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

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

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

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

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.

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)

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

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

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

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

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

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

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)

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…

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.

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

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]

NH3 + HCN/HNC Proton Transfer Reactions in Ice

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

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

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…

Ion-Ice Reactions

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

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

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

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

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…

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

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