E. I. Davydova

Do solid-state structures reflect Lewis acidity trends of heavier group 13 trihalides? Experimental and theoretical case study.

Lewis acidity trends of aluminum and gallium halides have been considered on the basis of joint X-ray and density functional theory studies. Structures of complexes of heavier group 13 element trihalides MX(3) (M = Al, Ga; X = Cl, Br, I) with monodentate nitrogen-containing donors Py, pip, and NEt(3) as well as the structure of the AlCl(3)·PPh(3) adduct have been established for the first time by X-ray diffraction studies. Extensive theoretical studies (B3LYP/TZVP level of theory) of structurally characterized complexes between MX(3) and nitrogen-, phosphorus-, arsenic-, and oxygen-containing donor ligands have allowed us to establish the Lewis acidity trends Al > Ga, Cl ≈ Br > I. Analysis of the experimental and theoretical results points out that the solid state masks the Lewis acidity trend of aluminum halides. The difference in the Al-N bond distances between AlCl(3)·D and AlBr(3)·D complexes in the gas phase is small, while in the condensed phase, shorter Al-N distances for AlBr(3)·D complexes are observed with 9-fluorenone, mdta, and NEt(3) donors. The model based on intermolecular (H···X) interactions in solid adducts is proposed to explain this phenomenon. Thus, the donor-acceptor bond distance in the solid complexes cannot always be used as a criterion of Lewis acidity.

Quantum-Chemical Study of the Adducts of Silicon Halides with Nitrogen-containing Donors: I. Adducts with Ammonia

Ab initio and DFT methods (RHF and B3LYP) were used to calculate the structural and thermodynamic characteristics of adducts SiX4·NH3 and SiX4·2NH3 (X = H, F, Cl, Br). With the resulting data, the enthalpies and entropies of sublimation of SiF4·2NH3 were estimated for the first time. The effect of the nature of heteroatom X on the Si-N and Si-X bond lengths and atomic charges in the adducts and their dissociation enthalpies was analyzed. The hydride system was shown to differ from the halide systems, and the adducts with X = F have some peculiar features compared with those with X = Cl, Br. The Si-X bond is found to be sensitive to the charge redistribution produced by complex formation.

Quantum-Chemical Study of Adducts of Silicon Halides with Nitrogen-containing Donors: IV. Adducts with Pyridine

The structural and thermodynamic characteristics of SiX4·Py and SiX4·2Py adducts (X = H, F, Cl, Br) were calculated by ab initio and DFT methods (RHF and B3LYP). The resulting data were used to estimate for the first time the enthalpies of sublimation of trans-SiX4·2Py complexes. The distortion energies of the donor and acceptor fragments and the energies of the Si-N bonds in the 1:1 and 1:2 halide complexes were calculated. The high distortion energy makes thermodynamically unfavorable equatorial monopyridine adducts with Si-N bond energies of 150-200 kJ/mol. In trans 1:2 complexes, pyridine acts as a weaker donor than ammonia with respect to silicon tetrahalides.

Structure and stability of M6N8 clusters (M = Si, Ge, Sn, Ti).

The structures and stabilities of the M(6)N(8) clusters (M = Si, Ge, Sn, Ti) have been theoretically studied at DFT and ab initio levels of theory. Two new isomers have been considered: cage-like molecules and propeller-like molecules. It is shown that only for M = Si are both isomers true minima on the potential energy surface. The thermodynamics of the dissociation process (1/6)M(6)N(8) --> (1/3)M(3)N(4) is discussed. For each M(3)N(4) molecule, four structures with different multiplicity are considered. The thermodynamic analysis shows that independently of the multiplicity of M(3)N(4) nitrides all M(6)N(8) clusters are stable in the gas phase in a wide temperature range and could be potential intermediates in chemical vapor deposition of the nitride materials.

Quantum-chemical study of adducts of germanium halides with nitrogen-containing donors

Structural and thermodynamic characteristics of adducts GeX4 · nL (n = 1, 2; X = F, Cl, Br; L = NH3, py, bipy, phen) have been calculated by the B3LYP density functional theory method. The enthalpies of sublimation of complexes trans-GeX4 · 2py and the adduct GeCl4 · bipy have been estimated for the first time. The rearrangement energies of the donor and acceptor fragments and the Ge-N bond energies for the 1:1 and 1:2 complexes have been calculated. While the rearrangement energy for germanium halides is lower by 19–63 kJ mol−1 than that for silicon halides, the energy of the donor-acceptor bond in the former case is slightly lower. As a result, germanium adducts are slightly more stable than silicon adducts.

