Electronic Structure of the Ground and Excited States of β‐Carboline

Coupled‐cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of β‐carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.     

Ab initio and semiempirical computational studies on 1‐{(2E)‐2‐[(aminocarbonothioyl)hydrazono]‐2‐(3‐mesityl‐3‐methylcyclobutyl)ethyl}‐pyrrolidine‐2,5‐dione

The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) ¹H‐ and ¹³C NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree‐Fock (HF) and density functional theory (DFT) methods with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and ¹H‐ and ¹³C NMR chemical shift values show good agreement with experimental data. To determine conformational flexibility, the molecular energy profile of the title compound was obtained by semiempirical (AM1) calculations with respect to the selected torsion angle, which was varied from ?180° to +180° in steps of 10°. The energetic behavior of the title compound in solvent media was examined using the B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Qnsager's method. By using TD‐DFT method, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD‐DFT method and the experimental one is determined. The predicted nonlinear optical properties of the title compound are much greater than ones of urea. In addition, the molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, NBO analysis and thermodynamic properties of the title compound were investigated using theoretical calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Application of the Perimeter Model to the Assignment of the Electronic Absorption Spectra of Gold(III) Hexaphyrins with [4n+2] and [4n] π‐Electron Systems

Expanded porphyrins : The electronic excited states of two forms of meso‐hexakis(pentafluorophenyl)‐substituted gold(III) hexaphyrin(1.1.1.1.1.1), such as that depicted, have been investigated by density functional calculations and magnetic circular dichroism spectroscopy to assign their low‐energy excited singlet states.

     

Retro‐Diels–Alder Approach to the Synthesis of π‐Expanded Azuliporphyrins and Their Porphyrinoid Aromaticity

Bicyclo[2.2.2]octadiene (BCOD) fused azuliporphyrins were synthesized by 3+1 porphyrin synthesis of azulitripyrranes with diformylpyrroles. Subsequent retro‐Diels–Alder reaction of the BCOD‐fused azuliporphyrins afforded azulibenzo‐, azulidibenzo‐, and azulitribenzoporphyrins 1 – 5 . NMR and UV/Vis spectra, as well as nucleus‐independent chemical shift (NICS) calculations revealed that 1 – 5 and their diprotonated dications exhibit relatively low porphyrinoid aromaticity, which was dependent on the position and number of fused benzene rings present.     

Theoretical study of the low‐lying electronic states of the molecular ion KRb+

The potential energy curves of the molecular ion KRb⁺ have been investigated for the 60 lowest molecular states of symmetry ²Σ⁺, ²Π, ²Δ, and Ω = 1/2, 3/2, and 5/2. Using an ab initio method, the calculation has been done in a one active electron approach based on nonempirical pseudopotentials with core valence effects taken into account through parameterized l‐dependent polarization potentials. Using the canonicals functions approach a rovibrational study is done by calculating the eigenvalues E_v, the rotational constants B_v, the centrifugal distortion constants D_v (up to 135 vibrational levels), and the spectroscopic constants ω_e and B_e for the five electronic states (1)²Σ⁺, (3)²Σ⁺, (1)²Π, (1)Ω = 1/2, and (1)Ω = 3/2. No comparison of these values with other results is yet possible because they are given here for the first time. Extensive tables of energy values of E_v, B_v, and D_v are displayed at http://hplasim2.univ‐lyon1.fr/allouche . © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Ultrafast Deactivation Mechanism of the Excited Singlet in the Light‐Induced Spin Crossover of [Fe(2,2′‐bipyridine)3]2+

The mechanism of the light‐induced spin crossover of the [Fe(bpy)₃]²⁺ complex (bpy=2,2′‐bipyridine) has been studied by combining accurate electronic‐structure calculations and time‐dependent approaches to calculate intersystem‐crossing rates. We investigate how the initially excited metal‐to‐ligand charge transfer (MLCT) singlet state deactivates to the final metastable high‐spin state. Although ultrafast X‐ray free‐electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high‐spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d⁶(${{\rm t}{{5\hfill \atop {\rm 2g}\hfill}}}$ ${{\rm e}{{1\hfill \atop {\rm g}\hfill}}}$ ) configuration either directly or by means of a triplet MLCT state. This triplet ligand‐field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower‐lying triplet state and subsequent intersystem crossing to the high‐spin state. The deactivation rate to the low‐spin ground state is much smaller, which is in line with the large quantum yield reported for the process.     

