Density Functional Theory Studies On The Static Structures And Water-Exchange Reactions Of Aluminum Dimer-Sulphate And Aluminum Dimer-Salicylate Complexes

As a kind of important metal element in earth's crust,investigations about aluminum's(Al)different forms and characteristics have always been the priority subject.Apart from existing in aqueous solution in the form of Al3+,Al can combine with many elements,such as oxygen(O),potassium(K)and silicon(Si),to form various kinds of A1 minerals,such as Gibbsite,Boehmite,Kaolinite(Al4[Si4O10]·(OH)8)and Illite(K0.75(Al1.75R)[Si3.5Al0.5O10](OH)2).The first complex generated from Al3+ in aqueous solution is Al(H2O)63+.Researches about its structure and water-exchange reaction are helpful to further understanding the structure and characteristics of Al polymers.In natural environment,inorganic acid(sulphate acid)and organic acid(salicylate acid)can adsorb on Al minerals and affect the transformation of nutrition and dissolution of various Al minerals.The researches about Al(H2O)63+ and adsorption between sulphate acid/salicylate acid and Al minerals have significant scientific meaning.There are a lot of researches about these two fields,and there are two problems existing in these fields at present:(1)Investigations about Al(H2O)63+ only consider two hydration shells surrounding Al3+,and there is evidence that the third hydration shell of Al3+exists,but there are a few studies about the effects of the third hydration shell on the structure and water-exchange reaction of Al(H2O)63+;(2)It has been proved that there are many kinds of complexes generated from the adsorption of sulphate acid and salicylate acid on Al minerals,and the dominant complex under lower pH is Al dimer,but there are seldom researches about the structure and water-exchange reactions.In this dissertation,the effects of the third hydration shell on the static structure and water-exchange reaction of Al(H2O)63+,as well as the static structures and water-exchange reactions of Al-sulphate dimer and Al-salicylate dimer are systematically investigated from the following four chapters:1.Introduction.The hydrolysis and polymerization mechanisms of Al3+(aq)in aqueous solution have always been the hot subject of aluminum chemistry in the environment.The Al monomer is the first product of A13+(aq)hydrolysis in aqueous solution.There are many hydration shells surrounding Al3+(aq),most of the investigations concentrated on the first and second hydration shells presently,and there are few reports about the third hydration shell.Two adjacent monomer hydroxyl-aluminum complexes can form Al dimer under certain conditions.The adsorption between Al dimer and different oxyanions can be used as the typical representatives and simple model of the reactions between mineral surface(interface)and acids.The studies about the static structures and water-exchange reactions of Al3+(aq)and Al dimer are scientific meaningful.This chapter illustrates the environmental chemistry significances of investigations about the water-exchange reactions of Al3+(aq)and Al dimer on mineral surface.The research status of these two fields and the characteristics of dihydroxyl-bridge in Al dimer are elucidated.Finally,the major questions and solutions that we have encountered during the Gauss-DFT calculations are summarized.2.DFT study of the static structural and 27Al-/17O-/1H-NMR characteristics for the third hydration shell of Al3+(aq)complexes.Density functional theory(DFT)calculations are performed on the static structural characteristics for the third hydration shell of Al3+(aq)complexes.Based on Al(H2O)63+·12H2O structure proposed by Bock including the first and the second hydration shell,the effect of the third hydration shell is considered.Systematically reserches are carried out on the influences of the third and the second hydration shell to the bond lengths,bond angles and NPA charges of the first hydration shell.The characteristics of the hydrogen bond network and 27Al-/17O-/1H-NMR for different Al3+(aq)complexes corresponding to three hydration shells are also investigated.3.DFT study of the influences of the third hydration shell on the dynamic characteristics for the water-exchange reaction of Al(H2O)63+(aq)complexes in aqueous solution.Density functional theory(DFT)calculations are performed on the influences of the third hydration shell on the dynamic characteristics for the water-exchange reaction of Al(H2O)63+(aq)complexes in aqueous solution.The effects of the third hydration shell on the water-exchange reaction rate constants between the first and second hydration shells kex1-2 and the water exchange rate constants between the second and third hydration shells kex2-3 are inspected.DFT calculation results show that the activation energy barriers for both kex1-2 and kex2-3 have small changes when different numbers of waters are considered in the third hydration shell.This indicates that the third hydration shell has some influence on the water-exchange rate constants kex1-2 and kex2-3 but not significant.It is realistic to account the explicit solvent effects with two hydration shells for Al(H2O)63+(aq)complexes in aqueous solution.4.Density functional theory studies on the static structures and water-exchange reactions of aluminum dimer-sulphate and aluminum dimer-salicylate complexes.Density functional theory(DFT)calculations and supermolecule-polarizable continuum model(GP-SM-PCM)are performed on the structures and water-exchange reactions of aluminum(Al)dimer-sulphate complexes(Al2(OH)4(H2O)4SO40 and Al2(OH)2(H2O)6SO42+)and Al dimer-salicylate complexes(Al2(OH)4(H2O)4(HSal)+and Al2(OH)2(H2O)6(HSal)3+),meanwhile,dihydroxy-bridged Al dimer complexes(Al2(OH)4(H2O)62+and Al2(OH)2(H2O)84+)are compared as the backgrounds.Based on the four models(gas phase(GP),polarizable continuum model(GP-PCM)which considers the bulk solvent effect,supermolecule model(GP-SM)which estimates the explicit solvent effects,and supermolecule-polarizable continuum model(GP-SM-PCM)which considers the bulk and explicit solvent effects),two different examinations are systematically conducted:(1)the static structures of Al dimer-sulphate and Al dimer-salicylate complexes as well as the dihydroxy-bridged Al dimer backgrounds are constructed and optimized to get the structural parameters,NPA charges,27Al NMR chemical shifts and energies;(2)the water-exchange reactions of Al dimer-sulphate and Al dimer-salicylate complexes as well as the dihydroxy-bridged Al dimer backgrounds are modeled with GP-SM-PCM model.The logarithm values of the water-exchange reaction rate constants(log kex)are predicted using three different methods.The log kex values of Al dimer-sulphate(Al2(OH)4(H2O)4SO40)and Al dimer-salicylate(Al2(OH)4(H2O)4(HSal)+ complexes as well as the dihydroxy-bridged Al dimer(Al2(OH)4(H2O)62+)background predicted in the correlation of "log kex-?G?" are 5.2,5.6 and 5.9,respectively,these indicate that sulphate and salicylate ligands tend to inhibit the water-exchange reaction rates of the dihydroxy-bridged Al dimer backgrounds.The log kex values of Al2(OH)2(H2O)6SO42+,Al2(OH)2(H2O)6(HSal)3+and Al2(OH)2(H2O)84+are 1.0(cis to OH bridge)/-0.3(trans to OH bridge)?0.8(cis to OH bridge)/-2.6(trans to OH bridge)and 6.2(cis to OH bridge)/1.4(trans to OH bridge),respectively,implying that the protonation of Al dimer mineral surface inhibits the water-exchange reaction of the dihydroxy-bridged Al dimer complexes,similar to the Al monomer's behaviors.