DANIEL P. ZALESKI, JUSTIN L. NEILL, MATT T. MUCKLE, AMANDA L. STEBER, RYAN A. LOOMIS, BRENT J. HARRIS,

and BROOKS H. PATE

Department of Chemistry, University of Virginia, McCormick Rd, Charlottesville, VA. 22904,USA.

JOANNA F. CORBY

Department of Astronomy, University of Virginia, McCormick Rd, Charlottesville, VA 22904, USA.

ANTHONY J. REMIJAN

National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22904-2475.

VALERIO LATTANZI and MICHAEL C. MCCARTHY

Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, and School of Engineering & Applied Sciences, Harvard University, 29 Oxford St., Cambridge MA

02138.

Nitrile Chemistry: Comparison of Laboratory Reaction Chemistry and Interstellar Observations

The Ohio State 66th International Symposium on Molecular Spectroscopy, June 22nd, 2011.

Advances in Radio Astronomy

The next-generation radio telescope arrays (ALMA and EVLA) will provide high spatial resolution observations of chemical species that will uncover new insights into chemical reaction processes.

The broadband capabilities of ALMA and EVLA make it possible to observe the overall chemical composition of different spatially resolved interstellar environments rather than traditional methods of mapping a single transition of a single molecular species.

This information about local chemical composition provides more stringent tests of reaction chemistry hypotheses.

There are lab-based broadband techniques in place suitable for screening broadband interstellar data

New Strategy Get away from: think of a molecule, go look for it in the ISM

Propose: screen reaction chemistry, deep average and drive the sensitivity way up, then analyze the reaction mixture

Identify known molecules using databases like Splatalogue.net

Then identify molecules based on related chemistry and quantum mechanics

Screen the unassigned transitions vs broadband interstellar data

If there are matches, put the effort into determining those molecular carriers

Does this strategy pay off?

A Reactions Approach to Interstellar Chemistry

Nitriles have high dipole moments allowing for greater chance of interstellar detection.

Nitriles (and isonitriles) are the largest class of known interstellar molecules.

Isomer ratios are a potential test of proposed formation mechanisms.

Pulsed discharge nozzles have had great success in producing interstellar species - possible indication of common chemistry.

Start with high abundance species and identify reaction products:

CH3CN + H2S CH2CN + CH3 + CN + SH + S + H products

Is there support for similar reaction chemistry in the interstellar medium?

Experiment

Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate. Rev. Sci. Instrum. 79, 053103, (2008).

M.C. McCarthy, W. Chen, M.J. Travers, and P. Thaddeus, Ap. J. Supp. Series, 129, 611-623 (2000).

x3

No Helmholtz coil

24 Gs/s AWG

Broadband rotational spectrum of H2S and CH3CN

The signal level shown in the red spectrum is about 40x weaker than the 13C level of the starting material CH3CN

x4000

x600

Laboratory Reaction Chemistry

The chemical composition produced from the pulsed discharge suggests the main processes are:

- Radical Formation (directly detected or inferred) - Radical Recombination followed by H2 loss and/or isomerization

24 molecules have been identified in the laboratory spectrum, 18 are interstellar

Accounts for ~10% of all known interstellar molecules without intentionally adding oxygen

- O-bearing species likely resulting from atmospheric H2O in the line

Still hundreds of unassigned laboratory transitions- Over 50% of the remaining lines

Molecule Number Atoms Interstellar

SH 2 Y

SSH 3 N

NCS 3 N

SO2 3 Y

OCS 3 Y

HSCN 4 Y

HNCS 4 Y

HCNS 4 N

H2CS 4 Y

CCCS 4 Y

HCCCN 5 Y

HCCNC 5 Y

CH2CN 5 Y

H2CCS 5 N

HCSCN 5 N

CH3NC 6 Y

CH3SH 6 Y

H2CCNH 6 Y

CH2CHCN 7 Y

CH2CHNC 7 N

HSCH2CN 7 N

CH3CCH 7 Y

CH3CCCN 8 Y

H2CCCHCN 8 Y

CH3CH2CN 9 Y

Dehydrogenation

∙CH3 + ∙CH2CN CH3CH2CN -421 kJ/mol

CH3CH2CN CH2CHCN + H2 +163 kJ/molCH2CHCN HCCCN + H2 +207 kJ/mol-----------------------------------------------------------------------CH3CH2CN HCCCN + 2H2 +370 kJ/mol

The energy released by the radical combination reaction could potentially be enough to sequentially dehydrogenate.

