Cis-trans Isomerization Reaction Research Articles

Theoretical study on the isomerization mechanism of visible light-driven azobenzene-based materials

This article investigates the geometric structure and isomerization mechanism of 4,4-acetamidazobenzene molecules with halogen atoms (F, Cl, Br) substituted at four ortho positions through density functional theory (DFT) calculations. The calculation method was b3lyp/g, and the base set was def2-tzvp. According to DFT calculations, the ortho substitution of the halogen atoms differently distorted the molecular plane of the trans isomers, resulting in obvious absorption peaks in the visible range, giving them visible-light driven properties. In addition, the transition state (TS) of the cis–trans isomerization reaction in the ground state was also found. To study the molecular isomerization mechanism, the potential energy surfaces (PESs) of the ground state (S0), first excited state (S1), and second excited state (S2) of halogen substituted azobenzene derivatives were scanned separately. Through the potential energy surface curve of the ground state, the mechanism of isomerization reaction in the ground state of the four molecules is realized through a rotation-assisted inversion pathway, which is consistent with the calculation of TS. When the molecules are in the excited states, the isomerization pathways are realized through rotation pathways.

Liquid-phase hydrogenation of sunflower oil over platinum and nickel catalysts: Effects on activity and stereoselectivity

This study presents the results of comparative hydrogenation of sunflower oil over a platinum catalyst supported on activated diatomite and an industrial nickel catalyst (Pricat-9910). Catalysts are characterized using physical methods, such as transmission electron microscopic, temperature programmed hydrogen desorption, Brunauer–Emmett–Teller and inductively coupled plasma atomic emission spectrometry, enabling the determination of the size of catalyst particles, forms of adsorbed hydrogen, and specific surface area. Depending on the process conditions, hydrogenation reaction rates on platinum catalysts have been found to be 2–5 times higher than cis-trans isomerization reaction rates. Using a nickel catalyst, the hydrogenation and isomerization rates at low temperatures (110°C-130 °C) and 0.5 MPa pressure are almost similar, while, the hydrogenation rate is 1.4–1.8 times lower than the isomerization rate at high temperatures and pressures, leading to the formation of a large number of trans isomers. The difference in catalysts selectivity may be attributed the speciation and different adsorption energies of surface hydrogen, leading to the possibility of simultaneous hydrogenation of linoleic and oleic acids on the nickel catalyst.

Synergy between Lewis acidic metal sites and silanols in Sn- and Ti-Beta for conversion of butenes by DRIFTS

The synergistic effect of Lewis acidic Sn/Ti sites and silanols in Sn- and Ti-Beta zeolites on the reaction pathways and mechanisms of the conversion of butenes was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy. The double bond migration reaction of n-butene to cis-2-butene followed an AB–AD mechanism, in which hydrogen abstraction (AB) from the methylene group at Lewis acidic Sn/Ti sites afforded an allyl intermediate, followed by synergistic hydrogen addition (AD) on silanols. In contrast, the following cis–trans isomerization reaction between cis-2-butene and trans-2-butene, and the activation of isobutene, followed an AD–AB mechanism via hydrogen addition to the double bond of butenes on silanols, forming an alkoxyl group rather than allyl group as the intermediate owing to the absence of a methylene group in 2-butene and isobutene.

Modification of Thermal Chemical Rates in a Cavity via Resonant Effects in the Collective Regime.

The modification of thermal chemical rates in Fabry-Perot cavities, as observed in experiments, still poses theoretical challenges. While we have a better grasp of how the reactivity of isolated molecules and model systems changes under strong coupling, we lack a comprehensive understanding of the combined effects and the specific roles played by activated and spectator molecules during reactive events. In this study, we investigate an ensemble of randomly oriented gas-phase HONO molecules undergoing a cis-trans isomerization reaction on an ab initio potential energy surface. One thermally activated molecule can overcome the reaction barrier, while the other molecules are nonactivated but coupled to the cavity as well. Using the classical reactive flux method, we analyze the transmission coefficient and determine the conditions that lead to accelerated rates within the collective regime. We identify two main mechanistic aspects: First, nonactivated molecules enhance the cavity's ability to dissipate excess energy from the activated molecule postreactive event. Second, the activated molecule couples with the polaritonic resonance created by the nonactivated molecules and the cavity at a shifted resonance frequency with respect to the bare cavity.

