Observations on the Crystal Structures of Electron Donor-Acceptor Complexes

The theoretical interpretation of electron donor-acceptor complex formation in terms of charge transfer interactions has stimulated many structure determinations for these complexes. These fall into three classes, depending on the type of orbitals involved in charge transfer. In σ-σ complexes, intermolecular bonds become shorter and intramolecular bonds become longer as charge transfer increases. Relative orientations correspond to overlap of donor and acceptor molecules in directions of “preferred polarizability”. Intermolecular bond lengths in σ-π complexes show similar trends, and the axial orientation in the benzene-halogen complexes is probably the result of the best compromise between orbital overlap and energy factors. π-π Complexes contain stacks of alternate plane-to-plane donor and acceptor molecules, arranged in three characteristic ways. There is little correlation between interplanar spacing in these stacks and charge transfer properties. The relative orientations of donor and acceptor molecules within the stacks are determined by a combination of charge transfer interactions (maximized when aromatic rings of donor and acceptor molecules are displaced by half a ring diameter) and dipole-induced dipole interactions (maximized, for example, when a polar bond of one molecule overlaps a polarizable region of another). Crystal packing requirements and dispersion forces modify these effects, and no satisfactory theoretical treatment of this complex combination of interactions is yet available.     

Synthesis and characterisation of tetra-tetrazole macrocycles

The syntheses of tetra-tetrazole macrocycles, containing two bis-tetrazole units linked by a variety of alkyl-chain lengths from four to eight carbons, are described. The crystal structures of three of these derivatives are reported, and the molecular conformation in the solid state is compared to that of the previously reported tetra-tetrazole macrocycle and to other bis- and tris(tetrazole)benzene structures. The macrocycle conformation is influenced by the length of the alkyl-chain linker, the relative orientation of the tetrazole rings on the benzene ring and by intermolecular interactions. In the macrocycles based on 1,2-bis(tetrazole)benzene, the adjacent tetrazole rings on the benzene ring are prevented from becoming co-planar on intramolecular (steric) grounds. In the 1,3- and 1,4-bis(tetrazole)benzene derivatives, there is no such impediment, and a co-planar arrangement is observed where intra- and/or intermolecular stacking interactions exist. Deviations from co-planarity are associated with optimisation of intermolecular interactions between the tetrazole rings and adjacent alkyl chains. In the macrocycle based on 1,4-bis(tetrazole)benzene with four-carbon linkers, an intramolecular stacking interaction exists, which precludes the presence of any cavity. In the macrocycle based on 1,3-bis(tetrazole)benzene with six-carbon linkers, a cavity of 10.8×9.4 Å is observed for each molecule in the solid state, although the packing of adjacent molecules is such that there are no extended channels running through the crystal.     

The intricacies of the stacking interaction in a pyrrole–pyrrole system

Extensive calculations of potential energy surfaces for parallel-displaced configurations of pyrrole–pyrrole systems have been carried out by the use of a dispersion-corrected density functional. System geometries associated with the energy minima have been found. The minimum interaction energy has been calculated as ?5.38 kcal/mol. However, bonding boundaries appeared to be relatively broad, and stacking interactions can be binding even for ring centroid distances larger than 6 Å. Though the contribution of the correlation energy to intermolecular interaction in pyrrole dimers appeared to be relatively small (around 1.6 smaller than it is in a benzene–benzene system), this system’s minimum interaction energy is lower than those calculated for benzene–benzene, benzene–pyridine and even pyridine–pyridine configurations. The calculation of the charges and energy decomposition analysis revealed that the specific charge distribution in a pyrrole molecule and its relatively high polarization are the significant source of the intermolecular interaction in pyrrole dimer systems.     

Heteroatom influence in substituted benzenes and pyridine-like heterocycles in terms of direct and indirect intramolecular interactions

The general expression for the one-electron density matrix obtained previously for saturated organic molecules (V. Gineityte, J. Mol. Struct. (Theochem), 343 (1995) 183) is shown to be applicable also to substituted benzenes and pyridine-like heterocycles. On this basis, a new interpretation of the influence of a heteroatom (substituent) upon the remaining fragment of an aromatic molecule is suggested. To this end, the occupation number of a 2p_z AO of the aromatic ring has been expressed as a sum of five terms, two of them describing the intramolecular charge transfer and the remaining ones representing the secondary (induced) dipole moments arising within the ring under the influence of heteroatom, viz. the so-called ipso–ortho (para–meta), para–ipso and ortho–meta dipole moments. Just the latter two moments proved to play the principal role in the formation of the observed picture of the electron density distribution, viz. of an increase (reduction) of occupation numbers of 2p_z AOs in the ortho and para positions after introducing an electron-donating (accepting) substituent. For pyridine-like heterocycles and substituted benzenes, these dipole moments are determined mostly by the direct and the indirect interactions, respectively, between the highest occupied and the lowest vacant MO of benzene. Orbitals of the heteroatom (substituent) play the role of mediators in the above-mentioned indirect interaction.     

