Theoretical Study The Structural Characteristics Of Hydration Of Ca²⁺, Mg²⁺ And Magnesium Sulfate Saline Solutions

Hydration structure characteristics of calcium and magnesium ions, and their ionassociating rules are of paramount importance in the fields of chemistry and biochemistry asthey participate in animals and plant life activities as trace element. And thus great interests inhydrated calcium and magnesium ions have stimulated numerous studies on structuralparameters （such as coordination number, intermolecular distance et. al） by theory andexperiment. However, detail information about the first hydration shell of Mg²⁺and Ca²⁺isstill uncertain. And what is the difference for the first hydration shells of Mg²⁺and Ca²⁺hydrates? In addition, study of the hydration structure in MgSO₄solution is considerablepractical importance as MgSO₄is encountered in many industrial process such as brineevaporation and fertilizer industries as well as in mineral processing industries using aqueouschemistry. However, the hydrated structures of MgSO₄aqueous and the characteristics ofRaman spectra are still controversial issues. Therefore, it is quite necessary to investigatehydration characteristics of Mg²⁺, Ca²⁺and magnesium sulfate.Hydrates of Ca²⁺, Mg²⁺and magnesium sulfate were chosen as the object of this studyunder hydrated cluster model, and searching for stable structures of [M（H₂O）_n]²⁺（M=Mg，Ca）and [MgSO₄（H₂O）_n] hydrated clusters were performed using B3LYP/aVDZ method. On thebasis of clusters/polarizable continuum model and approximatively saturated hydration ofcentral ions, the stable structures of Mg²⁺, Ca²⁺, and MgSO₄species in dilute solution wereanalysed, and spectral feature of possible dominant species were also investigated. In addition,[M（H₂O）_6¹0]²⁺（M=Mg, Ca） clusters were calculated using both B3LYP/aVDZ andMP2/aVDZ method to check the validation of B3LYP/aVDZ method. Moreover, the structuralcharacteristics of the first hydration shell of [M（H₂O）_n]²⁺（M=Mg, Ca） clusters, and theirstability and change of characteristics as temperature change for were investigated using abinitio molecular dynamics （AIMD） and methods of transition state theory. The main resultsare as follows.It is found that the hydration of Ca²⁺prefers to6-coordated structures in the gas phase,and7-and8-coordianted structures become favorable as hydration proceeds in the aqueousphase, whereas the hydration of Mg²⁺generally prefers six-coordinated structures both in thegas and aqueous phases. Our transition state calculations and AIMD simulations attemperatures of100³00K show the CN value of Ca²⁺reduces from8to6⁷astemperature increases, which indicates that the hydration of Ca²⁺prefers6-and7-coordinatedstructures at temperature of200and300K, while the CN transformation of Ca²⁺from6or7to8is much more difficult and slow in our MD simulations. However, such transformationsbetween those isomers among different CNs are more difficult and slow for Mg²⁺hydrates, and the hydration of Mg²⁺always favor six-coordinat structures during the simulation at100³00K. Moreover, the charge transfer from central cation to water molecules is found to belocated nearly in the first hydration shell for Mg²⁺, whereas in the first and second hydrationshells for Ca²⁺. Combined with hydration characteristics, approximatively saturated ionhydration shells for the hydration of Mg²⁺and Ca²⁺were proposed. Our calculated[M（H₂O）_n]²⁺(M=Mg²⁺, Ca²⁺；n>18) infrared spectra indicate that the absorption peaks ofO–H stretching at the range of3400³550cm^-1, and those at roughly3300cm^-1providecharacteristics for the first hydration shell and the second hydration shell, respectively.In the gas phase, the hydration of MgSO₄prefers basic contact ion pair structure (basiccontact ion pair of Mg²⁺and HSO₄^-, CIP-OH), and contact ion pair （CIP） structures becomemore stable as hydration proceeds. In the aqueous phase, however, CIP conformers of[MgSO₄（H₂O）_n] clusters are generally more stable, while SSIP （solvent-shared ion pair）conformers will become more stable in dilute aqueous solution. The calculated Raman spectraindicate that the frequencies of the S O stretching vibrational peaks of HSO₄^-, MgSO₄and[SO₄（H₂O）₆]^2- all blue-shift compared with those of SO₄2-, such blue-shift is less obvious for[SO₄（H₂O）₆]^2- cluster. The calculated O H vibrational peaks of [MgSO₄（H₂O）_n] clusters forbidentate-CIP, monobidentate-CIP, SSIP-OH (basic SSIP of HSO₄^-and Mg²⁺) and SSIP are⁸50cm^-11050cm^-1;⁸70cm^-1,⁹60–1000cm^-1;⁹30–960cm^-1,1000cm^-1and⁸40cm^-1,980cm^-1. The calculated characteristics of spectra can be used to identify the types conformerof [MgSO₄（H₂O）_n] solutions, and thus provides a theoretical reference for structure analysis ofexperimental research.