Magnetic Inequivalency, Electron Paramagnetic Resonance, Electronic Structure, Optimal Geometry, and Electronic Spectra of the 4,5-Bis(trifluoromethyl)-1,3,2-dithiazol-2-yl Radical†

Abstract

The geometry of the 4,5-bis(trifluoromethyl)-1,3,2-dithiazolyl radical, predicted by full geometry optimization, is found to be in very good agreement with that determined experimentally by electron diffraction. Post-SCF configuration-interaction (CI) computations must be performed to generate the weak transitions in the spectrum which are responsible for its characteristic blue color. The energy barrier due to the rotation of the two CF3 groups along their C−C axes is estimated to be approximately 60 kcal/mol. Thus, they are not expected to rotate freely at low temperatures and the six 19F atoms should be magnetically inequivalent. This is verified by electron paramagnetic resonance (EPR) spectroscopy of the isolated radical in an Ar matrix at 12 K and is the first experimental evidence of magnetic inequivalency in a main group inorganic cyclic complex. The inhomogeneous broadening due to the magnetic inequivalency of the 19F is large enough to mask their hyperfine splittings. The specific expressions that give rise to these magnetic inequivalencies in conjunction with the g and hyperfine tensors, as a function of the molecular orbital (MO) coefficients, are derived and are required to fully comprehend and accurately simulate the EPR spectra. The magnitudes and signs of the MO coefficients are independently estimated by computing its electronic structure using the B1LYP hybrid density functional 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 has a 2B ground state. Its spin Hamiltonian parameters are found to be gxx = 2.0020, gyy = 2.0004, gzz = 2.0124, Axx(14N) = 29.097 G, Ayy(14N) = 2.717 G, and Azz(14N) = 2.246 G. The high intensity at the low magnetic field end of the EPR spectrum is due to an extra “off-principal axes” resonance occurring in the xz molecular plane.