Porphyrin-naphthodiimide interactions as a structural motif in foldamers and supramolecular assemblies

pi-pi Stacking interactions between electron deficient naphthalenediimides (NDI) and electron-rich porphyrins (POR) leading to charge transfer are shown to be prevalent in linked NDI-POR and POR-NDI-POR structures. For flexibly-linked systems, intramolecular interactions lead to S-shaped foldamers in solution, whereas intermolecular association is predominant in more rigid systems. The foldamer structures can be interrupted by competing aromatic solvents, by six-coordination of metallated porphyrin derivatives, by protonation of the free base porphyrin in non-metallated structures, and in facially sterically hindered porphyrins.     

Dynamic nature of the intramolecular electronic coupling mediated by a solvent molecule: a computational study

We present a combined Molecular Dynamics/Quantum Chemical study of the solvent-mediated electronic coupling between an electron donor and acceptor in a C-clamp molecule. We characterize the coupling fluctuations due to the solvent motion for different solvents (acetonitrile, benzene, 1,3-diisopropyl-benzene) for the charge separation and the charge recombination processes. The time scale for solvent-induced coupling fluctuation is approximately 0.1 ps. The effect of these fluctuations on the observed rate is discussed using a recently developed theoretical model. We show that, while the microscopic charge transfer process is very complicated and its computational modeling very subtle, the macroscopic phenomenology can be captured by the standard models. Analyzing the contribution to the coupling given by different solvent orbitals, we find that many solvent orbitals mediate the electron transfer and that paths through different solvent orbitals can interfere constructively or destructively. A relatively small subset of substrate-solvent configurations dominate contributions to solvent-mediated coupling. This subset of configurations is related to the electronic structure of the C-clamp molecule.     

An attempt at interpreting competitive Friedel-Crafts reactions : Role of solvation energy

Results of Olah et al. on benzylation of benzene and toluene are studied theoretically. The evolution of intermolecular k_T/k_B and intramolecular o/2p selectivities is interpreted with the aid of a model involving electron affinities of electrophilic entities. Our electrophilic affinities are calculated taking solvation into account, which reverses the order of affinities computed on the naked cations.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Temperature-dependent structure of methyltributylammonium bis(trifluoromethylsulfonyl)amide: X ray scattering and simulations

We report the combined results of computational and x ray scattering studies of amorphous methyltributylammonium bis(trifluoromethylsulfonyl)amide as a function of temperature. These studies included the temperature range for the normal isotropic liquid, a deeply supercooled liquid and the glass. The low q peaks in the range from 0.3 to 1.5 A(-1) in the structure function of this liquid can be properly accounted for by correlations between first and second nearest neighbors. The lowest q peak can be assigned to real space correlations between ions of the same charge, while the second peak arises mostly from nearest neighbors of opposite charge. Peaks at larger q values are mostly intramolecular in nature. While our simulated structure functions provide an excellent match to our experimental results and our experimental findings agree with previous studies reported for this liquid, the prior interpretation of the experimental data in terms of an interdigitated smectic A phase is not supported by our simulations. In this work, we introduce a set of general theoretical partitions of real and reciprocal space correlations that allow for unambiguous analysis of all intra- and interionic contributions to the structure function and coherent scattering intensity. We find that the intermolecular contributions to the x ray scattering intensity are dominated by the anions and cross terms between cations and anions for this ionic liquid.     

Attraction between electrophilic caps: A counterintuitive case of noncovalent interactions

Intermolecular attractive interaction between electrophilic sites is a counterintuitive phenomenon, as the electrostatic interaction therein is repulsive and destabilizing. Here, we confirm this phenomenon in four representative complexes, using state-of-the-art quantum mechanical methods. By employing the block-localized wavefunction (BLW) method, which can turn off intermolecular charge transfer interactions, we profoundly demonstrated the significance of charge transfer interactions in these seemingly counterintuitive complexes. Indeed, after being “turned off” the intermolecular charge transfer interaction in, for example, the FNSi···BrF complex, the originally attractive intermolecular interaction turns to be repulsive. The energy decomposition approach based on the BLW method (BLW-ED) can partition the overall stability gained on the formation of intermolecular noncovalent interaction into several physically meaningful components. According to the BLW-ED analysis, the electrostatic repulsion in these counterintuitive cases is overwhelmed by the stabilizing polarization, dispersion interaction, and most importantly, the charge transfer interaction, resulting in the eventual counterintuitive overall attraction. The present study suggests that, predicting bonding sites of noncovalent interactions using only the “hole” concept may be not universally sufficient, because other significant stabilizing factors will contribute to the stability and sometimes, play even bigger roles than the electrostatic interaction and consequently govern the complex structures. © 2018 Wiley Periodicals, Inc.     

Enthalpies of elementary chain propagation steps in the oxidation of organic compounds

A calculation of the enthalpies of elementary steps of the intra- and intermolecular chain propagation for model oxidation reactions of ethers, esters, ketones and hydrocarbons has been carried out. The heats of the intermolecular and intramolecular transfer of free valence with participation of peroxy radicals and C−H bond of the oxygen-containing compounds are shown to be comparable.     

