Search

Shear selectivity Chemical reactions typically proceed by distributing energy statistically among all accessible molecular vibrations. Liu et al. report that external shear forces can sometimes pry open strained carbon rings without dissipating energy into adjacent bond rotations. Through careful design and synthesis of polymer-embedded cyclobutyl rings, the authors showed that certain relative substituent geometries are preserved when sonication induces ring opening. Accompanying simulations support the instigation of “flyby” trajectories that channel energy narrowly to cleave the cyclic sigma bonds and then rapidly form acyclic pi bonds. Science , abi7609, this issue p. 208

Channelrhodopsin 2 (ChR2) is the most commonly used tool in optogenetics. Because of its faster photocycle compared to wild-type (WT) ChR2, the E123T mutant of ChR2 is a useful optogenetic tool when fast neuronal stimulation is needed. Interestingly, in spite of its faster photocycle, the initial step of the photocycle in E123T (photoisomerization of retinal protonated Schiff base or RPSB) was found experimentally to be much slower than that of WT ChR2. The E123T mutant replaces the negatively charged E123 residue with a neutral T123 residue, perturbing the electric field around the RPSB. Understanding the RPSB photoisomerization mechanism in ChR2 mutants will provide molecular-level insights into how ChR2 photochemical reactivity can be controlled, which will lay the foundation for improving the design of optogenetic tools. In this work, we combine ab initio nonadiabatic dynamics simulation, excited state free energy calculation, and reaction path search to comprehensively characterize the RPSB photoisomerization mechanism in the E123T mutant of ChR2. Our simulation agrees with previous experiments in predicting a red-shifted absorption spectrum and significant slowdown of photoisomerization in the E123T mutant. Interestingly, our simulations predict similar photoisomerization quantum yields for the mutant and WT despite the differences in excited-state lifetime and absorption maximum. Upon mutation, the neutralization of the negative charge on the E123 residue increases the isomerization barrier, alters the reaction pathway, and changes the relative stability of two fluorescent states. Our findings provide new insight into the intricate role of the electrostatic environment on the RPSB photoisomerization mechanism in microbial rhodopsins.

Woodward and Hoffman once jested that a very powerful Maxwell demon could seize a molecule of cyclobutene at its methylene groups and tear it open in a disrotatory fashion to obtain butadiene (Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry.

New tungsten and molybdenum alkylidyne complexes bearing mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been synthesized. Formation of unprecedented structures in complexes bearing ...

The dynamics of proton transfer to the aprotic solvent 1-methylimidazole (MeIm, proton acceptor) from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was investigated using fast fluorescence measurements. The closely related molecule, 8-methoxypyrene-1,3,6-trisulfonic acid trisodium salt (MPTS), which is not a photoacid, was also studied for comparison. Following optical excitation, the wavelength-dependent population dynamics of HPTS in MeIm resulting from the deprotonation process were collected over the entire fluorescence emission window. Analysis of the time-dependent fluorescence spectra revealed four distinct fluorescence bands that appear and decay on different time scales. We label these four states as protonated (P), associated I (AI), associated II (AII), and deprotonated (D). We find that the simple kinetic scheme of P → AI → AII → D is not consistent with the data. Instead, the kinetic scheme that describes the data has P decaying into AI, which mainly goes on to deprotonation (D), but AI can also feed into AII. AII can return to AI or decay to the ground state, but does not deprotonate within experimental error. Quantum chemistry and excited state QM/MM Born-Oppenheimer molecular dynamics simulations indicate that AI and AII are two H-bonding conformations of MeIm to the HPTS hydroxyl, axial, and equatorial, respectively.

In silico reaction discovery using ab initio molecular dynamics shows that the chemistry of life could have originated from two simple inorganic molecules (HCN and water).

