Molecular Structure,Conformation And Decomposition Of Reactive Phosphorous-containing Species

Phosphorous-containing pseudohalogen,as a significant branch of the pseudohalogen chemistry,has been broadly used in functional materials(such as high energy density materials,HEDM)and flame-retardant materials,and become one of the most popular subjects in recent years.Now,the phosphorous-containing azides and phosphorous-containing isocyanates are the two more studied in phosphorous-containing pseudohalogens,play an important role in synthesis and characterization of other phosphorous-containing pseudohalogens.In this thesis,we performed experimental and theoretical study of the two molecules,FP(O)(N3)2 and Cl2PNCO.We have investigated the photolysis and thermal decomposition of FP(O)(N3)2 and its isotopic isomers by matrix-isolated IR spectroscopy,analyzed the experimental results with the aid of theoretical calculations,and predicted the possible decomposition mechanism.We also successfully synthesized and separated the pure Cl2PNCO by using three U-traps,and investigated the conformational and structure properties of Cl2PNCO in experiment and theory.1.The photolytic and thermal decomposition of fluorophosphoryl diazide,FP(O)(N3)2,was studied by matrix isolation spectroscopy.Upon ArF laser photolysis(? =193 nm)of normal,18O labeled,15N enriched FP(O)(N3)2,FPO and a new germinal azido nitrene FP(O)(N3)N were identified by matrix IR spectroscopy.To determine the nature of the electronic ground state of the nitrene intermediate,a matrix EPR was recorded for the products obtained by ?=193 nm photolysis of FP(O)(N3)2.The nitrene shows a triplet ground state.Further decomposition of the nitrene into FPO was observed under irradiation of ?>335 nm.In contrast,no nitrene but only FPO was identified after flash vacuum pyrolysis of the diazide.Thus,flash vacuum pyrolysis of FP(O)(N3)2 offers an efficient way of producing FPO in the gas phase.To reveal the decomposition mechanism,quantum chemical calculations on the potential energy surface(PES)of the diazide using DFT methods were performed.On the singlet PES four conformers of the nitrene were predicted.The two conformers(syn and anti)showing intramolecular Nnitrene…N?.azide interactions are much lower in energy(ca.40 kJ mol-1,B3LYP/6-311+G(3df))than the other two exhibiting Nnitrene…O interactions.Syn/anti refers to the relative orientation of the P=O bond and the N3 group.The syn conformer of triplet FP(O)(N3)N is lowest in energy.The interconversion of these species and the decomposition into FPO via a novel three-membered ring diazo intermediate cyclo-FP(O)N2 were computationally explored.The calculated low dissociation barrier of 45 kJ mol-1(B3LYP/6-311+G(3df))of this cyclic intermediate rationalizes why it could not be detected in our experiments.2.Dichlorophosphanyl isocyanate,Cl2PNCO,was synthesized and characterized by IR,Raman and 31P NMR spectroscopy.The conformational properties and molecular structures were studied by using gas electron diffraction(GED),X-ray crystallography and quantum-chemical calculations.Extensive DFT and ab initio calculations show that the potential energy surface of Cl2PNCO upon rotating the P-N bond is rather flat;three conformers,namely syn,anti and gauche between the NCO group and the bisector of the C1PC1 angle,were theoretically predicted.For the evaluation of the existence of different conformers in the gas phase,the potential energy curve for rotating the isocyanate group around the P-N bond was calculated on different levels of theory with different basis sets.As shown above for the potential energy scans,the predicted relative stabilities of the conformers depend on the applied theoretical methods and basis sets.Experimentally,only one conformer was indicated by gas-phase IR spectroscopy and the preference for a gauche conformation in both,gas phase and solid state,was unambiguously ascertained by gas electron diffraction and X-ray crystallographic data.In the solid state,the Cl2PNCO molecules adopt a gauche conformation with two distinct dihedral angles C1P-NC of-121.3(2)and 137.4(2)°and form polymeric chains through weak intermolecular C …O contacts.