环聚炔苯和环聚炔吡啶组成的盘状液晶中的电荷转移

用电子转移的半经典模型在量子化学B3LYP/6-31G(d)水平(对单体)和B3LYP/STO-3G水平(对二聚物)对环聚炔苯和环聚炔吡啶组成的盘状液晶体系的电荷转移性质进行了研究. 盘状液晶体系的电荷转移速率主要依赖于重组能和电荷转移矩阵元, 重组能越小, 电荷转移矩阵元越大, 则电荷转移速率常数越大. 计算结果表明, 这些大环化合物比目前广泛研究和应用的苯并菲衍生物组成的液晶有较小的重组能, 所以有更好的电荷转移性质. 计算结果对有效地设计和合成高效的光导材料和载流子输送材料是有帮助的.     

Electron charge redistribution following electrophilic attack on heterocycles: nitrogen as a charge transducer

It is shown that an electrophilic attack on the ring nitrogen in pyrrole or tryptamine produces an electron charge redistribution that is qualitatively different from the redistribution caused by an attack on the oxygen in furan. The electrophilic attacks are represented theoretically as interactions with a positive point charge and calculated by an ab-initio LCAO-SCF method with gaussian basis sets. Results show that an attacked nitrogen responds to the perturbation by moving electronic charge to the adjacent carbons whereas oxygen retains most of the charge polarized by the interaction. The nitrogen also acts as a charge transducer in other systems that are structurally very different. As a consequence of the charge redistribution, the comparative susceptibility of various sites in the heterocyclic molecules to an electrophilic attack may also depend on the response of the molecule to a prior attack on the heteroatom. The results indicate the need for dynamical reactivity considerations which reflect the variability in the molecular response to an incipient attack and the possibility that enhanced reactivity can be induced at certain sites by this response.     

Lone-pair orbital interactions in polythiaadamantanes

The electronic structures of a series of polythiaadamantanes from thiaadamantane through 2,4,6,8,9,10-hexathiaadamantane (HTA) have been analyzed using density functional theory calculations in conjunction with Hückel and natural bond orbital analysis. The effects of multiple sulfur p-type lone-pair orbital interactions on ionization potentials, hole mobilities, and electronic coupling have been determined. An overall increase in the average energy of the lone-pair orbitals as the number of sulfur atoms increases is predicted, with the exact positioning of the HOMO depending on specific lone-pair interactions. Separation of through-bond (TB) and through-space (TS) interactions between intramolecular sulfur atoms has been performed using localized molecular orbitals and model systems based on interacting hydrogen sulfide molecules. TB interations were found to reduce orbital splitting, while TS interactions were found to increase orbital splitting. TS interactions were more or less constant from one polythiaadamantane to the next, and the contributions of TB effects to individual orbital energies vary depending on the relative orientation of sulfur atoms as determined by the sigma molecular framework. Electronic coupling between intermolecular sulfur lone-pair orbitals was determined by investigating unique dimer pairs observed in the crystal structure of HTA. Electronic coupling is not as strong as expected given the short intermolecular S-S distances observed in the crystal structure. In general, B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) give very similar orbital energies and splittings.     

A quantum-chemical study of para/ortho-toluene alkylation by adsorbed methoxy species on zeolites

MNDO and PM3 semiempirical methods have been employed to study the mechanism of toluene alkylation by methanol on zeolites, by means of the cluster (H)₃SiO(CH₃)T(H)₂OSi(H)₃ in were T = Al, B, Ga. Two possible reaction mechanisms have been found. The first one corresponds to the classical process of aromatic electrophilic substitution, in which the transition state of the reaction shows distances and angles according to the precursor of the Wheland intermediate. The second mechanism proceeds in one concerted step, in which the transfer of the proton from the toluene to the basic oxygen of the zeolite is simultaneous to the transfer of the methyl cation from the zeolite to the toluene. The effect of chemical composition of the cluster on regioselectivity has been studied, and perturbed molecular orbital (PMO) analysis of the system at the ab initio HF/STO-3G level has shown that covalent interactions are important. The energy of the covalent interaction between the molecular orbitals of the toluene and the cluster mainly HOMO-LUMO, are more important in para than in ortho positions.     

Electronic basis of the comparable hydrogen bond properties of small H2CO/(H2O)n and H2NO/(H2O)n systems (n = 1, 2)

The electronic and structural properties of dihydronitroxide/water clusters are investigated and compared to the properties of formaldehyde/water clusters. Exploring the stationary points of their potential energy surfaces (structurally, vibrationally, and energetically) and characterizing their hydrogen bonds (by both atoms in molecules and natural bond orbitals methods) clearly reveal the strong similarity between these two kind of molecular systems. The main difference involves the nature of the hydrogen bond taking place between the X-H bond and the oxygen atom of a water molecule. All the properties of the hydrogen bonds occurring in both kind of clusters can be easily interpreted in terms of competition between intermolecular and intramolecular hyperconjugative interactions.     

Potassium ion controlled switching of intra- to intermolecular electron transfer in crown ether appended free-base porphyrin-fullerene donor-acceptor systems

Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads.