Rationalizing Aggregate Structures with Orbital Contributions to the Exchange-Repulsion Energy.

It is shown that repulsive interactions have a crucial influence on the structure of prototypical non-covalently bonded systems. To explain this, we propose a molecular orbital-based model for the exchange-repulsion contribution to the total interaction energy. As a central result, our model shows that energetically preferred aggregate structures frequently exhibit reduced exchange repulsion, which can be deduced from the nodal structure of certain occupied orbitals. This is used to explain key features of the intermolecular potentials of the Cl2 -He, benzene-benzene, and benzene-hexafluorobenzene aggregates, which are not correctly reproduced by commonly applied electrostatic models.

Coincidence study of core-ionized adamantane: site-sensitivity within a carbon cage?

We investigate the fragmentation dynamics of adamantane dications produced after core-ionization at the carbon edge followed by Auger decay. The combination of high-resolution electron spectroscopy, energy-resolved electron-ion multi-coincidence spectroscopy and different theoretical models allows us to give a complete characterization of the processes involved after ionization. We show that energy- and site-sensitivity is observed even for a highly-symmetric molecule that lacks any unique atomic site.

Pnictide-Capped Butterfly Cluster in the Crystal Structure of Nb4PnX11 (Pn = N, P; X = Cl, Br, I).

Niobium pnictide halides Nb4PnX11 (Pn = N, P; X = Cl, Br, I) are reported with their average crystal structures. Individual pnictide-capped butterfly cluster cores [Nb4P] in the structure are interconnected into two-dimensional layers, with their electronic and magnetic properties being analyzed.

Accurate property prediction by second order perturbation theory: The REMP and OO-REMP hybrids.

The prediction of molecular properties such as equilibrium structures or vibrational wavenumbers is a routine task in computational chemistry. If very high accuracy is required, however, the use of computationally demanding ab initio wavefunction methods is mandatory. We present property calculations utilizing Retaining the Excitation Degree - Møller-Plesset (REMP) and Orbital Optimized REMP (OO-REMP) hybrid perturbation theories, showing that with the latter approach, very accurate results are obtained at second order in perturbation theory. Specifically, equilibrium structures and harmonic vibrational wavenumbers and dipole moments of closed and open shell molecules were calculated and compared to the best available experimental results or very accurate calculations. OO-REMP is capable of predicting bond lengths of small closed and open shell molecules with an accuracy of 0.2 and 0.5 pm, respectively, often within the range of experimental uncertainty. Equilibrium harmonic vibrational wavenumbers are predicted with an accuracy better than 20 cm-1. Dipole moments of small closed and open shell molecules are reproduced with a relative error of less than 3%. Across all investigated properties, it turns out that a 20%:80% Møller-Plesset:Retaining the Excitation Degree mixing ratio consistently provides the best results. This is in line with our previous findings, featuring closed and open shell reaction energies.

Auger electron spectroscopy of fulminic acid, HCNO: an experimental and theoretical study.

HCNO is a molecule of considerable astrochemical interest as a precursor to prebiotic molecules. It is synthesized by preparative pyrolysis and is unstable at room temperature. Here, we investigate its spectroscopy in the soft X-ray regime at the C 1s, N 1s and O 1s edges. All 1s ionization energies are reported and X-ray absorption spectra reveal the transitions from the 1s to the π* state. Resonant and normal Auger electron spectra for the decay of the core hole states are recorded in a hemispherical analyzer. An assignment of the experimental spectra is provided with the aid of theoretical counterparts. The latter are using a valence configuration interaction representation of the intermediate and final state energies and wavefunctions, the one-center approximation for transition rates and band shapes according to the moment theory. The computed spectra are in very good agreement with the experimental data and most of the relevant bands are assigned. Additionally, we present a simple approach to estimate relative Auger transition rates on the basis of a minimal basis representation of the molecular orbitals. We demonstrate that this provides a qualitatively good and reliable estimate for several signals in the normal and resonant Auger electron spectra which have significantly different intensities in the decay of the three core holes.

