Double Bonds? Studies on the Barrier to Rotation about the Cumulenic C=C Bonds of Tetraaryl[ n ]cumulenes ( n= 3, 5, 7, 9)

An algorithm to find the optimal oriented external electrostatic field for annihilating a reaction barrier in a polarizable molecular system

The use of oriented external electric fields (OEEFs) to promote and control chemical reactivity has motivated many theoretical and computational studies in the last decade to model the action of OEEFs on a molecular system and its effects on chemical processes. Given a reaction, a central goal in this research area is to predict the optimal OEEF (oOEEF) required to annihilate the reaction energy barrier with the smallest possible field strength. Here, we present a model rooted in catastrophe and optimum control theories that allows us to find the oOEEF for a given reaction valley in the potential energy surface (PES). In this model, the effective (or perturbed) PES of a polarizable molecular system is constructed by adding to the original, non-perturbed, PES a term accounting for the interaction of the OEEF with the intrinsic electric dipole and polarizability of the molecular system, so called the polarizable molecular electric dipole (PMED) model. We demonstrate that the oOEEF can be established by locating a point in the original PES with unique topological properties: the optimal barrier breakdown or bond-breaking point (oBBP). The essential feature of the oBBP structure is the fact that this point maintains its topological properties for all the applied OEEFs, also for the unperturbed PES, thus becoming much more relevant than the commonly used minima and transition state structures. The PMED model proposed here has been implemented in an open access package and is shown to successfully predict the oOEEF for two processes: an isomerization reaction of a cumulene derivative and the Huisgen cycloaddition reaction.

Anti-ohmic nanoconductors: myth, reality and promise

The recent accomplishment in the design of molecular nanowires characterized by increasing conductance with length has led to the origin of an extraordinary new family of molecular junctions referred to as "anti-ohmic" wires. Herein, this highly desirable, non-classical behavior, has been examined for molecules long-enough to exhibit pronounced diradical character in their ground state within the unrestricted DFT formalism with spin symmetry breaking. We demonstrate that highly conjugated acenes signal higher resistance in an open-shell singlet (OSS) configuration as compared to their closed-shell counterparts. This anomaly has been further proven for experimentally certified cumulene wires, which reveals phenomenal modulation in the transport characteristics such that an increasing conductance is observed in the closed-shell limit, while higher cumulenes in the OSS ground state yield regular decay of conductance.

Advances in Polyynes to Model Carbyne

The formation and study of molecules that model the sp-hybridized carbon allotrope, carbyne, is a challenging field of synthetic physical organic chemistry. The target molecules, oligo- and polyynes, are often the preferred candidates as models for carbyne because they can be formed with monodisperse lengths as well as defined structures. Despite a simple linear structure, the synthesis of polyynes is often far from straightforward, due in large part to a highly conjugated framework that can render both precursors and products highly reactive, i.e., kinetically unstable. The vast majority of polyynes are formed as symmetrical products from terminal alkynes as precursors via an oxidative, acetylenic homocoupling reaction based on the Glaser, Eglinton-Galbraith, and Hay reactions. These reactions are very efficient for the synthesis of shorter polyynes (e.g., hexaynes and octaynes), but yields often drop dramatically as a function of length for longer derivatives, usually starting with the formation of decaynes. The most effective approach to circumvent unstable precursors and products has been through the incorporation of sterically demanding end groups that serve to "protect" the polyyne skeleton. This approach was arguably identified in the early 1950s by Bohlmann and co-workers with the synthesis of tBu-end-capped polyynes. During the next 50 years, a polyyne with 14 contiguous alkyne units remained the longest isolated derivative until 2010, when the record was extended to 22 alkyne units. The record length was broken again in 2020, when a polyyne consisting of 24 alkynes was isolated and characterized. Beyond polyynes, there have been several reports describing the potential synthesis of carbyne, but conclusive characterization and proof of structure have been tenuous. The sole example of synthetic carbyne arises from synthesis within carbon nanotubes, when chains of thousands of sp carbon atoms have been linked to form polydisperse samples of carbyne. Thus, model compounds for carbyne, the polyynes, remain the best means to examine and predict the experimental structure and properties of this carbon allotrope.This Account will discuss the general synthesis of polyynes using homologous series of polyynes with up to 10 alkyne units as examples (decaynes). The limited number of specific syntheses of series with longer polyynes will then be presented and discussed in more detail based on end groups. The monodisperse polyynes produced from these synthetic efforts are then examined toward providing our best extrapolations for the expected characteristics for carbyne based on ¹³C NMR spectroscopy, UV-vis spectroscopy, X-ray crystallography, and Raman spectroscopy.

