NOx Linkage Isomerization in Metal Complexes. NOx Related Chemistry. Elsevier; 2015. pp. 1-86. [DOI: 10.1016/bs.adioch.2014.11.003]

Exploring the electronic structure of [RuF5NO]2- ion by the generation of a metastable state (MS1)

Some mononitrosyl complexes of transition metals exhibit one or two metastable states (linkage isomers, MS1 and MS2) when irradiated at low temperatures with appropriate wavelengths. In this work, the generation of metastable state one (MS1) (or Ru-ON linkage isomer) in K2[RuF5NO].H2O at 77 K was studied by sample excitation using laser light in a wide range of wavelengths. The effects of irradiation was monitored by infrared spectroscopy. ν(NO) in the ground state was shifted by -161 cm-1 when the complex was excited to MS1, a magnitude similar to that observed in other transition metal nitrosyls for a such state. We report on the excitation and deactivation of metastable states by using a wide variety of laser lines. A novel method for exploring the electronic structure of [RuF5NO]2- through the generation of MS1 is proposed. For this purpose, a sample was carefully irradiated with the same intensity of light for all laser lines in the spectral region 260-1064 nm. The integrated area under the ν(NO)MS1 band was used as a measure of MS1 population. The profile peaks of the MS1 population (ν(NO)MS1 band area) vs. the irradiation wavelength fit well with those of the electronic spectrum of the [RuF5NO]2- ion in an aqueous solution. The onset temperature for MS1 decay in K2[RuF5NO].H2O, at approximately 180 K, is slightly lower than the average reported for other ruthenium-nitrosyl systems.

Projection-Based Density Matrix Renormalization Group in Density Functional Theory Embedding

The density matrix renormalization group (DMRG) method has already proved itself as a very efficient and accurate computational method, which can treat large active spaces and capture the major part of strong correlation. Its application on larger molecules is, however, limited by its own computational scaling as well as demands of methods for treatment of the missing dynamical electron correlation. In this work, we present the first step in the direction of combining DMRG with density functional theory (DFT), one of the most employed quantum chemical methods with favorable scaling, by means of the projection-based wave function (WF)-in-DFT embedding. On two proof-of-concept but important molecular examples, we demonstrate that the developed DMRG-in-DFT approach provides a very accurate description of molecules with a strongly correlated fragment.

Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst

Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron-nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm-2 h-1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.

Structural and Optical Properties of Metal-Nitrosyl Complexes

The electronic, structural and optical properties (including Spin–Orbit Coupling) of metal nitrosyl complexes [M(CN)₅(NO)]²⁻ (M = Fe, Ru or Os) are investigated by means of Density Functional Theory, TD-DFT and MS-CASPT2 based on an RASSCF wavefunction. The energy profiles connecting the N-bound (η¹-N), O-bound (η¹-O) and side-on (η²-NO) conformations have been computed at DFT level for the closed shell singlet electronic state. For each structure, the lowest singlet and triplet states have been optimized in order to gain insight into the energy profiles describing the conformational isomerism in excited states. The energetics of the three complexes are similar—with the N-bound structure being the most stable—with one exception, namely the triplet ground state of the O-bound isomer for the iron complex. The conformation isomerism is highly unfavorable in the S₀ electronic state with the occurrence of two energy barriers higher than 2 eV. The lowest bands of the spectra are assigned to MLCT_NO/LLCT_NO transitions, with an increasing MLCT character going from iron to osmium. Two low-lying triplet states, T1 (MLCT_NO/LLCT_NO) and T2 (MLCT_NO/IL_NO), seem to control the lowest energy profile of the excited-state conformational isomerism.