UREMP, RO-REMP, and OO-REMP: Hybrid perturbation theories for open-shell electronic structure calculations.

An accurate description of the electron correlation energy in closed- and open-shell molecules is shown to be obtained by a second-order perturbation theory (PT) termed REMP. REMP is a hybrid of the Retaining the Excitation degree (RE) and the Møller-Plesset (MP) PTs. It performs particularly encouragingly in an orbital-optimized variant (OO-REMP) where the reference wavefunction is given by an unrestricted Slater determinant whose spin orbitals are varied such that the total energy becomes a minimum. While the approach generally behaves less satisfactorily with unrestricted Hartree-Fock references, reasonable performance is observed for restricted Hartree-Fock and restricted open-shell Hartree-Fock references. Inclusion of single excitations to OO-REMP is investigated and found-as in similar investigations-to be dissatisfying as it deteriorates performance. For the non-multireference subset of the accurate W4-11 benchmark set of Karton et al. [Chem. Phys. Lett. 510, 165-178 (2011)], OO-REMP predicts most atomization and reaction energies with chemical accuracy (1 kcal mol-1) if complete-basis-set extrapolation with augmented and core-polarized basis sets is used. For the W4-11 related test-sets, the error estimates obtained with the OO-REMP method approach those of coupled-cluster with singles, doubles and perturbative triples [CCSD(T)] within 20%-35%. The best performance of OO-REMP is found for a mixing ratio of 20%:80% MP:RE, which is essentially independent of whether radical stabilization energies, barrier heights, or reaction energies are investigated. Orbital optimization is shown to improve the REMP approach for both closed and open shell cases and outperforms coupled-cluster theory with singles and doubles (CCSD), spin-component scaled Møller-Plesset theory at second order (SCS-MP2), and density functionals, including double hybrids in all the cases considered.

OO-REMP: Approaching Chemical Accuracy with Second-Order Perturbation Theory.

We present a perturbation theory (PT) providing second-order energies that reproduce main group chemistry benchmark sets for reaction energies, barrier heights, and atomization energies with mean absolute deviations below 1 kcal mol-1. The PT is defined as a constrained mixture of the unperturbed Hamiltonians of the Retaining the Excitation degree (RE) and the Møller-Plesset (MP) PTs. The orbitals of the reference wave function, a single unrestricted Slater determinant, are iteratively optimized to minimize the total energy. For all benchmark sets, good and near optimal performance of OO-REMP was observed for an unperturbed Hamiltonian consisting of 25% MP and 75% RE contributions.

REMP: A hybrid perturbation theory providing improved electronic wavefunctions and properties.

We propose a new perturbation theoretical approach to the electron correlation energy by choosing the zeroth order Hamiltonian as a linear combination of the corresponding "Retaining the Excitation degree" (RE) and the Møller-Plesset (MP) operators. In order to fulfill Kato cusp conditions, the RE and MP contributions are chosen to sum up to one. 15% ± 5% MP contribution is deduced to be in an optimal range from a fit of the first order REMP wavefunction to near full configuration interaction reference data. For closed shell systems, the same range of MP weights shows best performance for equilibrium bond distances and vibrational wavenumbers of diatomic molecules, the reaction energies in the spin component scaled MP2 fit set, the transition energies of the BHPERI test set, and the parameterized coupled cluster with singles and doubles (pCCSD) fit set. For these properties, REMP outperforms all other tested perturbation theories at second order and shows equal performance as the best coupled pair approaches or pCCSD methods as well as the best double hybrid density functionals. Furthermore, REMP is shown to fulfill all required fundamental boundary conditions of proper wavefunction based quantum chemical methods (unitary invariance and size consistency).

Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B',B''-trichloro-N,N',N''-tri((4-fluoro)phenyl)borazine.