New Lab Detections Predicted From This Chemistry

SH + CH2CN HSCH2CN -313 kJ/molHSCH2CN HCSCN + H2 +135 kJ/mol

MP2/6-31+G(d,p)

HCSCN Spectral ParametersEXP B3LYP/6-31G

A (MHz) 42909.959(15) 42118.002

B (MHz) 3195.3928(37) 3081.115

C (MHz) 2970.1222(37) 2871.109

ΔJ (kHz) 1.145(30) 1.067

ΔJK (kHz) -106.23(47) -86.3931

δJ (kHz) 0.216(31) .184

1.5Xaa (MHz) -5.291(51)

0.25(Xbb-Xcc) (MHz) -0.483(41)

22 lines17 kHz

M. Bogey et al. J. Am. Chem. Soc., 111, (1989), 7399-7402.

Previous room temperature mm-wave study

ISM Analysis from PRIMOSMolecule

Column Density (cm-2)

Rotational Temperature (K)

HCCCN ~6*1013 6*

CH2CHCN ~10*1013 3.4*

CH3CH2CN ~2*1014 7.6*

CH2CN ~1015 3.2†

Numerous product species present from our experiment, including CH2CHCN, CH3CH2CN, HC3N and CH2CN.

Common spectral features (absorption in the 18 GHz range) and evidence for a low-temperature velocity component (64 km/s and 82 km/s, 73 km/s warmer).

Cold rotational temperatures do not suggest thermal desorption from grains.

If this chemistry is occurring in Sgr B2(N), we expect that product molecules will be co-spatial and rotationally cold.

* 64 km/s† 68 km/s

Spatial Maps

CH3CH2CN - VLA in DnC around 43.5 GHz toward the Sgr B2(N)Solid contours – emission

Dashed contours - absorptionColor scale - continuum emission around 43.5 GHz

Unusual position for nitrile rich chemistry?

J.M. Hollis et al., ApJ, 596, L235-L238, (2003)

Common Lineshapes

Definitely not the conventional 3 velocity components

(Z)-Ethanimine – CH3CHCNH

R. D. Brown, P. D. Godfrey, and D. A. Winkler. Aust. J. Chem. 33, (1980), 1-7.

(Z)-Ethanimine

(E)-Ethanimine

303-212

Conclusions The current testable hypothesis is that these species may have a

similar formation chemistry in regions toward Sgr B2(N).

Broadband reaction screening of interstellar molecules

Screen for chemical processes

Identified 2 new molecules by rotational spectroscopy (HSCH2CN and HCSCN)

- Not including previously reported HSCN isomers

Because of the discharge bias to synthesize interstellar species, these are potential candidates for interstellar detection, even though they don’t appear in the PRIMOS data

25 molecules detected in discharge, 19 interstellar

Cont.

Emergence of broadband techniques in the lab and interstellar observations

Determine molecules coupled by reaction chemistry

Family of molecules in the lab and the ISM in absorption

Once ALMA/EVLA are on, double screen broadband laboratory data with broadband interstellar data

Shown that this approach is fruitful:• (Z)-ethanimine and (E)-ethanime

Acknowledgments

Centers for Chemical Innovation

Award Number 0847919

Isomerization: Isonitriles and Hydride Shifts

CH2CHCN → CH2CHNC HCCCN → HCCNC

CH3CN ←→ CH2CNH

∆Ea 273 kJ/mol

Energy

∆Ea 74 kJ/mol

↓- H2 219 kJ/mol

J. B. Moffat. J. Phys. Chem. 81, No. 1 (1977), 82-86.X. Yang, S. Maeda, K. Ohno. Chem. Phys. Lett. 418, (2006), 208-216.A. Doughty, G. B. Bacskay, and J. C. Mackle. J. Phys. Chem. 98, (1994), 13546-13555. MP2/6-31+G(d,p)

Rough HSCH2CN Spectral ParametersEXP M062X/6-311++G(d,p)

A (MHz) 22716.3 23133.822

B (MHz) 3104.7952 3105.6788

C (MHz) 2820.8348 2825.770

1.5Xaa (MHz) -4.061

0.25(Xbb-X) (MHz) 1.85

10 lines68 kHz

Fitting 1 state

Pyrolysis

K. D. King and R. D. Goddard. J. Phys. Chem., 82, (1978), 1675-1679.

kT(1000K) = ~8 kJ/mol

Recall: CH3∙ + CH2CN∙ CH3CH2CN yielded 421 kJ/mol

Key intermediate to detect

c

c