G2 Molecular Orbital Investigation of Torsional Barriers in H 2 AldXHCH 3 and H 2 AldYCH 3 (X ) N, P, and As; Y ) O, S, and Se) Systems Abraham F. Jalbout Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148-2820 Abderrahim Boutalib* De ´ partement de Chimie, UniVersite ´ Cadi Ayyad, Faculte ´ des Sciences Semlalia, B.P. 2390 Marrakech, Morocco ReceiVed: March 28, 2003; In Final Form: May 28, 2003 Structures of H 2 AldXHCH 3 and H 2 AldYCH 3 (X ) N, P, and As; Y ) O, S, and Se) systems were investigated using ab initio method at the G2 level to study the conformational preferences of the methyl group. In all of the molecules, the eclipsed C s symmetry arrangement (one of the C-H bonds of the methyl group eclipses AldX(Y)) conformer is found to be more stable than the staggered C s symmetry arrangement (the C-H bond is trans to AldX(Y))conformer. The G2 energetic results show that the 3-fold methyl rotational barrier is found to decrease as the electronegativity of X(Y) increases. They also show that this 3-fold methyl rotational barrier decreases when descending in the corresponding periodic table column, from nitrogen (or oxygen) to arsenic (or selenium) atoms. A qualitative argument based on the interaction of the fragment orbitals is used to rationalize the observed trends. The thermodynamic values of the methyl transfer reactions are examined. The possible dissociation processes of H 2 AldXH(Y)CH 3 systems into HAlXH(Y) and CH 4 or into HAlXCH 3 and molecular H 2 are also examined and reported. 1. Introduction Chemical vapor deposition (CVD) of aluminum and alumi- num compounds is of interest for a variety of technologies. For example, CVD remains today one of the most attractive methods to prepare AlN solid films which has many interesting properties such as high thermal conductivity, good oxidation resistance and hard coatings for abrasion and corrosion resistance, among others, which make it a promising material for future years. 1-5 Therefore, reactions of base-stabilized AlH 3 with secondary amines R 2 NH were studied in detail in order to obtain structural information on as-prepared oligomeric aminoalanes. The reac- tion temperature, the stoichiometry of the reactants, and the steric demand of the substituent bound to N were found to play key roles in what degree of oligomerization was attained. Monomeric aminoalanes of the type base Al(H 2 )NR 2 as well as oligomeric aminoalanes such as [H 2 AlNR 2 ] x were obtained, mainly in the form of a four-membered heterocycle. 6-14 In contrast, equimolecular reactions of H 3 AlNR 3 donor acceptor complex with primary amines RNH 2 preferentially yielded iminoalanes rather than aminoalanes [H 2 AlN(H)R] x , depending on their instability toward further H 2 elimination reactions. Aminoalanes of the type H 2 AlN(H)R are only known in the form of intramolecular stabilized heterocycles. 13,14 On the other hand, donor acceptor complexes of Lewis acids AlH 3 and AlX 3 (X ) halogen atom) with various Lewis bases have been the subject of many experimental and theoretical studies. 15-28 Recently, we reported detailed ab initio molecular orbital studies of a series of donor-acceptor complexes of AlH 3 . 29-34 We showed that the stability of these complexes does not depends on the charge transfer. We have also shown that the donor- acceptor coordination was not based on a simple HOMO- LUMO interaction. In this work, we have now extended our investigation to the structures and energetics of the conformations of H 2 AldXHCH 3 and H 2 AldYCH 3 (X ) N, P, and As; Y ) O, S, and Se) systems by ab initio calculations. The relative stability and 3-fold methyl rotational barrier of these systems are examined. The possible dissociation of the H 2 AldXH(Y)CH 3 systems into HAlXH(Y) and CH 4 and the dissociation of the H 2 AldXHCH 3 systems into HAlXCH 3 and molecular H 2 are examined. In addition, methyl transfer reactions are also examined. To the best of our knowledge, no comparative ab initio study of these systems has been carried out. 2. Computational Details Ab initio calculations were performed using the Gaussian 98 program. 35 Geometry optimizations were performed at the MP2- (full)/6-31G(d) level. 36 No symmetry constraints were imposed during the optimization process, and the geometry searches were carried out for a number of possible isomers to ensure the location of the global minimum. The zero-point vibrational energies (ZPE) are obtained from scaled HF/6-31G (d) calcu- lated frequencies (scaled by the factor 0.893). 37 For improved energy, the Gaussian-2 (G2) energies 38 were computed. 3. Results and Discussion The methyl group in 1-7 (C s symmetry systems) has two conformational eclipsed and staggered orientations (Figure 1). In 1a-7a, one of the C-H bonds of the methyl group eclipses the double bond (eclipsed conformation), and in 1b-7b, the C-H bond is trans to the double bond (staggered conformation). 1a-3a, 5a, and 7a correspond to minima and 1b-3b, 5b, and * To whom correspondence should be addressed. E-mail: boutalib@ ucam.ac.ma. 5488 J. Phys. Chem. A 2003, 107, 5488-5491 10.1021/jp034809k CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003