Quantum Chemical Study of the Reaction of C+ with Interstellar Ice: Vibrational and Electronic Spectra of Reaction Products David E. Woon RI15

Low-Energy Deposition of Cations on Ice Quantum chemical calculations demonstrate that some cations will undergo barrierless reactions when deposited on icy grain mantles even at ambient cloud or disk temperatures (~15 K). E.g., HCO+ + 2H2O (ice)  HCOOH + H3O+ (ice) CH3+ + 2H2O (ice)  CH3OH + H3O+ (ice) OH+ + CO + H2O (ice)  CO2 + H3O+ (ice)

ALMA image of CO snow line Interstellar Ice Volatile molecules can accrete onto refractory solids (mineral and carbonaceous dust) at cold interstellar temperatures and beyond the snow line in protostellar nebulae. SOURCE: ESO press release 7/18/2013, http://www.eso.org/public/news/eso1333 Artist conception of snow line in the TW Hydrae disk ALMA image of CO snow line Qi et al., Science 341, 630, 2013.

Interstellar Ice Ice formed at interstellar cloud temperatures (<100K) is amorphous, not crystalline, and is microporous. Some compaction occurs, however, even at very low temperatures. Bulk water molecules are generally coordinated via hydrogen bonds to four other water molecules, two as proton donors and two as proton acceptors. Surface waters can have dangling H’s and/or dangling O’s (or more appropriately, dangling O lone pairs).

Why Study C+ Reactions in Ice? In diffuse clouds, carbon is predominantly ionized to C+ and is an important player in ion-neutral reactions. C+ is used a tracer (for H2) of star formation and has been detected in some protoplanetary disks. In the form of cosmic rays, C+ can penetrate deep into quiescent dark clouds. C+ reactions with molecules in icy grain mantles have received very little attention to date.

A Bump in the Road Another group has recently published the results of their study of the C+ + H2O reaction in ice: JPCA 118, 6991 (2014)

A Bump in the Road Similarities between the present work and MMK:  Clusters treated with density functional theory in both.  HOC, CO–, and HCO are observed products of the reaction of C+ with H2O in ice (at least transiently). (MMK saw other products as well, including CO.) Differences:  Optimizations at 0 K (present work) vs dynamics at finite deposition energies (MMK)  CO– is stabilized in the present work, but not in MMK.  The present work predicts both vibrational and electronic spectra and uses them to offer means to determine if reactions have occurred and what products are produced.

4 keV can dissociate 19H2O twenty times over. Experimental Studies of C+ + H2O in Ice 4 keV can dissociate 19H2O twenty times over.

Water Benchmarks: De and Long Range Behavior CCSD(T)/AVTZ overshoots due to basis set super-position error (BSSE). Pople sets (6-311+G**, etc.) vastly overestimate De due to large BSSE. B3LYP/MVDZ does quite well (albeit because of canceling errors). MVDZ: AVDZ on O, VDZ on H (Tschumper JCP 116, 690, 2009)

Water Benchmarks: Vibrational Spectrum Functional-dependent scaling factors were determined using experimental data for the H2O bending mode near 1600 cm-1 in H2O, 2H2O, and 3H2O. For B3LYP/MVDZ, the scaling factor is 0.9824. The computed IR spectra of nH2O clusters can be compared against an experimental spectrum. 6 clusters were optimized: 16H2O (2), 17H2O, 18H2O (2), and 19H2O.  Summing the spectra of the six cases provides an “average” spectrum: common features are enhanced, outliers are smoothed away.

B3LYP/ MVDZ 16H2O(1) cluster Not much dependence on the functional is observed.

Calculated (scaled harmonic) Water Benchmarks: Vibrational Spectrum Calculated (scaled harmonic) (a) crys ice at 12K; (b) amorph ice at 12K after annealing at 130K; (c) 100K aerosol of 3 nm H2O clusters; (d) microporous amorph ice at 12K Devlin, JPCA 105, 974, 2001 a b c d B3LYP/MVDZ open region

Water Benchmarks: Electronic Spectrum Excited state spectra were computed with time-dependent DFT (TDDFT) for the six clusters. 300 excited states were computed for each cluster, yielding maximum excitation energies over 10 eV.

