chemistry journal of moldova

I. Ogurtsov, A. Tihonovschi / Chem.J.Mold. 2008, 3 (1), 112-117For such a task binuclear systems with equivalent magnetic centers can represent a good choice. The simplestcase is d 1 -d 1 systems but for these complexes the only question that arises is which state is the ground from two possible– singlet or triplet. Much more interesting are d 2 -d 2 systems. There can be singlet, triplet and quintet states for thesecomplexes and it is interesting not only which state is the ground one but also what is the order of states and which aretheir energies and energy gaps between them. From the other point of view these systems are relative simple and easyto study.The binuclear complex [V 2O(bipy) 4Cl 2] 2+ (Fig.1) was chosen for study in this work. First of all it is a representativeof the d 2 -d 2 family and there are available experimental magnetic data for this complex to compare results with.This work goals are the study of the [V 2O(bipy) 4Cl 2] 2+ binuclear complex electronic structure and the interpretationof its magnetic properties using the ab initio approach.Geometry optimizationThe [V 2O(bipy) 4Cl 2] 2+ complex theoretically may have three different geometrical configurations with the mirroranalogues. However only one geometrical configuration given in Fig. 1 was observed in experimental works [10].In this work this system configuration was initially built and optimized using MM+ methods. In the next step of theoptimization the gradient methods realized in PC GAMESS [11] program package was used. Total energy calculationswere performed in the ROHF (Restricted Open Hartree - Fock) and UHF (Unrestricted Hartree – Fock) approximations.Geometry optimizations using ROHF method have been carried out for three different spin states (S = 2, 1, 0). Processof the optimization using UHF method was converged only for the state with S=2. For further electronic structurestudies there were selected two optimized geometries with lowest energies: one obtained in the UHF optimization andone in the ROHF (both optimized in the states with S=2 and with approximately coincided values of the total energy:-4833.76627647 Hartree (ROHF) and -4833.7689578 Hartree (UHF).List of significant parameters both calculated and observed experimentally is given in Table 1. It is seen thatgeometry obtained after ROHF optimization is satisfactorily good in distances and angles between different parts ofmolecule. However the dihedral angle Cl-V-V * -Cl * was calculated to be about 113° while reported experimental value isabout 62° [10]. The geometry parameters obtained by the UHF optimization are sufficiently close to that given by X-raydata. Therefore below this conformation will be used in study of the [V 2O(bipy) 4Cl 2] 2+ electronic structure.Experimental p and calculated values of the g geometry yp parametersParameter [V 2 O(bipy) 4 Cl 2 ] 2+(ROHF)[V 2 O(bipy) 4 Cl 2 ] 2+(UHF)[V 2 O(bipy) 4 Cl 2 ] 2+(experimental [10])rVO,Å 1.661 1.664 1.787r 2.263 2.150 2.381VClr 1.954 1.970 2.126VN 1rVN 21.924 1.935 2.140rVN 32.139 2.142 2.178rVN 42.089 2.079 2.106rVV3.301 3.315 3.568 , deg 166.8 170.2 173.5VOV * , deg 113.4 65.5 61.6ClVV *Cl* , deg 94.0 98.6 100.2OVClTable 1Electronic structure calculationsElectronic structure studies were performed both using the ROHF method with further taking into considerationCI (Configuration Interaction) and the UHF method. The basis set STO-6G (Slater Type Orbital approximated by6 Gauss-type orbitals) was used for all calculations. The calculations were carried out for geometry configurationobtained using ROHF as well as for geometry configuration obtained using UHF.All the UHF calculations were performed for states with spin values equal to S=2, 1, 0. When the ROHF+CImethod was used the active space was selected according to reasons described below.113