Methyl and Ethyl Formate Spectroscopy, Spatial Distributions, and Reaction Proceses
J. L. Neill, A. L. Steber, M. T. Muckle, D. P. Zaleski, V. Lattanzi, S. Spezzano, M. C. McCarthy,
A. J. Remijan, D. N. Friedel, S. L. Widicus Weaver, & B. H. Pate
Using Interferometers to Understand Interstellar Chemistry
To date, most observations of molecular clouds in the Interstellar Medium (ISM) have been made with single dish telescopes. These observations allow astronomers to catalog the molecular species in the region as a whole, but do not allow them to distinguish between different molecular environments. This is due to the large beam size of the single dish observations. Thus a complete picture of the reaction chemistry of these clouds cannot be determined.
With interferometric arrays such as ALMA, the eVLA, and CARMA coming online, each individual environment can be imaged separately, creating a high spatial resolution contour map of the molecular species present.
Figure 1 shows a CARMA contour map of methyl formate and formic acid in the Orion molecular cloud. Because of the smaller beam size, the distributions of both molecules could be precisely mapped. However, the beam size of the GBT is much larger so the spatial resolution is lost.
Spatial Mapping of Methyl Formate
In the figure below, we see that in the KL Region of the Orion Molecular Cloud, methyl formate is shown to have a peak abundance where formic acid is depleted. This result indicates that methyl formate may be formed at the expense of formic acid. By analyzing maps such as these, we can begin to formulate reaction pathways that lead to the production of more complex molecules from simpler ones. These proposed reactions can then be tested through ab initio calculations, laboratory experiments, and further observational tests.
Possible Production Routes of Protonated Methyl Formate
The anti-correlation of the concentrations of methyl formate and formic acid could be explained by a reaction of formic acid and methanol to produce methyl formate. Thus two new potential gas phase reactions were proposed in which formic acid is a reactant. Because there is such a high activation energy barrier for the reaction of neutral methanol with neutral formic acid, one reactant must be protonated to make a more energetically favorable reaction. Panel (a) shows the Fischer esterification reaction in which formic acid is protonated. In Panel (b) the methyl cation transfer reaction is displayed, where methanol is protonated. Conformational specification is shown in both reactions and both provide a pathway for the high abundance formation of the trans conformer of protonated methyl formate that is seen in the ISM, suggesting it as a target for interstellar observations.
Conformational Isomerism in the Interstellar Medium
Ethanol is previously the only molecule for which two conformers have been unambiguously detected in the ISM; its conformers are very close in energy (40 cm-1) so both could be present under equilibrium conditions. Methyl and ethyl formate have two possible ester geometries, and while the energy difference between these isomers is large (with the cis favored), the barrier to isomerization could prevent thermal equilibrium, making interstellar detection of these meta-stable conformers possible.
At thermal equilibrium, the higher-energy conformers of methyl and ethyl formate have very low abundances (~1/30000 that of the more stable conformers at 300 K), so any interstellar detection must have a kinetic origin. Additionally, due to their low populations in the laboratory, they provide a challenge for laboratory rotational spectroscopy.
Laboratory Detection of trans-Methyl and Ethyl Formates
Chirped-pulse Fourier transform microwave spectroscopy from 6-18 and 25-40 GHz (UVa) was used to find transitions of these species. To obtain higher frequency resolution data, a coaxially-oriented cavity FT microwave spectrometer (CfA) was employed as well. For all measurements, a pulsed discharge nozzle was used to enhance the population of weak conformers.
Because both trans-methyl and ethyl formate have low-barrier internal motions, and because of the high line density of the observed spectra, microwave-microwave double resonance spectroscopy was used to find new transitions and to confirm spectral assignments.
The laboratory assignments were confirmed through comparison to electronic structure theory (see tables below).
The detection of trans-methyl formate in Sagittarius B2(N) using the Green Bank Telescope at ~1% of the abundance of the more stable cis conformer suggests kinetically controlled formation chemistry in this region. Additionally, the trans seems to be significantly colder than the cis, suggesting that the two conformers have different spatial distributions: further interferometric observations, coupled with laboratory spectroscopy of trans-methyl formate in the millimeter wave region, will be important for fully understanding the chemistry of methyl formate in this region.
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ALMA Array from NRAO / AUI / NSF