears tricoordinated. It must be observed, however, that the 0-Al+ bond is similar to those found for other species with a C=O group and that the AI+-H bonds are also analogous to that in species 16. Isomer 18 is the third most stable of the whole series, even though our topological analysis shows that both the &Al+ and the AI+-H bonds are particularly weak. Nevertheless this is not a surprising result, since it reflects the relative high stability of the two neutrals involved, (methane and carbon monoxide). A rotation of the
