Accuracy Assessment of <i>GW</i> Starting Points for Calculating Molecular Excitation Energies Using the Bethe–Salpeter Formalism

The performance of the Bethe–Salpeter equation (BSE) approach for the first-principles computation of singlet and triplet excitation energies of small organic, closed-shell molecules has been assessed with respect to the quasiparticle energies used on input, obtained at various levels of <i>GW</i> theory. In the corresponding <i>GW</i> computations, quasiparticle energies have been computed for <i>all</i> orbital levels by means of using full spectral functions. The assessment reveals that, for valence excited states, quasiparticle energies obtained at the levels of eigenvalue-only self-consistent (ev<i>GW</i>) or quasiparticle self-consistent theory (qs<i>GW</i>) are required to obtain results of comparable accuracy as in time-dependent density-functional theory (TDDFT) using a hybrid functional such as PBE0. In contrast to TDDFT, however, the BSE approach performs well not only for valence excited states but also for excited states with Rydberg or charge-transfer character. To demonstrate the applicability of the BSE approach, computation times are reported for a set of aromatic hydrocarbons. Furthermore, examples of computations of ordinary photoabsorption and electronic circular dichroism spectra are presented for (C₆₀)₂ and C₈₄, respectively

Approaching Phosphorescence Lifetimes in Solution: The Two-Component Polarizable-Embedding Approximate Coupled-Cluster Method

Theoretical description of phosphorescence lifetimes in the condensed phase requires a method that takes into account both spin–orbit coupling and solvent–solute interactions. To obtain such a method, we have coupled our recently developed two-component coupled-cluster method with singles and approximated doubles to a polarizable environment. With this new method, we investigate how different solvents effect the electronic phosphorescence energies and lifetimes of 4<i>H</i>-pyran-4-thione

Non-covalent Interactions of CO₂ with Functional Groups of Metal–Organic Frameworks from a CCSD(T) Scheme Applicable to Large Systems

The strength of interactions between CO₂ and 23 building blocks of metal–organic frameworks are reported in this paper. This theoretical study is based on an incremental, explicitly correlated coupled-cluster scheme with interference effects. This scheme allows the accurate calculation of molecular complexes such as zinc acetate (32 non-hydrogen atoms) at the CCSD­(T) level, close to the basis set limit. Higher CO₂ affinity for complexes with nitrogen-containing heterocycles is predicted from the calculated interaction energies. The good agreement between the interaction energies obtained from the CCSD­(T) scheme and DFT-D3 is discussed

<i>ortho</i>-Perfluoroalkylation and Ethoxycarbonyldifluoromethylation of Aromatic Triazenes

A robust protocol for perfluoroalkylation and ethoxycarbonyldifluoromethylation of functionalized aromatic triazenes is described. Using silver­(I)-fluoride and different fluorinated (tri­methyl)­silyl substituted species, it was possible to synthesize various <i>ortho</i>-fluorinated triazenes in good yields via simple <i>CH</i>-substitution. Initial reactions under solvent-free (neat) conditions indicate a stabilizing interaction between “AgR_f” and the triazene moiety, which may be responsible for the good yields and regioselectivity. The transformation possibilities of the triazene moiety make these reactions interesting for the synthesis of fluorinated building blocks. In addition, quantum chemical calculations suggest that the stabilization of the radical intermediate in the <i>ortho</i>-position is distinctly more favored for aromatic triazenes than for other aromatic substrates

Structure Revision of Plakotenin Based on Computational Investigation of Transition States and Spectroscopic Properties

We show that the previously [<i>Tetrahedron Lett.</i> <b>1992</b>, <i>33</i>, 2579] proposed structure of natural plakotenin must be revised. Recently, the total synthesis of plakotenin was achieved via an intramolecular Diels–Alder reaction from a (<i>E,E,Z,E</i>)-tetraene as linear precursor. Using density functional theory, the computation of the four possible transition states for this reaction shows that the previously proposed structure could only have been formed via an energetically high-lying transition state, which is very unlikely. Instead, we suggest that the structure of plakotenin corresponds to the product formed via the lowest transition state. A comparison of experimental and theoretical optical rotation, circular dichroism, and two-dimensional nuclear Overhauser enhancement spectra conclusively proves that the structure of plakotenin is the one that is suggested by the transition state computations. Moreover, the simulation of the nuclear Overhauser enhancement spectra suggests that it is most likely that the misassignment of the ¹H chemical shifts of two methyl groups has led to the wrong structure prediction in the 1992 work. The previously proposed structure of <i>iso</i>-plakotenin remains unaffected by our structure revision, but the structures of <i>homo</i>- and <i>nor</i>-plakotenin must also be revised. The present work shows how the total synthesis of a natural product, together with the theoretical determination of the barrier heights of the reactions involved, can be of great help to assign its structure. It appears that intramolecular Diels–Alder reactions can be modeled accurately by today’s first-principles methods of quantum chemistry

Effect of Proton Substitution by Alkali Ions on the Fluorescence Emission of Rhodamine B Cations in the Gas Phase

