Theoretical Study On The Molecular Structures And Bonding Properties Of Xenon Compounds

Rare gas?Rg?including six elements owns stable octet electronic structure,so that it is difficult for them to lose electrons,occur the chemical reaction,and form compounds.Therefore,it is also known as noble gas?Ng?.Until half a century after the Rg element was discovered,the first rare gas compound,Xe?Pt F₆?_n?1?n?2?was successfully synthesized.So far,the rare gas can form chemical bonds with a number of elements,yielding the variety of rare gas compounds which are abundant,such as the hydrides,complexes and organic compounds containing Rg atoms.However,Among most of these compounds,the rare gas atoms form chemical single bonds.It is a very interesting and unsolved scientific question whether these atoms can form multiple bonds.Xenon compounds have always been the breakthrough of new rare gas compounds.Most of the hundreds of synthesized rare gas compounds are xenon compounds.Therefore,in the present study,we predicted the molecular structure and bonding nature of a series of xenon involved multiple bond compounds by performing quantum chemical calculations.In addition,as the bonds formed with Rg in rare gas compounds are generally weak,the molecule system HRg NH₃⁺?Rg=He,Ne,Ar,Kr and Xe?with strong bonds were studied.The main results are as following:By using four methods,B3 LYP,BMK,MP2 and CCSD?T?,the geometry structure of the rare gas hydride molecule,HRg NH₃⁺,was optimized,followed by frequency calculation and stability analysis.The Rg bonding nature was analyzed by performing AIM,NBO and NEDA calculations.The predicted Rg-N bond lengths are 1.889,2.468,2.577,2.604 and 2.687?at CCSD?T?level,respectively,which are much less than the corresponding values in the previously predicted ionic bond species.It indicates that there is relatively strong interaction between Rg and N atoms in HRg NH₃⁺.The bonding energy analysis obtained from NEDA calculation pointed out that among the Rg-N chemical bonding interaction,the charge transfer mainly contributes to the bonding energy for the light Rg elements?such as He and Ne?,and polarization effect for the heavy Rg elements?such as Kr and Xe?.The bonding nature results showed that Rg forms a covalent bond with H and an ionic bond with N.The bonding nature of the compounds is that the lone pair electrons of N atom in NH₃ are packed into the antibonding orbital of H-Rg⁺bond.The determined reaction path showed that HRg NH₃⁺?Rg=He,Ar,Kr and Xe?is metastable in thermodynamics,however their dissociation energy barrier s are 4.6,6.7,9.5 and 12.8 kcal mol^-1,respectively,which are higher than the corresponding values for the speices containing H-Rg bond.By using B3 LYP,DSD-BLYP and MP2 methods,the molecules F₂Xe MF₂?M=Mo and W?with Xe double bond were studied.In the optimized geometric structures,the Xe-Mo and Xe-W bond lengths in the two molecules are 2.518 and2.504?at CCSD?T?level,respectively.By comparing with the standard Xe=Mo and Xe=W double bonds with the lengths of 2.56 and 2.55?,respectively,one can see that both of them are chemical double bonds.Molecular orbital analysis showed that the bond consists of one?bond and one?bond.The former is formed by the interaction between the 5p_z orbital of Xe and the d_z2orbital of the transition metal,and the later is formed between the 5p_y orbital of Xe and the d_yzorbital of the transition metal.In addition,FXe Mo F₃ and FXe WF₃,which are the isomers of F₂Xe MF₂,were also found.In these two molecules,the Xe-Mo and Xe-W bond lengths are 2.692 and 2.703?,respectively,showing that they are chemical single bonds.It can also imply the Xe-M bond in F₂Xe MF₂ is double bond.According to the stability analysis,the decomposition energy barriers of F₂Xe MF₂ are 6.5 and9.2 kcal mol^-1,respectively,suggesting their stability in dynamics.Further study found that the molecule,F₄Xe Os F₄,contains Xe triple bond.The novel configurations of four isomers were characterized in theory.The Xe-Os bond length is 2.444?in one of the four isomers,larger than the standard triple bond length of 2.31?and shorter than the standard double bond length of 2.51?.The calculated bond length is very close to the triple bond length,implying that there is a Xe-Os triple bond in the isomer.The further bonding nature analysis supported the Xe-Os triple bond,the two p orbitals of xenon are involved in the formation of the two?bond,respectively.The study results showed that the antibonding orbital of F-Xe-F in the molecules is the key in the formation of multiple bonds,and the?bond is formed between this orbital and the 5d orbital of Os atom.In the other three isomers,the Xe-Os bond lengths are 2.679,2.469 and2.607?,respectively.and the bonding nature analysis showed that they are double bond or single bond.