Single-crystal EPR study of a mixed-valence ruthenium dimer ion: the ground state of the creutz-taube complex

Single-crystal electron paramagnetic measurements have been carried out on an orthorhombic chlorotetrabromo salt of the mixed-valence μ-pyrazinc-decamminediruthenium(5⁺) ion (Creutz-Taube ion) Below ≈ 200 K the spectrum consists of two lines arising from two magnetically inequivalent but crystallographically equivalent centres. The resonances can be described by a g tensor with principal axes aligned with

and x₁.y₁:x₂.y₂ in the ab plane. At 113 K the comp of the g tensor are g_x = 2.779(5). g_y = 2.489(5). g_z = 1.334(10). The point-group symmetry about the centre of unit is C₂h (m/2) with the c axis being the two-fold axis. The g_z component is aligned perpendicular to the plane of the pyrazine ring and perpendicular to the Ru-Ru axis (X). The remaining components should be aligned closely wilh X.Y. The angle between the X axes for the two sites as determined by X-ray measurements, is 85.8° compared with 82.5 ± 1° between the x₁.x₂g tensor axes. Magnetic exchange coupling between the dimer ions is very small. The tetragonal approximation to the observed g tensor is g₁ = 2.632 ± 0.005. g₁₁ = 1.334 ± 0.010. The theory of the g tensor for this complex is discussed, it is shown that these values are within the range predicted for an ion with a symmetrical delocalized ground state, the orientation of the g₁₁ component normal to the plane of the pyrazine ring is also predicted for a delocalized ground state. The EPR results arc thus consistent with a stable single-minimum ground state for the Creutz-Taube ion Powder EPR experiments on a tosylate salt of this ion have previously been reported by Bunker et al. A g₁₁ signal was not observed and it was assumed to be very small (? 0.6) and oriented along the Ru-Ru axis, however the absence of a signal at g ≈ 1.3 (in fact oriented perpendicular to the Ru-Ru axis) was presumably due to poor statistics for g₁₁ in EPR measurements on powder samples. The proposal of these authors that EPR measurements provide evidence for a trapped-valence ground state for the Creutz-Taube ion thus cannot be accepted.     

Switchable electronic coupling in model oligoporphyrin molecular wires examined through the measurement and assignment of electronic absorption spectra

The use of a quinone functionality in the linkage unit of laterally bridged oligoporphyrins as a switch for controlling electronic coupling between the termini is examined. The quinone-bridged bisporphyrin P(2)TA-O(2) was synthesized by condensation of 2 equiv of the dione 2,3-dioxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin with 2,3,5,6-tetraamino-1,4-benzoquinone. The electronic absorption spectra of P(2)TA-O(2) and its conjugated benzenoid analogue P(2)TA are measured and assigned, in conjunction with the spectra of the fragment monomers and porphyrin-bridge compounds. Band homologies and CASPT2 calculations are used to make the assignments. Chemically, the dimer in one case is bridged by a through-conjugated, pi-delocalized 1,4,5,8-tetraazaanthracene molecule. This is shown to display significant inter-porphyrin coupling, with an observed difference in the exciton couplings of the B(x) and B(y) bands being ca. 0.18 eV. However, the other dimer is bridged using a derivative in which the central ring is converted to a cross-conjugated, pi-localized quinonoid form; this molecule displays no observable inter-porphyrin coupling. This scenario provides a paradigm for the use of molecular electronic devices in sensing, control, and high-capacity relatively low-speed data storage applications.     

Calculation of atomic and molecular photoionization cross sections using the ground state inversion potential method

Calculations of atomic photoionization cross sections to near Hartree-Fock accuracy can be achieved very simply if the non-local Hartree-Fock potential experienced by the photoelectron is replaced by a local potential obtained by inversion of the corresponding ground state orbital. Subshell cross sections for atomic photoionizatim for representative atoms in the range 0–1500 eV calculated in this way are in good general agreement with Hartree-Fock values and with experiment. The extension to molecular systems and the use of an independent atomic centre approach to calculation of molecular photoionization cross sections at low photon energies is outlined.     

Chain-branching control of the atomic structure of alkanethiol-based gold-sulfur interfaces

Density functional theory structure calculations at 0 K and simulations at 300 K of observed high-resolution in situ scanning tunneling microscopy (STM) images reveal three different atomic-interface structures for the self-assembled monolayers (SAMs) of three isomeric butanethiols on Au(111): direct binding to the Au(111) surface without pitting, binding to adatoms above a regular surface with extensive pitting, and binding to adatoms with local surface vacancies and some pitting. Thermal motions are shown to produce some observed STM features, with a very tight energy balance controlling the observed structures. Variation of the degree of substitution on the α carbon is found to significantly change the relative energies for interaction of the different types of adatom structures with the surface, while the nature of the surface cell, controlled primarily by inter-adsorbate steric interactions, controls substrate reorganization energies and adsorbate distortion energies. Most significantly, by manipulating these features, chemical control of the adsorbate can produce stable interfaces with surface pitting eliminated, providing new perspectives for technological applications of SAMs.