Al-F-Bindungen gehoren zu den starksten bekannten σ-Bindungen und weisen sogar hohere Bindungsenergien als Si-F-Bindungen auf. Trotz eines Stabilitatsvorteils und einer potenziell hohen Lewis-Aciditat von Al-F-Komplexen, wurden diese bisher nicht als strukturell definierte Katalysatoren fur enantioselektive Reaktionen beschrieben. Hier zeigen wir, dass Al-F-Salen-Komplexe mit gekoppelten Ammonium-Einheiten eine ausergewohnlich hohe katalytische Aktivitat in asymmetrischen Carboxycyanierungen aufweisen. Neben aromatischen Aldehyden wurden auch Enale und aliphatische Substrate effizient umgesetzt. Umsatzzahlen bis zu ca. 104 wurden erreicht, wohingegen sie mit vorherigen Katalysatoren ublicherweise im Bereich von 101–102 lagen. Anders als Al-Me- und Al-Cl-Salen-Komplexe sind die analogen Al-F-Spezies bemerkenswert stabil gegenuber Luft, Wasser und Hitze, und sie konnten nach der Katalyse unverandert zuruckgewonnen werden. Sie zeichnen sich laut DFT-Rechnungen durch eine betrachtlich erhohte Lewis-Aciditat aus.

Al−F bonds are among the most stable σ bonds known, exhibiting an even higher bond energy than Si−F bonds. Despite a stability advantage and a potentially high Lewis acidity of Al−F complexes, they have not been described as structurally defined catalysts for enantioselective reactions. We show that Al−F salen complexes with appended ammonium moieties give exceptional catalytic activity in asymmetric carboxycyanations. In addition to aromatic aldehydes, enal and aliphatic substrates are well accepted. Turnover numbers up to around 104 were achieved, whereas with previous catalysts 101–102 turnovers were typically attained. In contrast to Al−Me and Al−Cl salen complexes, the analogous Al−F species are remarkably stable towards air, water, and heat, and can be recovered unchanged after catalysis. They possess a considerably increased Lewis acidity as shown by DFT calculations.

Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom-tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. On the contrary, using computational investigations the influence of tunneling on the reaction rates can directly be monitored. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.

The vibrational spectra of simple CH3+-Rg (Rg = He, Ne, Ar, Kr) complexes have been studied by vibrational configuration interaction theory relying on multidimensional potential energy surfaces (PESs) obtained from explicitly correlated coupled cluster calculations, CCSD(T)-F12a. In agreement with experimental results, the series of rare gas atoms leads to rather unsystematic results and indicates huge zero point vibrational energy effects for the helium complex. In order to study these sensitive complexes more consistently, we also introduce configuration averaged vibrational self-consistent field theory, which is a generalization of standard vibrational self-consistent field theory to several configurations. The vibrational spectra of the complexes are compared to that of the methyl cation, for which corrections due to scalar-relativistic effects, high-order coupled-cluster terms, e.g., quadruple excitations, and core-valence correlation have explicitly been accounted for. The occurrence of tunneling splittings for the vibrational ground-state of CH3+-He has been investigated on the basis of semiclassical instanton theory. These calculations and a direct comparison of the energy profiles along the intrinsic reaction coordinates with that of the hydronium cation, H3O+, suggest that tunneling effects for vibrationally excited states should be very small.

This paper investigates the impact of the increased reaction rate constant due to tunneling effects on planet-forming disks. Our aim is to quantify the astrophysical implications of atom tunneling for simple molecules that are frequently used to infer disk structure information or to define the initial conditions for planet (atmosphere) formation. We explain the tunneling effect on reaction rates by using a scholarly example in comparison to previous UMIST2012 rate constants. In a chemical network with 1299 reactions, we identify all reactions that could show atom tunneling. We devise a simple formulation of reaction rate constants that overestimates tunneling and screen a standard T Tauri disk model for changes in species abundances. For those reactions found to be relevant, we find values of the most recent literature for the rate constants including tunneling and compare the resulting disk chemistry to the standard disk models. The rate constants in the UMIST2012 database in many cases already capture tunneling effects implicitly. A rigorous screening procedure identified three neutral-neutral reactions where atom tunneling could change simple molecule abundances. However, by adopting recent values of the rate constants of these reactions and due to the layered structure of planet-forming disks, the effects are limited to a small region where cold neutral-neutral chemistry dominates. Abundances of water close to the midplane snowline can increase by a factor of two at most compared to previous results with UMIST2012 rates. Observables from the disk surface, such as high excitation (> 500 K) water line fluxes, decrease by 60% at most when tunneling effects are explicitly excluded. On the other hand, disk midplane quantities relevant for planet formation such as the C-to-O ratio and also the ice-to-rock ratio are clearly affected by these gas-phase tunneling effects., 17 pages, 12 Figures in main Text. Accepted at A&A

Der quantenmechanische Tunneleffekt von Atomen wird immer haufiger als wichtiger Aspekt in chemischen Reaktionen identifiziert. Experimentell kann man Atomtunneln nur indirekt nachweisen; beispielsweise durch temperaturunabhangige Reaktionsgeschwindigkeitskonstanten oder durch sehr grose kinetische Isotopeneffekte. Im Gegensatz dazu konnen theoretische Methoden den Einfluss des Tunneleffekts auf die Reaktionsgeschwindigkeit direkt bestimmen. Der Tunneleffekt andert beispielsweise Reaktionspfade und Verzweigungsverhaltnisse, ermoglicht chemische Reaktionen in astrochemischer Umgebung, die thermisch nicht stattfinden, und beeinflusst auch biochemische Prozesse.

Asymmetric 1,2-additions of cyanide yield enantioenriched cyanohydrins as versatile chiral building blocks. Next to HCN, volatile organic cyanide sources are usually used. Among them, cyanoformates are more attractive on technical scale than TMSCN for cost reasons, but catalytic productivity is usually lower. Here, the development of a new strategy for cyanations is described, in which this activity disadvantage is overcome. A Lewis acidic Al center cooperates with an aprotic onium moiety within a remarkably robust bifunctional Al-F-salen complex. This allowed for unprecedented turnover numbers of up to 104 . DFT studies suggest an unexpected unique trimolecular pathway in which the ammonium bound cyanide attacks the aldehyde, which itself is activated by the carbonyl group of the cyanoformate binding to the Al center. In addition, a novel practical carboxycyanation method was developed that makes use of KCN as the sole cyanide source. The use of a pyrocarbonate as carboxylating reagent provided the best results.

The title reaction is involved in the formation of ammonia in the interstellar medium. We have calculated thermal rates including atom tunnelling using different rate theories. Canonical variational theory with microcanonically optimised multidimensional tunnelling was used for bimolecular rates, modelling the gas-phase reaction and also a surface-catalysed reaction of the Eley–Rideal type. Instanton theory provided unimolecular rates, which model the Langmuir–Hinshelwood type surface reaction. The potential energy was calculated on the CCSD(T)-F12 level of theory on the fly. We report thermal rates and H/D kinetic isotope effects. The latter have implications for observed H/D fractionation in molecular clouds. Tunnelling causes rate constants to be sufficient for the reaction to play a role in interstellar chemistry even at cryogenic temperature. We also discuss intricacies and limitations of the different tunnelling approximations to treat this reaction, including its pre-reactive minimum.

Lewis pair polymerization employing N-Heterocyclic olefins (NHOs) and simple metal halides as co-catalysts has emerged as a useful tool to polymerize diverse lactones. To elucidate some of the mechanistic aspects that remain unclear to date and to better understand the impact of the metal species, computational methods have been applied. Several key aspects have been considered: (1) the formation of NHO-metal halide adducts has been evaluated for eight different NHOs and three different Lewis acids, (2) the coordination of four lactones to MgCl2 was studied and (3) the deprotonation of an initiator (butanol) was investigated in the presence and absence of metal halide for one specific Lewis pair. It was found that the propensity for adduct formation can be influenced, perhaps even designed, by varying both organic and metallic components. Apart from the NHO backbone, the substituents on the exocyclic, olefinic carbon have emerged as interesting tuning site. The tendency to form adducts is ZnCl2 > MgCl2 > LiCl. If lactones coordinate to MgCl2, the most likely binding mode is via the carbonyl oxygen. A chelating coordination cannot be ruled out and seems to gain importance upon increasing ring-size of the lactone. For a representative NHO, it is demonstrated that in a metal-free setting an initiating alcohol cannot be deprotonated, while in the presence of MgCl2 the same process is exothermic with a low barrier.

