Modeling gas-surface processes. Theory and Experiments for adsorption energies determination of nitrile / isonitrile isomers.

Transcription

1 Modeling gas-surface processes Theory and Experiments for adsorption energies determination of nitrile / isonitrile isomers. Y. Ellinger, A. Markovits, F. Pauzat, M. Doronin (Laboratoire de Chimie Théorique - LCT) M. Bertin, M. Doronin, J.-H. Fillion (Observatoire de Paris LERMA) M16 by NASA

2 Nitriles and Isonitriles Molecules in the ISM (as of 02/2016) Almost 20% of the ~ 200 organics detected have a CN triple bond. 37 species split in 30 R-CN 7 R-NC If R is saturated the list is reduced to CH 3 CN (Solomon et al 1971) CH 3 CH 2 CN (Johnson et al 1977) n-c 3 H 7 CN and i-c 3 H 7 CN (Belloche et al 2009; 2014) CH 3 NC (Cernicharo et al 1988)

3 Abundance ratios as constraints to chemical models HCN/HNC (the most studied) Variations from ~1 in cold molecular clouds to ~n x 1000 in PDRs CH 3 CN/CH 3 NC Variations negligibles ~50 in all situations Why?

4 Possible reasons Observational bias: Radio observations are blind to molecules depleted on the grains Observational fact: The abundance ratios are different for the gas phase and for the depleted species (Dipole moment: µ ~ 4 D for CH 3 CN/CH 3 NC) Possible origin: Selective adsorption between isomers

5 Interstellar surfaces Original work by Greenberg (1976) Silicates: Molster et al (2002) Water ices: Schutte (1999); Gigg et al (2004) Watanabe & Kouchi (2008); Boogert et al (2015) Carbonaceous matter: Leger & Puget (1984); Allamandola et al (1985); Papoular et al (1995) Experiments TPD: Temperature Programmed Desorption Numerical simulations First principle periodic DFT calculations M. Lattelais, M. Bertin, et al. A&A, 532, A12 (2011) M. Bertin, M. Doronin, et al; A&A, 598, A18 (2017) M. Bertin, M. Doronin, et al; A&A, in press (2017)

6 TPD: Temperature Programmed Desorption (ices) Mass spectrometer RAIRS RAIRS H 2 O ice Polycrystalline Au T = 80 K T 1 K.min - 1 Adsorption/desorption behaviours of each isomer Estimation of adsorption energy in the single molecule limit Measure of the ability to bind with water surface via hydrogen bonding

7 Experimental check è adsorption (ices) Vola%les : Below 100 K (H 2, CO, CH 4, CO 2 ) E Ads ~ 0.1 ev Weakly bound : Before H 2 O (HCOOCH 3, CH 3 OCH3...) E Ads ~ 0.3 ev Strongly bound : With H 2 O (CH 3 COOH, CH 3 CH 2 OH...). Adapted from M. Lattelais, M. Bertin, et al. A&A, 532, A12 (2011) E Ads ~ 0.6 ev

8 Numerical simulation è adsorption (ices) E (kcal/mol) HOCH 2 CHO 27 [CH 3 COOH] [HCOOCH 3 ] 17 0 HCOOCH 3 CH 3 COOH = 0,01 à 0,1 [CH 3 COOH] [HOCH 2 CHO] = 0,05 E CH3COOH = 17,2 kcal/mol E HOCH2CHO = 11,6 kcal/mol E HCOOCH3 = 7,3 kcal/mol. M. Lattelais, M. Bertin, et al. A&A, 532, A12 (2011)

9 Theoretical approaches for weak interactions with systems of large dimensions Ab initio / DFT molecule molecule ρ 1 ρ molecule support support ρ 2 support «supermolecule» approach: è BSSE artifact Tran et al. J. Phys. Chem. B (2002); Heine et al. PCCP (2004) KSCED approach Tran et al. J. Phys. Chem. B (2002) Periodic approach: plane waves