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B',B''-trichloro-N,N',N''-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNH2BCl3, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k1 = 3.03(7) × 10-2 min-1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k1 = 3.18 × 10-2 min-1) from computed activation parameters (ΔH‡373.15 = 28.1 kcal mol-1, ΔS‡373.15 = 1.56 eu, ΔG‡373.15 = 27.6 kcal mol-1). The product of the first hydrogen chloride evolution is anilinodichloroborane ArNHBCl2 (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of formation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.

A model hamiltonian tuned toward high level ab initio calculations to describe the character of excitonic states in perylenebisimide aggregates.

On the example of an aggregate of two perylenebisimide (PBI) molecules the character of the lowest excited electronic states in terms of charge transfer (CT) and Frenkel exciton (FE) configurations is investigated as a function of the intermolecular arrangement. A minimal model Hamiltonian based on two FE and two CT configurations at the frontier-orbitals CIS (FOCIS) level is shown to represent a simple and comprehensible approach providing insight into the physical significance of the model Hamiltonian matrix elements. The recently introduced analysis and diabatization procedure (Liu et al., J. Chem. Phys. 2015, 143, 084106 ) method is used to extract the energies of the configurations and their interactions (the model Hamiltonian parameters) also from the accurate CC2 approach. An analysis in terms of diabatic energy profiles and their interactions shows that the FOCIS parameters give a qualitatively correct description of the adiabatic excited state energy profiles. Comparison with CC2 reveals, however, the presence of avoided crossings at FOCIS level, associated with a large character change (CT/FE) of the excited states as a function of the aggregate structure, which represents the major drawback of FOCIS results. We show that proper amendment of the FOCIS-derived parameters allows to accurately represent the potential energy surfaces and crossings of the excited dimer states as a function of the aggregate structure. © 2018 Wiley Periodicals, Inc.

Comparison of the periodic slab approach with the finite cluster description of metal-organic interfaces at the example of PTCDA on Ag(110).

We present a comparative study of metal-organic interface properties obtained from dispersion corrected density functional theory calculations based on two different approaches: the periodic slab-supercell technique and cluster models with 32-290 Ag atoms. Fermi smearing and fixing of cluster borders are required to make the cluster calculation feasible and realistic. The considered adsorption structure and energy of a PTCDA molecule on the Ag(110) surface is not well reproduced with clusters containing only two metallic layers. However, all clusters with four layers of silver atoms and sufficient lateral extension reproduce the adsorbate structure within 0.04 Å with respect to the slab-supercell structure and provide adsorption energies of ( -4.45± 0.08 eV) consistent with the slab result of -4.47 eV. Thus, metal-organic adsorbate systems can be realistically represented by properly defined cluster models. © 2018 Wiley Periodicals, Inc.

Heptacene: Characterization in Solution, in the Solid State, and in Films.

Acenes comprise an important class of organic semiconducting materials. As graphene nanoribbons of ultimate width, they are valuable atom-precise model systems for studying the properties of this form of nanoscale carbon materials. Heptacene is the smallest member of the acene series that could only be studied under matrix isolation conditions. Its existence in bulk had never been positively confirmed, despite efforts dating back more than 70 years. We report that the reduction of 7,16-heptacenequinone produces a mixture of two diheptacene molecules. The diheptacenes undergo thermal cleavage to heptacene at high temperatures in the solid state. Monitoring this cycloreversion by solid state 13C cross-polarized magic angle spinning NMR reveals that solid heptacene has a half-life time of several weeks at room temperature. The diheptacenes are valuable precursors for generating films of heptacene by vapor phase deposition that can be studied below or at room temperature.

Why does MP2 work?

