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The unconventional carbon dioxide insertion reaction of a gold-aluminyl [ t Bu 3 PAuAl(NON)] complex has been recently shown to be related to the electron-sharing character of the Au-Al bond that acts as a nucleophile and stabilizes the insertion product through a radical-like behavior. Since a gold-diarylboryl [IPrAuB( o -tol) 2 ] complex with similar reactivity features has been recently reported, in this work we computationally investigate the reaction of carbon dioxide with [LAuX] (L = phosphine, N-heterocyclic carbene (NHC); X = Al(NON), B( o -tol) 2 ) complexes to get insights into the Al/B anionic and gold ancillary ligand effects on the Au-Al/B bond nature, electronic structure, and reactivity of these compounds. We demonstrate that the Au-Al and Au-B bonds possess a similar electron-sharing nature, with diarylboryl complexes displaying a slightly more polarized bond as Au(δ + )-B(δ - ). This feature reduces the radical-like reactivity toward CO 2 , and the Al/B anionic ligand effect is found to favor aluminyls over boryls, despite the greater oxophilicity of B. Remarkably, the ancillary ligand of gold has a negligible electronic trans effect on the Au-X bond and only a minor impact on the formation of the insertion product, which is slightly more stable with carbene ligands. Surprisingly, we find that the modification of the steric hindrance at the carbene site may exert a sizable control over the reaction, with more sterically hindered ligands thermodynamically disfavoring the formation of the CO 2 insertion product.

In this study, we benchmark density functional theory gauge-including projector-augmented-wave (GIPAW) chemical shieldings against molecular shieldings for which basis set completeness has been achieved [Jensen et al., Phys. Chem. Chem. Phys. 18, 21145 (2016)]. We demonstrate the importance of two-center corrections for GIPAW hydrogen shieldings. For the other nuclei studied, standard GIPAW is sufficiently accurate. We find that GIPAW can be pushed to closely approach the basis set limit. The only source of small inaccuracies lies in the contribution to the shielding that is caused by surface currents, which we estimate comparing GIPAW susceptibilities to converged molecular magnetizabilities.

For many years, fullerene derivatives have been the main n-type material of organic electronics and optoelectronics. Recently, fullerene derivatives functionalized with ethylene glycol (EG) side chains have been showing important properties such as enhanced dielectric constants, facile doping and enhanced self-assembly capabilities. Here, we have prepared field-effect transistors using a series of these fullerene derivatives equipped with EG side chains of different lengths. Transport data show the beneficial effect of increasing the EG side chain. In order to understand the material properties, full structural determination of these fullerene derivatives has been achieved by coupling the X-ray data with molecular dynamics (MD) simulations. The increase in transport properties is paired with the formation of extended layered structures, efficient molecular packing and an increase in the crystallite alignment. The layer-like structure is composed of conducting layers, containing of closely packed C 60 balls approaching the inter-distance of 1 nm, that are separated by well-defined EG layers, where the EG chains are rather splayed with the chain direction almost perpendicular to the layer normal. Such a layered structure appears highly ordered and highly aligned with the C 60 planes oriented parallel to the substrate in the thin film configuration. The order inside the thin film increases with the EG chain length, allowing the systems to achieve mobilities as high as 0.053 cm 2 V -1 s -1 . Our work elucidates the structure of these interesting semiconducting organic molecules and shows that the synergistic use of X-ray structural analysis and MD simulations is a powerful tool to identify the structure of thin organic films for optoelectronic applications., Competing Interests: The authors declare that they have no competing interests., (This journal is © The Royal Society of Chemistry.)

Per iodoannulene molecules and ions C x I x in planar geometry offer examples of systems with the potential for outer σ and inner π ring-current double aromaticity, given a sufficient overlap of tangential p q in planar geometry offer examples of systems with the potential for outer σ and inner π ring-current double aromaticity, given a sufficient overlap of tangential p σ -orbital manifolds on the large atoms of the outer cycle. Previous theoretical work indicated concentric diatropic currents in the dication C 6 I 6 2+ . Ab initio ipsocentric calculations support an account in terms of frontier-orbital selection rules for current contributions in C 6 I 6 2+ iodobenzene). A σ/π analogue of the annulene-within-an-annulene model is applied here to 6 I 6 + , implicated in recent experimental work on the oxidation of per = 0, +1, +2, +4, -2 and structures constrained to a planar per iodo systems based on cyclooctatetraene. Model species C 8 I 8 q with charges q = 0, +1, +2, +4, -2 and structures constrained to a planar D 4 h symmetry exhibit maps with all combinations of σ/π con- and counter-rotation, comprising global σ ring currents on the iodine perimeter and central π ring currents on the carbocycle. All can be rationalized by the separate application of the tropicity selection rules to the two subsystems, whether in singlet or triplet states.