The trans/cis ratio of formic (HCOOH) and thioformic (HC(O)SH) acids in the interstellar medium

Context. Observations of the different isomers of molecules in the interstellar medium (ISM) have revealed that both low- and high-energy isomers can be present in space despite the low temperature conditions. It has been shown that the presence of these isomers may be due to tunneling effects. Aims. We carried out a theoretical study of the cis–trans isomerization reactions of two astrophysically relevant acids, formic acid (HCOOH) and thioformic acid (HC(O)SH), where the latter has recently been discovered in space. We also searched for these molecules towards the hot core G31.41+0.31 to compare their abundances with the expected theoretical isomerization results. Methods. We employed high-level ab initio calculations to study the reaction rate constants of the isomerization reactions. We used the canonical variational transition state theory with the multidimensional small curvature tunneling approximation in the temperature range of 10–400 K. Moreover, we used the spectrum obtained from the ALMA 3mm spectral survey GUAPOS (GUAPOS: G31 Unbiased ALMA sPectral Observational Survey), with a spectral resolution of ~0.488 MHz and an angular resolution of 1.′′2×1.′′2 (~4500 au), to derive column densities of HCOOH and HC(O)SH towards G31.41+0.31. Results. Our results demonstrate that these isomerizations are viable in the conditions of the ISM due to ground-state tunneling effects, which allow the system to reach the thermodynamic equilibrium at moderately low temperatures. At very low temperatures (Tkin ~ 10 K), the reaction rate constants for the cis-to-trans isomerizations are very small, which implies that the cis isomers should not be formed under cold ISM conditions. This is in disagreement with observations of the cis/trans isomers of HCOOH in cold cores where the cis isomer is found to be ~5–6% the trans isomer. At high temperatures (~150–300 K), our theoretical data not only match the observed behavior of the trans/cis abundance ratios for HCOOH (the cis form is undetected), but they support our tentative detection of the trans and – for the first time in the insterstellar medium – the cis isomer of HC(O)SH towards the hot molecular core G31.41+0.31 (with a measured trans/cis abundance ratio of ~3.7). Conclusions. While the trans/cis ratio for HC(O)SH in the ISM depends on the relative stability of the isomers, the trans/cis ratio for HCOOH cannot be explained by isomerization, and is determined by other competitive chemical processes.

Does urea modify microheterogeneous nature of ionic amide deep eutectics? Clues from non-reactive and reactive solute-centered dynamics

The impact of urea on the microheterogeneous nature of acetamide + LiNO3 deep eutectic solvents (DESs) was investigated via following the viscosity coupling of the average timescales of solute rotation, solvation and cis–trans isomerization reaction occurring in such media. Picosecond resolved fluorescence measurements were carried out to access these average relaxation timescales and the solutes employed were the non-reactive probe, coumarin153 (C153), and a reactive probe, trans- 2-[4′-(Dimethylamino) styryl]benzothiazole (DMASBT). DMASBT undergoes fast cis–trans isomerization reaction upon photo-excitation. The coupling to the solution viscosity (η) of the experimentally determined average timescales was then analyzed via the relation, 〈τx〉∝ηp, x denoting rotation, solvation or reaction, and a fractional p considered as solution microheterogeneity reflector. A comparison of the findings between the urea containing and non-urea ionic DESs revealed that addition of urea did not alter the p value for C153 rotation and DMASBT reaction times, while the C153 average solvation times showed a stronger decoupling. This stronger decoupling has been ascribed to the faster rate of solvation upon addition of urea via the participation of the amide group through the collective H-bond excitations. Addition of electrolyte at a fixed amide concentration, on the other hand, makes the fractional viscosity dependence more pronounced for both solvation and rotational dynamics, although the viscosity coupling (p value) of isomerization reaction timescale remains unchanged. DMASBT reveals, through steady state fluorescence measurements, stronger spatial heterogeneity in these solutions than that by C153, and DMASBT better senses the urea or electrolyte induced modulation of spatial heterogeneity.

