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Quantum mechanical tunneling of atomsplays a significant rolein many chemical reactions. The crossover temperature between classicaland quantum movement is a convenient preliminary indication of theimportance of tunneling for a particular reaction. Here we show, usinginstanton theory, that quantum tunneling is possible significantlyabove this crossover temperature for specific forms of the potentialenergy surface. We demonstrate the effect on an analytic potentialas well as a chemical system. While protons move asynchronously alonga Grotthuss chain in the classical high-temperature range, the onsetof tunneling results in a synchronization of their movement. [ABSTRACT FROM AUTHOR]

We performed surface hopping simulations of Al + H2Ocollisions by a direct semiempirical method, reproducing the conditionsof previous beam-gas experiments.(1, 2) We observedthe formation of the HAlOH species, that dissociates to AlOH + H aftera lifetime of about 0.6 ps. This species undergoes nonadiabatic transitionsto its first excited state and is responsible for chemiluminescencein the visible range, while the Al–H2O complex emitsin the infrared. The computed emission band in the visible is red-shiftedwith respect to the experimental one,(1, 2) because ofslight inaccuracies of the potential energy surfaces. However, collisionswith more water molecules and exciplex formation with excited Al(2S, 4P) atoms may also contribute to the short wavelengthemission, as we show by accurate ab initio calculations. [ABSTRACT FROM AUTHOR]

The first step of the reaction of two relatively largeAlmclusters (m= 17,28) with a fewwater molecules has been studied by electronic structure methods.The complexes Alm·(H2O)n(n= 1–2) have beencharacterized, and the saddle points corresponding to the first stepin the reaction, namely, formation of HAlmOH·(H2O)n−1systems,have been located. The Al28cluster is special in the senseit has two electronic states, singlet and triplet, which are veryclose 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 clusterdistortion being moderately significant in the stability of the complexes.The interaction with water does not appear, in general, to bring thetriplet state of the Al28·(H2O)2adducts below the singlet; not even the corresponding saddle pointsappear to be lower in energy. The rate coefficients, tunneling transmissionfactors, and activation free energies have been computed and comparedwith those of the Al13and Al3clusters, evenwith those of the Al atom. It turns out the rates are quite closeto those of Al3and much larger than those of Al and Al13. There is no dramatic difference between the reactivityof the singlet and triplet state of Al28; however, thereare very significant differences between different sites. Finally,we studied the interaction between the lowest-lying singlet and tripletstates of Al28through multireference configuration interaction(MRCI) spin–orbit computations. The vertical excitation energiescorresponding to a number of low-lying singlet and triplet statesare also determined by MRCI computations. It turns out that the spin–orbitinteraction is very weak, which suggests that both states, the lowest-lyingsinglet and triplet, could evolve somehow independently, at leastwhen interacting with closed-shell molecules. It is suggested thatthe situation could be quite different in a reaction with molecularradicals or if external fields are applied. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of the Al+H2O reaction in the gas phase has been studied by means of TST and RRKM theories, including the treatment of tunneling. The main reaction path involves isomerization into a HAlOH species, which dissociates into AlOH+H. There is a second mechanism which involves hydrogen atom elimination from a Al-OH2 complex, but it only becomes relevant at very high temperatures. Our theoretical results for the variation of the rate coefficient with temperature are coherent with the experimental findings [R.E. McClean, H.H. Nelson, M.L. Campbell, J. Phys. Chem. 97 (1993) 9673] in the sense that they can be fitted to a double exponential form. Our explanation for that behavior is the variation of the transmission coefficient with temperature. We find formation of AlO+H2 to be insignificant and the lifetime of Al-OH2 much larger than that of HAlOH. [Copyright &y& Elsevier]

Abstract: The interaction of individual Al atoms with a few water molecules or water clusters may result in the generation of HAlOH·(n −1)H2O or AlOH·(n −1)H2O+H. It is shown how these species may undergo further reactions and produce molecular hydrogen as well as Al hydroxides and related species in the gas phase and inert matrices by means of an analysis of the potential energy surfaces. The most likely reaction schemes and the corresponding intermediates are identified. HAlOH·(n −1)H2O can generate both Al(OH)2·(n −2)H2O+H2 and AlOH·(n −1)H2O+H; generation of AlO·(n −1)H2O+H2 is not competitive. AlOH·(n −1)H2O (n >2) can easily recombine with a hydrogen atom producing H2 and hydrated Al(OH)2. The stability of a single hydrated AlO molecule is examined; we have found some reaction paths leading to Al(OH)2·(n −2)H2O with very small or no barrier. There is no indication that ionic mechanisms may play a significant role. [ABSTRACT FROM AUTHOR]

Abstract: An accurate study of the potential energy surface of the ground state of the AlOH2 system has been performed together with a multi-configuration SCF and multi-reference CI analysis of the excited electronic states, in order to help understand the hydration experiments in which the Al atom has been found to react with water. Two critical transition states for the Al+H2O reaction appear to lie above the ground state reactants. On the other hand, the computed electronic spectrum of HAlOH and the behavior of the excited electronic states with some reaction coordinates are coherent with the experimental results. [Copyright &y& Elsevier]

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 10 4 . 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.