Computationally obtaining structural parameters along a reaction coordinate is commonly performed with Kohn-Sham density functional theory which generally provides a good balance between speed and accuracy. However, CPU times still range from inconvenient to prohibitive, depending on the size of the system under study. Herein, the tight binding GFN2-xTB method [C. Bannwarth, S. Ehlert, S. Grimme, J. Chem. Theory Comput. 2019, 15, 1652] is investigated as an alternative to produce reasonable geometries along a reaction path, that is, reactant, product and transition state structures for a series of transformations involving gold complexes. A small mean error (1 kcal/mol) was found, with respect to an efficient composite hybrid-GGA exchange-correlation functional (PBEh-3c) paired with a double-ζ basis set, which is 2-3 orders of magnitude slower. The outlined protocol may serve as a rapid tool to probe the viability of proposed mechanistic pathways in the field of gold catalysis., (© 2021 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

In this contribution, we studied the OC-C bond in carbon suboxide and related allene compounds using the valence bond method. The nature of this bond has been the subject of debate, whether it is a regular, electron sharing bond or a dative bond. We compared the nature of this bond in carbon suboxide with the gold-CO bond in Au(CO) 2 , which is a typical dative bond, and we studied its charge-shift bond character. We found that the C-CO bond in carbon suboxide is unique in the sense that it cannot be assigned as either a dative or electron sharing bond, but it is an admixture of electron sharing and dative components, together with a high contribution of ionic character. These findings provide a clear basis for distinguishing the commonly found dative bonds between ligands and transition metals and the present case of what may be described as coordinative bonding to carbon.+ , which is a typical dative bond, and we studied its charge-shift bond character. We found that the C-CO bond in carbon suboxide is unique in the sense that it cannot be assigned as either a dative or electron sharing bond, but it is an admixture of electron sharing and dative components, together with a high contribution of ionic character. These findings provide a clear basis for distinguishing the commonly found dative bonds between ligands and transition metals and the present case of what may be described as coordinative bonding to carbon.

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 µW m -1 K -2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., (© 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Δ O ) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T 1/2 above room temperature (300-368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin ( 6 ) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approximate "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the 5 -substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand σ- and π-bonding.para -substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand σ- and π-bonding.

Electron and charge transfers are part of many vital processes in nature and technology. Ab initio descriptions of these processes provide useful insights that can be utilized for applications. A combination of the embedded cluster material model and nonorthogonal configuration interaction (NOCI), in which the cluster wave functions are expanded in many-electron basis functions (MEBFs) consisting of spin-adapted, antisymmetrized products of multiconfigurational wave functions of fragments (which are usually molecules) in the cluster, appears to provide a compromise between accuracy and calculation time. Additional advantages of this NOCI-Fragments approach are the chemically convenient interpretation of the wave function in terms of molecular states, and the direct accessibility of electronic coupling between diabatic states to describe energy and electron transfer processes. Bottlenecks in this method are the large number of two-electron integrals that have to be handled for the calculation of an electronic coupling matrix element and the enormous number of matrix elements over determinant pairs that have to be evaluated for the calculation of one matrix element between the MEBFs. We show here how we created a reduced common molecular orbital basis that is utilized to significantly reduce the number of two-electron integrals that need to be handled. The results obtained with this basis do not show any loss of accuracy in relevant quantities like electronic couplings and vertical excitation energies. We also show a significant reduction in computation time without loss in accuracy when matrix elements over determinant pairs with small weights are neglected in the NOCI. These improvements in the methodology render NOCI-Fragments to be also applicable to treat clusters of larger molecular systems with larger atomic basis sets and larger active spaces, as the computation time becomes dependent on the number of occupied orbitals and less dependent on the size of the active space.

Incorporating ethylene glycols (EGs) into organic semiconductors has become the prominent strategy to increase their dielectric constant. However, EG's contribution to the dielectric constant is due to nuclear relaxations, and therefore, its relevance for various organic electronic applications depends on the time scale of these relaxations, which remains unknown. In this work, by means of a new computational protocol based on polarizable molecular dynamics simulations, the time- and frequency-dependent dielectric constant of a representative fullerene derivative with EG side chains is predicted, the origin of its unusually high dielectric constant is explained, and design suggestions are made to further increase it. Finally, a dielectric relaxation time of ∼1 ns is extracted which suggests that EGs may be too slow to reduce the Coulombic screening in organic photovoltaics but are definitely fast enough for organic thermoelectrics with much lower charge carrier velocities.