Structural and thermodynamic properties of molecular complexes of aluminum and gallium trihalides with bifunctional donor pyrazine: decisive role of Lewis acidity in 1D polymer formation

Solid state structures of group 13 metal halide complexes with pyrazine (pyz) of 2:1 and 1:1 composition have been established by X-ray structural analysis. Complexes of 2:1 composition adopt molecular structures MX3·pyz·MX3 with tetrahedral geometry of group 13 metals. Complexes of AlBr3 and GaCl3 of 1:1 composition are 1D polymers (MX3·pyz)∞ with trigonal bipyramidal geometry of the group 13 metal, while the weaker Lewis acid GaI3 forms the monomeric molecular complex GaI3·pyz, which is isostructural to its pyridine analog GaI3·py. Tensimetry studies of vaporization and thermal dissociation of AlBr3·pyz and AlBr3·pyz·AlBr3 complexes have been carried out using the static method with a glass membrane null-manometer. Thermodynamic characteristics of vaporization and equilibrium gas phase dissociation of the AlBr3·pyz complex have been determined. Comprehensive theoretical studies of (MX3)n·(pyz)m complexes (M = Al, Ga; X = Cl, Br, I; n = 1, 2; m = 1-3) have been carried out at the B3LYP/TZVP level of theory. Donor-acceptor bond energies were obtained taking into account reorganization energies of the fragments. Computational data indicate that the formation of (MX3·pyz)∞ polymers with coordination number 5 is only slightly more energetically favorable than the formation of molecular complexes of type MX3·pyz for X = Cl, Br. It is expected that on melting (MX3·pyz)∞ polymers dissociate into individual MX3·pyz molecules. This dovetails with low melting enthalpies of the (MX3·pyz)∞ complexes. Polymer stability decreases in the order AlCl3 > AlBr3 > GaCl3 > AlI3 > GaBr3 > GaI3. For MI3·pyz complexes computations predict that the monomeric structure motif is more energetically favorable compared to the catena polymer. These theoretical predictions agree well with the experimentally observed monomeric complex GaI3·pyz in the solid state. Thus, the Lewis acidity of the group 13 halides may play a decisive role in the formation of 1D polymeric networks.

Quantum-Chemical Study of Silicon Halide Adducts with Nitrogen-containing Donors: III. Energy Effects of Ammonolysis and Complex-Formation Reactions in Silicon Tetrahalide-Ammonia Systems

Thermodynamic characteristics of hydrogen halide elimination from SiX4-NH3 systems were calculated by the B3LYP density functional method. The role of adducts and oligomeric forms of compounds containing Si-N bonds, as intermediates in the chemical vapor deposition of silicon nitride, was considered. It was shown experimentally that the reaction between silicon tetrachloride and ammonia in nonaqueous solvents involves ammonolysis.

Quantum-Chemical Study of the Adducts of Silicon Halides with Nitrogen-containing Donors: II.1 Calculation of a Donor-Acceptor Bond Energy. Complex trans-SiH4·2NH3

Structural and thermodynamic characteristics of the trans-SiH4·2NH3 adduct were obtained by ab initio and DFT (RHF and B3LYP) calculations. Scanning the potential energy surface (PES) of the com plex showed that its structure corresponds to a local minimum, whereas the global minimum corresponds to the free fragments. The energy of the silicon-nitrogen chemical bond was calculated with inclusion of fragment rearrangement energies and basis set superposition error. The procedure offered for calculating the Si-N bond energy was extended to adducts of silicon halides with ammonia. It was found that the energy of SiX4 rearrangement contributes most to the energies of donor-acceptor bonds in mono- and diammoniates of silicon tetrahalides.

Structure and stability of molecular and ionic complexes of AlCl3 with pyrazine and 4,4′-bipyridyl

Structural and thermodynamic characteristics of molecular and ionic complexes of aluminum trichloride with pyrazine (pyz) and 4,4′-bipyridyl (bipy) are calculated at the RI-BP86/def2-SVP level. It is found that for molecular 2AlCl3·3L and 4AlCl3·3L complexes an energy difference between isomers does not exceed 4 kJ/mol, and the rotation barrier of the AlCl3 moiety relative to the N-Al-N bond does not exceed 24 kJ/mol. A comparison of the stability of molecular and ionic complexes of aluminum in the gas phase shows that the maximum energy difference is ∼60 kJ/mol. For L = pyz the molecular complex is more stable whereas for L = bipy it is the ionic one.

Gas phase reaction between MCl4 and NH3: Monomers or oligomers?

The technology of nitride films production is based on the reaction of metal tetrachloride with ammonia at high temperatures. The knowledge of the mechanism of the CVD processes is crucial for optimizing experimental conditions to achieve high quality materials. Formation of polymer forms during SiCl4 ammonolysis was observeded experimentally by mass spectrometry.1 In this work we theoretically investigate gas phase oligomer formation in course of MCl4 (M = Si, Ge, Sn) ammonolysis. All calculations have been carried out using the GAUSSIAN 98 program package. The geometries of all compounds were optimized by B3LYP/DZP method and correspond to minima on the potential energy surface. The following scheme shows possible pathways of gas phase reactions during MCl4 ammonolysis at high temperatures: