Computational Quantum Chemical Investigation On Heterospin System Consisting Of Metal(Ⅱ) Ion And Aminoxyl Radicals By Using Broken Symmetry Approach And Density Functional Theory

Quantum chemistry is an important tool in the design of molecules with desired chemical and physical features and both the structures and properties of molecules can be predicted with the current ab initio technology. In this work quantum chemical methods have been used to predict the magnetic properties (magnetic coupling constants) of paramagnetic organic radical complexes. This thesis deals with modelling of hetero-spin system such as bis(hexafluoroacetylacetonato)Metal(ΙΙ) [M(hfac)₂] ligated with 3- and 4-(N-oxyl- tert-butylamino)-pyridines (3NOPy and 4NOPy) complexes. The methods used are density functional theory and Hartree-Fuck combined with Broken Symmetry Approach developed by Noodleman, which is undoubtedly most commonly applied, to clusters containing several paramagnetic metal centers or to paramagnetic organic radical complexes. The magnetic coupling constants were obtained and compared with the available experimental data. We find that density functional theories combined with Broken Symmetry Approach are suitable for determine the magnetic properties of paramagnetic organic radical complexes than Hartree-Fock Methods. In this dissertation also we investigate the suitability of using different basis sets and expressions for estimating the magnetic coupling constant. Different hetero-spin systems have been studied in the three papers of this thesis. An extensive overview of computational methods used is also included.Firstly the magnetic-structural correlation in metal-radical hetero-spin system such as bis(hexafluoroacetylacetonato)copper(ΙΙ) [Cu(hfac)₂] ligated with 3- and 4-(N-oxyl- tert-butylamino)-pyridines (3NOPy and 4NOPy) complexes has been investigated on the basis of unrestricted density functional theory (UDFT) and unrestricted Hartree–Fock (UHF) calculations combined with the Broken Symmetry (BS) Approach, in order to study the magnetic properties of the complexes [Cu(hfac)₂(4NOPy)₂] and [Cu(hfac)₂(3NOPy)₂]. The effective exchange coupling constants J1 have been obtained by different methods (UPBE, UB3PW91, UB3LYP, UB3P86, UPBE0 and UHF). The best calculated exchange coupling constant for complex [Cu(hfac)₂(4NOPy)₂] was found to be J = 55.8 K, while the smallest magnitude calculated exchange coupling constant for complex [Cu(hfac)₂(3NOPy)₂] was found to be J = -33.1 K. For both two complexes UPBE0, UB3PW91, UB3LYP, and UB3P86 methods are suitable for calculation of exchange coupling constant.Secondly the performance of the different mixed basis set and the density functional theory (DFT) in determining the magnetic properties of metal containing systems is presented. The uses of different exchange correlation functional and mixed basis sets are proposed to determine exchange coupling constant (J) for bis(hexafluoro- acetylacetonato)copper(II) [Cu(hfac)₂] ligated with 3- and 4-(N-oxyl-tert-butylamino)- pyridines (3NOPy and 4NOPy) complexes. In the present thesis, a critical comparison between the calculated data of the magnetic interactions obtained by DFT with different basis set used on the atoms of the target system is reported in order to find the best compromise between accuracy and reliability in the calculations of the J values of the complexes. Comparison with available experimental data and our previous theoretical results, we provide the reliability of the PBE and PBE0 methods with the SDD/6-311G(d) and LanL2DZ/6-311G(d) basis set for complexes [Cu(hfac)₂(4NOPy)₂] (1) and [Cu(hfac)₂(3NOPy)₂] (2), respectively. The influence of different basis sets on the calculations of J is limited.Finally we have investigated the magnetic properties of metal-radical heterospin system such as bis(hexafluoroacetylacetonato)manganese(II) [Mn(hfac)₂] ligated with 4-(N-oxyl-tert-butylamino)-pyridines (4NOPy) complex by means of broken symmetry (BS) with density functional theory (DFT). We obtained the magnitude and sign of the magnetic coupling constant (J) for the complex under investigation [Mn(hfac)₂(4NOPy)₂] by using different methods (UPBE, UB3PW91, UB3LYP, UB1LYP, UB3P86, and UPBE0) and basis sets (DGDZVP and LANL2DZ). The best calculated J value was found to be J = -12.5 K, which are in excellent agreement with the experiment value (J = -12.4±0.1K).