Asymmetric Carboxycyanation of Aldehydes by Cooperative AlF/Onium Salt Catalysts: from Cyanoformate to KCN as Cyanide Source.

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.

Atom tunnelling in the reaction NH3+ + H2 → NH4+ + H and its astrochemical relevance.

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.

Can alumina particles be formed from Al hydroxide in the circumstellar media? A first-principles chemical study.

AlOH has been detected in the circumstellar envelope of an oxygen-rich supergiant star (VY CMa) and is an abundant Al-containing system. Water molecules have also been detected, even in a vibrationally excited state. The coalescence of AlOH units and other processes involving AlOH could be the source of alumina-type particles. The results indicate that the formation of (AlOH)2 dimers is barrier-free but (HAlO)2 systems are far more stable. The (AlOH)2 → (HAlO)2 transformation is hindered by substantial energy barriers but is probably moderately fast at very high temperatures. Water catalysis by relay (or Grotthuss-like) mechanisms substantially reduces those barriers to the point that, in the (AlOH)2·(H2O)2 system, the critical transition states lie clearly below 2AlOH + 2H2O. A surface or nucleation environment may favor the (AlOH)2 → (HAlO)2 conversion as to be kinetically competitive with water elimination ((AlOH)2·(H2O)n → (AlOH)2 + nH2O) in the hydrated systems. The hydrated (HAlO)2 structures can easily produce very stable hydrogenated Al2O3 and Al2O4 frames, which, to the same extent, can eliminate molecular hydrogen by exothermic processes. The remaining hydrogen atoms are exterior to the frames and perhaps could be removed by reaction with atomic hydrogen. The possible role of the coalescence of the undetected HAl(OH)2 and Al(OH)2 or AlO2H molecules is discussed. Al(OH)2 can easily be formed by reaction of AlO with a water molecule in exothermic barrier-free processes.

Tunneling above the crossover temperature.

Quantum mechanical tunneling of atoms plays a significant role in many chemical reactions. The crossover temperature between classical and quantum movement is a convenient preliminary indication of the importance of tunneling for a particular reaction. Here we show, using instanton theory, that quantum tunneling is possible significantly above this crossover temperature for specific forms of the potential energy surface. We demonstrate the effect on an analytic potential as well as a chemical system. While protons move asynchronously along a Grotthuss chain in the classical high-temperature range, the onset of tunneling results in a synchronization of their movement.

Theoretical study of the chemiluminescence of the Al + H2O reaction.

We performed surface hopping simulations of Al + H(2)O collisions by a direct semiempirical method, reproducing the conditions of previous beam-gas experiments. We observed the formation of the HAlOH species, that dissociates to AlOH + H after a lifetime of about 0.6 ps. This species undergoes nonadiabatic transitions to its first excited state and is responsible for chemiluminescence in the visible range, while the Al-H(2)O complex emits in the infrared. The computed emission band in the visible is red-shifted with respect to the experimental one, because of slight inaccuracies of the potential energy surfaces. However, collisions with more water molecules and exciplex formation with excited Al((2)S, (4)P) atoms may also contribute to the short wavelength emission, as we show by accurate ab initio calculations.

Size, adsorption site, and spin effects in the reaction of Al clusters with water molecules: Al17 and Al28 as examples.

The first step of the reaction of two relatively large Alm clusters (m = 17, 28) with a few water molecules has been studied by electronic structure methods. The complexes Alm·(H2O)n (n = 1-2) have been characterized, and the saddle points corresponding to the first step in the reaction, namely, formation of HAlmOH·(H2O)n-1 systems, have been located. The Al28 cluster is special in the sense it has two electronic states, singlet and triplet, which are very close in energy and also have quite similar equilibrium structures. The preferred adsorption and reaction sites have been determined. We find quite clear preferences toward some sites, the effect of cluster distortion being moderately significant in the stability of the complexes. The interaction with water does not appear, in general, to bring the triplet state of the Al28·(H2O)2 adducts below the singlet; not even the corresponding saddle points appear to be lower in energy. The rate coefficients, tunneling transmission factors, and activation free energies have been computed and compared with those of the Al13 and Al3 clusters, even with those of the Al atom. It turns out the rates are quite close to those of Al3 and much larger than those of Al and Al13. There is no dramatic difference between the reactivity of the singlet and triplet state of Al28; however, there are very significant differences between different sites. Finally, we studied the interaction between the lowest-lying singlet and triplet states of Al28 through multireference configuration interaction (MRCI) spin-orbit computations. The vertical excitation energies corresponding to a number of low-lying singlet and triplet states are also determined by MRCI computations. It turns out that the spin-orbit interaction is very weak, which suggests that both states, the lowest-lying singlet and triplet, could evolve somehow independently, at least when interacting with closed-shell molecules. It is suggested that the situation could be quite different in a reaction with molecular radicals or if external fields are applied.

Atom Tunneling in the Hydroxylation Process of Taurine/α-Ketoglutarate Dioxygenase Identified by Quantum Mechanics/Molecular Mechanics Simulations.

Taurine/α-ketoglutarate dioxygenase is one of the most studied α-ketoglutarate-dependent dioxygenases (αKGDs), involved in several biotechnological applications. We investigated the key step in the catalytic cycle of the αKGDs, the hydrogen transfer process, by a quantum mechanics/molecular mechanics approach (B3LYP/CHARMM22). Analysis of the charge and spin densities during the reaction demonstrates that a concerted mechanism takes place, where the H atom transfer happens simultaneously with the electron transfer from taurine to the Fe═O cofactor. We found the quantum tunneling of the hydrogen atom to increase the rate constant by a factor of 40 at 5 °C. As a consequence, a quite high kinetic isotope effect close to 60 is obtained, which is consistent with the experimental value.