Rapid formation of H3^+ from ammonia and methane following ionization by fast protons (original) (raw)
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Ammonia as a case study for the spontaneous ionization of a simple hydrogen-bonded compound
Nature Communications, 2014
Modern ab initio calculations predict ionic and superionic states in highly compressed water and ammonia. The prediction apparently contradicts state-of-the-art experimentally established phase diagrams overwhelmingly dominated by molecular phases. Here we present experimental evidence that the threshold pressure of B120 GPa induces in molecular ammonia the process of autoionization to yet experimentally unknown ionic compoundammonium amide. Our supplementary theoretical simulations provide valuable insight into the mechanism of autoionization showing no hydrogen bond symmetrization along the transformation path, a remarkably small energy barrier between competing phases and the impact of structural rearrangement contribution on the overall conversion rate. This discovery is bridging theory and experiment thus opening new possibilities for studying molecular interactions in hydrogen-bonded systems. Experimental knowledge on this novel ionic phase of ammonia also provides strong motivation for reconsideration of the theory of molecular ice layers formation and dynamics in giant gas planets.
Isotope Effects on Chemical Shifts in the Study of Hydrogen Bonds in Small Molecules
Molecules, 2022
This review is giving a short introduction to the techniques used to investigate isotope effects on NMR chemical shifts. The review is discussing how isotope effects on chemical shifts can be used to elucidate the importance of either intra-or intermolecular hydrogen bonding in ionic liquids, of ammonium ions in a confined space, how isotope effects can help define dimers, trimers, etc., how isotope effects can lead to structural parameters such as distances and give information about ion pairing. Tautomerism is by advantage investigated by isotope effects on chemical shifts both in symmetric and asymmetric systems. The relationship between hydrogen bond energies and two-bond deuterium isotope effects on chemical shifts is described. Finally, theoretical calculations to obtain isotope effects on chemical shifts are looked into.
Tetrahedron Letters, 1989
Very accurate frequency measurements on 500 MHz JH NJviR spectra of CHq, CH3D, CHzD2, and CHD3 are reported. There are relatively large (up to 10%) solvent and temperature effects on the isotope shifts (2A =-13.998 to-15.555 ppb for CH3D), which are non-additive by ca 0.2'ppb/deuterium; 2J~~ (-1.929 + 0.001 Hz, sign assumed, equivalent to 2J~~ =-12.57 Hz) is almost independent of solvent, temperature, and the number of deuterons present. The effects of partial deuterium substitution on the chemical shifts of the remaining protons in a molecule have been extensively studied, both experimentally and theoretically. 1-S In rigid and certain other molecules, where deuterium substitution does not perturb any rotameric or conformational equilibria, the observed shifts can be assigned to intrinsic isotope effects, in contrast to other cases where contributions from equilibrium isotope efsects can be important.2+6 Questions concerning intrinsic isotope effects include (a) the additivity of the isotope effects when more than one deuterium is introduced in the molecule, (b) the temperature dependence of the isotope effect, and (c) the solvent dependence of the isotope effect.
Chemical Physics, 2006
The time dependent real wave packet method using the helicity decoupling approximation was used to calculate the cross section evolution with collision energy (excitation function) of the O++H2(v=0,j=0)→OH++H reaction and its isotopic variants with D2 and HD, using the best available ab initio analytical potential energy surface. The comparison of the calculated excitation functions with exact quantum results and experimental data showed that the present quantum dynamics approach is a very useful tool for the study of the selected and related systems, in a quite wide collision energy interval (approximately 0.0-1.1eV), involving a much lower computational cost than the quantum exact methods and without a significant loss of accuracy in the cross sections.
2015
The results of the laboratory study of reaction rate coefficients of several ion-molecule reactions with atomic and molecular hydrogen and molecular deuterium at low temperatures are presented in the thesis. The reaction rate coefficients of the N+ and H+ reaction with H2 were measured with respect to the nuclear spin configuration and rotational excitation of H2. The reactions of anions were a subject of the isotope exchange and isotope effect study. The measurements of the rate coefficients of H2O and D2O formation in the reaction of O– with H2 and D2, isotope exchange reactions OH– + D2 and OD– + H2, and associative detachment and charge transfer channels of D– + H interaction were performed. Experiments were carried out using an AB-22PT instrument with an ion trap. It has producing, guiding, trapping, and detecting systems for ions and a separate source of atomic H. The cooling system allowed to measure the temperature dependencies of the reaction rate coefficients at temperatur...
