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Myhre, Einar Uggerud. Reactions of microhydrated ozonide with methyl chloride. International Journal of Mass Spectrometry , , Mass spectrometry of aerosol particle analogues in molecular beam experiments. Mass Spectrometry Reviews , 37 5 , Feng Yu. The Journal of Chemical Physics , 1 , Hierl, Albert A. Viggiano, William L. Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism.
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E Wilkinson, T. B McMahon. These effects are most likely electronic in origin: water stabilizes formaldehyde as well as the transition states TSs through dipolar interactions. Thus, the addition TS is slightly more stabilized than formaldehyde, lowering the barrier, while the abstraction TS is less stabilized than formaldehyde, resulting in a larger barrier. Our calculations thus predict that for the reaction of hydrogen atoms with formaldehyde in water-rich ices addition is preferred over abstraction, thus making D enrichment via abstraction—addition reactions less efficient.
Lines are to guide the eye only. The experiments by Hiraoka et al.
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We thus also investigated the effect of another formaldehyde molecule on reactions 1 and 2. The second formaldehyde has a much smaller effect on the activation barriers than water, with both barriers slightly increased [ 1 : K, 2 : K] with respect to the gas phase. Consequently, an additional formaldehyde molecule does not affect the preference of H abstraction over addition Fig. However, in the lab experiments as wall as in the ISM, the surface can affect the apparent reactivity in several ways.
Through the same effect, possibly augmented by a tunnel effect, D atoms diffuse slower on and in the ice.
In the lab experiments the hydrogen fluxes are about eight orders of magnitude larger than in molecular clouds, and consequently the majority of the hydrogen atoms dimerise Fuchs et al. To comprehensively model the reactivity of H and D atoms with a formaldehyde ice both in lab and in space, all these effects should be taken into account with a kinetic Monte Carlo method Fuchs et al.
In the gas phase and in solid formaldehyde H atoms are preferentially abstracted from H 2 CO by either D or H, which could be an efficient route for deuterium enrichment in both interstellar formaldehyde and methanol. Conversely, H atoms preferentially add to D 2 CO so that once deuterated formaldehyde is formed it will likely continue to react to form deuterated methanol. When the reacting formaldehyde is surrounded by two water molecules, however, addition is preferred over abstraction, and consequently deuterium enrichment via H—D exchange in formaldehyde is predicted to be less efficient in water-rich polar ices.
Any H atoms attached to the C atom in methanol can be further exchanged for D atoms but not in the OH group Nagaoka et al. However, OD can be exchanged for H atoms via protic exchange with H 2 O in the melting and evaporating ice in the warm-up phase near young stellar objects Ratajczak et al. The two combined mechanisms of H—D exchange via abstraction of H atoms but not D atoms from formaldehyde and methanol and the protic exchange of OD for OH in the melting ice could explain why formaldehyde and interstellar methanol are heavily deuterated but relatively less so in the alcoholic group of methanol.
We fitted our gas phase rate coefficients between 20 and K for the abstraction and addition reactions to the standard modified Arrhenius form that is incorporated in astrochemical reaction networks and kinetic Monte Carlo codes. Because of the large tunnelling effect below T c we fitted the low and high temperature separately. The low- T parameters are quite different from parameters usually employed and cannot be applied below the fitted temperature range.