Influence Of Orbital Delocalization On The Molecular Stability And Explanation Of CO Activation In Cobalt Surfaces From Orbital Interaction Point Of View

This thesis mainly includes two parts.In the first part,we employed the density functional theory to calculate and analyze two molecular systems[NHC-B]_n and[NHB-C]_n?n=2-6?,which clarified the relationship between occupying the excessively delocalized orbitals and the molecular stability.In the second part,the influence of the orbital interaction between cobalt and carbon monoxide on the CO activation was systematically studied using the first-principle calculation based on plane wave pseudopotential method.In the first part,we find that it is elusive to understand the stability of[NHC?B]_n?NHC=N-heterocyclic carbene?relative to[NHB–C]_n?NHB=N-heterocyclic boranide??n=2–6?species simply based on electron delocalization.Thermodynamically,the former were much less stable than the latter.However,electronic structure analyses suggested that the orbitals in the former are more delocalized than the latter.This is contrary to the conventional idea that"the orbital decentralization can help stabilize the system".Therefore,we performed the orbital analyses and found that though the proper delocalization is beneficial to stabilization,the excessive delocalization is negative to stabilization.The main results and innovations in the first part1.The bond situations of ^NHCC-B in[NHC-B]_n and ^NHBB-C in[NHB-C]_n are quite different,The former is a dative bond and the latter is a standard electron-sharing bond.2.Since the ^NHCC-B bond of system[NHC-B]_n is a dative bond,the central B_n moiety must play the role of back-donation,forming a highly delocalized 2nc-2e or 4nc-2e orbitals,corresponding to the CMO at high orbital energy.However,the ^NHBB-C bond of[NHC-B]_n is the electron sharing bonds so that orbitals in the central C_n moiety are localized 2c-2e or less delocalized nc-2e in nature,corresponding to CMO at low orbital energy.3.When the delocalization area of an orbital greatly exceeds the region its electrons coming from,it is the excessively delocalized orbital.Occupying one or more such orbitals may be a sign of energetically less favourable molecules.In the second part,the interpretation of the mechanism of CO dissociation in the initial reaction for Fischer-Tropsch synthesis is very controversial.In the present work,we had performed the first-principle calculationstostudytheabsorption,directdissociation,and hydrogen-assisted dissociation of CO on six typical cobalt?Co?surfaces,which gave the results consistent with other studies,and we use frequency analysis to further prove the accuracy of the results.Furthermore,we carried out the orbital analyses?including DOS and COHP?to systematically investigate the influence of Co-CO orbital interactions on CO activation.The results provide a new perspective for clarifying the mechanism of CO activation in FTS.The main results and innovations in the second part1.The adsorbed CO interacts with the metal to form new hybrid orbitals.2.For direct CO dissociation,the C-O bond can activate in higher degree when the the Co-C and Co-O bonding become stronger,as well as when the proportion of electrons provided by the hybrid atoms become greater in the newly formed d_?hybrid orbital.3.Hydrogen atoms help reduce the activation energy.When a hydrogen atom is bonded to oxygen atom,the composition of carbon atom in d_?hybrid orbital increases,while it is bonded to carbon atoms,the Co-O bond was enhanced obviously.Both cases can significantly decrease the activation energy of the C-O bond.