Isoxazolinones are biologically and synthetically interesting densely functionalized heterocycles, which for a long time were not accessible in enantioenriched form by asymmetric catalysis. Next to the deficit of enantioselective methods, the functionalization of isoxazolinones is often plagued by regioselectivity issues due to the competition of various nucleophilic centers within the heterocycles. We report the first regio- and enantioselective C-allylations of isoxazolinones. These occur with high regioselectivity in favor of the linear allylation products, although Ir phosphoramidite catalysts were used, which commonly results in branched isomers. Our studies suggest that this outcome is the result of a reaction cascade via an initial regio- and enantioselective N-allylation to provide a branched allyl intermediate, followed by a spontaneous [3,3]-rearrangement resulting in chirality transfer.

The first synthetic protocol to high oxidation state molybdenum(VI) N-heterocyclic carbene (NHC) alkylidyne complexes (NHC=1,3-diisopropylimidazol-2-ylidene, 1,3-dimethyl-4,5-R2-imidazol-2-ylidene, R2=H, Cl, CN) is reported. Steric limitations of the NHCs and the benzylidyne are described. All novel complexes were characterized by single crystal X-ray diffraction and solution NMR techniques. It was shown that all complexes presented here show activity in the self-metathesis of 1-phenyl-1-propyne at room temperature. To identify mechanistic differences, an experimental sequence to detect dissociation of ligands was developed. Results reveal dissociation of less electron-donating NHCs in course of the reaction. Mechanistic and reactivity differences were attributed to electronic and steric effects through Tolman's electronic parameter and the percentage of buried volume. Furthermore, Mo-1 containing the 1,3-dimethylimidazol-2-ylidene ligand showed good activity in self-metathesis reactions of p-substituted 1-phenyl-1-propynes with electron-donating moieties at room temperature.

Atom tunneling in the hydrogen atom transfer reaction of the 2,4,6-tri-tert-butylphenyl radical to 3,5-di-tert-butylneophyl, which has a short but strongly curved reaction path, was investigated using instanton theory. We found the tunneling path to deviate qualitatively from the classical intrinsic reaction coordinate, the steepest-descent path in mass-weighted Cartesian coordinates. To perform that comparison, we implemented a new variant of the predictor-corrector algorithm for the calculation of the intrinsic reaction coordinate. We used the reaction force analysis method as a means to decompose the reaction barrier into structural and electronic components. Due to the narrow energy barrier, atom tunneling is important in the abovementioned reaction, even above room temperature. Our calculated rate constants between 350 K and 100 K agree well with experimental values. We found a H/D kinetic isotope effect of almost 106 at 100 K. Tunneling dominates the protium transfer below 400 K and the deuterium transfer below 300 K. We compared the lengths of the tunneling path and the classical path for the hydrogen atom transfer in the reaction HCl + Cl and quantified the corner cutting in this reaction. At low temperature, the tunneling path is about 40% shorter than the classical path.

The rapidly growing area of asymmetric imine reduction by imine reductases (IREDs) has provided alternative routes to chiral amines. Here we report the expansion of the reaction scope of IREDs by showing the stereoselective reduction of 2,2,2-trifluoroacetophenone. Assisted by an in silico analysis of energy barriers, we evaluated asymmetric hydrogenations of carbonyls and imines while considering the influence of substrate reactivity on the chemoselectivity of this novel class of reductases. We report the asymmetric reduction of C=N as well as C=O bonds catalysed by members of the IRED enzyme family.

International audience; Photoionization can create a nonstationary electronic state and therefore initiates coupled electron-nuclear dynamics in molecules. Using a CASSCF implementation of the Ehrenfest method, we study the nuclear dynamics following vertical ionization of toluene, starting close to the conical intersection between ground and first excited states of its cation. The results show how the initial nuclear dynamics is controlled by the nonstationary electronic state character. In particular, ionization of this system leading to an equal superposition of the two lowest energy states can initiate nuclear dynamics in an orthogonal direction in the branching space to dynamics on the ground or first excited state potential energy surfaces alone.

We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling were applied using a fitted potential energy surface [J. Chen et al., J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval 4 ⋅ 10(-20) to 4 ⋅ 10(-17) cm(3) s(-1), demonstrating that even deuterated versions of the title reaction are possibly relevant to astrochemical processes in molecular clouds. The transferred hydrogen atom dominates the kinetic isotope effect at all temperatures.