Theoretical Study On The Structures And Properties For The Azide-Clusters Of Boron And Aluminium

The structures and properties of several series of azide-clusters of boron and aluminum are systematically studied using density functional theory (DFT). The process of searching for single source precursors to BN and AlN materials has been completed. The whole work can be divided into two parts:The first part is concentrated on the theoretical studies on the structures and properties of boron azide clusters.The fully optimized structures of the clusters (BH2N3)n(n=1 to 4) are obtained using DFT theory at B3LYP level with different basis sets. All the cyclic structures (BH2N3)2-4 contain Bn(Nα)n skeleton structures. The boat-like and chair-like trimers are found to exhibit two minima with very similar binding energies. The tetramers have four structures that correspond to minima with slightly different binding energies. The Nα-Nβand B-H bond lengths increase with the increasing oligomerization degree n, however, the Nβ-Nγbond lengths decrease. Which shows it could easily eliminate N2 (Nβ-Nγ) and H2 groups to yield BN materials. The calculated IR spectra in the gas phase are assigned, and related with the oligomerization degree are discussed. The thermodynamic properties (C0p,m S0m and H0m) all increase with increasing temperature and oligomerization degree n. As for isomers, their thermodynamic functions are all close. The isobaric oligomerizations can occur spontaneously for n=3 and 4 at 298.2 K, but is unfavorable for the case of n=2. The results with the 6-311+G*,6-311++G** and aug-cc-pVTZ basis sets show it is suitable to choose the economic 6-311+G* basis set to study the clusters (BH2N3)n(n=1 to 4).The geometric configurations, electronic structures, IR and thermodynamic properties of the clusters (BN3X2)n(n=1 to 4; X=F, Cl, Br) have been studied at the B3LYP/6-311+G* level of DFT. The Nα-Nβ, B-X bond lengths and the B-Nα-B, Nα-B-Nαbond angles all increase with the size of the cluster growing, however, Np-Ny bond lengths decrease. Owing to the strain of the four-membered ring, the B-Nαbonds in the dimer are longer than those in trimers and tetramers. The charge transfer is discussed during oligomerization. The calculated IR spectra are reliable compared with the experimental results. They have three main characteristic regions:the N3 asymmetric stretching, the N3 symmetric stretching and the complicated fingerprint region. The Gibbs free energies (ΔG) of the oligomerizations at 298.2 K are positive, which indicates the isobaric oligomerizations can not occur spontaneously.The simplest models for the organoboron azide clusters (BMe2N3)n(n=1 to 3) have been studied similarly at the DFT-B3LYP/6-311+G* level. The trends in the B-Na, Nα-Nβ, Nβ-Nγ and B-C bond lengths with oligomerization degree n are discussed. The B-Nα-B angles in the cyclic oligomers are consistently larger than the Nα-B-Nαones. Frequency calculations have been carried out for each optimized structure, and their IR spectra are assigned. The calculated frequencies agree reasonably with experiment ones. Both the energies and thermodynamic properties demonstrate that twist-boat conformation is preferred over chair conformation in the gas phase for the (BMe2N3)3. Thermodynamic analysis of the gas-phase reaction shows that the trimerization is exothermic at 298.2 K, however, the dimerization is endothermic. With the increasing temperature, the oligomerization becomes less favorable.The second part focuses on the theoretical studies of the structures and properties for the aluminum azide clusters.DFT/B3LYP method with 6-311+G* basis set was used to calculate the clusters AlN3F2 consisting of up to four molecules, and it predicts that (AlN3F2)n(n=2 to 4) are cyclic-like clusters with Al-Nα-Al linkages. The Nα-Nβand Al-F bond lengths all increase slightly with the size of the cluster growing, however, Nβ-Nγbond lengths decrease, showing it could easily yield AlN materials. With the oligomerization degree n increasing, the charge transfers for the clusters are large. As for isomers, their binding energies are all close, respectively. The ZPE correction has little effect on the binding energy. Thermodynamic properties demonstrate that the isobaric formations of dimer, trimers and tetramers are favorable thermodynamically even up to 800 K. The calculated equilibrium constants for the formation more stable dimer, trimer and tetramer, based on the equation AG=-RTInKp, are large. It reveals that the AIN3F2 systems occur dimer-trimer—tetramer equilibriums. By the vibrational analysis, the simulated infrared (IR) spectra for the AIN3F2 monomer and its clusters are obtained and assigned. The N3 asymmetric stretching moves to higher frequency (hypsochromic phenomenon) as the cluster becomes larger, however, the N3 symmetric stretching moves to lower frequency (bathochromic phenomenon).The fully optimized structures for the (AIN3X2)n(n=1 to 4, X=Cl, Br) are obtained similarly at the DFT-B3LYP/6-311+G* level, respectively. The trends in the Al-Nα, Na-Np, Nβ-Nγand Al-X bond lengths with oligomerization degree n are discussed. Al-Nα-Al angles in the cyclic oligomers are consistently larger than the Nα-Al-Nαones. The trimers and tetramers (AlN3X2)3-4 (X=Cl, Br) have not been found experimentally and theoretically. The calculated IR spectra have three main characteristic regions:two of them are very strong. One corresponds to the N3 asymmetric stretching, and the other is associated with the N3 symmetric stretching. And the remaining one is in the lower frequency range and corresponds to the complicated fingerprint region. With the oligomerization degree n increasing, the N3 asymmetric and symmetric stretchings move to higher frequency (hypsochromic phenomenon) and lower frequency (bathochromic phenomenon), respectively. To testify the reliability of the theoretical IR, the experimental and calculated vibrational frequencies are compared. It is evident that the calculated frequencies agree reasonably with experiment ones. Thermodynamic properties all increase linearly with increasing temperature and oligomerization degree n. The gradients of C0p,m and S0m to the temperature decrease, but that of H0m increases. All the isobaric oligomerizations can occur spontaneously at 298.2 K.In a word, the systemic studies on the structures, properties and the molecular designs have been investigated for several series of azide-clusters of boron and aluminum using DFT method, which explains a great deal of the experimental facts and predicts many unknown results. The abundance of information and the rules provided are used to instruct the experimental synthesis for the precursors to the currently hot BN and AlN materials.