Achieving long-range order with surface-supported supramolecular assemblies is one of the pressing challenges in the prospering field of non-covalent surface functionalization. Having access to defect-free on-surface molecular assemblies will pave the way for various nanotechnology applications. Here we report the synthesis of two libraries of naphthalenediimides (NDIs) symmetrically functionalized with long aliphatic chains (C 28 and C 33 ) and their self-assembly at the 1-phenyloctane/highly oriented pyrolytic graphite (1-PO/HOPG) interface. The two NDI libraries differ by the presence/absence of an internal double bond in each aliphatic chain (unsaturated and saturated compounds, respectively). All molecules assemble into lamellar arrangements, with the NDI cores lying flat and forming 1D rows on the surface, while the carbon chains separate the 1D rows from each other. Importantly, the presence of the unsaturation plays a dominant role in the arrangement of the aliphatic chains, as it exclusively favors interdigitation. The fully saturated tails, instead, self-assemble into a combination of either interdigitated or non-interdigitated diagonal arrangements. This difference in packing is spectacularly amplified at the whole surface level and results in almost defect-free self-assembled monolayers for the unsaturated compounds. In contrast, the monolayers of the saturated counterparts are globally disordered, even though they locally preserve the lamellar arrangements. The experimental observations are supported by computational studies and are rationalized in terms of stronger van der Waals interactions in the case of the unsaturated compounds. Our investigation reveals the paramount role played by internal double bonds on the self-assembly of discrete large molecules at the liquid/solid interface.

A procedure is described for unbiased identification of all π-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into a singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a π-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of | T A , the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. Several of the optimized pair geometries are somewhat unexpected, but all are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of | 2 , the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. Several of the optimized pair geometries are somewhat unexpected, but all are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of | T , consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements | A | 2 , consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements | T *| 2 and | T **| 2 for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k , the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the | T *| 2 values are compared with those obtained from high-level ab initio nonorthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether, 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard-sphere approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (five-dimensional) surfaces of seven six-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on | T **| 2 values are compared with those obtained from high-level ab initio nonorthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether, 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard-sphere approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (five-dimensional) surfaces of seven six-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on | T A | 2 , | T *| 2 , or k . It is available as freeware at https://cloud.uochb.cas.cz/simple .

We report the formation of one- and two-dimensional metal-organic coordination structures from para -hexaphenyl-dicarbonitrile (NC-Ph 6 -CN) molecules and Cu atoms on graphene epitaxially grown on Ir(111). By varying the stoichiometry between the NC-Ph 6 -CN molecules and Cu atoms, the dimensionality of the metal-organic coordination structures could be tuned: for a 3:2 ratio, a two-dimensional hexagonal porous network based on threefold Cu coordination was observed, while for a 1:1 ratio, one-dimensional chains based on twofold Cu coordination were formed. The formation of metal-ligand bonds was supported by imaging the Cu atoms within the metal-organic coordination structures with scanning tunneling microscopy. Scanning tunneling spectroscopy measurements demonstrated that the electronic properties of NC-Ph 6 -CN molecules and Cu atoms were different between the two-dimensional porous network and one-dimensional molecular chains., Competing Interests: The authors declare no competing financial interest.

We report on the self-assembly of a conformational flexible organic compound on Au(111) using scanning tunneling microscopy and low-energy electron diffraction measurements. We observed different conformers of the compound upon adsorption on the reconstructed Au(111) surface. Increasing the molecular coverage enhanced the lateral pressure, that is, parallel to the surface, favoring a coverage-controlled transition from a supramolecular network displaying only one molecular organization, into a polymorphic array with two coexisting arrangements. Our results give insights into the role of substrate-induced conformational changes on the formation of polymorphic supramolecular networks., Competing Interests: The authors declare no competing financial interest.

Recent efforts and progress in polymer solar cell research have boosted the photovoltaic efficiency of the technology. This efficiency depends not only on the device architecture but also on the material properties. Thus, insight into the design of novel semiconductor materials is vital for the advancement of the field. This paper looks from a theoretical viewpoint into two of the factors for the design of semiconductor materials with applications to bulk heterojunction solar cells: the charge transfer exciton binding energy and the nanoscale arrangement of donor and acceptor molecules in blend systems. Being aware that the exciton dissociation of local excitons in charge transfer states initiates the charge generation process, the excited state properties of four oligomers (one donor-type: PEO-PPV; and three donor-acceptor-types: PTFB, PTB7, and PTB7-Th) and two fullerene derivatives ([60]PCBM and [70]PCBM), previously reported in the literature as having high electrical conductance, are studied. With such a study, the donor molecules, either of donor-type or donor-acceptor type, are screened as candidates for [60]PCBM- and/or [70]PCBM-based bulk heterojunctions. The charge transfer energy and charge transfer exciton binding energy of suitable donor:acceptor bulk heterojunctions, some of them not yet fabricated, are studied. Further, the charge transfer exciton binding energies of [60]PCBM- and [70]PCBM-based blends are compared. A combination of molecular dynamics simulations with calculations based on Kohn-Sham density functional theory (KS-DFT) and its time-dependent extension (KS-TDDFT) is used. An important feature of this work is that it incorporates the effect of the environment of the quantum chemical system in KS-DFT or KS-TDDFT calculations through a polarizable discrete reaction field (DRF). Our predictions in terms of the influence of the nanoscale arrangement of donor and acceptor molecules on the performance of organic solar cells indicate that bulk heterojunction morphologies for donor-acceptor-type oligomers lead to their lowest excited states having charge transfer character. Further, we find that in terms of favorable charge transfer exciton binding energy, the PTB7-Th:[70]PCBM blends outperform the other blends.

