Theoretical Investigation Of The Structural And Electronic Properties Of The Compounds Containing Phosphorus

Aromaticity is an abstract concept. Its definition varies with the development of chemistry. On one hand, the knowledge concerning aromaticity has gone beyond the region of orgnic chemistry. Since Boldyrev and Wang found the first aromatic metallic cluster Al42-, a new research fielde merged. Addtionally, more and more new aromatic compounds are found and sythsised. The proliferation in classifications of aromaticity, going far beyond the conventional confines of benzenoid hydrocarbons and their related heteroarenes, continue as the following examples. Homoaromaticity, spheric aromaticity, metal aromaticity, Mobius aromaticity, and Y aromaticity etc. On the other hand, aromaticity is a muti-dimensional phenomenon. No absolute criterion can be defined. But there are some measure used to descript aromaticity quantitatively, e. g. geometry, aromaticity stable energy, chemical activeity, and magnetic property etc.Recently, the bimeta-clusters containing phosphorus have caught people's attention. Almost all transition metals can form phosphorus compounds. There are more than 200 transition bimetallic phosphorus clusters. These transition bimetallic phosphorus clusters can form many linear, cyclo, and caged structures. In small molecules, some bond type are kown: (1)σ2,λ3-phosphorus e.g. H2C=PH (2)σ3,λ3-phosphorus e.g. HP(CH3)2 (3)σ3,λ5-phosphorus e.g. CH2=PH=CH2 (4)σ4,λ5-phosphorus e.g. H3P=CH2. In the present work, the structural and electronic properties of Ti2P6+ cation cluster and some molecules containing the tri-coordinated phosphorus are investigated.1. Cationic cluster Ti2P6+ has been studied within density functional theory. The structure of this cluster is predicted to be a slightly distorted tetragonal prism. According to the homolytic dissociation reactions and heterolytic dissociation reactions, this cationic cluster has a very high bonding energy. In Ti2P6+, the hybridization of P atoms of the ring is SP3. The bonding between the metal atoms and the P ring containsπ-πandσcontributions. The nuclear independent chemical shift (NICS) indicates that Ti2P6+ is a three-dimensional aromatic molecule. The electronic delocalization is another stabilizing factor for Ti2P6+.2. Calculations with B3LYP within quantum chemical density functional theory have been carried out for 1-H-phosphininium cation and a series of 1-R-phosphininium molecules (R= cyclopentadiene,σandβpyrroles,σandβphosphole, C5BH5-and CH2-). The negative NICS values and the positive aromatic stabilization energies confirm that they are aromatic compounds. In particular, the 1-H-phosphininium cation even exhibits stronger aromatic character than the well-known aromatic phosphinine. The aromatic substituents have strong capability to attract electrons. It is the conjugation and aromaticity that keeps the stability and conformations of the molecules investigated. Owing to the perturbation of the aromatic substitutent groups, the predicted large T values and the enlarged HOMO-LUMO gap of the phosphininium cation indicate that these compounds are expected in experiment. 3.By using the B3LYP/6-31+G(2d,2p) level of theory, the structures and electronic properties of phosphanyl ketones have been studied. The results show that the planar meta-phosphanyl ketone is a minimum on the potential energy surface (PES), but the planar structures of the ortho- and para-phosphanyl ketones are transition states. The large negative NICScπvalues show that their planar conformations are aromatic. The delocalization of the tricoordinated phosphorus lone pair electrons strongly depends on the relative position of the two important groups (the carbonyl group and the tricoordinated phosphorus atom) in the molecules and determines the nature of the stationary point.4.We have compared the structure, electronic properties, and aromaticity of pyridones to those of phosphanyl ketones. Analysis of the electron density using the electron localization function (ELF) rationalizes the structural difference. In the meta- pyridone and phosphanyl ketones, no lone pair basin appears in the tri-coordinated nitrogen and phosphorus. The meta-phosphanyl ketone adopts a planar minimum. In contrast to this, the large lone pair in the ortho- and para- phosphanyl ketone forces the tir-coordiated phosphorus atom to be pyramidal. The natural resonance therory (NRT) analyses reveal that the adoption of molecular shape is a balance among electron delocalization (aromatic stabilization), the octet rule, and pyramidality of the tri-coordinated nitrogen atom and phosphorus atom in such molecules.