J. Phys. Chem. 1993,97, 5553-5561
5553
Structures and Stabilities of [C2H5NAI]+ Molecular Ions. An ab Initio Molecular Orbital Study 0. M6,'vt M. Yhfiez,t A. Total,$ J. Tortajada,# and J. P. Morizur# Departamento de Qulmica, C-XZV, Universidad Autbnoma de Madrid, Cantoblanco, 28049 Madrid, Spain, and Laboratoire de Chimie Organique Structurale. Universitb Pierre et Marie Curie, CNRS URA 455,4 Place Jussieu 75252 Paris Cedex 05,France Received: November 5, I992
An exploration of the CzHsNAl+ potential energy surface has been carried out by means of ab initio molecular orbital theory at the SCF 6-31G* level. The relative stabilities of the corresponding stationary points are calculated at the MP4/6-31+G(d,p) level. Our results show that the most stable CzHsNAl+ cations in the gas phase correspond to the nitrogen adducts of C-methylmethylenimine, N-methylmethylenimine, vinylamine, and aziridine. Another stable species corresponds to the carbon association to the carbene C H ~ C N H Zwhich , has been identified as a minimum of the C Z H ~ potential N energy surface. The AF-carbon adduct of vinylamine and the CHAl+-NCH complex are more stable than the nitrogen adduct of aziridine. Cationization of aziridine and C-methylmethylenimine by Al+ parallels their gas-phase protonations. This is not the case for vinylamine since N-A1+ association is slightly preferred to C-AP association while protonation takes place at the carbon atom. Our results present significant discrepancies with the experimental outcomes regarding the relative stabilities of some of the aforementioned neutrals. As a consequence, the relative basicity of N-methylmethylenimine vs Al+ predicted by our calculations is in contrast with its relative experimental gas-phase proton affinity. The five neutrals show sizable C-N bond activations upon Al+ association. There are significant differences between the general features of the CzHsNAl+ potential energy surface and that of the analogous oxygen-containing systems [CzHdOAl]+ due to the fact that while 0-Al+ association is clearly preferred to C-Al+ association, C-Al+ adducts are only slightly less stable than the N-AI+ ones. Another main difference is related to the low stability of the transition state corresponding to the opening of the aziridine ring, which lies much higher in energy than the transition state corresponding to the opening of oxirane.
Introduction The nature of the bonding of ligands to metal ions has been an active research area in contemporarygas-phase ion chemistry.' These studiesnot only provide intrinsicthermodynamicand kinetic information in the absence of complicatingsolvent effects but are also useful for understanding the energetics of metal chemistry and catalysis. Pursuing our investigations of bond activation of organic molecules by metallic monocations, we have focused our attention on complexes with Al+ for several reasons: Firstly, although the gas-phase chemistry of alkali and transition metal ions has been studied thoroughly, thechemistry of Al+has received less attention, even though relative binding energies of .Al+ to different molecules2-8 and a basicity scale9JO relative to Al+ have been published. Secondly, the existence of aluminum hydrocarbon complexes has been recently reportede5 Thirdly, Al+ is a closedshell cation with a highly polarizable 3sz core that is responsible for some of its peculiarities7JJ' with respect to other closed-shell monocations such as Li+ or Na+. Fourthly, there is considerable biological interest since aluminum seems to be involved in a number of diseases.l2 In discussing the chemistry of specific Al+-molecule adducts, an ultimate aim must be to describethe complete potential energy surface over which the speciescan react to undergo isomerization or bond-cleavage processes. The aim of the present paper is to present an abinitioMOstudyofthe [C2H~NAl]+potentialenergy surface. The [C2HsNA1]+cations can be considered the result of Al+ association to aziridine (a) and its isomers: vinylamine (b), N-methylmethylenimine (c), C-methylmethylenimine (a), and the aminoethylidene C H ~ C N H(e). Z Hence, this study will be a natural progression in our study of bond activation of small cycles and related compounds by Al+ cationization in the gas phase, and therefore, the results presented here will be related +
Universidad Aut6noma de Madrid. Pierre et Marie Curie.
