Theoretical Calculation And Infrared Spectroscopic Study On The Structure Of Rh₂O_n⁺（n=7-10）Species

The transition metal rhodium（Rh）as the active center of catalyst plays a vital role in many organic synthesis,industrial catalysis and exhaust gas treatment.What’s more,both rhodium oxide,complexes,and supported rhodium oxide nanoparticles,the rhodium active site has high catalytic activity and reaction selectivity.So,it is significant to study the relationship between the structure and the catalytic activity of those rhodium compounds.A large number of reports have been carried on the supported rhodium oxide catalysts and rhodium complex catalysts.However,the research about binary rhodium multi-oxide is relatively less than them.The rhodium oxides and its clusters in this paper are prepared by the reaction of laser ablation of rhodium atoms with O₂/He to produce Rh₂O_n⁺（n=2-10）.The infrared spectrum of Rh₂O_n⁺（n=8-10）were obtained by infrared dissociation experiment.The focus of our work is to apply the density functional theoretical（DFT）to calculate the possible isomer structures of Rh₂O_n⁺（n=7-10）based on the infrared dissociation spectrum of Rh₂O_n⁺（n=8-10）,to analyze those structures and bonding characteristics so as to get the most stable geometric structure of each cluster.The main results and conclusions are as follows:（1）Theoretical calculations are performed on the geometric structure of Rh₂O₇⁺and Rh₂O₉⁺cluster.First of all,a total of 29 geometries of Rh₂O₇⁺were obtained including ring,ring-opening and linear structures with doublet,quartet,sextet,and octet states.The energy of cyclic Rh₂O₃structure is lower than cyclic Rh₂O₂structure,which is lower than the Rh₂O₄linear structure,and open ring structure is the highest.After the calculations of single point energy at the CCSD（T）level,a kind of cyclic Rh₂O₃structure with sextet is found to be the lowest energy in the 29 geometric structures.For the same geometric configuration,the lowest energy is the structure only with O₂unit,and Rh₂O₇⁺tends to be unstable when the coordination groups change from O₂to O₂^-unit.The bonding analysis of these 29 structures revealed that no Rh-Rh bond was observed for all the optimized Rh₂O₇⁺isomers including oxides,peroxides,superoxides,and oxygen groups.The localized orbital bonding analysis（LOBA）indicates that Rh has low oxidation states of 1,2 or 3 in ring structures.Secondly,a total of 43 geometries of Rh₂O₉⁺were obtained including ring structures and linear structures with doublet,quartet,sextet,and octet states.The energy of five-numbered ring structure is lower than four-numbered ring structure,which is lower than linear structure.After the accurate calculations of single point energy,a kind of linear structure with sextet is found to be the lowest energy in the 43geometric structures,which is also extremely fit the experimental results and then be regarded as the experimental results of Rh₂O₉⁺.The results show that for the same geometric configuration,the lowest energy is the structure with O₂^-unit,which is different with the result of Rh₂O₇⁺.The energy is increased by the number of superoxide unit in the structure,and the coordination mode of the unit（whether located in the same Rh）also affect the structural stability.（2）Based on the experimental spectrum of Rh₂O₈⁺,four isomers were optimized,and the spectrum of Rh₂O₈⁺was explained by frequency analysis.Apart from the structures observed in the experiment,we also calculated 38 geometries including ring structures and linear structures with different spin states.The ring structure includes a four-membered ring structure and a five-membered ring structure,the former can be divided into diamond ring and trapezoid ring structure.The energy of four-membered ring structure is lower than linear structure,which is lower than five-membered ring structure.After the accurate calculations of CCSD（T）single point energy,it is found that the lowest energy of the 38 geometric structures is the linear structure cooperated with two O₂units,which is one of the isomers consistent with the experimental results of Rh₂O₈⁺（a）.For the same geometric configuration,Rh₂O₈⁺tends to be unstable when the coordination groups change from O₂to O₂^-unit and the coordination mode of the unit（whether exist the ended Rh-O bond）will also affect the structure energy.For the 38 species,the bonding analysis revealed except the Rh-O unit and O^2-,O₂^-,O₂^2-unit,Rh-Rh single bond is also observed in some structures.（3）Combined with the infrared spectrum of Rh₂O⁺₁₀,three isomers were optimized,and the experimental spectrum was explained by frequency analysis.In addition,there are 20 geometries with different spin states.LOBA show that the oxidation state of Rh is also low oxidation state in ring structures.