We show analytically and numerically that the performance of second order Møller-Plesset (MP) perturbation theory (PT), coupled-cluster (CC) theory, and other perturbation theory approaches can be rationalized by analyzing the wavefunctions of these methods. While rather large deviations for the individual contributions of configurations to the electron correlation energy are found for MP wavefunctions, they profit from an advantageous and robust error cancellation: The absolute contribution to the correlation energy is generally underestimated for the critical excitations with small energy denominators and all other doubly excited configurations where the two excited electrons are coupled to a singlet. This is balanced by an overestimation of the contribution of triplet-coupled double excitations to the correlation energy. The even better performance of spin-component-scaled-MP2 theory is explained by a similar error compensation effect. The wavefunction analysis for the lowest singlet states of H2O, CH2, CO, and Cu+ shows the predicted trends for MP methods, rapid but biased convergence of CC theory as well as the substantial potential of linearized CC, or retaining the excitation-degree (RE)-PT.

Charge carrier mobilities in organic semiconductor crystals based on the spectral overlap.

The prediction of substance-related charge-transport properties is important for the tayloring of new materials for organic devices, such as organic solar cells. Assuming a hopping process, the Marcus theory is frequently used to model charge transport. Here another approach, which is already widely used for exciton transport, is adapted to charge transport. It is based on the spectral overlap of the vibrational donor and acceptor spectra. As the Marcus theory it is derived from Fermi's Golden rule, however, it contains less approximations, as the molecular vibrations are treated quantum mechanically. In contrast, the Marcus theory reduces all vibrational degrees of freedom to one and treats its influence classically. The approach is tested on different acenes and predicts most of the experimentally available hole mobilities in these materials within a factor of 2. This represents a significant improvement to values obtained from Marcus theory which is qualitatively correct but frequently overestimates the mobilities by factors up to 10. Furthermore, the charge-transport properties of two derivatives of perylene bisimide are investigated. © 2016 Wiley Periodicals, Inc.

Influence of a polarizable surrounding on the electronically excited states of aggregated perylene materials.

To tune the efficiency of organic semiconductor devices it is important to understand limiting factors as trapping mechanisms for excitons or charges. An understanding of such mechanisms deserves an accurate description of the involved electronical states in the given environment. In this study, we investigate how a polarizable surrounding influences the relative positions of electronically excited states of dimers of different perylene dyes. Polarization effects are particularly interesting for these systems, because gas phase computations predict that the CT states lie slightly above the corresponding Frenkel states. A polarizable environment may change this energy order because CT states are thought to be more sensitive to a polarizable surrounding than Frenkel states. A first insight we got via a TD-HF approach in combination with a polarizable continuum model (PCM). These give limited insights because TD-HF overestimates excitation energies of CT states. However, SCS-CC2 approaches, which are sufficiently accurate, cannot easily be used in combination with continuum solvent models. Hence, we developed two approaches to combine gas phase SCS-CC2 results with solvent effects based on TD-HF computations. Their accuracies were finally checked via ADC(2)//COSMO computations. The results show that for perylene dyes a polarizable surrounding alone does not influence the energetic ordering of CT and Frenkel states. Variations in the energy order of the states only result from nuclear relaxation effects after the excitation process. © 2016 Wiley Periodicals, Inc.

A Facile Method for the Synthesis of Binary Tungsten Iodides.

The preparation of tungsten iodides in large quantities is a challenge because these compounds are not accessible using an easy synthesis method. A new, remarkably efficient route is based on a halide exchange reaction between WCl6 and SiI4. The reaction proceeds at moderate temperatures in a closed glass vessel. The new compounds W3I12 (W3I8 ⋅2 I2) and W3I9 (W3I8 ⋅½I2) containing the novel [W3I8] cluster are formed at 120 and 150 °C, and remain stable in air. W3I12 is an excellent starting material for the synthesis of other metal-rich tungsten iodides. At increasing temperature these trinuclear clusters undergo self-reduction until an octahedral tungsten cluster is formed in W6I12 . The synthesis, structure, and an analysis of the bonding of compounds containing this new trinuclear tungsten cluster are presented.

Isomerization and fragmentation pathways of 1,2-azaborine.