(Invited) Chemistry in a Nanoscale Gap

In the realm of the Single-Molecule Electronics, a suite of advanced electrical characterization approaches have emerged allowing measuring charge transport in an electric contact made out of an individual molecule[1]. The field has (and is still) drawn(ing) an scenario where individual molecules can be chemically modified to deliver a particular electrical function in a nanoscale circuit, e.g. variable resistors[2], diodes[3], switches[4], etc. Along this excursion, we have observed that single molecules trapped in a nanoscale tunneling junction experience conformational structural changes and changes in molecule/electrode contact geometries, which are usually accompanied by large conductance variations and can be easily detected electrically[1]. Such changes are induced by the imposed forces fields experienced by the molecules within the nanoscale gap, namely, a mechanical force and/or an electric field. In the past decade, we have learnt that under certain force field conditions, the individual molecules wired in a nanoscale junction undergo chemical transformations. Here, we will present a couple of illustrative examples of the use of single-molecule junction to study and control reactivity at the nanoscale using electric fields, a concept that is inherent in the natural enzymatic molecular machinery[5]. Tunable, well-oriented electric fields can be easily delivered along the main junction axis of a nanoscale electrical device[6]. We exploited the latter to study; (1) a simple monomolecular cis-trans isomerization reaction[7], and (2) a bi-molecular Diels-Alder reaction[8]. Along with the experimental design and fundamental chemistry aspects, we will discuss the advantages these examples can bring to possible technological applications.[1]. L. Sun et al. Chem. Soc. Rev., 2014,43, 7378-7411.[2]. A. C. Aragones et al. Chem. Eur.J.2015, 21,7716 –7720.[3]. I. Diez-Perez et al. Nature Chemistry 1, 635–641 (2009).[4]. N. Darwish et al. Nano Lett. 2014, 14, 12, 7064–7070.[5]. Stephen D. Fried et al. Annu Rev Biochem. 2017; 86: 387–415.[6]. N. Darwish et al. “Principles of Molecular Devices Operated by Electric Fields” in Effects of Electric Fields on Structure and Reactivity: New Horizons in Chemistry, Royal Society of Chemistry, Ed. Sason Shaik.[7]. C. S. Quintans et al. Appl. Sci. 2021, 11(8), 3317.[8]. A. C. Aragones et al. Nature 531, 88–91 (2016).

Distribution of Peptidyl-Prolyl Isomerase (PPIase) in the Archaea.

Cis-trans isomerization of the peptide bond prior to proline is an intrinsically slow process but plays an essential role in protein folding. In vivo cis-trans isomerization reaction is catalyzed by Peptidyl-prolyl isomerase (PPIases), a category of proteins widely distributed among all the three domains of life. The present study is majorly focused on the distribution of different types of PPIases in the archaeal domain. All the three hitherto known families of PPIases (namely FKBP, Cyclophilin and parvulin) were studied to identify the evolutionary conservation across the phylum archaea. The basic function of cyclophilin, FKBP and parvulin has been conserved whereas the sequence alignment suggested variations in each clade. The conserved residues within the predicted motif of each family are unique. The available protein structures of different PPIase across various domains were aligned to ascertain the structural variation in the catalytic site. The structural alignment of native PPIase proteins among various groups suggested that the apo-protein may have variable conformations but when bound to their specific inhibitors, they attain similar active site configuration. This is the first study of its kind which explores the distribution of archaeal PPIases, along with detailed structural and functional analysis of each type of PPIase found in archaea.