Benzothiazole-substituted 1,3-diaza-2-oxophenoxazine as a luminescent nucleobase surrogate for silver(I)-mediated base pairing

A benzothiazole-substituted derivative (X) of 1,3-diaza-2-oxophenoxazine was evaluated with respect to its ability to engage in Ag(I)-mediated homo base pair formation in two different DNA duplexes. The metal binding was determined by a combination of temperature-dependent UV spectroscopy, CD spectroscopy, and fluorescence spectroscopy, indicating the incorporation of two Ag(I) ions to generate a dinuclear X-Ag(I)₂-X base pair. Interestingly, a luminescence increase was observed upon metal binding. Theoretical luminescence spectra were calculated using time-dependent density functional theory (TDDFT) for all possible Ag(I)-mediated X : X base pair geometries to identify the species responsible for the increase in luminescence. The study shows that even bulky non-planar artificial nucleobases can be applied to form stabilizing metal-mediated base pairs.

Synthesis and exploration of configurational dynamics in equilibrating E/Z 2-aryliminothiazolidin-4-ones using NMR and estimation of thermodynamic parameters

NMR based in-depth exploration of stereodynamic behavior in equilibrating E/Z 2-aryliminothiazolidin-4-ones and determination of kinetic and thermodynamic parameters.

Quantification of the Helicality of Helical Molecular Orbitals

The frontier molecular orbital (MO) topology of linear carbon molecules, such as polyynes, can be visually identified as helices. However, there is no clear way to quantify the helical curvature of these π-MOs, and it is thus challenging to quantify correlations between the helical curvature and molecular properties. In this paper, we develop a method that enables us to compute the helical curvature of MOs based on their nodal planes. Using this method, we define a robust way of quantifying the helical nature of MOs (helicality) by their deviation from a perfect helix. We explore several limiting cases, including polyynes, metallacumulenes, cyclic allenes, and spiroconjugated systems, where the change in helical curvature is subtle yet clearly highlighted with this method. For example, we show that strain only has a minor effect on the helicality of the frontier orbitals of cycloallenes and that the MOs of spiroconjugated systems are close to perfect helices around the spiro-carbon. Our work provides a well-defined method for assessing orbital helicality beyond visual inspection of MO isosurfaces, thus paving the way for future studies of how the helicality of π-MOs affects molecular properties.

Spin-Spin Interactions in One-Dimensional Assemblies of a Cumulene-Based Singlet Biradical

The synthesis of phenalenyl-endcapped [5]cumulene as a cumulene-based singlet biradical and the spin correlation changes of one-dimensional aggregates are described. The high propensity for self-aggregation of phenalenyl rings and the introduction of bulky substituents into the appropriate positions led to the formation of a one-dimensional chain assembly. Single-crystal X-ray structural analysis indicated that the bond length alternation of the cumulene chain increased with decreasing temperature, along with improved overlapping of the phenalenyl rings. Variable-temperature Raman spectroscopy and magnetic susceptibility measurements revealed that a localized spin pair within the molecule decouples at low temperatures, and a continuum spin system involving intra- and intermolecular spin-spin interactions emerges in the one-dimensional chain.

