Teller Distortion Research Articles

Second-Order Jahn−Teller Distortions in Group 17 Fluorides EF3 (E = Cl, Br, I, and At). Large Relativistic Bond Angle Changes in AtF3

Group 17 element fluorides EF3 (E = Cl, Br, I) are well-known to undergo second-order Jahn−Teller symmetry breaking toward a T-shaped C2v arrangement mainly due to a1‘HOMO ⊗ e‘LUMO mixing at the expected symmetric trigonal planar D3h state. For heavy elements, the a1‘ HOMO is relativistically stabilized because of large element s-orbital participation. Hence, relativistic effects diminish the second-order Jahn−Teller term. This results in a large relativistic change in the Feq−E−Fax bonding angle of αeR − αeNR = 5.5° in the case of AtF3 and causes an anomaly in the bond angle behavior down the group 17 compounds, α(ClF3) > α(BrF3) > α(AtF3) > α(IF3). Furthermore, the difference between the symmetric D3h and the distorted C2v structure of AtF3 is only 10 kJ/mol at the coupled cluster level of theory, indicating that the measured Feq−At−Fax angle αe will be very sensitive upon the temperature applied in gas phase diffraction studies. Vibrational frequencies are predicted for all group 17 fluorides EF3. As a consequence of the second-order Jahn−Teller distortion, the A1 symmetric bending mode is strongly influenced by relativistic effects and becomes much lower in frequency compared to the B1 out of plane mode for the heavier elements. With the exception of IF3, the symmetric D3h structure represents a (metastable) weak local minimum at the MP2 level, rather than a transition state as expected. The D3h point represents, however, a second-order saddle point at the HF level, and therefore, electron correlation seems to be responsible for changing the nature of the trigonal planar structure. Extended basis sets at the MP2 level as well as coupled cluster calculations were applied in order to obtain more accurate information for the energetics and structure of ClF3. These studies show, however, that the nature of the D3h point is critically dependent upon the basis set (and the electron correlation procedure) applied.

Studies on Aerobic Reactions of Ammonia/3,5-Di-tert-butylcatechol Schiff-Base Condensation Products with Copper, Copper(I), and Copper(II). Strong Copper(II)−Radical Ferromagnetic Exchange and Observations on a Unique N−N Coupling Reaction

Stoichiometric quantities of 3,5-di-tert-butylcatechol and aqueous ammonia react in pyridine solution to form 2-amino-4,6-di-tert-butylphenol. Under an atmosphere of dioxygen the aminophenol is oxidized to either the corresponding iminosemiquinone or iminobenzoquinone. In the presence of Cu(II) iminosemiquinone condensation with the aminophenol gives the Cat-N-SQ radical ligand obtained as the Cu(py)2(Cat-N-SQ) complex. Metal and ligand magnetic orbitals are orthogonal and couple ferromagnetically to give a S = 1 molecular spin state at temperatures up to 300 K. In nonpolar solvents the complex undergoes ligand oxidation and disproportionation to give Cu(Cat-N-BQ)2. Crystallographic characterization on crystals obtained as the i-propanol solvate [orthorhombic, C2221, a = 19.548(3) Å, b = 24.536(5) Å, c = 23.655(5) Å, V = 11346(4) Å3, Z = 8, R = 0.068] show that the expected Jahn−Teller distortion appears in the trans Cu−O lengths of the equatorial plane rather than for the axial Cu−N lengths. Reactions carried out with both Cu(I) and Cu metal require metal oxidation to give the Cu(II) products obtained. With metallic Cu this occurs by a reaction with iminoquinone to give bis(iminosemiquinone)copper(II). Further reaction of this product with aminophenol gives Cu(py)2(Cat-N-SQ) by condensation, and, with O2, oxidation gives a coordinated azophenolate ligand in Cu(py)(azophenolate) by a unique N−N bond-forming reaction. Cu(py)(azophenolate) has been characterized crystallographically [monoclinic, P21/n, a = 10.990(2) Å, b = 10.736(3) Å, c = 26.848(4) Å, β = 98.09(1)°, V = 3136(1) Å3, Z = 4, R = 0.048].

Electron spin resonance study of the radical anions of substituted cyclooctatetraenes. The effects of Jahn-Teller distortions and vibronic mixing

The radical anions of methyl-, methoxy-, fluoro-, cyclopropyl-, and cyanocyclooctatetraene were generated by electrolytic reduction of the neutral compounds in liquid ammonia. The electron spin resonance spectrum of each radical anion is assigned, and a clean even--odd alternation of the spin densities is observed in each. This alternation can be correlated with the electronic properties of the substituent. For the radical anions of methyl-, methoxy-, and cyclopropylcyclooctatetraene, the temperature dependence of the coupling constants is reported. The results are accounted for in terms of vibronic mixing and Jahn--Teller distortions, and are discussed in relation to INDO calculations. 3 figures, 7 tables.

Multiphonon Vibronic TransitionT2g1−A2g3ofNi2+in MgO

The ${\ensuremath{\Gamma}}_{5g}(^{1}T_{2g},{t}_{2g}^{5}{e}_{g}^{3})\ensuremath{-}{\ensuremath{\Gamma}}_{5g}(^{3}A_{2g},{t}_{2g}^{6}{e}_{g}^{2})$ vibronic spectrum is recorded in absorption and emission---under x-ray excitation---at crystal temperatures of 77 and 5 \ifmmode^\circ\else\textdegree\fi{}K. The spectrum involves up to four phonon processes and a theoretical treatment of such a transition is presented. A possible explanation of the band is suggested and, by using imperfect-lattice Green's functions for the nearest-neighbor motion, it is constructed to give excellent agreement with experiment. The transition is forced (electric dipole) by one of the ${T}_{1u}$ vibrations of the nearest-neighbor complex. Additional ${E}_{g}$ vibrations couple in the higher phonon processes which suggest the presence of a Jahn- Teller distortion in the $^{1}T_{2g}$ excited state. However, it has not been possible to establish the presence of such a Jahn- Teller distortion from any other experimental data. A similar calculation is undertaken for the two-phonon band of the ${\ensuremath{\Gamma}}_{3g}(^{3}T_{2g},{t}_{2g}^{5},{e}_{g}^{3})\ensuremath{-}{\ensuremath{\Gamma}}_{5g}(^{3}A_{2g},{t}_{2g}^{6}{e}_{g}^{2})$ transition of ${\mathrm{Ni}}^{2+}$ in MgO. Again there is an indication of a coupling to ${E}_{g}$ vibrations in the two-phonon process.