The generation of 1,2-azaborine (4), the BN-analogue of ortho-benzyne, was recently achieved by elimination of tert-butyldimethylchlorosilane under the conditions of flash vacuum pyrolysis. The present investigation identifies by computational means pathways for the thermal isomerization and fragmentation of 1,2-azaborine. The computations were performed using single reference (hybrid/density functional, second order Møller-Plesset perturbation, and coupled cluster theories) as well as multiconfiguration methods (complete active space SCF based second order perturbation theory, multireference configuration interaction, and multiconfiguration coupled electron pair approximation) with basis sets up to polarized triple-ζ quality. The 1,2-azaborine is, despite the distortion of its molecular structure, the most stable C4H4BN isomer investigated. The formation of BN-endiyne isomers is highly unfavorable as the identified pathways involve barriers close to 80 kcal mol(-1). The concerted fragmentation to ethyne and 2-aza-3-bora-butadiyne even has a barrier close to 120 kcal mol(-1). The fragmentation of BN-enediynes has energetic requirements similar to enediynes.

From WCl6 to WCl2: Properties of Intermediate Fe-W-Cl Phases.

Phenomenological studies of WCl6 reduction with transition metal powders M = Mn, Fe, and Co have been recently reported. These reactions involve a series of reductive intercalation steps of M atoms into layered tungsten chloride arrangements, followed by exsolution of MCl2. In the series M = Fe, the presence of divalent iron is evidenced for Fe(x)WCl6, FeW2Cl10, Fe2W2Cl10, and (Fe,W)Cl2 by Mössbauer spectroscopy. Magnetic properties are reported. Bonding characteristics between tungsten atoms in edge-sharing [W2Cl10](n-) bioctahedra reveal that a double bond can be addressed to FeW2Cl10. A similar situation appears for Fe2W2Cl10, due to the localized and thus nonbonding character of the two electrons in the δ orbitals of this compound.

A general ansatz for constructing quasi-diabatic states in electronically excited aggregated systems.

We present a general method for analyzing the character of singly excited states in terms of charge transfer (CT) and locally excited (LE) configurations. The analysis is formulated for configuration interaction singles (CIS) singly excited wave functions of aggregate systems. It also approximately works for the second-order approximate coupled cluster singles and doubles and the second-order algebraic-diagrammatic construction methods [CC2 and ADC(2)]. The analysis method not only generates a weight of each character for an excited state, but also allows to define the related quasi-diabatic states and corresponding coupling matrix elements. In the character analysis approach, we divide the target system into domains and use a modified Pipek-Mezey algorithm to localize the canonical MOs on each domain, respectively. The CIS wavefunction is then transformed into the localized basis, which allows us to partition the wavefunction into LE configurations within domains and CT configuration between pairs of different domains. Quasi-diabatic states are then obtained by mixing excited states subject to the condition of maximizing the weight of one single LE or CT configuration (localization in configuration space). Different aims of such a procedure are discussed, either the construction of pure LE and CT states for analysis purposes (by including a large number of excited states) or the construction of effective models for dynamics calculations (by including a restricted number of excited states). Applications are given to LE/CT mixing in π-stacked systems, charge-recombination matrix elements in a hetero-dimer, and excitonic couplings in multi-chromophoric systems.

Identification of ultrafast relaxation processes as a major reason for inefficient exciton diffusion in perylene-based organic semiconductors.

The exciton diffusion length (LD) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials. Our model is based on simulations with a hybrid approach which combines high-level ab initio computations for the part of the system directly involved in the described processes with a force field to include environmental effects. The adequacy of the model is shown by detailed comparison with available experimental results. The model indicates that the short exciton diffusion lengths of α-perylene tetracarboxylicdianhydride (PTCDA) are due to ultrafast relaxation processes of the optical excitation via intermolecular motions leading to a state from which further exciton diffusion is hampered. As the efficiency of this mechanism depends strongly on molecular arrangement and environment, the model explains the strong dependence of LD on the morphology of the materials, for example, the differences between α-PTCDA and diindenoperylene. Our findings indicate how relaxation processes can be diminished in perylene-based materials. This model can be generalized to other organic compounds.