Marçal Capdevila-Cortada

The Nature of the [TTF].+⋅⋅⋅[TTF].+ Interactions in the [TTF]22+ Dimers Embedded in Charged [3]Catenanes: Room-Temperature Multicenter Long Bonds†

The properties of tetrathiafulvalene dimers ([TTF](2)(2+)) and the functionalized ring-shaped bispropargyl (BPP)-functionalized TTF dimers, [BPP-TTF](2)(2+), found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are energetically unstable towards dissociation. When enclosed in the 4(+)-charged central cyclophane ring of charged [3]catenanes (CBPQT(4+)), [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are also energetically unstable with respect to leaving the CBPQT(4+) ring; since the barrier for the exiting process is only about 3 kcal mol(-1), that is, within the reach of thermal energies at room temperature (neutral [TTF](2)(0) dimers are stable within the CBPQT(4+) ring). However, the [BPP-TTF](2)(2+) dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP-TTF(+) macrocycle. Finally, it was shown that the [TTF](2)(2+), [BPP-TTF](2)(2+) dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation-anion interactions. In these environmental conditions at room temperature the TTF units of the [BPP-TTF](2)(2+) dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room-temperature multicenter long bond is formed, similar to those previously found in other [TTF](2)(2+) salts and their solutions.

Entropic contributions enhance polarity compensation for CeO2(100) surfaces

Surface structure controls the physical and chemical response of materials. Surface polar terminations are appealing because of their unusual properties but they are intrinsically unstable. Several mechanisms, namely metallization, adsorption, and ordered reconstructions, can remove thermodynamic penalties rendering polar surfaces partially stable. Here, for CeO2(100), we report a complementary stabilization mechanism based on surface disorder that has been unravelled through theoretical simulations that: account for surface energies and configurational entropies; show the importance of the ion distribution degeneracy; and identify low diffusion barriers between conformations that ensure equilibration. Disordered configurations in oxides might also be further stabilized by preferential adsorption of water. The entropic stabilization term will appear for surfaces with a high number of empty sites, typically achieved when removing part of the ions in a polar termination to make the layer charge zero. Assessing the impact of surface disorder when establishing new structure-activity relationships remains a challenge.

Keys for the Existence of Stable Dimers of Bis-tetrathiafulvalene (bis-TTF)-Functionalized Molecular Clips Presenting [TTF]•+···[TTF]•+ Long, Multicenter Bonds at Room Temperature

A systematic theoretical and computational investigation is performed to determine the keys governing the existence, in acetonitrile solutions, of dimers of bis-tetrathiafulvalene (bis-TTF)-functionalized diphenylglycoluril molecular clips (clip2(n+)) that are stable at room temperature for n ≤ 4. Although the experimental structure of these dimers in solution is unknown, electronic absorption studies suggest that they have [TTF](l+)···[TTF](m+) interactions that are preserved at room temperature (note that when l = m = 1 these interactions become long, multicenter bonds). In good agreement with the interpretation of the experimental spectroscopic data, all clip2(n+) dimers whose charge is ≤4 present an optimum geometry that, in all cases, has three short interfragment [TTF](l+)···[TTF](m+) interactions. The computed ΔG(298 K) for these optimum structures matches the available experimental data on the stability of these dimers. Such optimum geometry, combined with the zwitterionic character of the electron distribution in monomers and dimers (most of the net positive charge is equally distributed among the TTF groups, while a 1- au charge is located in the central fused five-membered rings) allows the formation of a maximum of two long, multicenter [TTF](•+)···[TTF](•+) bonds when all TTF groups host a 1+ au of charge, as in clip2(4+). However, these long, multicenter bonds alone do not account for the stability of clip2(n+) dimers at room temperature. Instead, the studies carried out here trace the origin of their stability to (1) the zwitterionic character of their charge distribution, (2) the proper geometrical shape of the interacting monomers, which allows the intercalation of their arms, thus making possible the simultaneous formation of two short contacts, both involving the positively charged TTF group of one monomer and the negatively charged central ring of the other, (3) the simultaneous presence of three short contacts among the TTF groups in the optimum geometry of the clip2(n+) dimers, which become two long, multicenter bonds and one van der Waals interaction when the four TTF groups host a 1+ charge, and (4) the net stabilizing effect of the solvent.

Unusually Long, Multicenter, Cationδ+⋅⋅⋅Anionδ− Bonding Observed for Several Polymorphs of [TTF][TCNE]†

The α, β, and δ polymorphs of [TTF][TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) exhibit a new type of long, multicenter bonding between the [TTF](δ+) and [TCNE](δ-) moieties, demonstrating the existence of long, hetero-multicenter bonding with a cationic(δ+)···anionic(δ-) zwitterionic-like structure. These diamagnetic π-[TTF](δ+) [TCNE](δ-) heterodimers exhibit a transfer of about 0.5 e(-) from the TTF to the TCNE fragments, as observed from experimental studies, in accord with theoretical predictions, that is, [TTF(δ+)···TCNE(δ-)] (δ≅0.5). They have several interfragment distances <3.4 Å, and a computed interaction energy of -21.2 kcal mol(-1), which is typical of long, multicenter bonds. The lower stability of [TTF](δ+) [TCNE](δ-) with respect to typical ionic bonds is due, in part, to the partial electron transfer that reduces the electrostatic bonding component. This reduced electrostatic interaction, and the large interfragment dispersion stabilize the long, heterocationic/anionic multicenter interaction, which in [TTF(δ+)···TCNE(δ-)] always involves two electrons, but have ten, eight, and eight bond critical points (bcps) involving C-C, N-S, and sometimes C-S and C-N components for the α, β, and δ polymorphs, respectively. In contrast, γ-[TTF][TCNE] possesses [TTF](2)(2+) and [TCNE](2)(2-) dimers, each with long, homo-multicenter 2e(-)/12c (c=center, 2 C+4 S) [TTF](2)(2+) cationic(+)···cationic(+) bonds, as well as long, homo-multicenter 2e(-)/4c [TCNE](2)(2-) anionic(-)···anionic(-) bonding. The MO diagrams for the α, β, and δ polymorphs have all of the features found for conventional covalent C-C bonds, and for all of the previously studied multicenter long bonds, for example, π-[TTF](2)(2+) and π-[TCNE](2)(2-). The HOMOs for α-, β-, and δ-[TTF][TCNE] have 2c C-S and 3c C-C-C orbital-overlap contributions between the [TTF](δ+-) and [TCNE](δ-) moieties; these are the shortest intra [TTF···TCNE] separations. Thus, from an orbital-overlap perspective, the bonding has 2c and 3c components residing over one S and four C atoms.

The nature of the C–Cl⋯Cl–C intermolecular interactions found in molecular crystals: a general theoretical-database study covering the 2.75–4.0 Å range

The nature of C–Cl⋯Cl–C interactions in molecular crystals has been evaluated at the MP2/aug-cc-pVDZ computational level after test computations on simple model systems showed that such a computational level predicts for model dimers the same angular dependence and impact of the C(ipso) atom hybridization as those at the MP2/CBS computational levels. Thus MP2/aug-cc-pVDZ calculations predict a C(spn)–Cl⋯Cl–C(spn) strength of −0.73, −0.87, and −0.96 kcal mol−1 for n = 3, n = 2 (non-aromatic), and n = 1 (all BSSE-corrected values) at their most stable orientation, while for the same orientations and hybridization MP2/CBS calculations predict values of −1.14, −1.29, and −1.40 kcal mol−1. The first group of computations on the model dimers allows conclusion that the strength of the C–Cl⋯Cl–C interactions depends on (a) the number of short-distance Cl⋯Cl contacts involved, (b) the hybridization of the C(ipso) atom [E(Csp2) > E(Csp) > E(Csp3)], (c) the degree of chlorination of the C(ipso) atom, and (d) the relative orientation of the two C–Cl groups. Two types of minima were found in the E(θ1, θ2) potential energy surface [θ1 = <C(1)–Cl(2)⋯Cl(3) and θ2 = <Cl(2)⋯Cl(3)–C(4)]: type I minima, where θ1 = θ2 = 90°, and type II minima, which is energetically more stable, where θ1 = 180° and θ2 = 90° or θ1 = 90° and θ2 = 180° (the orientation where θ1 = θ2 = 155° (a type I geometry) is a saddle point in E(θ1, θ2) that connects the two type II minima). The interaction energy was also computed for 45 C–Cl⋯Cl–C containing dimers extracted from the Cambridge Crystallographic Database (CCDB) and having a Cl⋯Cl distance smoothly distributed within the 2.75–4.0 A range. The nature of these interactions was further characterized by looking at (a) the dominant component of the dimer interaction energy and (b) the characteristic properties of their only Cl⋯Cl bond critical point (evaluated from an atoms-in-molecules (AIM) analysis of the dimer electron density). Their interaction energy is dominated by the dispersion component, although a much weaker electrostatic component is also present in some cases. These interactions fail to fulfill the strength–length distribution that should correlate Eint and the Cl⋯Cl distance. A previously proposed correlation between the electron density at the Cl⋯Cl bond critical point and the strength of the C–Cl⋯Cl–C interaction is shown to fail for short Cl⋯Cl distances.

The nature of the C–Br⋯Br–C intermolecular interactions found in molecular crystals: a general theoretical-database study

The nature of the C–Br⋯Br–C intermolecular interactions has been evaluated by doing first-principles calculations on model dimers and on full-sized dimers extracted from crystals deposited in the Cambridge Structural Database presenting short-distance C–Br⋯Br–C contacts. First of all, the strength of the C(spn)–Br⋯Br–C(spn) interaction was determined at the MP2/CBS level on model dimers placed at their most stable orientation, getting values of −1.72, −1.83, and −1.91 kcal mol−1 for n = 3, 2 (non-aromatic), and 1, respectively. SAPT analyses of the interaction energy showed that it is dominated by the dispersion term, although a strong electrostatic component is also present, which depends on the fragment dipole moment (induced by the substituents attached to the two C atoms involved in the C–Br⋯Br–C interaction) and the σ-hole of the halogen atoms (induced by the asymmetry of the electron density around each bromine atom). The angular dependence of the C–Br⋯Br–C interactions was determined by computing the Eint(θ1,θ2) surface (where θ1 and θ2 are the ∠C–Br⋯Br and ∠Br⋯Br–C angles) for the (CH3Br)2 dimer. The most stable orientations were found at θ1⋯θ2 = 90° (a case of Type I orientation) and at θi ≈ 180° and θj ≈ 90° (Type II orientation). The Eint(θ1,θ2) surface also showed that the θ1 = θ2 = 150° orientation has the lowest energy among all Type I, but rather than a minimum, it should be considered as a saddle point between both Type II minimum energy orientations. Finally, in order to gain information on the properties of the C–Br⋯Br–C interactions in real cases, the interaction energy was evaluated for 39 dimers extracted from the Cambridge Structural Database that present C–Br⋯Br–C interactions smoothly distributed over the 3.0–4.5 A range. This allowed establishment of the overall stabilizing nature of these interactions in complex molecules (all have interaction energies that range from −2.35 to −0.38 kcal mol−1, with an average value of −1.26 kcal mol−1). The correlation between stronger interaction energies and higher electron density values at the Br⋯Br bond critical point was shown to be incorrect for sub-van der Waals C–Br⋯Br–C interactions.