A strong organic electron donor incorporating highly π-donating triphenylphosphonium ylidyl substituents

The π-electron donor strength of a triphenylphosphonium ylidyl group (Ph₃P[double bond, length as m-dash]CH-) was explored through its substitution onto a bispyridinylidene (BPY) scaffold. Electrochemical studies revealed that the new triphenylphosphonium ylidyl-substituted BPY is the most reducing di-substituted derivative reported to date (E_1/2 = -1.55 V vs. SCE). By using a previously established correlation between the redox potential of the substituted BPYs and the corresponding substituent, a Hammett constant for the Ph₃P[double bond, length as m-dash]CH- group was determined (σ_p⁺ = -2.33), establishing it as the most donating neutral substituent currently quantified. The BPY is readily oxidized by hexachloroethane to produce the corresponding dicationic bipyridinium salt as a mixture of isomers owing to hindered C_ylidyl-C_pyridyl bond rotation. In preliminary tests of the BPY as a reductant, dichlorotricyclohexylphosphorane and chlorodiphenylphosphine were reduced to the corresponding phosphine and diphosphine, respectively.

Enantioselective Control of Both Helical and Axial Stereogenic Elements though an Organocatalytic Approach

A highly diastereo- and enantio-selective method for the asymmetric synthesis of molecules containing helicenes and stereogenic axes was developed based on organocatalysis. Various compounds bearing both helical and axial stereogenic elements were obtained in excellent enantioselectivities. The mechanism study revealed that the reaction proceeded through two stages: 1) The first cyclization produces a reaction intermediate containing a stereogenic axis. 2) The dynamic kinetic resolution of helix reaction intermediate following with cyclization generates a helix and another stereogenic axis.

Directing isomerization reactions of cumulenes with electric fields

Electric fields have been proposed as having a distinct ability to catalyze chemical reactions through the stabilization of polar or ionic intermediate transition states. Although field-assisted catalysis is being researched, the ability to catalyze reactions in solution using electric fields remains elusive and the understanding of mechanisms of such catalysis is sparse. Here we show that an electric field can catalyze the cis-to-trans isomerization of [3]cumulene derivatives in solution, in a scanning tunneling microscope. We further show that the external electric field can alter the thermodynamics inhibiting the trans-to-cis reverse reaction, endowing the selectivity toward trans isomer. Using density functional theory-based calculations, we find that the applied electric field promotes a zwitterionic resonance form, which ensures a lower energy transition state for the isomerization reaction. The field also stabilizes the trans form, relative to the cis, dictating the cis/trans thermodynamics, driving the equilibrium product exclusively toward the trans.

Mechanosynthesis of Odd-Numbered Tetraaryl[n]cumulenes

A mechanochemical synthesis of one-dimensional carbon allotrope carbyne model compounds, namely tetraaryl[n]cumulenes (n=3, 5) was realized. Central for the mechanosynthesis of the cumulenic carbon nanostructures were the development of a mechanochemical Favorskii alkynylation-type reaction and the implementation of a solvent-free, acid-free reductive elimination with tin(II) chloride by ball milling.

Unusual Length Dependence of the Conductance in Cumulene Molecular Wires

Cumulenes are sometimes described as "metallic" because an infinitely long cumulene would have the band structure of a metal. Herein, we report the single-molecule conductance of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n=1, 2, 3, and 5. The [n]cumulenes with n=3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n=1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n=2) is much lower, because of its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n]cumulenes where n is even. The lack of length dependence in the conductance of [3] and [5]cumulenes is attributed to the strong decrease in the HOMO-LUMO gap with increasing length.

Structural and Electronic Effects of Stepwise Reduction of a Tetraaryl[3]Cumulene

The chemical reduction of a [3]cumulene ([3]TrTol) has been explored using alkali metals. Mono- and doubly reduced forms of [3]TrTol were isolated as solvent-separated ion pairs with {Na(18-crown-6)THF₂ }⁺ and {K(18-crown-6)THF₂ }⁺ counterions and crystallographically characterized. This allowed analysis of structural parameters of the "naked" anions of [3]TrTol without interference from metal binding. The dianion of [3]TrTol was also isolated as a contact-ion complex with {Cs(18-crown-6)}⁺ cations, thereby adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, [3]TrTol, outline monotonic increases in bond-length alternation (BLA) upon stepwise reduction. The greatest BLA value is found for the contact-ion complex, which shows an alternating sequence of short and long carbon-carbon bonds, consistent with the structure of an alkyne. In contrast to studies on tetraphenyl[3]cumulene, the cumulenic framework of [3]TrTol remains planar in all the derivatives.