The photophysics of chromophores is strongly influenced by their environment. Solvation, charge state, and adduct formation significantly affect ground and excited state energetics and dynamics. The present study reports on fluorescence emission of rhodamine B cations (RhBH⁺) and derivatives in the gas phase. Substitution of the acidic proton of RhBH⁺ by alkali metal cations, M⁺, ranging from lithium to cesium leads to significant and systematic blue shifts of the emission. The gas-phase structures and singlet transition energies of RhBH⁺ and RhBM⁺, M = Li, Na, K, Rb, and Cs, were investigated using Hartree–Fock theory, density functional methods, second-order Møller–Plesset perturbation theory, and the second-order approximate coupled-cluster model CC2. Comparison of experimental and theoretical results highlights the need for improved quantum chemical methods, while the hypsochromic shift observed upon substitution appears best explained by the Stark effect due to the inhomogeneous electric field generated by the alkali ions

Vibrational Coherence Controls Molecular Fragmentation: Ultrafast Photodynamics of the [Ag₂Cl]⁺ Scaffold

The recently introduced pump–probe fragmentation action spectroscopy reveals a unique observation of excited state vibrational coherence (430–460 fs) in the isolated metal complex [Ag₂(Cl)­(dcpm)₂)]⁺ (dcpm = bis­(dicyclohexylphosphino)­methane) containing the [Ag₂Cl]⁺ scaffold. After photoexcitation by an ¹XMCT transition (260 nm) in an ion trap, an unexpected correlation between specific fragment ions (loss of HCl/Cl^– vs loss of dcpm) and the phase of the wave packet is probed (1150 nm). Based on <i>ab initio</i> calculations, we assign the primary electronically excited state and ascribe the observed coherences (72–78 cm^–1) to contain predominantly Ag–Ag stretch character. We propose specific probe absorption and vibronic coupling at the classical turning points to switch remarkably early on between the different fragmentation pathways. The overall excited state dynamics are fitted to a multiexponential decay with time constants: 0.2–0.4/3–4/19–26/104–161 ps. These findings open new perspectives for further dynamics investigations and possible applications in photocatalysis

<i>Ab Initio</i> Study of the Adsorption of Small Molecules on Metal–Organic Frameworks with Oxo-centered Trimetallic Building Units: The Role of the Undercoordinated Metal Ion

The interactions of H₂, CO, CO₂, and H₂O with the undercoordinated metal centers of the trimetallic oxo-centered M₃^III(μ₃-O)­(X) (COO)₆ moiety are studied by means of wave function and density functional theory. This trimetallic oxo-centered cluster is a common building unit in several metal–organic frameworks (MOFs) such as MIL-100, MIL-101, and MIL-127 (also referred to as soc-MOF). A combinatorial computational screening is performed for a large variety of trimetallic oxo-centered units M₃^IIIO (M = Al³⁺, Sc³⁺, V³⁺, Cr³⁺, Fe³⁺, Ga³⁺, Rh³⁺, In³⁺, Ir³⁺) interacting with H₂O, H₂, CO, and CO₂. The screening addresses interaction energies, adsorption enthalpies, and vibrational properties. The results show that the Rh and Ir analogues are very promising materials for gas storage and separations

Gas-Phase Photoluminescence Characterization of Stoichiometrically Pure Nonanuclear Lanthanoid Hydroxo Complexes Comprising Europium or Gadolinium

Gas-phase photoluminescence measurements involving mass-spectrometric techniques enable determination of the properties of selected molecular systems with knowledge of their exact composition and unaffected by matrix effects such as solvent interactions or crystal packing. The resulting reduced complexity facilitates a comparison with theory. Herein, we provide a detailed report of the intrinsic luminescence properties of nonanuclear europium­(III) and gadolinium­(III) 9-hydroxy­phenalen-1-one (HPLN) hydroxo complexes. Luminescence spectra of [Eu₉(PLN)₁₆(OH)₁₀]⁺ ions reveal an europium-centered emission dominated by a 4-fold split Eu^III hypersensitive transition, while photoluminescence lifetime measurements for both complexes support an efficient europium sensitization via a PLN-centered triplet-state manifold. The combination of gas-phase measurements with density functional theory computations and ligand-field theory is used to discuss the antiprismatic core structure of the complexes and to shed light on the energy-transfer mechanism. This methodology is also employed to fit a new set of parameters, which improves the accuracy of ligand-field computations of Eu^III electronic transitions for gas-phase species

<i>Ab Initio</i> Study of the Adsorption of Small Molecules on Metal–Organic Frameworks with Oxo-centered Trimetallic Building Units: The Role of the Undercoordinated Metal Ion

The interactions of H₂, CO, CO₂, and H₂O with the undercoordinated metal centers of the trimetallic oxo-centered M₃^III(μ₃-O)­(X) (COO)₆ moiety are studied by means of wave function and density functional theory. This trimetallic oxo-centered cluster is a common building unit in several metal–organic frameworks (MOFs) such as MIL-100, MIL-101, and MIL-127 (also referred to as soc-MOF). A combinatorial computational screening is performed for a large variety of trimetallic oxo-centered units M₃^IIIO (M = Al³⁺, Sc³⁺, V³⁺, Cr³⁺, Fe³⁺, Ga³⁺, Rh³⁺, In³⁺, Ir³⁺) interacting with H₂O, H₂, CO, and CO₂. The screening addresses interaction energies, adsorption enthalpies, and vibrational properties. The results show that the Rh and Ir analogues are very promising materials for gas storage and separations