1. TF15 A Quantum Chemical Investigation of the Stability and Chemistry of the Anions of CO and H 2 CO in Astrophysical Ices (& the Mystery of OH – ) Lina Chen & David E. Woon

2. Outline  Summary of the Role of Ions in Interstellar Ice Chemistry  Electron Interactions with CO and H 2 CO in Ice  The Mystery of OH –

3. Ionic Character Enhances Ice Chemistry Ice chemistry involving ions or strong ionic character has been a frequent theme of our studies:  Acid-base chemistry: HCN+NH 3 +nH 2 O  (CN – )(NH 4 + )+nH 2 O (Woon, unpublished)  Cation-ice reactions: (HCO + )+ice  HCOOH+(H 3 O + )+ice (Woon, Astrophys. J. 728, 44, 2011)  Carbonyl-NH 3 reactions: H 2 CO – /NH 3 + character plays a role in the H 2 CO+NH 3 reaction in ice (Chen & Woon, J. Phys. Chem. A 115, 5166, 2011)

4. Ionic Character Enhances Ice Chemistry Kayi et al. recently published a study on the reaction of CO 2 with CH 3 NH 2 in ice that parallels our work: CO 2 – /CH 3 NH 2 + character plays a role. (Kayi, Kaiser & Head, Phys. Chem. Chem. Phys. 13, 11083, 2011) The presence of H 2 CO – and CO 2 – in these studies raises the question: What happens to electrons in ice?

5. The Origin and Fate of Electrons in Ice A. Sources of electrons:  Endogenous – photoionization (Woon & Park, Astrophys. J. 607, 342, 2004) (Woon, Adv. Space Res. 33, 44, 2004)  Exogenous – deposited as free electrons – deposited as anions (e. g., C 2n H – ) B. Interactions with ice:  Pure ice – electrons interact weakly with water  Ice mixtures – electrons seek sinks at impurity sites: (i) cations: H 3 O + + e –  H 2 O + H, etc. (ii) radicals: OH + e –  OH – (iii) closed-shell: CO 2 + e –  CO 2 –

6. Electron Interactions with CO and H 2 CO in Ice We chose to study CO and H 2 CO because they are common interstellar molecules and vital precursors to prebiotic chemistry and are therefore relevant to astrobiology. Neither CO nor H 2 CO will bind an electron in the gas phase, but as unsaturated species they are promising targets to have bound anions in ice. (CO 2 – is known to be metastable in the gas phase.) Methodology: B3LYP/6-31+G** and MP2/6-31+G** cluster calculations were performed with Gaussian 03/09. Molden was used to generate synthetic IR spectra.

7. Structures of (CO-6H 2 O) – While the e – adds to CO to form CO –, a reaction follows: B3LYP/6-31+G**MP2/6-31+G** 1.1351.192 2.31 1.501 1.613 1.563 1.930 Bond lengths (Å) in black; Mulliken populations in red. 1.118 1.204 2.412 1.637 1.623 1.520 1.993 2.073 0.40 0.13 -0.49 -1.11 -0.97 0.35 0.08 -0.40 -1.13 -0.96 0.17 0.18 HCO OH –

8. Structure of (CO-12H 2 O) – B3LYP/6-31+G** Once CO – forms, it quickly reacts with H 2 O to yield HCO and OH –. This is a potential alternative formation pathway for H 2 CO (instead of hydrogenation, which has a small barrier). HCO OH –

9. Structures of (H 2 CO-6H 2 O) – The e – adds to H 2 CO to form H 2 CO –, but results vary… B3LYP/6-31+G**MP2/6-31+G** Bond lengths (Å) in black; Mulliken populations in red. 1.358 1.006 1.657 1.994 1.371 1.630 1.621 -0.88 0.26 0.583 -1.09 0.18 1.342 1.843 1.432 1.814 0.38 -1.25 -0.97 H 2 COHH 2 CO – OH –

10. B3LYP/6-31+G** Structure of (H 2 CO-12H 2 O) – 0.992 1.373 1.860 1.513 1.575 1.716 1.828 1.515 -0.85 0.18 0.51 -1.07 -0.90 0.23 H 2 COH While H 2 CO – forms initially with both methods, it reacts with H 2 O to form H 2 COH and OH – with B3LYP but does not react with MP2. Further calculations are needed to ascertain which outcome occurs. OH –

11. The Mystery of OH – The hydroxide ion has appeared as a product in a number of our computational studies  carbonyl – NH 3 reactions in ice  CO – in ice  H 2 CO – in ice (perhaps) Also, if water ice is photolyzed to form OH and H, OH is expected to be a good sink to which electrons can bind. To date, however, OH – has not been observed in ice in lab studies (e.g., by M. Gudipati). Where is OH – ?

12. IR Spectra of HCO/H 2 COH – OH – – 5H 2 O Lorentzian lineshape; Scale factor: 0.97; Half-width: 10.0 cm -1. H 2 COH HCO The OH – feature is very weak and is obscured by the stronger OH stretches of H 2 O. Frequency/cm -1 Intensity/(KM/Mole)

13. Resolution Dependence of IR Spectra The OH – feature is distinct at higher resolution, but it remains very weak. Frequency/cm -1 Intensity/(KM/Mole) Low resolution Half-width: 10 cm -1 Frequency/cm -1 Intensity/(KM/Mole) High resolution Half-width: 2 cm -1

14. OH – Feature in Larger Clusters Lorentzian lineshape; Scale factor: 0.97; Half-width 2.0 cm -1. Labeled values reflects the position of (OH – ). H 2 COH HCO OH – / 12H 2 O The OH – feature is very regular, but high resolution is needed to observe it. Frequency/cm -1 Intensity/(KM/Mole)

15. Conclusions  Electrons deposited on ice or liberated in situ via photo- ionization will seek favorable binding sites, including radicals and unsaturated hydrocarbons.  In some cases, chemistry may ensue: CO – reacts with H 2 O to form HCO and OH – H 2 CO – may react with H 2 O to form H 2 COH and OH –  While OH – is expected to be formed in ice through various pathways, the associated IR feature was found to be very weak and would require high resolution spectroscopy to be detected.

16. Further Study  Identify characteristic IR features of anions formed in ice through electron capture by radical or closed-shell neutral species (e.g., H 2 CO – ) to provide a means by which the conclusions of this work can very verified experimentally.  Extend the study to include other unsaturated species with potential astrobiological significance: CO 2, HCN, C 2 H 2, etc.

17. Funding: NASA Exobiology grant NNX 07AN33G Acknowledgment