10 Adsorption on graphite Position of R-CN Orientation of RCN L: middle of a bond X X: to plane C: carbon atom O: center of the ring L O C Z Y Y: to plane Z: to plane

11 Periodic approach Vertical replication Vertical replication vacuum

12 Adsorption on graphene: vertical dimension of the cell vertical Dimension (Ǻ) ΔE (kcal/mol) Test of the adsorption energy of a dipolar system: HCN

13 Nitrile versus Isonitrile on perfect graphite CH 3 CN CH 3 NC E ads (Th) E ads (Exp) = 275 mev (30 mev) 440 ± 25 mev E ads (Th) E ads (Exp) = 255 mev (30 mev) 430 ± 25 mev No significant difference between nitrile and isonitrile on perfect HOPG M. Bertin, M. Doronin, et al; A&A, in press (2017)

14 Nitrile versus Isonitrile on damaged graphite CH 3 CN CH 3 NC E ads (Th) E ads (Exp) = 310 mev (40 mev) 440 ± 25 mev E ads (Th) E ads (Exp) = 300 mev (40 mev) 430 ± 25 mev No significant difference between nitrile and isonitrile on HOPG with 2C hole defect M. Bertin, M. Doronin, et al; A&A, in press (2017)

15 Nitrile versus Isonitrile on damaged graphite CH 3 CN CH 3 NC E ads (Th) E ads (Exp) = 390 mev (50 mev) 440 ± 25 mev E ads (Th) E ads (Exp) = 350 mev (50 mev) 430 ± 15 mev No significant difference between nitrile and isonitrile on HOPG with 2C hole hydrogenated defect M. Bertin, M. Doronin, et al; A&A, in press (2017)

16 Nitrile versus Isonitrile on damaged graphite CH3CN Eads (Th) = 480 mev (0 mev) Eads (Exp) 440 ± 25 mev CH3NC Eads (Th) = 440 mev (0 mev) Eads (Exp) 430 ± 25 mev No significant difference between nitrile and isonitrile on HOPG with hydrogenated step defect M. Bertin, M. Doronin, et al; A&A, in press (2017)

17 Nitrile versus Isonitrile on water ice CH 3 CN CH 3 NC E ads (The) E ads (Exp) = 558 mev 565 ± 25 mev E ads (The) E ads (Exp) = 545 mev 540 ± 15 mev No significant difference between nitrile and isonitrile Similar results on silicates M. Bertin, M. Doronin, et al; A&A, 598, A18 (2017)

18 What to conclude Theory Eads (mev) Perfect HOPG C hole CH hole q No significant difference between nitrile and isonitrile whatever substract and defects (within ~10%) q Systemetic shift of E ads towards lower values for CH 3 NC q CH 3 CN/CH 3 NC not affected by environment TPD Eads (mev) CH3CN CH3NC CH3CN CH3NC Step-H ± ± 25 α-quartz (0001) ± ± 25 Water ice (Ih) ± ± 15

19 The End

20 Adsorption of COMs and simulations Bertin, M., Doronin, M., Fillion, F.-H., Michaut, X.,Philippe, L., Lattelais, M., Markovits, A., Pauzat, F.,Ellinger, Y. & Guillemin, J.-C. 2017, A&A, 598, A18 and 2017, A&A, Part II in press Doronin, M., Bertin, M., Michaut, X., Philippe, P., &Fillion, J.-H. 2015, J. Chem. Phys., 143, Lattelais, M., Pauzat, F., Ellinger, Y., & Ceccarelli, C. 2015, A&A, 578, A62 Pauzat, F., Marloie, G., Markovits, A., & Ellinger, Y., 2015, Int.,J. Astro., 14, 563 Lattelais, M., Bertin, M., Mokrane, H., et al. 2011, A&A, 532, A12 Pauzat, F., Lattelais, M., Ellinger, Y., & Minot, C. 2011, MNRAS, 412, 2729