Theoretical Study of the Ionization of the H2O-H2O, NH3-H2O, and FH-H2O Hydrogen-Bonded Molecules
Journal of the American Chemical Society, 1994
The first two vertical ionization energies of H2GH20, NH3-H20, and FH-H20 hydrogen-bonded complexes and the rearrangement processes in their ionized states have been studied by ab initio calculations that include correlation energy. In all cases, the two lowest ionic states are the 2Af and 2A'f states depending on whether the ionization is located in the proton acceptor or in the proton donor molecule, respectively. If the ionization is produced in the proton donor, the 2A'' state, the dimer evolves to a proton-transfer complex. However, if the ionization is produced in the proton acceptor, the 2A' state, the dimer rearranges to a non-proton-transfer hydrogen-bonded structure in which the role of the two monomers is just the inverse of that of the original neutral dimer. For the water-water and water-ammonia dimers, relaxation of the ZA' state can also lead to three-electron hemibond complexes.
The Journal of Chemical Physics, 2011
The neutral muonic helium atom 4 Heμ, in which one of the electrons of He is replaced by a negative muon, may be effectively regarded as the heaviest isotope of the hydrogen atom, with a mass of 4.115 amu. We report details of the first muon spin rotation (μSR) measurements of the chemical reaction rate constant of 4 Heμ with molecular hydrogen, 4 Heμ + H 2 → 4 HeμH + H, at temperatures of 295.5, 405, and 500 K, as well as a μSR measurement of the hyperfine coupling constant of muonic He at high pressures. The experimental rate constants, k Heμ , are compared with the predictions of accurate quantum mechanical (QM) dynamics calculations carried out on a well converged Born-Huang (BH) potential energy surface, based on complete configuration interaction calculations and including a Born-Oppenheimer diagonal correction. At the two highest measured temperatures the agreement between the quantum theory and experiment is good to excellent, well within experimental uncertainties that include an estimate of possible systematic error, but at 295.5 K the quantum calculations for k Heμ are below the experimental value by 2.1 times the experimental uncertainty estimates. Possible reasons for this discrepancy are discussed. Variational transition state theory calculations with multidimensional tunneling have also been carried out for k Heμ on the BH surface, and they agree with the accurate QM rate constants to within 30% over a wider temperature range of 200-1000 K. Comparisons between theory and experiment are also presented for the rate constants for both the D + H 2 and Mu + H 2 reactions in a novel study of kinetic isotope effects for the H + H 2 reactions over a factor of 36.1 in isotopic mass of the atomic reactant.
Kinetic isotope effect in the hydrogen evolution reaction
Electrochimica Acta, 2001
In the framework of the medium reorganization-proton-tunneling mechanism, the dependence of the kinetic isotope effect (KIE) on the electrode potential for poorly adsorbing hydrogen cathodes has been considered. Two experimentally accessible quantities are discussed: the ratio of the rates of hydrogen evolution in light and heavy water (K H/D ) and the isotopes' separation factor S H/D . Their opposite dependencies on the electrode potential have been explained. With increasing polarization, many factors decrease the KIE. The opposite trend should result, in the normal (h: 0.5) region, from the effect of the electrode potential on the relative contribution of the products formation in the ground and vibrationally excited states. This explains the increase of K H/D with polarization. For the barrierless (h= 1) and the activationless (h= 0) processes, where the true value of h is constant in the whole range of potentials, the effect of vibrationally excited states is predicted to be independent of potential. In accordance with this prediction, the KIEs for the barrierless hydronium discharge and for activationless electrochemical desorption were found to decrease with increasing polarization. This presents the first experimental evidence of a substantial role for the formation of vibrationally excited final states in the total process of hydrogen evolution.
The Astrophysical Journal, 2019
Deuterated molecules are important chemical tracers of prestellar and protostellar cores. Up to now, the titular reaction has been assumed to contribute to the generation of these deuterated molecules. We have measured the merged-beams rate coefficient for this reaction as function of the relative collision energy in the range of about 10 meV to 10 eV. By varying the internal temperature of the reacting H + 3 molecules, we found indications for the existence of a reaction barrier. We have performed detailed theoretical calculations for the zero-point-corrected energy profile of the reaction and determined a new value for the barrier height of ≈ 68 meV. Furthermore, we have calculated the tunneling probability through the barrier. Our experimental and theoretical results show that the reaction is essentially closed at astrochemically relevant temperatures. We derive a thermal rate coefficient of < 1 × 10 −12 cm 3 s −1 for temperatures below 75 K with tunneling effects included and below 155 K without tunneling.