The isotropic Nucleus Independent Chemical Shift (NICSiso) is widely considered to be a suitable descriptor for aromaticity based on the correlations it exhibits with other aromaticity descriptors. To gain more insight into the origin of these correlations, we establish causal relations between the NICSiso and the underlying current density patterns by resolving the NICSiso into its underlying density. Our results indicate that the origin of the behavior of the NICSiso can be radically different from what is generally assumed. Not only does this bring into question the robustness of applying the NICSiso beyond the realms of where good correlations with other measures of aromaticity have been established, it also points to an inherent weakness in all interpretations of NICSiso values that are not based on additional data.

1,2,4-Benzenetriol (BTO), sourced from the carbohydrate-derived platform chemical 5-hydroxylmethylfurfural (HMF), is an interesting starting point for the synthesis of various biobased aromatic products. However, BTO readily undergoes dimerization and other reactions under mild conditions, making analysis and isolation challenging. To both control and utilize the reactivity of BTO to produce biobased building blocks, its reactivity needs to be better understood. Here it was found that specific BTO aromatic C-H bonds are reactive toward deuterium exchange with D 2 O, which appears pronounced under acidic conditions at room temperature and can lead to the selective formation of BTO with an aromatic ring that contains one or two deuterium atoms, the first at the five and the second at the three position. By exposure to air, it was shown that BTO forms a 5,5'-linked BTO dimer [1,1'-biphenyl]-2,2',4,4',5,5'-hexaol ( 1 ) and subsequently a hydroxyquinone containing dimeric structure 2',4,4',5'-tetrahydroxy-[1,1'-biphenyl]-2,5-dione ( 2 ). Additionally, condensed dimer dibenzo[ b , d ]furan-2,3,7,8-tetraol ( 3 ) can be relatively easily accessed. The controlled formation of these symmetric and asymmetric multifunctional dimers illustrates diverse possibilities for BTO to be converted to valuable biobased aromatic compounds. Deuterium exchange was attributed to electrophilic aromatic substitution because this reactivity was found to be independent of oxygen and acid mediated. On the contrary, the dimerization was dependent on the presence of oxygen and thus likely involves radical intermediates. Thus this report overall displays different accessible reaction pathways for BTO that can be exploited for the production of BTO-derived compounds., Competing Interests: The authors declare no competing financial interest.

A recent report on the crystal structure of the pentagonal-pyramidal hexamethylbenzene dication C 6 (CH 3 ) 6 by Malischewski and Seppelt [Angew. Chem. Int. Ed. 2017, 56, 368] confirmed the structural proposal made in the first report of this compound in 1973 by Hogeveen and Kwant [Tetrahedron Lett. 1973, 14, 1665]. The widespread attention that this compound quickly gained led us to reinvestigate its electronic structure. On the basis of intrinsic bond orbital analysis, effective oxidation state analysis, ring current analysis, and comparison with well-established coordination complexes, it is demonstrated that the central carbon atom behaves like a transition metal. The central (apical) carbon atom, although best described as a highly Lewis-acidic carbon atom coordinated with an anionic cyclopentadienyl ligand, is also capable of acting as an electron-pair donor to a formal CH 2+ by Malischewski and Seppelt [Angew. Chem. Int. Ed. 2017, 56, 368] confirmed the structural proposal made in the first report of this compound in 1973 by Hogeveen and Kwant [Tetrahedron Lett. 1973, 14, 1665]. The widespread attention that this compound quickly gained led us to reinvestigate its electronic structure. On the basis of intrinsic bond orbital analysis, effective oxidation state analysis, ring current analysis, and comparison with well-established coordination complexes, it is demonstrated that the central carbon atom behaves like a transition metal. The central (apical) carbon atom, although best described as a highly Lewis-acidic carbon atom coordinated with an anionic cyclopentadienyl ligand, is also capable of acting as an electron-pair donor to a formal CH 3 + group. The different roles of coordination chemistry are discussed., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Magnetic bistability in organic-radical based materials has attracted significant interest due to its potential application in electronic devices. The first-principles bottom-up study herein presented aims at elucidating the key factors behind the different magnetic response of the low and high temperature phases of four different switchable dithiazolyl (DTA)-based compounds. The drastic change in the magnetic response upon spin transition is always due to the changes in the JAB magnetic interactions between adjacent radicals along the π-stacks of the crystal, which in turn are driven mostly by the changes in the interplanar distance and degree of lateral slippage, according to the interpretation of a series of magneto-structural correlation maps. Furthermore, specific geometrical dispositions have been recognized as a ferromagnetic fingerprint in such correlations. Our results thus show that an appropriate substitution of the chemical skeleton attached to the DTA ring could give rise to new organic materials with dominant ferromagnetic interactions.