1 UniversitC
quite often, in our discussion, with those reported recently]' on the [C2H40A1]+systems, which correspond to complexes of oxirane and its isomers with Al+. This comparison will be particularly useful if one takes into account that the present investigation was restricted to a theoretical treatment due to the high toxicity of aziridine. As we shall show in forthcoming sections, many stable isomers of the potential energy surface arise from the attachment of Al+ either to nitrogen or to carbon atoms or from the insertion of this metal ion into the different bonds of the aforementioned neutrals, while other stable species involve other neutral compounds, such as methane, hydrogen cyanide, or acetylene. In this respect, we must emphasize that we have found that the carbene CH3CNH2 is a minimum of the C2HSN potential energy sttrface. This species would be the analogue of the carbene CH3COH, which is believed to be formed in the pyrolysis of pyruvic acidl3 and which has been found to be also a minimum of the CzH40potential energy ~urface.'~ We have investigated 25 [C2HsNAl]+ isomers in all, as summarized in Figure 1. Computational Details The geometries of the complexes included in this study and those of the neutral bases were fully optimized at the SCF level of theory employing suitable gradient techniques. For the particular case of carbene e, due to its biradical character, these calculationswerecarried out for both thesinglet and triplet states. We have found that the singlet lies 24.9 kcal/mol below the triplet state. Hence, in what follows, only the former was considered. These optimizations were carried out using a 6-3 lG* basis set.15 For species 21 and 24, which are minima of the potential energy surface with very weak vibrational modes, the optimization was performed by computing the analytical second derivativesat every point of the optimization. The harmonic vibrational frequencies were determined by analytical second-derivative techniques and used both to characterize stationary points of the potential surface and to evaluate zero-point energies (ZPE) (which were scaled by
0022-3654/93/2097-5553$04.00/00 1993 American Chemical Society
M6 et al.
5554 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
AE (kcal.mol-') +70
+60 +56.6
\I*
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S
+50
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+40
L
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3
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+30
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t24.0
+ II,C=CIINII~
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+
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1
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11 II
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Figure 1. Relativestabilities(in kcal/mol) of theC2HsNAI+complexes. Valuesobtainedat the MP4/6-3 1+G(d,p)//6-3lGs levelafter ZPEcorrections. These values have been repeated in Schemes I-VI.
the empirical factor 0.8916). Electron correlation effects have been taken into account by evaluating the energies at fourthorder Merller-Plesset theory1' including single, double, triple, and quadruple excitations (MP4SDTQ) keeping the coreelectrons frozen. Since it has been claimed18 that at this level the contributions from the diffuse functions may be significant, these post-SCF calculations were carried out employing the 6-3 1+G(d,p) basis set19at the SCF 6-3 1G*optimized geometries (MP4/
6-3l+G(d,p)//6-3lG*). As we shall show later, we have found some discrepancies between our theoretical estimates regarding the relative stabilities20 of the four known (a-d) neutrals and the experimental values. To check whether the aforementioned discrepancies were a consequence of the limitations of the theoretical model used, the geometries of these four species and that of e were refined at the MP2//6-31+G(d,p) level of theory and their energies were obtained at the QCISD(T)/6-3 1lG(d,p)
Structures and Stabilities of [C2H5NAl]+ level using these refined geometries. All calculations have been performed by using the Monstergausszl and Gaussian 8822 and 9OZ3series of programs. Al+ binding energies were evaluated by subtracting from the energy of each complex the energy of the neutral (in its most stable conformation) plus that of the Al+ cation. The binding energies defined in this way are affected by the so-called basis set superposition error (BSSE), which has been estimated using the counterpoise method of Boys and Bernardi.24 The characteristics of the Al+-base interactions were analyzed by means of the Laplacian of the electronic density (V2p). As has been shown by Bader and c o - w ~ r k e r s , ~V2p ~ - ~identifies ~ regions of space wherein the electronic charge of a given system is locally concentrated (V2p < 0) or depleted (V2p > 0). In general, negative values of V2p are typical of covalent bonds, while positive values of V2p are associated with interactions between closed-shellsystems, as in typical ionic bonds, hydrogen bonds, or van der Waals molecules. Therefore, an analysis of the topological properties of V 2 p ( r )will yield direct information on the nature of the interactions between the base and the Al+ ion. We have also located the relevant bond critical points (bcps), Le., points where the electronic charge density, p, has one positive curvature (A,) and two negative curvatures (Al,h2), because the values of p and V2p at these points permit us to characterize quantitatively the bonding between the base and the attaching ion and to calculate the elipticity of the bond (e = X1/X2 - 1). More importantly, we have also shown29 that an investigation of thevalues of the Laplacian of p within the bonding region permits the quantification of bond activations by cationization in the gas phase. The nonbonded maxima in the valence shell charge concentrations of a base may also provide information30about its relative base strength. These nonbonded maxima, which are associated with a lone pair of electrons, correspond to maxima in lV2pl. 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 (V2plfor them and for the most stable Al+ complexes.
Results and Discussion The calculated total energies of aziridine, vinylamine, N-methylmethylenimine, C-methylmethylenimine, the carbene CH3CNH2, and the 25 [C2H5NAl]+ molecular cations obtained at various theoretical levels are listed in Table I. The optimized geometries are available from the authors upon request. The values in Table I show that, for the five neutrals, there are no great discrepancies between the relative stabilities obtained at the MP4/6-3l+G(d,p)//6-31GZ level with respect to those obtainedat the MP4/6-31+G(d,p) levelusingtheMP2/6-31+G(d,p) optimized geometries. Hence, for the sake of consistency, the relative stabilitiespresented in Figure 1for all systems included in this study were obtained at the MP4/6-31+G(d,p)//6-31G* level, after ZPE corrections and taking isomer 1 as the reference. These values do not include the BSSE corrections. In this respect, two things should be noticed: (a) This error is small (