Multistep π Dimerization of Tetrakis(n‐decyl)heptathienoacene Radical Cations: A Combined Experimental and Theoretical Study

Radical cations of a heptathienoacene α,β-substituted with four n-decyl side groups (D4T7(.) (+) ) form exceptionally stable π-dimer dications already at ambient temperature (Chem. Comm. 2011, 47, 12622). This extraordinary π-dimerization process is investigated here with a focus on the ultimate [D4T7(.) (+) ]2 π-dimer dication and yet-unreported transitory species formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the π-dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one-electron oxidation: 1) a different transient conformation of the ultimate [D4T7(.) (+) ]2 π-dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]2 (.) (+) π-dimer radical cations formed prior to the fully oxidized [D4T7]2 (.) (+) π-dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems.

Orientational Preference of Long, Multicenter Bonds in Radical Anion Dimers: A Case Study of π-[TCNB]22− and π-[TCNP]22−†

The similar shape and electronic structure of the radical anions of 1,2,4,5-tetracyanopyrazine (TCNP) and 1,2,4,5-tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon-carbon bond in π-[TCNP]2 (2-) and in π-[TCNB]2 (2-) , in good accord with the Maximin Principle predictions. Instead, the two known structures of π-[TCNP]2 (2-) have a D2h (θ=0°) and a C2 (θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six-membered rings). The only known π-[TCNB]2 (2-) structure has a C2 (θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (Eint ) and the variation of the Eint components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the Eint (θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of Eint (θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the Eint (θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)(.-) ⋅⋅⋅cation(+) interactions are also included in the model aggregate are the experimental data reproduced computationally.

The Origin of the Room‐Temperature Stability of [TTF].+⋅⋅⋅[TTF].+ Long, Multicenter Bonds Found in Functionalized π‐[R‐TTF]22+ Dimers Included in the Cucurbit[8]uril Cavity

A computational study is performed to identify the origin of the room-temperature stability, in aqueous solution, of functionalized π-[R-TTF]2(2+) dimers (TTF=tetrathiafulvalene; R=(CH2OCH2)5CH2OH) included in the cavity of a cucurbit[8]uril (CB[8]) molecule. π-[R-TTF]2(2+) dimers in pure water are weakly stable, and are mostly dissociated at room temperature. Upon addition of CB[8] to an aqueous π-[R-TTF]2(2+) solution, a (π-[R-TTF]2⊂CB[8])(2+) inclusion complex is formed. The same complex is obtained after the sequential inclusion of two [R-TTF](.+) monomers in the CB[8] molecule. Both processes are thermodynamically and kinetically allowed. π-[R-TTF]2(2+) dimers dissolved in pure water present a [TTF](.+)⋅⋅⋅[TTF](.+) long, multicenter bond, similar to that already identified in π-[TTF]2(2+) dimers dissolved in organic solvents. Upon their inclusion in CB[8], the strength and other features of the [TTF](.+)⋅⋅⋅[TTF](.+) long, multicenter bond are preserved. The room temperature stability of the π-[R-TTF]2(2+) dimers included in CB[8] is shown to originate in the π-[R-TTF]2(2+)⋅⋅⋅CB[8] interaction, the strength of which comes from a strongly attractive electrostatic component and a dispersion component. Such a dominant electrostatic term is caused by the strongly polarized charge distribution in CB[8], the geometrical complementarity of the π-[R-TTF]2(2+) and CB[8] geometries, and the amplifying effect of the 2+ charge in π-[R-TTF]2(2+).

Formation of Long, Multicenter π‐[TCNE]22− Dimers in Solution: Solvation and Stability Assessed through Molecular Dynamics Simulations

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of π-[TCNE]22- (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π-[TCNE]22- dimers form dissociated ion pairs with the NBu4+ counterions, and their first solvation shell comprises approximately 20 CH2 Cl2 molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C-H⋅⋅⋅N contacts. The calculated free energy of dimerization of TCNE.- in solution at 175 K is -5.5 kcal mol-1 . These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.