Au(I) complexes are studied as precursors for focused electron beam induced processing (FEBIP). FEBIP is an advanced direct-write technique for nanometer-scale chemical synthesis. The stability and volatility of the complexes are characterized to design an improved precursor for pure Au deposition. Aurophilic interactions are found to play a key role. The short lifetime of ClAuCO in vacuum is explained by strong, destabilizing Au-Au interactions in the solid phase. While aurophilic interactions do not affect the stability of ClAuPMe 3 , they leave the complex non-volatile. Comparison of crystal structures of ClAuPMe 3 and MeAuPMe 3 shows that Au-Au interactions are much weaker or partially even absent for the latter structure. This explains its high volatility. However, MeAuPMe 3 dissociates unfavorably during FEBIP, making it an unsuitable precursor. The study shows that Me groups reduce aurophilic interactions, compared to Cl groups, which we attribute to electronic rather than steric effects. Therefore we propose MeAuCO as a potential FEBIP precursor. It is expected to have weak Au-Au interactions, making it volatile. It is stable enough to act as a volatile source for Au deposition, being stabilized by 6.5 kcal/mol. Finally, MeAuCO is likely to dissociate in a single step to pure Au.

We use a layered solution crystal growth method to synthesize high-quality single crystals of two different benzylammonium lead iodide perovskite-like organic/inorganic hybrids. The well-known (C 6 H 5 CH 2 NH 3 ) 2 PbI 4 phase is obtained in the form of bright orange platelets, with a structure comprised of single ⟨100⟩-terminated sheets of corner-sharing PbI 6 octahedra separated by bilayers of the organic cations. The presence of water during synthesis leads to formation of a novel minority phase that crystallizes in the form of nearly transparent, light yellow bar-shaped crystals. This phase adopts the monoclinic space group P2 1 /n and incorporates water molecules, with structural formula (C 6 H 5 CH 2 NH 3 ) 4 Pb 5 I 14 ·2H 2 O. The crystal structure consists of ribbons of edge-sharing PbI 6 octahedra separated by the organic cations. Density functional theory calculations including spin-orbit coupling show that these edge-sharing PbI 6 octahedra cause the band gap to increase with respect to corner-sharing PbI 6 octahedra in (C 6 H 5 CH 2 NH 3 ) 2 PbI 4 . To gain systematic insight, we model the effect of the connectivity of PbI 6 octahedra on the band gap in idealized lead iodide perovskite-derived compounds. We find that increasing the connectivity from corner-, via edge-, to face-sharing causes a significant increase in the band gap. This provides a new mechanism to tailor the optical properties in organic/inorganic hybrid compounds.

Cation-π interactions between tyrosine amino acids and compounds containing N,N,N-trimethylethanolammonium (N(CH 3 ) 3 ) are involved in the recognition of histone tails by chromodomains and in the recognition of phosphatidylcholine (PC) phospholipids by membrane-binding proteins. Yet, the lack of explicit polarization or charge transfer effects in molecular mechanics force fields raises questions about the reliability of the representation of these interactions in biomolecular simulations. Here, we investigate the nature of phenol-tetramethylammonium (TMA) interactions using quantum mechanical (QM) calculations, which we also use to evaluate the accuracy of the additive CHARMM36 and Drude polarizable force fields in modeling tyrosine-choline interactions. We show that the potential energy surface (PES) obtained using SAPT2+/aug-cc-pVDZ compares well with the large basis-set CCSD(T) PES when TMA approaches the phenol ring perpendicularly. Furthermore, the SAPT energy decomposition reveals comparable contributions from electrostatics and dispersion in phenol-TMA interactions. We then compared the SAPT2+/aug-cc-pVDZ PES obtained along various approach directions to the corresponding PES obtained with CHARMM, and we show that the force field accurately reproduces the minimum distances while the interaction energies are underestimated. The use of the Drude polarizable force field significantly improves the interaction energies but decreases the agreement on distances at energy minima. The best agreement between force field and QM PES is obtained by modifying the Lennard-Jones terms for atom pairs involved in the phenol-TMA cation-π interactions. This is further shown to improve the correlation between the occupancy of tyrosine-choline cation-π interactions obtained from molecular dynamics simulations of a bilayer-bound bacterial phospholipase and experimental affinity data of the wild-type protein and selected mutants., Competing Interests: AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B4(2-), B5(+/-), B6, B7(-), B8(2-), B9(-), B10(2-), B11(-), B12, B13(+), B14(2-) and B16(2-) are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B12 and B13(+) is confirmed. The π electrons of B18(2-), B19(-) and B20(2-) obey the disk aromaticity rule with an electronic configuration of [1σ(2)1π(4)1δ(4)2σ(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5(+), Li7B5H5 and M@BnHn(q) clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B7C(-), B8C, B6C2, B9C(-), B8C2 and B7C3(-) is in conflict with the Hückel electron count. This is also the case for the ions B11C5(+/-) whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.

Aromatic compounds are characterised by the presence of a ring current when in a magnetic field. As a consequence, current density maps are used to assess (the degree of) aromaticity of a compound. However, often a more discrete set of so-called Nucleus Independent Chemical Shift (NICS) values is used that is derived from the current density. It is shown here that there is no simple one-to-one relationship that allows reconstructing current density maps from only NICS-values. NICS values should therefore not be used as aromaticity indices without analysis of the ab initio computed current density map.

The fullerene-50 is a 'magic number' cage according to the 2(N + 1)(2) rule. For the three lowest isomers of C50 with trigonal and pentagonal symmetries, we calculate the sphericity index, the spherical parentage of the occupied π-orbitals, and the current density in an applied magnetic field. The minimal energy isomer, with D3 symmetry, comes closest to a spherical aromat or a superaromat. In the D5h bond-stretch isomers the electronic structure shows larger deviations from the ideal spherical shells, with hybridisation or even reversal of spherical parentages. It is shown that relative stabilities of fullerene cages do not correlate well with aromaticity, unlike the magnetic properties which are very sensitive indicators of spherical aromaticity. Superaromatic diamagnetism in the D3 cage is characterized by global diatropic currents, which encircle the whole cage. The breakdown of sphericity in the D5h cages gives rise to local paratropic countercurrents.

We show that charge recombination in ordered heterojunctions depends sensitively on the degree of coherent delocalization of charges at the donor-acceptor interface. Depending on the relative sign of the electron and hole transfer integrals, such delocalization can dramatically suppress recombination through destructive quantum interference. This could explain why measured recombination rates are significantly lower than predictions based on Langevin theory for a variety of organic bulk heterojunctions. Moreover, it opens up a design strategy for photovoltaic devices with enhanced efficiencies through coherently suppressed charge recombination.

We have investigated the dynamics in polycrystalline samples of l-methionine related to the structural transition at about 307K by incoherent inelastic and quasielastic neutron scattering, X-ray powder diffraction as well as ab-initio calculations. l-Methionine is a sulfur amino acid which can be considered a derivative of alanine with the alanine R-group CH3 exchanged by CH3S(CH2)2. Using X-ray powder diffraction we have observed at ~190K an anomalous drop of the c-lattice parameter and an abrupt change of the β-monoclinic angle that could be correlated to the anomalies observed in previous specific heat measurements. Distinct changes in the quasielastic region of the neutron spectra are interpreted as being due to the onset and slowing-down of reorientational motions of the CH3-S group, are clearly distinguished above 130K in crystalline l-methionine. Large-amplitude motions observed at low frequencies are also activated above 275K, while other well-defined vibrations are damped. The ensemble of our results suggests that the crystalline structure of l-methionine is dynamically highly disordered above 275K, and such disorder can be linked to the flexibility of the molecular thiol-ether group., (© 2013 Elsevier B.V. All rights reserved.)

Double aromaticity of neutral, planar rings of carbon atoms is demonstrated through visualisation of the induced ring currents, mapped at the ipsocentric B3LYP/6-31G(d)//B3LYP/6-31G(d) level for species C(6) to C(30), with onset of delocalised current in the in-plane pi system at C(10)/C(11). Both in-plane and conventional out-of-plane pi systems have diatropic/paratropic current in accordance with the Hückel rule, with 4 m+2 occupation of the out-of-plane pi system taking precedence, as predicted by simple nesting of Frost-Musulin diagrams. The current-density maps show characteristic double-doughnut and double-track topographies for out-of-plane and in-plane ring currents, respectively, both governed by a common framework of angular momentum rules.

