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

 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

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)

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

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

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 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

Experimental approach Use double resonance to extend frequency range ( 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

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

Frequency (MHz) Initial Searches c-SiO 2 H 2 GS 1v71v7 Theoretical Prediction Constant (MHz) 1v71v7 ExperimentTheory AA BB CC Constant (MHz) ExperimentTheoryDifference (%) A B C  amu Å … 90 min

K a =0K a =1 J= E/k (K) E/h (GHz) Offset (kHz) c- 29 SiH 2 O 2 c-SiH 2 O 2 0 = MHz 0 = MHz Doppler hfs FT DR min 1 min

K a =0K a =1 J= E/k (K) E/h (GHz) Offset (kHz) 0 = MHz Doppler FT DR 0 = MHz Doppler

K a =0K a =1 J= E/k (K) E/h (GHz) Offset (kHz) 0 = MHz Doppler Frequency (MHz) Intensity of MHz line FT DR hfs DR spectrum FT spectrum Doppler hfs 0 = MHz Offset (kHz) D

K a =0K a =1 J= E/k (K) E/h (GHz) Offset (kHz) 0 = MHz Doppler FT DR Intensity of MHz line Frequency (MHz)

Structural Determinations r e SE Å Å 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) = Å r(OO) = Å r(SiH) = Å  (HSiH)=113.31°;  (OSIO)=58.74° current best theoretical prediction: (CCSD(T)/cc-pCVQZ): r(Si c O) = Å r(Si t O) = Å  (OSiO) = 99.58° r(H c Si) = Å  (H c SiO) = ° r(H t Si) = Å  (H t SiO) = ° 99.55° Å Å Å Å ° °

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

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 ?

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)

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

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

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