The rôle of surfaces in prebiotic chemistry. Piero Ugliengo University of Torino Dip. Chimica Via P. Giuria, 7 - Torino

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1 The rôle of surfaces in prebiotic chemistry Piero Ugliengo University of Torino Dip. Chimica Via P. Giuria, 7 - Torino 1

2 A collaborative work Dip. Chimica - NIS Centre Torino University TO BCN Dep. Quimica UAB Barcelona J. Navarro M. Sodupe A. Rimola 2

3 Chemical processes of prebiotic relevance Pillars of Creation Chemistry in ISM (gas, dust) Chemistry in Comets Chemical evolution on primordial Earth Chemistry on primordial atmosphers (Titan) 3

4 Chemical scenarios Omogeneous chemistry Gas-phase reactions H 2 +hn H 2 + H H 2 H H Models of primordial atmospheres Titan atmosphere Heterogeneous chemistry Reactions at the grain surfaces (ice, olivine, silica, PAH, IDP) Reactions at the mineral surfaces (feldspar, clays, FeS 2, etc.) Can computer simulation contribute? 4

5 Grains in molecular clouds Diffuse Clouds Dust Particles + Gaseous Atoms and Molecules N n at 10 2 cm -3 T K OH O CO n at 10 4 cm -3 T K Celestial Valentine W5 star-forming region Portrait from NASA s Spitzer Space Telescope H 2 CO CO 2 NH 3 Dense Clouds Dust Particles + Interstellar Ices Silicates/carbonaceous C H 2 H H 2 O CH 4 CH 3 OH HCN CO CORE CORE+MANTLE 5

6 Current approaches to H 2 on interstellar dust molar fraction Spectroscopic Measurements H Chemical Reactivity Lab. IR Obs. IR core grain HD + D 2 H atoms D atoms H 2 olivine Molster, F. et al., Space Sci. Rev. 2005, 119, 3-28 Perets, H. B. et al., Astrophys. J. Lett. 2007, 661, L163-L166 Numerical Astrochemical Models Rate Equation Models 1 OH H + H H O 2 O + O O 2 0 H 2 H + O OH Log n(h) [cm -3 ] Caselli, P. et al., Astrophys. J. 1998, 495,

7 Relevance of chemical modeling Spectroscopic Measurements Chemical Reactivity Experiments Numerical Astrochemical Models Average Data Lack of Atomic-Scale Information Molecular Simulations Based on QUANTUM MECHANICAL METHODS The Schrödinger Equation Ĥ = E If we can solve this equation we know everything about the systems Spectroscopic Measurements Chemical Reactivity Experiments Quantum Chemistry Numerical Astrochemical Models 7

8 The ab-initio simulation: attacking the SE Schrödinger equation Te + Tn + Ven + Vee + Vnn Hartree-Fock (thousands atoms) Møller-Plesset: (MP2 ~ atoms) CI-SD, CCSD, CCSD(T) (CCSD(T) ~ atoms) Density Functional Methods HK1964 H 2 O: 10 e - and 3 nuclei ( r,, r ; R,, R ) H O ( x, y, z) (x,y,z) is a function of three spatial variables irrespective of the system complexity and contains all relevant ingredients of a given physical system. F is unfortunately unknown. F [] = B-LYP B3-LYP PW91 PBE 2 E = F [ (x,y,z)] 8

9 From molecules to crystals and surfaces Gas-phase processes DFT MP2 CCSDT(T) Locate TS accuracy Gaussian09, ORCA, Gamess Crystals and surfaces Well defined Bloch theorem bulk surfaces Only DFT (B3LYP with CRYSTAL09) Harder to locate TS??? Bonds cuts. How? How to terminate? Reconstruction? VASP, CPMD, QMespresso, CRYSTAL09, CP2K 9

10 Termination at surfaces: ionic, covalent, molecular crystals IONIC COVALENT MOLECULAR YES NO Surfaces exhibiting net dipole across the surface are unstable (yellow lines) Cutting bonds leave unsatisfied dangling bonds which are very reactive Only inter-molecular bonds are cut. No dangling bonds left. Cations at the surface tend to move inwards. The reverse for anions. (surface relaxation) Dangling bonds tend to self-heal by surface reconstruction Large molecular displacements may stabilize polar surfaces 10

11 Quantum mechanical codes for periodic systems Treatment of 0D, 1D, 2D and 3D structures HF, LDA, GGA, HYBRID Gaussian Basis Set Treatment of 3D structures Gaussian and Plane Waves approaches HF, LDA, GGA, HYBRID, MP2, RPA Geometry Optimizations Frequency Calculations Electrostatic Potential Maps Molecular Dynamics 11