To investigate the consequences of cyclometalation for electronic communication in dinuclear ruthenium complexes, a series of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridged diruthenium complexes was prepared and studied. These complexes have a central tppz ligand bridging via nitrogen-to-ruthenium coordination bonds, while each ruthenium atom also binds either a monoanionic, N,C,N'-terdentate 2,6-bis(2'-pyridyl)phenyl (R-N(wedge)C(wedge)N) ligand or a 2,2':6',2''-terpyridine (tpy) ligand. The N,C,N'-, that is, biscyclometalation, instead of the latter N,N',N''-bonding motif significantly changes the electronic properties of the resulting complexes. Starting from well-known [{Ru(tpy)}(2)(mu-tppz)](4+) (tpy = 2,2':2'',6-terpyridine) ([3](4+)) as a model compound, the complexes [{Ru(R-N(wedge)C(wedge)N)}(mu-tppz){Ru(tpy)}](3+) (R-N(wedge)C(H)(wedge)N = 4-R-1,3-dipyridylbenzene, R = H ([4a](3+)), CO(2)Me ([4b](3+))), and [{Ru(R-N(wedge)C(wedge)N)}(2)(mu-tppz)](2+), (R = H ([5a](2+)), CO(2)Me ([5b](2+))) were prepared with one or two N,C,N'-cyclometalated terminal ligands. The oxidation and reduction potentials of cyclometalated [4](3+) and [5](2+) are shifted negatively compared to non-cyclometalated [3](4+), the oxidation processes being affected more significantly. Compared to [3](4+), the electronic spectra of [5](2+) display large bathochromic shifts of the main MLCT transitions in the visible spectral region with low-energy absorptions tailing down to the NIR region. One-electron oxidation of [3](4+) and [5](2+) gives rise to low-energy absorption bands. The comproportionation constants and NIR band shape correspond to delocalized Robin-Day class III compounds. Complexes [4a](3+) (R = H) and [4b](3+) (R = CO(2)Me) also exhibit strong electronic communication, and notwithstanding the large redox-asymmetry the visible metal-to-ligand charge-transfer absorption is assigned to originate from both metal centers. The potential of the first, ruthenium-based, reversible oxidation process is strongly negatively shifted. On the contrary, the second oxidation is irreversible and cyclometalated ligand-based. Upon one-electron oxidation, a weak and low-energy absorption arises.

The 12pi cation (3) and 14pi anion (4) derived from the phenalenyl radical (2) support diatropic ("aromatic") perimeter ring currents, but isoelectronic replacement of the central atom by either boron (5) or nitrogen (6) leads to paratropic ("antiaromatic") current; the ipsocentric approach to molecular magnetic response accounts for all four patterns in terms of competition between translationally and rotationally allowed virtual pi-pi* excitations.

Apparent contradictions between ipsocentric, Nucleus-Independent Chemical Shift (NICS) and Gauge-Including Magnetically Induced Current (GIMIC) evaluations of ring current aromaticity in Al(4)(2-) are resolved in favour of the description of this all-metal system as sigma aromatic on the magnetic criterion.

The bacterial mutagenic response (Ames-assay, Salmonella typhimurium strain TA98+/-S9-mix) of a series of monocyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) identified in combustion exhausts, viz. cyclopenta[cd]pyrene (1), acephenanthrylene (2), aceanthrylene (3) and cyclopenta[hi]chrysene (4), is re-evaluated. The mutagenic effects are compared with those exerted by the corresponding partially hydrogenated derivatives, 3,4-dihydrocyclopenta[cd]pyrene (5), 4,5-dihydroacephenanthrylene (6), 1,2-dihydroaceanthrylene (7) and 4,5-dihydrocyclopenta[hi]chrysene (8). It is shown that the olefinic bond of the externally fused five-membered ring of 1, 3 and 4 is of importance for a positive mutagenic response. In contrast, whilst CP-PAH 2 is found inactive, its dihydro analogue (6) shows a weak metabolism-dependent response. The importance of epoxide formation at the external olefinic bond in the five-membered ring is substantiated by the bacterial mutagenic response of independently synthesized cyclopenta[cd]pyrene-3,4-epoxide (9), acephenanthrylene-4,5-epoxide (10), aceanthrylene-1,2-epoxide (11) and cyclopenta[hi]chrysene-4,5-epoxide (12). Their role as ultimate, active mutagenic forms, when CP-PAHs 1, 3 and 4 exhibit a positive mutagenic response, is confirmed. Semi-empirical Austin Model 1 (AM1) calculations on the formation of the CP-arene oxides (9-12) and their conversion into the monohydroxy-carbocations (9a-12a and 9b-12b) via epoxide-ring opening support our results. For 2 and 4, which also possess a bay-region besides an annelated cyclopenta moiety, the calculations rationalize that epoxidation at the olefinic bond of the cyclopenta moiety is favoured.

Why are some (4n+2)pi systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four-electron diatropic (aromatic) ring current for (4n+2)pi carbocycles and a two-electron paratropic (antiaromatic) current for (4n)pi carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier-orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen-localised currents in alternating azabora-heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic pi-conjugated systems. Ab initio valence-bond calculations confirm the localisation predicted by the naive model, and coupled-Hartree-Fock calculations give current-density maps that exhibit the predicted delocalised-to-localised/carbocycle-heterocycle transition.

