1. Quantum Chemical Studies of Low-Energy Pathways to
Organic Molecules on Interstellar Icy Grain Mantles
David E. Woon & Lina Chen
RF16

2. Outline
We use quantum chemical cluster calculations to characterize organic reactions of astrobiological importance that might occur in interstellar ices. We also predict spectra of products and intermediates.
1. NH3 + Carbonyl Reactions in Ice
2. NH3 + HCN/HNC Proton Transfer Reactions in Ice
3. Ion-Ice Reactions

3. NH3 + Carbonyl Reactions in Ice (L. Chen)
formaldehyde
acetaldehyde
acetone

4. Barrier Height (kcal mol-1)
NH3 + H2CO Reaction
Studies at NASA-Ames observed that H2CO reacts with NH3 in ice below 70 K [Schutte et al., Science 259:1143 (1993); Icarus 104:118 (1993)].
Small cluster quantum chemical calculations suggested 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
Barrier Height (kcal mol-1) 1 2 4 8 12 16 20 24 28 32
Cluster 36
IPCM
NH3+H2CO

5. NH3 + Carbonyl Reactions
Objective: Revisit the NH3+H2CO reaction in larger clusters (up to 12H2O) and also study acetaldehyde and acetone.
 B3LYP/6-31+G** calculations (G03)
Concerted proton transfer mechanism is NOT favorable! But an alternative pathway is very favorable.

6. NH3 + Carbonyl Reactions – C-N Bond Formation
In 4H2O and larger clusters, NH3 undergoes nucleophilic addition to formaldehyde with no barrier:
2.542 Å
1.587 Å
NH3 (+0.50)
H2CO (-0.47)

7. NH3 + Carbonyl Reactions – C-N Bond Formation
Acetaldehyde and acetone are just as reactive. Both stereo-isomers of acetaldehyde can be formed (in equal amounts).
CH3CHO
S-isomer,
type a & b TS’s
(CH3)2CO TS

8. NH3 + Carbonyl Reactions – Nucleophilic Addition
NH3 lone pair
H2CO p bond
Re(CN)+1.0 Å

9. … delocalizes toward C …
NH3 + Carbonyl Reactions – Nucleophilic Addition
… delocalizes toward C …
… localizes onto O …
Re(CN)+0.5 Å

10. NH3 + Carbonyl Reactions – Nucleophilic Addition
CN s bond
2p2 pair on O (O–)
Re(CN)

11. NH3 + Carbonyl Reactions – Nucleophilic Addition
CN s bond
2p2 pair on O (O–)

12. NH3 + Carbonyl Reactions – Larger Clusters
With 9H2O, a proton transfers without a barrier to yield protonated hydroxymethylamine + OH–.
9H2O
NH3CH2OH+ (+0.81)
OH– (-0.80)
NH3CH2OH+ (+0.85)
OH– (-0.78)
12H2O

13. Barrier Height (kcal/mol)
NH3 + Carbonyl Reactions – Barrier Heights
While the C-N bond forms spontaneously, a significant barrier persists to the large clusters for concerted proton transfer.
Cluster
All of the barriers for concerted proton transfer are prohibitively high at low T.
Barrier Height (kcal/mol)
PCM (Ice)

14. NH3 + Carbonyl Reactions – Alternative Mechanism
Hydroxymethylamine and the analogous products of the reactions of NH3 with acetaldehyde and acetone can be formed from the nucleophilic addition intermediate simply by pulling off one of the extra H atoms on the N with a stronger base.
Try calculations with 2NH3…

15. NH3 + Carbonyl Reactions – Alternative Mechanism
For this configuration from acetaldehyde+2NH3+9H2O, a very small equilibrium barrier (0.5 kcal/mol) is removed when ZPE is included.

16. NH3 + Carbonyl Reactions – Spectroscopy
In addition to characteristic features of NH3 and H2CO losing intensity or shifting in frequency, new vibrational features appear on formation of the C-N bond.
B3LYP/6-31+G**
scaled by 0.97
1368
1384
1386
CH2 wag
1482
1478
1477
CH2 scis
1499
1583
1590
NH3 umb
NH3
1100 cm-1

17. NH3 + Carbonyl Reactions – Conclusions
In H2CO:NH3:H2O ice, reactivity is expected to exhibit strong dependence on NH3 concentration:
Nucleophilic addition of NH3 to carbonyls is barrierless.
Deprotonation can yield a neutral product if sufficient strong base molecules are present.
modest
C-N addition
NH3CH2OH+
large
deprotonation
NH2CH2OH
low
Reactions
IR Features
none
H2CO, etc.
[NH3]

18. NH3 + HCN/HNC Proton Transfer Reactions in Ice

19. NH3+HCN/HNC+14H2O
Like other acids (HNCO, HOCN, HCOOH), HCN and HNC transfer a proton to NH3 in ice with no barrier to yield a (CN– )(NH4+) complex.
2098
CN– str
H2O bends
NH4+ bends
2112
B3LYP/6-31+G**
scaled by 0.97

20. NH3+HCN+14H2O – Subsequent Reactions
What happens if another molecule approaches CN– from the gas phase?
CO, H2CO – non-reactive
Try a cation – Mg+
MgNC

21. HCO+ reacts, but not with CN–… Was CN– necessary? Try using pure ice…
NH3+HCN+14H2O – Subsequent Reactions
What happens if another molecule approaches CN– from the gas phase?
CO, H2CO – non-reactive
Try a cation – Mg+
Try another cation – HCO+
HCO+ reacts, but not with CN–…
Was CN– necessary? Try using pure ice…

22. Ion-Ice Reactions

23. Ion-Ice Reactions – HCO+ + H2O(ice)  HCOOH
Even if deposited with very low energy, some ions react with ice with no barrier and then lose a proton to the ice.
HCO+ + 17H2O

24. Protonated formic acid (HCOOH2+) forms…
Ion-Ice Reactions – HCO+ + H2O(ice)  HCOOH
Protonated formic acid (HCOOH2+) forms…

25. … but it rapidly deprotonates…
Ion-Ice Reactions – HCO+ + H2O(ice)  HCOOH
… but it rapidly deprotonates…

26. … to yield formic acid and H3O+.
Ion-Ice Reactions – HCO+ + H2O(ice)  HCOOH
… to yield formic acid and H3O+.

27. Ion-Ice Reactions – Summary and Other Cases
HCO+ reacts with ice to form formic acid with no barriers (but H+ abstraction is competitive).
CH3+ reacts with ice to form methanol with no barriers.
OH+ reacts with CO on ice to form CO2 with no barriers.
Only the beginning…

28. Acknowledgments www.astrochymist.org
Funding: NASA Exobiology grant NNX 07AN33G
Thom Dunning, Jr.

29. Ionization Energy (eV) Electron Affinity (eV)
NH3 + Carbonyl Reactions – C-N Bond Formation
Why does a C-N bond form so readily in ice-bound ammonia-carbonyl systems?
The large amount of charge transfer is a good clue.
NH3+
H2CO–
spin(C)=0.84
spin(N)=1.10
NH3
H2CO
Ionization Energy (eV)
Electron Affinity (eV)
Gas Phase
In Ice
10.07† …
6.84‡
2.23‡
†NIST WebBook ‡B3LYP/6-311+G** with PCM solvation