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

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 –

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has been a frequent theme of our studies:  Acid-base chemistry: HCN+NH 3 +nH 2 O  (CN – )(NH 4 + )+nH 2 O (Woon, unpublished)  Cation-ice reactions: (HCO + )+ice  HCOOH+(H 3 O + )+ice (Woon, Astrophys.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)

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?

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 –

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.

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** Bond lengths (Å) in black; Mulliken populations in red HCO OH –

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 –

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 H 2 COHH 2 CO – OH –

B3LYP/6-31+G** Structure of (H 2 CO-12H 2 O) – 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 –

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 – ?

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)

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

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)

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.

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.

Funding: NASA Exobiology grant NNX 07AN33G Acknowledgment