Quantum Chemical Cluster Studies of Ice-Bound

Reactions ofFormaldehyde (H2CO),

Acetaldehyde (CH3CHO), or Acetone (CH3COCH3) with

Ammonia (NH3)

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 H2CO reacts with NH3 in ice below 70 K [Science 259:1143 (1993); Icarus 104:118 (1993)].Quantum chemical calculations indicated that a substantial gas-phase barrier for H2CO + NH3 NH2CH2OH is largely removed when the reaction occurs in ice [Woon, Icarus 142:550 (1999)].

(H2O)n

Barr

ier

Heig

ht

(kcal m

ol-

1)

0 1 20

4

8

12

16

20

24

28

32

Cluster

36

IPCM

NH3+H2CO

MP2/6-31+G**

Revisit H2CO+NH3…

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+NH3

HCOOH+NH3

OCN-

COOH-

Recent History (Prior Work)

Objective and Methodology

formaldehyde

acetaldehyde

acetone

Characterize ice-bound reactions of carbonyl species with NH3…

energetics

vibrational features

… using quantum chemical calculations

self-consistent reaction field solvent calculationsIPCM: single point through 4H2O

PCM: opt through 4H2O, single point for 9 and 12 H2O

cluster calculationsMP2/6-31+G**: up to 4 H2O

B3LYP/6-31+G**: through 12H2O

cluster

SCRFsingle

SCRFopt

4H2O: A New Reactant Complex

Clu

ste

r O

nly

2.542 Å 1.587 Å

PC

M

(reop

t)

1.554 Å1.576 Å

NH3

(+0.50)

H2CO (-0.47)

Mechanism: Concerted Proton Transfer

same product:hydroxymethylamine

Reaction coordinate

En

erg

y

Big Clusters: 9H2O and 12 H2O

Reactant complex is now protonated hydroxymethylamine, which forms with no barrier.NH3CH2OH (+0.81)OH (-0.80)

9H2O

NH3CH2OH (+0.85)OH (-0.78) 12H2O

The first proton

transfer is free!

H2CO+NH3+nH2O: Energetics

nH2O Method Cluster PCM IPCM

2 MP2 10.0 -0.8 3.12 B3LYP 7.1 -0.3 -0.1

4 MP2 3.6 3.7 3.94 B3LYP 2.9 2.9

9 B3LYP 1.9 2.6

12 B3LYP 5.1 2.6

Barrier Height (kcal/mol)

A small but significant barrier remains. Does the reaction go to NH2CH2OH or stop at NH3CH2OH+?

IR Spectroscopy of H2O:H2CO:NH3 Ice

Identify any pronounced vibrational features that could be used for astronomical or laboratory detections

Moderately shifted features. The NH3 “umbrella” mode around 1110 cm-1 [as observed in H2O:H2CO (100:3) ice by Moore et al., Icarus 190:260 (2007)] is expected to shift in NH3CH2OH+ 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 H2O:H2CO (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 NH3CH2OH+ or NH2CH2OH.

IR Spectroscopy of H2O:H2CO:NH3 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

1238

657

4H2O

OH stretch

-

NH3 umbr 1500

1050

856

9H2O

2268

1583

1040

900

12H2O

3100

1590

reasonable

erratic

depends on OH-

reasonable

The CO stretch and NH3 umbrella modes appear to be good candidates, but the former is weak and the latter falls close to the strong H2O bending feature.

Acetaldehyde+NH3 (work in progress)

Does CH3CHO+NH3

have a distinct stereochemist

ry?

CH3CHO+NH3 with 4H2O

cis TS

trans TS

ETS

cis: 3.4 kcal/mol

B3LYP/6-31+G** w/single-point

PCM

trans: 2.7 kcal/mol

Acetone+NH3 (work in progress)

Acetone+NH3 with 9H2O

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 + NH3 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 H2O:H2CO:NH3 ice due to NH2CH2OH or to NH3CH2OH+?

• The energetics of the acetaldehyde and acetone reactions with NH3 are comparable to those of H2CO+NH3.

• The acetaldehyde+NH3 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.

www.astrochymist.orgwww.astrochymist.org

Acknowledgments