B3LYP MVDZ The electronic spectrum of ice is much more sensitive to the functional used.

Water Benchmarks: Electronic Spectrum Kobayashi, JPC 87, 4317, 1985 A larger discrepancy is found for the prediction of the electronic spectrum of amorphous ice than for its vibrational spectrum, but it is qualitatively accurate and finds a threshold for excitation.

B3LYP/MVTZ / RCCSD(T)/AVTZ) Gas-Phase H2O + C+ Reaction HOC + H+ +36 H2O + C+ H2OC+ 0 / 0 -94 / -84 B3LYP/MVTZ / RCCSD(T)/AVTZ) (kcal/mol) eTS1 HOC+ + H -73 / -56 -86 / -81 iTS1 trans-HCOH+ -66 / -54 -147 / -136

Starting structure(s) Reaction of C+ with Small nH2O (n=2–4) Clusters B3LYP / RCCSD(T) (kcal/mol) Starting structure(s) Product -140 / -122 HOC–H3O+ n = 2 case (i) case (ii) n = 3 -173 / -151 HOC–H3O+–H2O n = 4 -176 HOC–H3O+–2H2O

C+ Reaction with H2O in nH2O Clusters Two principal outcomes were observed: HOC–H3O+–(n-2)H2O CO––2H3O+–(n-3)H2O CO– is bound in ice, and there is additional stabilization from ion-H2O interactions.

C+ Reaction with H2O in nH2O Clusters CLUSTER CASE SITE TYPE PRODUCTS 16H2O(1) 1 ddaa HOC + H3O+ + 14H2O 17H2O(1) 2 da HOC + H3O+ + 15H2O 18H2O(1) 2 dda HOC + H3O+ + 16H2O 18H2O(2) 1 dda HOC + H3O+ + 16H2O 17H2O(1) 1 dda CO– + 2H3O+ + 14H2O 18H2O(1) 1 dda CO– + 2H3O+ + 15H2O 18H2O(2) 2 ddaa CO– + 2H3O+ + 15H2O 18H2O(1) 3 ddaa CO– + 2H3O+ + 15H2O 16H2O(1) 2 dda CO– + 2H3O+ + 13H2O 18H2O(2) 3 dda HCO + H3O+ + 16H2O

C+ Reaction with H2O in nH2O Clusters No correlation is evident between initial site type and product. Are there distinguishing spectroscopic features in the mid IR? HOC/HCO: bend and CO stretch CO–: CO stretch all outcomes generate H3O+

Vibrational Spectra of Various Outcomes HOC–H3O+ outcomes CO––2H3O+ outcomes + : H3O+ mode b : HOC bend s : CO stretch  HOC s,b erratic  CO– s under H2O bend

Assessment of Summed Vibrational Spectra H3O+ features are clearly present, indicating that reactions have occurred. However, no features make it possible to discern HOC from CO–. Perhaps the electronic spectra are more useful.

Electronic Spectra of Various Outcomes HOC–H3O+ outcomes CO––2H3O+ outcomes  HOC: nothing <4 eV  CO–: down to 2 eV

Assessment of Summed Vibrational Spectra HOC and CO– both have excitations below the threshold for amorphous ice. CO– excitations begin at lower energies than HOC excitations – perhaps even in the visible.

Conclusions and Future Work Reaction studies in modestly sized clusters provide insight into ice chemistry relevant to protostellar nebulae and the interstellar medium. Predictions of both vibrational and electronic spectra can be used to assess what products might be discernable in lab studies. There’s room to improve the prediction of electronic spectra with a better choice of functional and/or further developments in TDDFT methodology. Both gas phase and ice surface chemistry of C+ with benzene and PAHs is expected to introduce a number of previously unknown and unstudied compounds.

Thank you! Acknowledgment Funding provided via NASA Exobiology grant NNX 10AR82G. Thank you!

Why Does H2OC+ Readily Lose H+ in Ice? H2OC+ – H2O  HOC – H3O+ The DE for this reaction is mostly due to: H2OC+ – H+  HOC –PA(HOC) = 124.1 kcal/mol H2O + H+  H3O+ PA(H2O) = -171.5 kcal/mol -47.4 kcal/mol