To rationalize the high bacterial mutagenic response recently found for the (di-) cyclopenta-fused pyrene congeners, viz. cyclopenta[cd]-(1), dicyclopenta[cd,mn]-(2), dicyclopenta[cd,fg]-(3) and dicyclopenta[cd,jk]pyrene (4), in the presence of a metabolic activation mixture (S9-mix), their (di-)epoxides at the externally fused unsaturated five-membered rings were previously proposed as the ultimate mutagenic active forms. In this study, cyclopenta[cd]pyrene-3,4-epoxide (5) and the novel dicyclopenta[cd,mn]pyrene-1,2,4,5-di-epoxide (6), dicyclopenta[cd,fg]pyrene-5,6,7,8-di-epoxide (7) and dicyclopenta[cd,jk]pyrene-1,2,6,7-di-epoxide (8) were synthesised from 1 to 4, respectively, and subsequently assayed for bacterial mutagenicity in the standard microsomal/histidine reverse mutation assay (Ames-assay with Salmonella typhimurium strain TA98). The di-epoxides 6-8 are present as a mixture of their cis- and trans-stereo-isomers in a close to 1:1 ratio ((1)H NMR spectroscopy and ab initio IGLO/III//RHF/6-31G** calculations). The direct-acting mutagenic activity and the strong cytotoxicity exerted by 5-8 both in the absence or presence of an exogenous metabolic activation system (+/-S9-mix) demonstrate that the ultimate mutagenic active forms are the proposed (di-)epoxides of 1-4.

Quenching/survival of ring currents in pi-clamped conjugated monocycles is controlled by the match or mismatch in parity between the frontier orbitals of the central pi-conjugated 4n+2/4n monocycle and those of the clamps. Changes in ring current are not primarily caused by bond alternation or 'Mills-Nixon' effects; current and geometry changes on clamping are both consequences of electronic structure.

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X(2) (X=N, P, As, and Sb) and the (4)S/(2)D and (4)S/(2)P splittings for the corresponding atoms, the (1)A(1)-(3)B(1) splittings for CH(2) and SiH(2), and the absorption spectra of pyrrole and of Cu(Imidazole)(2)(SH)(SH(2))(+), which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the (4)S/(2)D and (4)S/(2)P splittings for the group fifteen atoms. This is due to the different treatment of orbitals belonging to the inactive or active spaces, making the NEVPT2 not invariant for the choice of active space, even in cases where the multiconfiguration self-consistent field energy is invariant.

Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S(4) (2+), Se(4) (2+), S(2)N(2), P(5) (-) and As(5) (-) with 6 pi electrons; S(3)N(3) (-), S(4)N(3) (+) and S(4)N(4) (2+) with 10 pi electrons; and S(5)N(5) (+) with 14 pi electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic pi currents, reinforced by sigma circulations in P(5) (-), As(5) (-), S(3)N(3) (-), S(4)N(3) (+), S(4)N(4) (2+) and S(5)N(5) (+), but opposed by them in S(4) (2+), Se(4) (2+) and S(2)N(2). The opposition of pi and sigma effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts).

Ab initio current-density maps for the (4n + 2)-electron transition states of two thermally allowed [pi2s + pi2s + pi2s] trimerisations, of ethyne to benzene (1), and of ethene to cyclohexane (2), show that both support sigma diatropic ring currents, dominated by four-electron sigma --> sigma* virtual excitation of HOMO electrons.

Direct visualization of the pi current density maps of highly strained annelated benzenes containing cyclopropa, cyclobuta, and cyclobutadieno clamps, alone and in combination, using a reliable distributed-origin, coupled Hartree-Fock method, shows the robustness of the classical benzene diatropic pi ring current. When only saturated clamps are used, the benzene ring current is essentially unchanged. In contrast, annelation with one or more cyclobutadieno clamps disrupts the benzene ring current. Analysis of orbital contributions to the current density maps gives a unified account of these observations in terms of the nature of the HOMO-LUMO transition.

Current-density maps at the coupled Hartree-Fock level calculated in the CTOCD (continuous transformation of origin of current density) approach demonstrate the magnetic response of the hypothetical planar hexacoordinate carbon species, CB(2-)(6). In contrast with the empty CB(2-)(6) framework, which supports paramagnetic currents, the carbon-containing species has a typical diamagnetic pi-ring current that circulates undisturbed by the central atom. In spite of the unconventional nature of the species, the properties of 6pi CB(2-)(6) and 4pi CB(2-)(6) can be rationalised with the same orbital model that accounts for the diamagnetic pi current of benzene and the paramagnetic pi current of planar cyclooctatetraene.