A New Conformation With an Extraordinarily Long, 3.04 Å Two‐Electron,Six‐Center Bond Observed for the π‐[TCNE]22− Dimer in [NMe4]2[TCNE]2 (TCNE=Tetracyanoethylene)

[NMe₄]₂[TCNE]₂ (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe₄)CN in MeCN has ν_CN IR absorptions at 2195, 2191, 2172, and 2156 cm^?1 and a ν_CC absorption at 1383 cm^?1 that are characteristic of reduced TCNE. The TCNEs have an average central C?C distance of 1.423 Å that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non‐eclipsed, centrosymmetric π‐[TCNE]₂^2? dimer with nominal C₂ symmetry, 12 sub van der Waals interatomic contacts <3.3 Å, a central intradimer separation of 3.039(3) Å, and comparable intradimer C???N distances of 3.050(3) and 2.984(3) Å. The two pairs of central C???C atoms form a ?C?C???C?C of 112.6° that is substantially greater than the 0° observed for the eclipsed D_2h π‐[TCNE]₂^2? dimer possessing a two‐electron, four‐center (2e^?/4c) bond with two C???C components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc‐pVTZ and atoms‐in‐molecules (AIM) calculations indicates that the non‐eclipsed, C₂ π‐[TCNE]₂^2? dimer exhibits a new type of a long, intradimer bond involving one strong C???C and two weak C???N components, that is, a 2e^?/6c bond. The C₂ π‐[TCNE]₂^2? conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/k_B=1000 K (700 cm^?1; 86.8 meV; 2.00 kcal mol^?1; H=?2 JS_a?S_b); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal mol^?1 higher in energy than the closed‐shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc‐pVTZ calculations indicate that the C₂ and D_2h conformers have two different local metastable minima with the C₂ conformer being 1.3 kcal mol^?1 less stable. The different natures of the C₂ and D_2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e^?/6c bond with one C???C and two C???N bonding components for the C₂ conformer as compared to the 6e^?/4c bond for the D_2h conformer with two C???C bonding components.     

12‐ to 22‐Membered Bridged β‐Lactams as Potential Penicillin‐Binding Protein Inhibitors

As potential inhibitors of penicillin‐binding proteins (PBPs), we focused our research on the synthesis of non‐traditional 1,3‐bridged β‐lactams embedded into macrocycles. We synthesized 12‐ to 22‐membered bicyclic β‐lactams by the ring‐closing metathesis (RCM) of bis‐ω‐alkenyl‐3(S)‐aminoazetidinone precursors. The reactivity of 1,3‐bridged β‐lactams was estimated by the determination of the energy barrier of a concerted nucleophilic attack and lactam ring‐opening process by using ab initio calculations. The results predicted that 16‐membered cycles should be more reactive. Biochemical evaluations against R39 DD‐peptidase and two resistant PBPs, namely, PBP2a and PBP5, revealed the inhibition effect of compound 4d , which featured a 16‐membered bridge and the N‐tert‐butyloxycarbonyl chain at the C3 position of the β‐lactam ring. Surprisingly, the corresponding bicycle, 12d , with the PhOCH₂CO side chain at C3 was inactive. Reaction models of the R39 active site gave a new insight into the geometric requirements of the conformation of potential ligands and their steric hindrance; this could help in the design of new compounds.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

Effects of the Au(I)–Au(I) closed‐shell attraction on the electronic and phosphorescent properties in a series of coordination compounds: A theoretical study

The Au(I)–Au(I) closed‐shell or aurophilic attraction has been the subject of interest in the experimental and theoretical chemistry fields, due to the intriguing properties associated to it. The presence of phosphorescence in “aurophilic” compounds has been addressed to a wide range of applications, but it has not yet been fully understood. A theoretical study on the electronic and phosphorescent properties of the following series of dinuclear gold complexes has been performed: [Au₂(dmpm) (i‐mnt)] ( 1 ), [Au₂(μ‐Me‐TU) (μ‐dppm)] ( 2 ), and [Au₂(μ‐G)(μ‐dmpe)] ( 3 ). Full geometry optimizations at the second‐order Møller–Plesset perturbation theory (MP2) were carried out for each of the species. These calculations made evident that, at the ground‐state geometry, the Au(I) cations allocated at the center of the ring show a short Au–Au distance below the sum of the van der Waals radii, at the range of the aurophilic attraction. An intermolecular Au(I)–Au(I) closed‐shell attraction for a pair of the systems under study is found. This attraction is comparable to that of the hydrogen bonds. The phosphorescent properties experimentally observed for this series were also characterized through ab initio techniques. The obtained results allow to fit reasonably the excitation energies with the experimental data and to identify a correlation between the strength of the Au(I)–Au(I) interaction and the phosphorescent behavior. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Enhanced Li Adsorption and Diffusion in Single‐Walled Silicon Nanotubes: An ab Initio Study

