Magnetic inequivalency, electron paramagnetic resonance spectroscopy, and electronic structure of the matrix-isolated (η6-C6H3F3)V half-sandwich

Abstract

Magnetic inequivalency of spatially equivalent atoms, in randomly oriented molecules, is demonstrated by isolating (η6-C6H3F3)V in an Ar matrix and recording its electron paramagnetic resonance (EPR) spectrum. This is the first experimental evidence of magnetic inequivalency in an organometallic complex. Although this phenomenon should exist in all EPR spectra of rigid polyatomic molecules, the extent of its effects crucially depends on the unpaired electron distribution. The analysis of the complete EPR spectrum yields the following spin Hamiltonian parameters: gxx=gyy=1.967(3), gzz=2.012(3), |Axx(V)|=|Ayy(V)|=103.00(5)×10−4 cm−1 and |Azz(V)|=10.20(5)×10−4 cm−1. While the principal axes of the g and vanadium hyperfine tensors are parallel to the molecular symmetry axes, those of the three 19F hyperfine tensors are not aligned with one another. Consequently, their 19F hyperfine resonance positions are different and a detailed theoretical treatment of this phenomenon is required to fully comprehend and accurately simulate their EPR line shapes. The specific expressions that give rise to these magnetic inequivalencies and the g, 51V, 19F, and 1H hyperfine tensors are derived as a function of the molecular orbital (MO) coefficients. The magnitudes and signs of the MO coefficients are independently estimated by computing electronic structure of this transient using the local density functional (LDF) method. The simulation of the experimental EPR spectra followed by the comparison of the experimental and computed spin Hamiltonian tensor components reveal that the complex, when trapped in an Ar matrix at 15 K, has a 2A1 ground state. The LDF computations also predict that the molecule is metastable in the gas phase with the 2E and 2A1 states being nearly degenerate. Thus, the influence of the matrix trapping site is a decisive factor in isolating this molecule in a stable form and in determining its ground state. This also explains why the (η6-C6H3F3)V transient is difficult to isolate in high concentrations and to characterize by infrared and electronic absorption spectroscopy.