TEMPERATURE-INDEPENDENT PARAMAGNETISM IN CLOSED-SHELL OXOANIONS OF 1ST-ROW TRANSITION-METALS

A consistent picture of the magnetism of the series of isoelectronic molecules VO43-, CrO42- and MnO4- is given by coupled Hartree-Fock theory. An external magnetic field induces a paramagnetic circulation around each nucleus that decreases in strength and extent from MnO4- to VO43-. The presence of low-lying empty orbitals derived from splitting of the partially occupied valence shell leads to a significant paramagnetic contribution to the total magnetizability. As the central charge falls, the bonding becomes more ionic, the metal-oxygen bond grows longer and the diamagnetic susceptibility increases in magnitude. At the same time, in Hartree-Fock theory, the virtual orbitals become more diffuse and the paramagnetic contribution falls: 160.7 au (1 au = e2a0(2)/m(e) almost-equal-to 7.89104 x 10(-29) J T-2) in MnO4-, which is about twice the estimated experimental value, 86.0 au in CrO42-, which is 10% too high, and 80.8 au in VO43- (no experimental value). As a result the anions change from 'strongly' paramagnetic to magnetically neutral; at the experimental bond lengths total magnetizabilities are 93.2 au in MnO4- (exp. 8.3), 13.6 au in CrO42- (exp. 6.3) and -0.8 au in VO43-. Lattice effects on the computed values are very small. Agreement with experiment is better at the SCF bond lengths. It is concluded that the Hartree-Fock model becomes more appropriate as the system becomes more ionic, and gives an accurate description of the magnetism in VO43-.