Quantum Chemical Cluster Studies of Ice-Bound Reactions of Formaldehyde (H 2 CO), Acetaldehyde (CH 3 CHO), or Acetone (CH 3 COCH 3 ) with Ammonia (NH 3 ) David E. Woon and Dorothy J. Miller FD08

Gas and Ice in Protoplanetary Disks SOURCE: Ciesla, Science 319:1488 (2008). Chemistry occurring on or within icy grain mantles can make important and unique contributions to the composition of cold molecular clouds. Much of the chemistry is organic and thus of great interest to the astrobiological study of the origin of life.

Ice is Observed in the Infrared SOURCE: adapted from Gibb et al., ApJ 536, 347, 2000 high-mass embedded protostar What’s out there? How does it form?

Ancient History (Prior Work) Schutte et al. observed that H 2 CO reacts with NH 3 in ice below 70 K [Science 259:1143 (1993); Icarus 104:118 (1993)]. Quantum chemical calculations indicated that a substantial gas-phase barrier for H 2 CO + NH 3  NH 2 CH 2 OH is largely removed when the reaction occurs in ice [Woon, Icarus 142:550 (1999)]. (H 2 O) n Barrier Height (kcal mol -1 ) Cluster 36 IPCM NH 3 +H 2 CO MP2/6-31+G**

Revisit H 2 CO+NH 3 … 1)B3LYP performs well. 2)Much bigger clusters can be treated. 3) Reliable ice-bound vibrational frequencies can be predicted. Studies on charge transfer reactions demonstrated: HNCO+NH 3 HCOOH+NH 3 OCN - COOH - Recent History (Prior Work)

Objective and Methodology formaldehydeacetaldehydeacetone Characterize ice-bound reactions of carbonyl species with NH 3 …  energetics  vibrational features … using quantum chemical calculations self-consistent reaction field solvent calculations IPCM: single point through 4H 2 O PCM: opt through 4H 2 O, single point for 9 and 12 H 2 O cluster calculations MP2/6-31+G**: up to 4 H 2 O B3LYP/6-31+G**: through 12H 2 O cluster  SCRF single  SCRF opt

4H 2 O: A New Reactant Complex Cluster Only Å Å PCM (reopt) Å Å NH 3 (+0.50) H 2 CO (-0.47)

Mechanism: Concerted Proton Transfer same product: hydroxymethylamine Reaction coordinate Energy

Big Clusters: 9H 2 O and 12 H 2 O Reactant complex is now protonated hydroxymethylamine, which forms with no barrier. NH 3 CH 2 OH (+0.81) OH (-0.80) 9H 2 O NH 3 CH 2 OH (+0.85) OH (-0.78) 12H 2 O The first proton transfer is free!

H 2 CO+NH 3 +nH 2 O: Energetics nH 2 OMethodClusterPCMIPCM 2MP B3LYP MP B3LYP2.9 9B3LYP B3LYP Barrier Height (kcal/mol) A small but significant barrier remains. Does the reaction go to NH 2 CH 2 OH or stop at NH 3 CH 2 OH + ?

IR Spectroscopy of H 2 O:H 2 CO:NH 3 Ice Identify any pronounced vibrational features that could be used for astronomical or laboratory detections Moderately shifted features. The NH 3 “umbrella” mode around 1110 cm -1 [as observed in H 2 O:H 2 CO (100:3) ice by Moore et al., Icarus 190:260 (2007)] is expected to shift in NH 3 CH 2 OH + due to the influence of the new C-N bond. Vanishing or greatly shifted features. The characteristic C=O stretch around 1720 cm -1 [as observed in H 2 O:H 2 CO (100:3) ice by Schutte et al.] is expected to be replaced by a lower frequency CO stretching feature. New features. Possibilities include the C-N stretch, OH stretch, COH bend, other bending or torsional features of NH 3 CH 2 OH + or NH 2 CH 2 OH.

IR Spectroscopy of H 2 O:H 2 CO:NH 3 Ice Identification of observable features can be hindered by mode delocalization and cluster dependent behavior. (raw frequencies scaled by 0.97) CO stretch CN stretch H 2 O OH stretch- NH 3 umbr H 2 O H 2 O reasonable erratic depends on OH - reasonable The CO stretch and NH 3 umbrella modes appear to be good candidates, but the former is weak and the latter falls close to the strong H 2 O bending feature.

Acetaldehyde+NH 3 (work in progress) Does CH 3 CHO+NH 3 have a distinct stereochemistry? CH 3 CHO+NH 3 with 4H 2 O cis TS trans TS  E TS cis: 3.4 kcal/mol B3LYP/6-31+G** w/single-point PCM trans: 2.7 kcal/mol

Acetone+NH 3 (work in progress) Acetone+NH 3 with 9H 2 O cluster: 2.3 kcal/mol PCM: 2.5 kcal/mol

Observations and Conclusions Adding additional water molecules to the model reveals new structural intermediates in XYCO + NH 3 reactions; in the most realistic model, spontaneous C-N bond formation and proton transfer occurs. A small but significant barrier exists even in large clusters. Is the behavior observed in H 2 O:H 2 CO:NH 3 ice due to NH 2 CH 2 OH or to NH 3 CH 2 OH + ? The energetics of the acetaldehyde and acetone reactions with NH 3 are comparable to those of H 2 CO+NH 3. The acetaldehyde+NH 3 reaction may exhibit a preference for one of the stereoproducts. Is biological handedness driven by basic chemistry?

Funding: NASA Exobiology grant NNX 07AN33G. Thom Dunning, Jr. Ibon Alkorta (Instituto de Química Médica): suggested that stereochemistry might be important in the reaction between acetaldehyde and ammonia. Acknowledgments