Molecular insight into photoresponsive surfactant regulated reversible emulsification and demulsification processes

Photoresponsive emulsions have attracted growing attention and exhibited enormous potential in a wide range of applications related to their green and efficient characteristics. Herein, molecular dynamics simulations were performed to elaborate the reversible emulsification/demulsification processes regulated by a mixture of an anionic surfactant sodium dodecyl sulfate (SDS), and the photoresponsive cationic surfactant 2-(4-(4-butylphenyl)diazenylphenoxy) ethyltrirnethylammonium bromide (AzoTAB). The simulation results indicate that the oil/water interface dominated by trans-AzoTAB molecules was more stable than that dominated by cis-AzoTAB molecules at specific concentrations, during which emulsification/demulsification can be reversibly regulated by the trans–cis isomerization reaction. The van Hove function results suggest that the cis-AzoTAB molecules possessed a faster diffusion rate than the trans isomer, contributing to the mutual dissolution of the oil and water phases. Furthermore, the role of SDS in regulating emulsification/demulsification processes was revealed. The addition of a small amount of SDS inhibited the desorption of AzoTAB from the oil/water interface and highlighted the influence of the structural difference between trans- and cis-AzoTAB molecules on interfacial stability. This work provides a molecular understanding of the interfacial stability and fluidity of photoresponsive emulsions, which are fundamental principles for the rational design of emulsions and other dispersion systems.

Light-responsive surfactants: Photochromic properties of water-soluble azobenzene derivatives

Light-responsive self-assembly systems have emerged as a promising tool for controlling a multitude of molecular and macroscopic processes. A number of sophisticated applications of surfactants based on the azobenzene moiety have already been demonstrated including drug delivery and induction of functional changes in biologically active compounds. In most of these applications the switching time between the two compound states is substantial. However, a small number of systematic studies on the relationship between the chemical structure of the compound on the kinetics of photoisomerization and the influence of experimental conditions led us to undertake the studies described in this paper. Changes in the spectral characteristics resulting from reversible photoinduced trans-cis isomerization reaction of selected azobenzene-based surfactants, differing in the hydrophobic tail length, were determined in aqueous solutions employing UV–Vis absorption spectroscopy. The effect of the hydrophobic tail length, surfactant concentration, molecular environment and light intensity on the efficiency and kinetics of photoisomerization were investigated. Due to the thermal stability of both photochromic forms, it is possible to reversibly switch between two different photostationary states of the light-responsive surfactant in a controlled manner using light of two wavelengths (ultraviolet and blue). The results demonstrates that both the increasing concentration of azobenzene amphiphile and the increasing hydrophobic pendant tail length slow down the trans-cis photoisomerization, while the photochromic efficiency remains constant.

Poly(amic acid)s vs. polyimides with π-conjugated –N[dbnd]N- units: Cis-trans isomerization reaction and kinetics of thermal imidization

The paper describes the synthesis and characterization of “T-type” polyimides and their precursors – poly(amic acid)s. Investigated polymers contained azobenzene derivatives with no substituent or with fluorine atom in para position to the NN linkage. The influence of the main chain architecture (open or close imide rings), location and content of azo chromophores in polymers chains on physicochemical properties were studied. To the best of our knowledge, for the first time, the cis-trans isomerization reaction was studied for azo poly(amic acid)s, that was 1–18% after 3-minutes of 405 nm-light irradiation. Poly(amic acid)s characterized higher stability of isomerization than their polyimide counterparts. The studied azo polyimides showed high glass transition temperatures (174–206 °C) and high thermal stability with the beginning of the decomposition at 246–345 °C. The imidization kinetic of the studied poly (amic acid)s were investigated by differential scanning calorimetry (DSC) using Kissinger and Ozawa models. It was found that the studied azo polymers characterized high activation energies (275.4–357.1 kJ mol−1) and frequency factors (6.5 × 1029–1.3 × 1037 s−1).

Modeling of surface waves in photo-responsive viscoelastic liquid crystal thin films under a moving light source

In this paper we present a computational model to simulate surface waves on photo-responsive liquid crystal thin films under dynamic illumination. The influence of light attenuation is included to model the dynamic photo-mechanical response as a result of the forward (trans to cis) and the backward (cis to trans) isomerization of the embedded azobenzene molecules. The viscoelastic nature of the liquid crystal films is also estimated and incorporated in this study. The temporal response of the material under static spot illumination is analyzed first to understand the underlying mechanisms behind the observed response. The relation between the fraction of transformed molecules and the macroscopic light induced strains and stresses through the thickness is investigated to establish the significance of light attenuation. Subsequently, the model liquid crystal film is exposed to a rectangular illumination waveform to simulate a moving light source and thereby generate surface waves. Results are presented that relate the wave attributes, (i.e., the amplitude, frequency and phase lag) to the time scales associated with the moving light source, the molecular trans-cis isomerization reaction and the viscoelastic response of the material. A design guideline for producing surface waves with specific features (amplitude, phase) is presented.

