Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex.

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT, a detailed picture of environmental effects on charge-transfer-induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(µ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of water at constant temperature causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day class II/III high spin state to a charge-localized, class I low spin state consistent with DFT. In the presence of a H-bonding solvent, the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives an MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

Electronic structural comparison of the reactions of dioxygen and alkenes with nitrogen-chelated palladium(0).

The reaction of molecular oxygen with palladium(0) centers is a key step in Pd-catalyzed aerobic oxidation reactions. The present study provides a density functional theory (DFT) computational analysis of the mechanism and electronic structural features of the reversible, associative exchange between O(2) and ethylene at an ethylenediamine (en)-coordinated palladium(0) center. Salient features of the mechanism include: (1) the near thermoneutrality of the O(2)-alkene exchange reaction, consistent with experimentally observed reversible exchange between O(2) and alkenes at well-defined Pd centers, (2) end-on activation of triplet O(2) at an apical site of the trigonal Pd(0) center, resulting in formation of a Pd(I)(η(1)-superoxide) species, (3) rearrangement of the Pd(I)(η(1)-superoxide) species into a pseudo-octahedral (en)Pd(η(2)-O(2))(η(2)-C(2)H(4)) species with concomitant crossing from the triplet to singlet energy surfaces, and (4) release of alkene from an axial face of (en)Pd(II)(η(2)-peroxo) with a geometry in which the alkene leaves with an end-on trajectory (involving an interaction of the Pd d(z(2)) and alkene π* orbitals). This study highlights the similar reactivity and reaction pathways of alkenes and O(2) with an electron-rich metal center, despite the different ground-state electronic configurations of these molecules (closed-shell singlet and open-shell triplet, respectively).

Simulations of infrared spectra of nanoconfined liquids: acetonitrile confined in nanoscale, hydrophilic silica pores.

The infrared spectrum of acetonitrile confined in hydrophilic silica pores roughly cylindrical and 2.4 nm in diameter has been simulated using molecular dynamics. Hydrogen bonding interactions between acetonitrile and silanol groups on the pore wall involve charge transfer effects that have been incorporated through corrections based on electronic structure calculations on a dimer. The simulated spectrum of confined acetonitrile differs most prominently from that of the bulk liquid by the appearance of a blue-shifted shoulder, in agreement with previous experimental measurements. The dominant peak is little changed in position relative to the bulk liquid case, but broadened by approximately 40%. A detailed analysis of the structure and dynamics of the confined liquid acetonitrile is presented, and the spectral features are examined in this context. It is found that packing effects, hydrogen bonding, and electrostatic interactions all play important roles. Finally, the molecular-level information that can be obtained about the dynamics of the confined liquid from the infrared line shape is discussed.

Umbrella sampling of solute vibrational line shifts in mixed quantum-classical molecular dynamics simulations.

An umbrella sampling approach for vibrational frequency line shifts is presented. The technique allows for efficient sampling of the solvent configurations corresponding to frequency shifts of a solute in mixed quantum-classical simulations. The approach is generally applicable and can also be used within traditional perturbation theory calculations of frequency shifts. It is particularly useful in the extraction of detailed mechanistic information about the solute-solvent interactions giving rise to the frequency shifts. The method is illustrated by application to the simple I2 in a liquid Xe system, and the advantages are discussed.

Molecular-level mechanisms of vibrational frequency shifts in a polar liquid.

A molecular-level analysis of the origins of the vibrational frequency shifts of the CN stretching mode in neat liquid acetonitrile is presented. The frequency shifts and infrared spectrum are calculated using a perturbation theory approach within a molecular dynamics simulation and are in good agreement with measured values reported in the literature. The resulting instantaneous frequency of each nitrile group is decomposed into the contributions from each molecule in the liquid and by interaction type. This provides a detailed picture of the mechanisms of frequency shifts, including the number of surrounding molecules that contribute to the shift, the relationship between their position and relative contribution, and the roles of electrostatic and van der Waals interactions. These results provide insight into what information is contained in infrared (IR) and Raman spectra about the environment of the probed vibrational mode.

Mixed quantum-classical molecular dynamics analysis of the molecular-level mechanisms of vibrational frequency shifts.

A detailed analysis of the origins of vibrational frequency shifts of diatomic molecules (I2 and ICl) in a rare gas (Xe) liquid is presented. Specifically, vibrationally adiabatic mixed quantum-classical molecular dynamics simulations are used to obtain the instantaneous frequency shifts and correlate the shifts to solvent configurations. With this approach, important mechanistic questions are addressed, including the following: How many solvent atoms determine the frequency shift? What solvent atom configurations lead to blue shifts, and which lead to red shifts? What is the effect of solute asymmetry? The mechanistic analysis can be generally applied and should be useful in understanding what information is provided by infrared and Raman spectra about the environment of the probed vibrational mode.

Insights into the spin-forbidden reaction between L2Pd0 and molecular oxygen.

The reaction between dioxygen and palladium(0) is a key step in palladium-catalyzed aerobic oxidation reactions. The spin-forbidden reaction between (en)Pd and O2 has been analyzed by spin-unrestricted density functional methods and shown to proceed by a stepwise process involving (1) formation of a triplet eta1-superoxo-PdI adduct, (2) spin-crossover from the triplet to the singlet surface, and (3) collapse of the singlet eta1-superoxo-PdI adduct into an eta2-peroxo-PdII complex. Delocalization of spin density from triplet O2 onto the palladium center in the first step reduces the exchange interaction between the unpaired spins and facilitates crossover from the triplet to singlet surface.

"Inverse-electron-demand" ligand substitution: experimental and computational insights into olefin exchange at palladium(0).

The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine-palladium(0) complexes bearing trans-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (3(X)), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3). Density functional theory calculations of model alkene-substitution reactions at a diimine-palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty pi orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an "inverse-electron-demand" ligand-substitution reaction.