Theoretical Modeling of Low-Energy Electronic Absorption Bands in Reduced Cobaloximes

The reduced Co^I states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task

Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(I) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies

Thermodynamic and kinetic parameters for the oxidative addition of H_2 to [Rh^I(bpy)_2]^+ (bpy = 2,2‘-bipyridine) to form [Rh^(III)(H)_2(bpy)_2]^+ were determined from either the UV−vis spectrum of equilibrium mixtures of [Rh^I(bpy)_2]^+ and [Rh^(III)(H)_2(bpy)_2]^+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh^(III)(H)_2(bpy)_2]^+ as a function of H_2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol^(-1) and −18 cal mol^(-1) K^(-1), respectively. The reaction enthalpy and entropy are −10.3 kcal mol^(-1) and −19 cal mol^(-1) K^(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (−15 and −16 cm^3 mol^(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (−15 cm^3 mol^(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H−H bond (0.823 and 0.810 Å, respectively) and forms only a weak Rh−H bond (1.866 and 1.915 Å, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process

CALCULATION OF GAS-PHASE ELECTRONIC SPECTRA OF TRANSITION-METAL COMPLEXES

This work was performed at Brookhaven National Laboratory and funded under contract DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences. The U.S Department of Energy is also gratefully acknowledged for funding under the BES Hydrogen Fuel Initiative.Author Institution: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000Despite widespread interest in the electronic structure and spectra of transition-metal complexes such as the starting and intermediate species of redox catalysts for water oxidation or hydrogen production, theoretical predictions of their electronic spectra generally come only from time-dependent density functional theory (TD-DFT) and are limited to one-electron excitations. Here we compare the results of TD-DFT to those of multi-reference configuration-interaction calculations for predicting the electronic spectra of some model and actual transition-metal-containing catalysts. Of particular interest are complexes with redox-active ligands that complicate the assignment of formal oxidation states, and complexes containing second- and third-transition-series metals

ANOMALOUS ISOTOPIC EFFECT IN THE H-Xe STRETCHING FREQUENCY OF HXeOH

Author Institution: Department of Chemistry, Brookhaven National LaboratoryThe rovibrational energy levels of HXeOH and its isotopomers have been calculated using a two-layer Lanczos algorithm in Radau-diatom-Jacobi coordinates based on a high level ab initio potential energy surface. The surface is obtained by fitting to 1229 RCCSD(T)/SDB-cc-pVQZ energy points. The equilibrium geometry of HXeOH is determined to have a trans configuration with a nearly collinear HXeO bond angle of 177.32 degrees. The well depth of this minimum is only 0.6123 eV with respect to the OH + Xe + H dissociation limit. The results show that the H-Xe stretching frequency of HXeOH and HXeOD has an anomalous isotopic shift upon $^{18}O$ isotope substitution, whereas the D-Xe stretch in DXeOH and DXeOD displays a normal isotopic shift. This trend is consistent with the experimental observations in a Xe solid matrix. The present results predict a lower frequency for the H-Xe stretch than was observed in the solid matrix expriment. Either these results are too low or there is a strong blue shift due to the matrix in the experimental values

THE PERMANENT ELECTRIC DIPOLE MOMENTS OF THE X3Δ,E3Π,A3ΦX^{3} \Delta, E^{3}\Pi, A^{3}\Phi AND B3ΠB^{3}\Pi STATES OF TITANIUM MONOXIDE, TiO

Author Institution: Department of Chemistry and Biochemistry, Arizona State University; Chemistry Department, Brookhaven National LaboratoryThe optical Stark spectrum of the origin bands of the $E^{3}\Pi_{0} - X^{3}\Delta_{1}, A^{3}\Phi_{2} - X^{3}\Delta_{1}$, and $B^{3}\Pi_{0} - X^{3}\Delta_{1}$ band systems of titanium monoxide, TiO, were analyzed to produce permanent electric dipole moments of $3.34(1)D, 3.2(4)D, 4.89(5)D$ and $4.9(2)D$ for the $X^{3}\Delta_{1}, E^{3}\Pi_{0}, A^{3}\Phi_{2}$ and $B^{3}\Pi_{0}$ states, respectively. The observations are compared with a simple molecular orbital description for the low-lying states and electronic structure calculations