12 Questions to be addressed by QM methods Physicochemical properties of the silicates Which are the actual mechanistic steps? What is the effect of metal ions? H H + H H 2 Mg 2+ Fe 2+ Structural Electronic Vibrational Dielectric Mg Si O silicate Mg 2+ : [Ne] silicate Fe 2+ : [Ar] 3d 6 How important is the role of surface morphology? (110) surface Mg O Si silicate (001) surface Mg Si O 12

13 Periodic approach to core grain simulation M1, M2 = Mg, Fe SiO 4 Mg/Fe Forsterite(x=0)/Fayalite(x=2) Mg 2-x Fe x SiO 4 13

14 Reflectance spectrum of crystalline forsterite Mg 2 SiO 4 B3LYP mass adsorption spectrum (red: spherical; black: continuous distribution of ellipsoids) B3LYP reflectance spectra with different damping factors M. De La Pierre et al, J. Comp. Chem., (2011), 32,

15 Crystalline surfaces of forsterite Mg 2 SiO 4 Crystal morphology of Forsterite Mg 2 SiO 4 bulk Mg Si O Surface energies of Mg 2 SiO 4 surfaces Surface γ (J m -2 ) (010) 1.22 (101) 1.78 (001) 1.78 (021) 1.90 (110) 2.18 Bruno, M. et al., J. Phys. Chem. C 2014, 118, (010) (001) (110) 15

16 Relevance of the interstellar H2 ASTROPHYSICAL ASTROCHEMISTRY H H H 2 + c. r. H e + c. r. H H 2 H H The most abundant molecules in ISM An effective coolant for gases and m. clouds Does not form from gas-phase reaction Chemistry of oxygen H O OH + + H 2 OH + + H 2 H 2 O + + H H 2 O + + H 2 H 3 O + + H H 3 O + + e H 2 O + H Chemistry of carbon H C CH + + H 2 CH + + H 2 CH hν CH O HCO + + H 2 HCO + + e CO + H Does form on grains in m. clouds Most abundant molecule in the SOLID-phase 2 nd most abundant molecule in the GAS-phase 16

17 Adsorption of H at the (010) Mg 2 SiO 4 forsterite Geometry optimization: B3LYP-D2*/DZVP Energy refinement: TZVP//DZVP ZPE-corrected energies in kcal/mol BSSE corrected values Si Mg O Navarro-Ruiz, J. et al., Phys. Chem. Chem. Phys. 2014, 16, Phys-010-Mg1 Chem-010-O δ ads U ads U Phys-010-Mg2 Electr. energy Mg1 Mg2 O ads U B3LYP-D2* BLYP-D2* BHLYP-D2* PBE-D2*

18 Diffusion of H at the (010) Mg 2 SiO 4 forsterite ZPE-corrected B3LYP-D2*/TZVP//B3LYP-D2*-DZVP in kcal/mol +H Fo + H TS Mg1 Mg Kerkeni Goumans TS Mg2 O1 Method E(H jump ) B3LYP-D2* 4.1 Exp H 2.54 O 2.73 Mg 010 Mg1 Si Mg O Method E(H phys ) B3LYP-D2* -2.7 Kerkeni -1.4/-5.8 Goumans -2.5 Sidis -3.2 Exp

19 Adsorption of a second H at the (010) Mg 2 SiO 4 forsterite ZPE-corrected B3LYP-D2*/TZVP//B3LYP-D2*-DZVP in kcal/mol Phys-Mg Mg Chem-Mg O Chem-O O δ δ δ δ δ δ + ads U ads U ads U ads U H physisorbed states Spin densities on the H atoms O H + proton and Mg + H hydride character H bond between H and H + O H + proton and Mg + H hydride character 2H chemisorbed states Formation of a surface geminal Si (OH) 2 groups Spin densities on the bare Mg atoms 19

20 L-H formation of H 2 at the (010) Mg 2 SiO 4 forsterite ZPE-corrected B3LYP-D2*/TZVP//B3LYP-D2*-DZVP in kcal/mol Radical-Radical Hydride-Proton Proton-Proton

21 log (k SC-TST / s -1 ) Tunnel for H 2 formation at the (010) Mg 2 SiO 4 forsterite Rate Constants via classical Eyring Equation Tunneling Coefficients via Fermann & Auerbach Correction Final Semi-classical Rate Constant k kbt q h q U exp RT TST 0 REACT U 0 T ) exp kbt 2U exp hn 0 ( 2k BT 1 hn J. Phys. Chem, 2000, 112, 6787 k SCTST ( T ) k TST K 264 K H H coupling H + H - coupling H jump T high 1000/T (K -1 ) T low 21