Methyl Formate Modeling
Jacob C. Laas, Robin T. Garrod, Eric Herbst, and Susanna L. Widicus Weaver
Organic Chemistry Reactions in Interstellar Space
Consider two types of interstellar chemical reactions: 1) grain-surface reactions. 2) gas-phase reactions. In a grain-surface mechanism on cold grains (10 K), simple molecules form in ices by single-atom addition reactions because heavier molecules are not mobile. The simple molecules in the ice are photodissociated to form radicals. Upon warming, these radicals become mobile on the ice surface and react via recombination reactions to form larger organic species such as methyl formate (see right). Eventually, as the temperature reaches 200-300 K, the ices are evaporated and some gas-phase molecules become ionized. In gas-phase reactions, neutral molecules react with these gas-phase ions in ion-molecule reactions (see below).
These types of grain-surface and gas-phase reactions have been included in The Ohio State Gas/Grain model described below.
Background of the Gas/Grain Model
The Ohio State Gas/Grain networks involve a set of thousands of chemical reactions as well as adsorption and desorption processes. These networks are then paired with a physical model appropriate for the type of environment being studied. There are many different versions of the chemical networks, each focusing on the chemistry that is relevant for a particular type of interstellar environment. The chemical networks are based on models that date back to 1973. The original network (Herbst & Klemperer) was concerned primarily with gas phase reactions (formation and destruction of polyatomic molecules) in the interiors of cold dense interstellar clouds. This model incorporated over 100 reactions, half of which had rate constants measured in the laboratory. These reactions involved only exothermic two-body collisions free of activation barriers because of the low densities and temperatures found in the interstellar medium. It was assumed that grain-surface chemistry would be negligible under these conditions, because the dense clouds are at extremely low temperatures. Moving forward from this first model, scientists began incorporating pseudo-time dependence with fixed physical conditions, followed by the realistic time dependence of physical parameters. The number of reactions considered increased enormously from hundreds to thousands (many of which remain unmeasured in the laboratory). In 1992, Hasegawa, Herbst, and Leung made the first serious attempt to incorporate the chemical consequences of grain-surface reactions. In the last two decades, the model has become more sophisticated, intricate, and versatile. The most complete gas/grain network was used to model the chemistry of a high-mass star-forming region (Garrod, Widicus Weaver, and Herbst 2008). The physical model used in this study incorporates various phases of hot-core evolution, with variable temperatures and time-dependent accretion onto and evaporation from grain surfaces. The chemical network includes over 7,500 possible reactions, including photodissociation of surface-bound molecules, reactions of surface-bound species, and gas-phase ion-molecule reactions.
The CCU is using this most recent gas/grain chemical network in an attempt to elucidate how methyl formate and its structural isomers are formed in star-forming regions. Toward this goal, two new gas-phase reactions leading to methyl formate were added to the network, involving formic acid and protonated methanol, and methanol and protonated formic acid. These reactions lead to different branching fractions of the stereoisomers of protonated methyl formate, which recombine with electrons to form the cis and trans stereoisomers of neutral methyl formate. Furthermore, the branching ratios for methanol photodissociation:
CH3OH + hv => CH3 +OH (x%), CH2OH +H (y%), & CH3O + H (z%)
(which are suspected to govern grain-surface organic chemistry) were varied to test their influence on methyl formate chemistry.
What Does This Type of Research Tell Us?
This new network can help us begin to answer questions such as:
~ Does grain surface chemistry dominate methyl formate production, or can other previously unconsidered gas phase reactions account for its formation?
~ Do trans-methyl formate and its protonated counterpart play a role in enhancing the observed methyl formate abundance?
The grain surface chemistry explored in this project gave us insights into the influence that photodissociation-driven reactions have on the organic chemistry of the interstellar medium. When the methoxy branch of methanol photodissociation is favored [CH3OH + hv => CH3 +OH (5%), CH2OH +H (5%), & CH3O + H (90%)] the resulting calculated abundances are closest to what is observationally found in Sagittarius B2(N) for methyl formate. The ratio that best reproduces the observed abundances for all organic molecules is less obvious. However, the set of branching ratios favoring the methoxy branch best matches the results of gas-phase laboratory measurements. The additions of the gas-phase reactions for the formation of trans- and cis- methyl formate were found to have less influence on the abundances of various organic molecules than the changes in the grain-surface methanol photodissociation branching ratios (see below for the full set of results).
Though the addition of new gas-phase reactions for the formation of trans- and cis- methyl formate does not appear to affect the overall cis-methyl formate concentration in the model, the results indicate that trans-methyl formate should be abundant in interstellar clouds (see left). These results indicate that grain-surface chemistry dominates the formation of cis-methyl formate, and the new gas-phase reactions do not contribute significantly to its formation. However, the trans- conformer of methyl formate is formed efficiently through gas-phase reactions, and the gas-phase abundance of trans-methyl formate is enhanced over that of cis-methyl formate prior to its evaporation from ices. At hot core temperatures, the trans-methyl formate gas-phase abundance approaches that of the cis- conformer. As a result of these improvements to the chemical network, we predict a detectable abundance of trans-methyl formate at temperatures above 200 K.
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