Study on Preparation of Fluorinated Photosensitive Copolymer and Surface Wettability Light Regulation

Low surface energy and micro/nano structure are two key factors affecting the construction of hydrophobic surfaces. In this paper, a series of tetrafluoropropyl methacrylate (TFPMA) and 6- [4- [2- [4- (trifluoromethoxy) phenyl] diazenyl] phenoxy] hexyl acrylate (FAzo) random copolymer [P(TFPMA-co-FAzo)] of different monomer molar ratios were prepared by free radical solution polymerization; and the chemical structure of the copolymer was characterized in detail using FTIR and 1HNMR. The introduction of long-chain flexible acrylate components with high fluorine content can effectively reduce the surface energy of the material. When the molar ratio of TFPMA is increased to 70%, the static water contact angle on the surface of the copolymer film is increased to 90°. By doping 10 wt% micron-sized PTFE particles, the water contact angle on the surface of the copolymer film can be further increased to 105°; Meanwhile, based on the cis-trans isomerization reaction of the azobenzene component in the copolymer, light irradiation with different wavelengths can realize the rapid and reversible regulation of hydrophobic/hydrophilic properties on the surface of copolymer film. This method of combining low surface energy fluorine-containing component with photosensitive azobenzene to construct light-responsive hydrophobic surface has a good research prospect.

A family of azoquinoline derivatives: Effect of the substituent at azo linkage on thermal cis-trans isomerization based on an experimental and computational approach

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

Multiresponse Kinetic Modeling of Heat-Induced Equilibrium of β-Carotene cis-trans Isomerization in Medium-Chain Triglyceride Oil.

The kinetics and mechanism of the stepwise cis-trans isomerization reactions of all-trans-β-carotene dissolved in MCT (medium-chain triglyceride) oil at temperatures in the range of 80-160 °C have been analyzed using multiresponse modeling. Quantitation of the cis-isomers was performed using HPLC-DAD and quantitation at the reaction isosbestic point at 421 nm. Multiresponse kinetic modeling using the Bayesian criterion was initially performed at 120 °C to determine the best model. Subsequently, the reparametrized Arrhenius equation was used to calculate the activation energies of all reactions. The equilibrium constants for the individual isomerization reactions were determined from the rate constants and the final product distributions. The enthalpies and entropies of the isomerization reactions were determined from the temperature dependence of the equilibrium constants. The 13-cis and 13,13'-di-cis isomers were found to be the fastest formed isomers followed by the 9-cis, 9,13-di-cis, and 13,15-di-cis isomers, where the latter was found to be formed from 13-cis and not the 15-cis isomer. The relative free energies of the β-carotene isomers were determined as all-trans < 13-cis < 9-cis < 13,13'-di-cis < 9,13-di-cis ≈ 15-cis < 13,15-di-cis. The entropic contribution of each reaction was found to play an important role in the ordering. It is concluded that the β-carotene system is quite labile at temperatures ranging from 80 to 160 °C and resulting in equilibrium distributions of the cis-trans isomers.

Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF): Automatic Generation of Sum-of-Products Form Potential Energy Surfaces for Quantum Dynamical Calculations.