We report a first‐principles investigation of Li adsorption and diffusion in single‐walled Si nanotubes (SWSiNTs) of interest to Li‐ion battery anodes. We calculate Li insertion characteristics in SWSiNTs and compare them with the respective ones in carbon nanotubes (CNTs) and other silicon nanostructures. From our calculations, SWSiNTs show higher reactivity toward the adsorption of Li adatoms than CNTs and Si nanoclusters. Considering the importance of Li kinetics, we demonstrate that the interior of SWSiNTs may serve as a fast Li diffusion channel. The important advantage of SWSiNTs over their carbon analogues is a sevenfold reduction in the energy barrier for the penetration of the Li atoms into the nanotube interior through the sidewalls. This prepossesses easier Li diffusion inside the tube and subsequent utilization of the interior sites, which enhances Li storage capacity of the system. The improvements in both Li uptake and Li mobility over their analogues support the great potential of SWSiNTs as Li‐ion battery anodes.     

Na4IrO4: Square‐Planar Coordination of a Transition Metal in d5 Configuration due to Weak On‐Site Coulomb Interactions

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition‐metal complexes. For example, square‐planar coordination geometries found in transition‐metal oxometalates such as cuprates are usually associated with the d⁸ or d⁹ electron configuration. In this work, we address an unusual square‐planar single oxoanionic [IrO₄]^4? species, as observed in Na₄IrO₄ in which Ir^IV has a d⁵ configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir^IV center in ground‐state Na₄IrO₄ has square‐planar coordination geometry because of the weak Coulomb repulsion of the Ir‐5d electrons. In contrast, in its 3d counterpart Na₄CoO₄, the Co^IV center is tetrahedrally coordinated because of strong electron correlation. Na₄IrO₄ may thus serve as a simple yet important example to study the ramifications of Hubbard‐type Coulomb interactions on local geometries.     

Electronic Structure Analysis of the Oxygen‐Activation Mechanism by FeII‐ and α‐Ketoglutarate (αKG)‐Dependent Dioxygenases

α‐Ketoglutarate (αKG)‐dependent nonheme iron enzymes utilize a high‐spin (HS) ferrous center to couple the activation of oxygen to the decarboxylation of the cosubstrate αKG to yield succinate and CO₂, and to generate a high‐valent ferryl species that then acts as an oxidant to functionalize the target C? H bond. Herein a detailed analysis of the electronic‐structure changes that occur in the oxygen activation by this enzyme was performed. The rate‐limiting step, which is identical on the septet and quintet surfaces, is the nucleophilic attack of the distal O atom of the O₂ adduct on the carbonyl group in αKG through a bicyclic transition state (^{5, 7}TS1). Due to the different electronic structures in ^{5, 7}TS1, the decay of ⁷TS1 leads to a ferric oxyl species, which undergoes a rapid intersystem crossing to form the ferryl intermediate. By contrast, a HS ferrous center ligated by a peroxosuccinate is obtained on the quintet surface following ⁵TS1. Thus, additional two single‐electron transfer steps are required to afford the same Fe^IV–oxo species. However, the triplet reaction channel is catalytically irrelevant. The biological role of αKG played in the oxygen‐activation reaction is dual. The αKG LUMO (C?O π*) serves as an electron acceptor for the nucleophilic attack of the superoxide monoanion. On the other hand, the αKG HOMO (C1? C2 σ) provides the second and third electrons for the further reduction of the superoxide. In addition to density functional theory, high‐level ab initio calculations have been used to calculate the accurate energies of the critical points on the alternative potential‐energy surfaces. Overall, the results delivered by the ab initio calculations are largely parallel to those obtained with the B3LYP density functional, thus lending credence to our conclusions.     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.