Origin of the Rarely Reported High Performance of Mn-doped Carbon-based Oxygen Reduction Catalysts.

Recent efforts to develop durable high-performance platinum-group metal (PGM)-free oxygen reduction reaction (ORR) electrocatalysts have focused on Fe- and Co-based molecular and pyrolyzed catalysts. While Mn-based catalysts have advantages of lower toxicity and higher durability, their activity has been generally poor. Nevertheless, several examples of high-performance Mn-based catalysts have been reported. Thus, it is necessary to understand why Mn-based materials much more rarely show high catalytic ORR performance and to determine the factors that can lead to the achievement of such high performance in these rare cases. We have studied the effects of the changes in the macrocycle structure, axial ligand, distance between the active sites, interactions with the dopant N atoms and the presence of an extended carbon network on the ORR catalysis of various Mn-, Fe-, and Co-based systems through the comparison of the adsorption energies of the ORR intermediates. We find that the sensitivity to the local environment changes is the largest for Mn and is the smallest for Co, with Fe between Mn and Co. Our results showed that the strong binding of OH by Mn and the strong sensitivity of the Mn to the modification of its environment necessitate a precise combination of local environment changes to achieve a high onset potential (Vonset ) in Mn-based catalysts. By contrast, the weaker binding of OH by Fe and Co and their weaker sensitivity to local environment changes lead to a wide variety of local environments with favorable catalytic activity (Vonset >0.7 V) for Co- and Fe-based systems. This explains the scarcity of reported Mn-based pyrolyzed catalysts and suggests that precise material synthesis and engineering of the active site can achieve high-performance Mn-based ORR electrocatalysts with high activity and durability.

Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups.

In the search for replacement of the platinum-based catalysts for fuel cells, MN4 molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum-group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e- to 4e- pathway in the pH range of 3.5-6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pKa values of the protonation sites in the vicinity of the ORR active sites. Control of the electron-withdrawing nature of these groups characterized by the pKa values could switch the ORR from the H+ to e- rate-determining step mechanisms and from 2e- to 4e- ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pKa of the N atoms at the active sites can be used as a descriptor for the design of high-performance, durable PGM-free catalysts.

Vibronic Coupling in the X̃2Πg-Ã2Πu Band System of Diacetylene Radical Cation.

Vibronic interactions in the two energetically lowest electronic states (X̃2Πg-Ã2Πu) of the diacetylene radical cation (C4H2•+) are theoretically examined here. The spectroscopy of these two electronic states of C4H2•+ has been a subject of considerable interest and measured in the laboratory by various groups. Inspired by numerous experimental data, we attempt here a detailed investigation of vibronic interactions within and between the doubly degenerate Π electronic states and their impact on the vibronic structure of each state. A vibronic coupling model is constructed in a diabatic electronic basis and with the aid of ab initio quantum chemistry calculations. The vibronic structures of the electronic states are calculated by time-independent and time-dependent quantum mechanical methods. The progression of vibrational modes in the vibronic band is identified, assigned, and compared with the literature data. The nonradiative internal conversion dynamics is also examined and discussed.

Theoretical Study on Molecules of Interstellar Interest. II. Radical Cation of Compact Polycyclic Aromatic Hydrocarbons.

Radical cations of polycyclic aromatic hydrocarbons have been postulated to be molecular carriers of diffuse spectroscopic features observed in the interstellar medium. Several important observations made by stellar and laboratory spectroscopists motivated us to undertake a detailed theoretical study attempting to validate the recorded data. In continuation of our work on this subject, we here focus on a detailed theoretical study of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of phenanthrene, pyrene, and acenaphthene molecule. A multistate and multimode theoretical model in a diabatic electronic basis is developed here through extensive ab initio quantum chemistry calculations. Employing this model, first-principles nuclear dynamics calculations are carried out to unravel the spectral assignment, time-dependent dynamics, and photostability of the mentioned electronic states of the radical cations. The theoretical results compare well with the observed experimental data.

Theoretical study on molecules of interstellar interest. I. Radical cation of noncompact polycyclic aromatic hydrocarbons.

Polycyclic aromatic hydrocarbons (PAHs), in particular, their radical cation (PAH(+)), have long been postulated to be the important molecular species in connection with the spectroscopic observations in the interstellar medium. Motivated by numerous important observations by stellar as well as laboratory spectroscopists, we undertook detailed quantum mechanical studies of the structure and dynamics of electronically excited PAH(+) in an attempt to establish possible synergism with the recorded data. In this paper, we focus on the quantum chemistry and dynamics of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of tetracene, pentacene, and hexacene molecule. This study is aimed to unravel photostability, spectroscopy, and time-dependent dynamics of their excited electronic states. In order to proceed with the theoretical investigations, we construct suitable multistate and multimode Hamiltonians for these systems with the aid of extensive ab initio calculations of their electronic energy surfaces. The diabatic coupling surfaces are derived from the calculated adiabatic electronic energies. First principles nuclear dynamics calculations are then carried out employing the constructed Hamiltonians and with the aid of time-independent and time-dependent quantum mechanical methods. The theoretical results obtained in this study are found to be in good accord with those recorded in experiments. The lifetime of excited electronic states is estimated from their time-dependent dynamics and compared with the available data.

Influence of the substituents on the electronic and electrochemical properties of a new square-planar nickel-bis(quinoxaline-6,7-dithiolate) system: synthesis, spectroscopy, electrochemistry, crystallography, and theoretical investigation.

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E(1/2) = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni(X(2)6,7-qdt)(2)](-)/[Ni(X(2)6,7-qdt)(2)](2-). Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh(4)](2)[Ni(Ph(2)6,7-qdt)(2)]·3DMF (2b) for its structural characterization.