Large Jahn-Teller Distortion Research Articles

Experimental and theoretical study on cation radicals of cyclopropane, cyclobutane and cyclopentane

Abstract Cation radicals of cycloalkanes have been produced for the first time in γ-irradiated rigid solutions using fluoromethane as a matrix. Observed ESR spectra are analyzed by ab initio MO calculations; molecular geometries of the cations are optimized for several low-lying doublet states by the energy gradient method. Based on the optimized geometry thus determined the hyperfine splitting constants are calculated by the pseudo-orbital theory. Large Jahn-Teller distortions are calculated for the carbon rings and the H-C-H frames of the cation radicals. The distortions are consistent with the nodal picture of singly occupied orbitals. The calculated hfs constants are sensitive to the change in geometry due to a large contribution of spin-delocalization. The average of the calculated proton hfs constants is compatible with the observed ESR spectra which indicates that all the protons of the individual cations have become equivalent owing to the dynamic Jahn-Teller distortion.

Ne matrix spectra of the sym-C 6Br 3F 3+ radical cation

The electronic absorption and laser excited, wavelength resolved fluorescence spectra of the title cation have been observed in solid Ne matrix and vibrationally analysed. The vibrational structure of the excited B 2A 2 ″ state shows close similarity to the parent compound. The X 2E″ ground state structure is strongly perturbed and irregular owing to a large Jahn—Teller distortion. The data are analysed in terms of a recently developed, sophisticated multimode Jahn—Teller theoretical model. We have generated the sym-C 6Br 3F 3 + cations in solid Ne matrix and obtained their wavelength resolved emission and absorption spectra. T ground electronic X 2E″ state exhibits an irregular and strongly perturbed vibrational structure, which can be successfully modeled using sophisticated multimode Jahn—Teller theory.

Linear Combination of Atomic Orbital-Molecular Orbital Treatment of the Deep Defect Level in a Semiconductor: Nitrogen in Diamond

A deep defect level in a semiconductor is simulated by a large cluster of host atoms surrounding the defect. The electronic states of the entire system are then computed by linear combination of atomic orbital-molecular orbital techniques. The nitrogen donor in diamond is treated as an example. A large Jahn-Teller distortion is predicted which forces the donor state down close to the valence band, in good agreement with experiment. The calculated donor wave function agrees with EPR and electron-nuclear double resonence results.

Negative Ion Formation in Selected Hexafluoride Molecules

Negative ion formation in gaseous SeF6, TeF6, MoF6, ReF6, and UF6 has been studied as a function of incident electron energy in the region from 0 to ∼10 eV. Negative hexafluoride ions formed by direct electron attachment were observed only in the cases of MoF6 and ReF6. SeF6−, TeF6−, and UF6− were produced by charge exchange with thermal SF6−* and rates for these reactions and for the reaction UF5− + UF6→UF6− + UF5 were measured and are reported here. Electrons having energies close to zero are known to attach to SF6 with a cross section approaching the maximum for s-wave capture. The excited negative ion SF6−* formed in this way has a lifetime of 26 μsec against autodetachment of the electron. An argument based on molecular orbital theory is given for the existence of a Jahn–Teller effect in SF6−. This could provide a means for the coupling between electronic and nuclear motion needed to trap the low-energy electron. Selenium and tellurium lie directly under sulfur in the same column of the periodic table, but the large spin-orbit splittings expected in SeF6 and TeF6 should dominate any Jahn–Teller effect in these otherwise similar molecules. The suggestion that a Jahn–Teller effect provides the coupling mechanism between electronic and nuclear motion is partially supported by the observation that ReF6 (large Jahn–Teller distortion) strongly captures slow electrons to form ReF6−*.

The optical absorption of divalent chromium in CrCl2. 4H2O and CrSO4. 7H2O

Abstract The optical absorption spectrum of single crystals of CrCl2 . 4H2O has been studied at room temperature and at 77°k. The broad band at 14 300 cm-1 in the solution spectrum is resolved into three bands at 13 100 cm-1, 15 200 cm-1 and 18 800 cm-1 in the crystal spectrum at 77°k. Two of these bands are strongly polarized. Many weak absorption lines have also been detected in the region 16 000—24 000 cm-1 and the observed temperature and polarization effects are recorded. The spectrum is compared with the energy levels predicted by ligand field theory and the presence of a large Jahn—Teller distortion in this salt is confirmed. The spectrum of CrSO4. 7H2O has been found to be very similar to the chloride spectrum.