1. A Joint Theoretical and Experimental Study of the SiO 2 H 2 Isomeric System Michael C. McCarthy Harvard-Smithsonian Center for Astrophysics Jürgen Gauss Institut für Physikalische Chemie, Universität Mainz Talk TD01 ISMS, 70 th Meeting, June 2015 Champaign-Urbana, Illinois

2.  Systematic comparison to well-studied CO 2 H 2 isomers (e.g., formic acid, dioxirane, Criegee, etc.)  SiH 2 + O 2 reaction important in both silicon hydride oxidation processes and SiH 4 –O 2 explosions (!)  Interest in using dioxasiliranes (R 1 R 2 SiO 2 ) as oxidants, i.e. for O 2 activation Xiong et al. Nat. Chem., 2, 577 (2010)  May be intermediate in silicate formation Motivation

3. Previous theoretical & experimental work The SiH 2 + O 2 reaction has been the subject of at least three different direct rate studies; rate is ~10 9 s -1 Chu et al. (1998), Guo et al. (2003), and Becerra et al. (2007) Bottleneck for the overall process probably is 3 H 2 SiOO  1 H 2 SiOO; lack of pressure dependence suggests that secondary reaction barriers are low Formation of lowest energy product pair (SiO + H 2 O) is thought to be the main reaction channel, but no product distributions have been measured Two calculations of potential energy surface; characterized by a complicated set of pathways Nagase et al. (1989), Becerra et al. (2007)

4. Potential energy surface Becerra et al. PCCP, 7, 2900 (2007); enthalpies calculated at G3 level

5. Key findings of our work Detection of three SiO 2 H 2 isomers using rotational spectroscopy, guided by new CCSD(T) calculations Extensive isotopic spectroscopy undertaken for two of these, yielding: -Precise molecular structures, in combination with theory - Insight into formation pathways. Appears to be distinctly different for c-SiO 2 H 2 and HOSiOH, with evidence both for ‘prompt’ and secondary reactions

6. HOSiOH (3) Relative Stability of Isomers CO 2 H 2 SiO 2 H 2 HSi(O)OH (2) c-SiO 2 H 2 HC(O)OH (2) HOCOH (3) c-CO 2 H 2 H 2 COO H 2 SiOO thought to undergo facile ring closure CCSD(T)/cc-pCVQZ 100 200 300 400 500 kJ/mol 0 kJ/mol HEAT-345(Q) Lam et al. JPC A (2015) 0 kJ/mol 6.0 kJ/mol 10.3 kJ/mol

7. Experimental approach Use double resonance to extend frequency range (5- 200 GHz+) Cavity FTWM (5-43 GHz) + pin-hole nozzle + electrical discharge Combination: rapid formation and stabilization of rotationally cold molecules in multiple minima on PES; T vib may be much higher

8. Search Strategy Start with SiH 4 + O 2 discharge; well-known method to produce copious amounts of SiO CCSD(T)/cc-pCVQZ calculation augmented by vibrational corrections of relevant spectroscopic parameters Double resonance to link transitions and extend frequency range of measurements Use SiD 4 and 16 O 18 O to confirm atom connectivity, for structural determinations, and to investigate formation pathway

9. Frequency (MHz) Initial Searches c-SiO 2 H 2 GS 1v71v7 Theoretical Prediction Constant (MHz) 1v71v7 ExperimentTheory AA 94.889 BB -6.5-6 CC 31.232 Constant (MHz) ExperimentTheoryDifference (%) A2111021164 0.26 B1322013176-0.33 C89588980 0.25  amu Å 2 -5.969-5.957… 90 min

10. K a =0K a =1 J=0 1 2 1 3 2 1 0 E/k (K) 25 125 100 50 0 E/h (GHz) Offset (kHz) c- 29 SiH 2 O 2 c-SiH 2 O 2 0 =21955.3 MHz 0 =22202.8 MHz Doppler hfs 3 2 3 4 5 6 75 4 4 FT DR 7 8 150 175 10 min 1 min

11. K a =0K a =1 J=0 1 2 1 3 2 1 0 E/k (K) 25 125 100 50 0 E/h (GHz) Offset (kHz) 0 =15579.2 MHz Doppler 3 2 3 4 5 6 75 4 FT DR 0 =33761.9 MHz Doppler

12. K a =0K a =1 J=0 1 2 1 3 2 1 0 E/k (K) 25 125 100 50 0 E/h (GHz) Offset (kHz) 0 =15131.8 MHz Doppler 3 2 3 4 5 6 75 Frequency (MHz) Intensity of 15131 MHz line FT DR hfs DR spectrum FT spectrum Doppler hfs 0 =31066.8 MHz Offset (kHz) D

13. K a =0K a =1 J=0 1 2 1 3 2 1 0 E/k (K) 25 125 100 50 0 E/h (GHz) Offset (kHz) 0 =14126.5 MHz Doppler 3 2 3 4 5 6 75 4 FT DR Intensity of 14126 MHz line Frequency (MHz)

14. Structural Determinations r e SE 1.6373 Å 1.4630 Å 1.6115 Å  (HSiH)=113.30°  (OSIO)=58.96° current best theoretical prediction: (CCSD(T)/basis-set limit plus core correlation, CCSDT and CCSDTQ corrections): r(SiO) = 1.63776 Å r(OO) = 1.60657 Å r(SiH) = 1.46482 Å  (HSiH)=113.31°;  (OSIO)=58.74° current best theoretical prediction: (CCSD(T)/cc-pCVQZ): r(Si c O) = 1.63661 Å r(Si t O) = 1.65614 Å  (OSiO) = 99.58° r(H c Si) = 0.96107 Å  (H c SiO) = 117.31° r(H t Si) = 0.95804 Å  (H t SiO) = 115.84° 99.55° 0.9571 Å 0.9604 Å 1.654 Å 1.635 Å 117.71° 116.35°

15. Relative abundance of isomers Unsuccessful searches for both SiH 3 OO radical and 1 H 2 SiOO; in contrast, both C-analogs are readily observed

16. Formation pathway: D labeling c-SiO 2 H 2 c-SiO 2 HDc-SiO 2 D 2 HOSiODDOSiOHDOSiOD(H 2 O) 2 !HOSiOH SiD 4 + O 2  c –SiO 2 D 2 ? SiD 4 + O 2  DOSiOD ?

17. Formation pathway: 18 O labeling c-Si 18 O 2 H 2 c-Si 18 O 16 OH 2 c-SiO 2 H 2 HOSi 18 OHH 18 OSiOHHOSiOH(H 2 O) 2 ! SiH 4 + ~50% random 18 O-O 2  H 18 OSi 18 OH ? -2%+2% Frequency (MHz)

18. Prompt versus secondary formation Becerra et al. PCCP, 7, 2900 (2007); enthalpies calculated at G3 level prompt secondary low-level contamination

19. Future Work Searches for other isomers : 3 H 2 SiOO, c-HSiOHO, etc. Analogous isotopic spectroscopy starting with CH 4 + O 2 ; Criegee already studied in this manner, but not dioxirane, formic acid, etc. Astronomical searches; SiO + H 2 Oto form HOSiOH is exothermic, with small or no barrier

20. Acknowledgments Acknowledgments Lan Cheng Marie-Aline Martin-Drumel Kyle Crabtree Oscar Martinez, Jr. Carl Gottlieb Paul Antonucci Sam Palmer NASA Deutsche Forschungsgemeinschaft