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Nitrile Chemistry and Proposed Observations

Graduate Student Researchers Joanna Corby & Dan Zaleski

Nitriles make up a significant fraction of the known interstellar species, in part because their large dipole moments allow for easier detection. The presence of several nitriles in rich interstellar sources makes them a class of molecules that can be used to test proposal for molecule formation in the interstellar medium. We have performed a screening of the laboratory nitrile chemistry produced in a pulsed discharge source using the high abundance interstellar species CH3CN and H2S. The reaction products are identified by broadband rotational spectroscopy in a chirped-pulse FTMW spectrometer. So far 25 discharge-induced species, of which 18 are known interstellar molecules, have been identified in the laboratory spectrum. Because the column densities found in the GBT PRIMOS survey of Sgr B2(N) are similar to what is seen in laboratory relative population analysis, it seems to suggest the conditions created by the discharge nozzle may be similar to those found in the interstellar medium. Radicals of CH3CN and H2S are produced in high abundances in the laboratory spectrum and can explain many of the observed product species. These comparisons suggest that radical chemistry may dominate nitrile formation in some interstellar sources.

 

When this experiment was preformed in a supersonic expansion, the known interstellar molecules acetonitrile (CH3CN) and hydrogen sulfide (H2S) were discharged. The resulting spectrum was found to contain 25 discharge-induced species, of which 18 are known interstellar molecules. Some of these species are those seen below:

The molecules created in the laboratory have similar properties to those in the GBT spectrum. The nitriles that appear in the GBT survey have cold rotational temperatures and appear in absorption. This leads to the testable hypothesis that similar chemistry is happening in the lab and the K6 shell region of SgrB2(N) (see right). We are testing possible chemical formation routes by the chemical composition of different spatial positions of the interstellar medium. The idea is that if the chemistry proposal is right, not just one molecule should be expected, but several reactions should be taking place. In this case, HC3N, CH2CHCN, and CH3CH2CN form from radical recombination followed by energetically feasible H2 loss. Spatial distributions from next-generation radio telescope arrays (ALMA and EVLA) will provide new insight into chemical reaction processes(example left roll over to enlarge, J.M. Hollis et al., ApJ, 596, L235-L238, (2003)). The broadband capabilities of ALMA and EVLA make it possible to observe the chemical composition of different spatially resolved interstellar environments. Information about local chemical composition provides more stringent tests of reaction chemistry proposals.

 

Of the molecules detected in the laboratory we address specifically HC3N, CH2CHCN, CH3CH2CN for the production of CH2CN- ethyl cyanide. The populations found in the laboratory are in Table I.

 

Table I- Laboratory

Molecule

Normalized Poulation

HC3N

1.00

CH2CHCN

1.12

CH3CH2CN

0.21

CH2CN

0.39


We believe the following reactions occur in the discharge experiments:

 


This could potentially be enough energy to sequentially dehydrogenate.The dominant channel for the process is CH2CHCN


Table II shows the column density and rotational temperatures of these moleuces found in the Interstellar Medium.

Table II- Interstellar Medium

Molecule

Column Density (cm-2)

Rotational Temperature (K)

HC3N

~6*1014

8

CH2CHCN

~1.5*1013

~8*1013

4 (Low T Model)

6.2 (High T Model)

CH3CH2CN

~9*1013

7

 

The justification for belief in radical-radical chemistry lies in the direct detection and indirect evidence of radicals of the starting molecules CH3CN and H2S. Two of the main radicals that are created from discharging CH3CN are CH2CN, which is directly measured, and CH3 which has no transitions in our bandwidth. However, based on the detection of ethyl cyanide (CH3CH2CN), we believe ethyl cyanide is formed when the two radicals collide. The two decomposition products formed when H2S is discharged are H atoms and SH. Since we directly measure SH, we can infer the presence of H atoms. And since we detect HSCN, we can infer the presence of CN radical as well. In fact, many combinations of these radicals we detect in our screening (except HCN which has no transitions in our bandwidth). Furthermore, based on this chemistry two new molecules were predicted and later discovered in the laboratory spectrum – HSCH2CN and HCSCN. Mercaptoacetonitrile (HSCH2CN) forms from the combination of CH2CN and SH radicals. Cyanothioformaldehyde (HCSCN) then can form from dehydrogenation of mercaptoacetonitrile. And since we believe there is a correlation between our discharge chemistry and some regions of the ISM, these two new molecules are prime candidates for a dedicated search in the ISM. These new “library free” detections were done through the use of electronic structure theory and an automated fitting routine which with high confidence can determine the likelihood of a species being present. These results show that broadband reaction screening is a powerful tool for studying the chemistry that can occur by “unusual” mechanisms.

 

 

 

Charnley, S. B., Tielens, A. G. G. M., & Millar, T. J. ApJ, 399, L71, (1992).

Belloche, A., Garrod, R.T., Müller, H.S.P., Menten, K.M., Comito, C., Schilke, P. A&A 499, 215–232, (2009).