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a theoretical investigation of the plausibility of reactions between ammonia and carbonyl species (formaldehyde, acetaldehyde, and acetone) in interstellar ice analogs at ultracold temperaturesWe have reexamined the reaction between formaldehyde and ammonia, which was previously studied by us and other workers in modestly sized cluster calculations. Larger model systems with up to 12H2O were employed, and reactions of two more carbonyl species, acetaldehyde and acetone, were also carried out. Calculations were performed at the B3LYP/6-31+G** level with bulk solvent effects treated with a polarizable continuum model; limited MP2/6-31+G** calculations were also performed. We found that while the barrier for the concerted proton relay mechanism described in previous work remains modest, it is still prohibitively high for the reaction to occur under the ultracold conditions that prevail in dense interstellar clouds. However, a new pathway emerged in more realistic clusters that involves at least one barrierless step for two of the carbonyl species considered here: ammonia reacts with formaldehyde and acetaldehyde to form a partial charge transfer species in small clusters (4H2O) and a protonated hydroxyamino intermediate species in large clusters (9H2O, 12H2O); modest barriers that decrease sharply with cluster size are found for the analogous processes for the acetone-NH3 reaction. Furthermore, if a second ammonia replaces one of the water molecules in calculations in the 9H2O clusters, deprotonation can occur to yield the same neutral hydroxyamino species that is formed via the original concerted proton relay mechanism. In at least one position, deprotonation is barrierless when zero-point energy is included. In addition to describing the structures and energetics of the reactions between formaldehyde, acetaldehyde, and acetone with ammonia, we report spectroscopic predictions of the observable vibrational features that are expected to be present in ice mixtures of different composition.
Document ID
20110023634
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Chen, Lina
(Illinois Univ. at Urbana-Champaign Urbana, IL, United States)
Woon, David E.
(Illinois Univ. at Urbana-Champaign Urbana, IL, United States)
Date Acquired
August 25, 2013
Publication Date
May 2, 2011
Publication Information
Publication: Journal of Physical Chemistry
Volume: 115
Issue: 20
Subject Category
Inorganic, Organic and Physical Chemistry
Funding Number(s)
CONTRACT_GRANT: NNX07AN33G
Distribution Limits
Public
Copyright
Other