We present Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF), an automated methodology for the generation of global potential energy surfaces (PES), molecular properties surfaces, e.g., dipole, polarizabilities, etc. using a single random geometry as input. The SRP-MGPF workflow integrates: (i) a fully automated procedure for the global topographical characterization of a (intermolecular) PES based on the Transition State Search Using Chemical Dynamical Simulations (TSSCDS) family of methods;i (ii) the global optimization of the parameters of a semiempirical Hamiltonian in order to reproduce a given level of electronic structure theory; and (iii) a tensor decomposition algorithm which turns the resulting SRP-PES into sum of products (Tucker) form with the Multigrid POTFIT algorithm. The latter is necessary for quantum dynamical studies within the Multiconfiguration Time-Dependent Hartree (MCTDH) quantum dynamics method. To demonstrate our approach, we have applied our methodology to the cis-trans isomerization reaction in HONO in full dimensionality (6D). The resulting SRP-PES has been validated through the computation of classical on-the-fly dynamical calculations as well as calculations of the lowest vibrational eigenstates of HONO as well as high-energy wavepacket propagations.

&lt;em&gt;Cis-trans&lt;/em&gt; isomerization of unsaturated fatty acids in edible oils to prepare &lt;em&gt;trans&lt;/em&gt; fat

Trans fats, unsaturated fatty acids with at least one double bond in the trans configuration, have received a great amount of attention in the field of oleo chemistry and safety. In this work, the cis-trans isomerization of unsaturated fatty acids in edible oils has been successfully developed using simple and cheap p-toluenesulfinic acid as catalyst. The effects of reaction time, temperature, catalyst loading, and the amounts of water and antioxidants on the cis-trans isomerization have been systematically investigated. The results indicate that the amount of water (0–20 wt %) and antioxidants (0–200 mg/kg) had no significant effect on this cis-trans isomerization of unsaturated fatty acids in edible oils. The final products (trans fats) from the cis-trans isomerization reaction were characterized by both GC and React IR. The yield of trans double bonds from the isomerized fat can reach 79.6% after heating at 100 oC for 90 min without changing either the location of the double bonds or the degree of unsaturation. In addition, this convenient method has been applied to various vegetable oils (e.g., olive oil, camellia seed oil, corn oil, sesame oil, sunflower oil and soybean oil) and nearly 80.0% yields of TFA were generated, which shows a promising method to provide trans-fat products for research related to oleo and food chemistry.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP+) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP+ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP+ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S1 state and triplet T1 state is found to play an important role in the photoisomerization process of BTImP+. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP+ is calculated to be 10.49kcalmol−1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP+.

Introducing the mean field approximation to CDFT/MMpol method: Statistically converged equilibrium and nonequilibrium free energy calculation for electron transfer reactions in condensed phases

A new theoretical method to study electron transfer reactions in condensed phases is proposed by introducing the mean-field approximation into the constrained density functional theory/molecular mechanical method with a polarizable force field (CDFT/MMpol). The method enables us to efficiently calculate the statistically converged equilibrium and nonequilibrium free energies for diabatic states in an electron transfer reaction by virtue of the mean field approximation that drastically reduces the number of CDFT calculations. We apply the method to the system of a formanilide-anthraquinone dyad in dimethylsulfoxide, in which charge recombination and cis-trans isomerization reactions can take place, previously studied by the CDFT/MMpol method. Quantitative agreement of the driving force and the reorganization energy between our results and those from the CDFT/MMpol calculation and the experimental estimates supports the utility of our method. The calculated nonequilibrium free energy is analyzed by its decomposition into several contributions such as those from the averaged solute-solvent electrostatic interactions and the explicit solvent electronic polarization. The former contribution is qualitatively well described by a model composed of a coarse-grained dyad in a solution in the linear response regime. The latter contribution reduces the reorganization energy by more than 10 kcal/mol.

Twist and Shout: Single-Molecule Mechanochemistry

Chemical reactions can be accelerated by various means, including applied mechanical forces. If the direction of the force does not project well onto the desired reaction coordinate, then only poor acceleration is achieved. Recent developments in single polymer mechanics illustrate how to overcome this limitation, in a simple cis-trans isomerization reaction. Generalizing the approach, synthetic chemistry can be used to attach tethers to different parts of reacting molecular fragments to direct the force usefully. This Perspective explores the prospects for using applied mechanical forces to create exciting new chemistries. For example, it is possible to imagine making polymers that sense mechanical forces within hard-to-reach places, like biological cells, or using mechanical forces to make nanoscale electrical devices using conjugated polymers.