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With Methylacetylene (CH3CCH; X(1)A1) and D4-Methylacetylene (CD UC Berkeley UC Berkeley Previously Published Works Title A Combined Experimental and Theoretical Study on the Formation of the 2-Methyl-1- silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne Radical (SiH; X(2)Π) with Methylacetylene (CH3CCH; X(1)A1) and D4-Methylacetylene (CD... Permalink https://escholarship.org/uc/item/11x6w8jp Journal The journal of physical chemistry. A, 120(27) ISSN 1089-5639 Authors Yang, Tao Dangi, Beni B Kaiser, Ralf I et al. Publication Date 2016-07-01 DOI 10.1021/acs.jpca.5b12457 Peer reviewed eScholarship.org Powered by the California Digital Library University of California The Journal of Physical Chemistry This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies. A Combined Experimental and Theoretical Study on the Formation of the 2-Methyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne 2 1 Radical (SiH; X Π) with Methylacetylene (CH 3CCH; X A1) and 1 D4-Methylacetylene (CD 3CCD; X A1) Journal: The Journal of Physical Chemistry Manuscript ID jp-2015-12457n.R1 Manuscript Type: Special Issue Article Date Submitted by the Author: 27-Jan-2016 Complete List of Authors: Yang, Tao; University of Hawaii at Manoa, Chemistry Dangi, Beni; University of Hawaii at Manoa, Chemistry Kaiser, Ralf; University of Hawaii at Manoa Bertels, Luke; University of California at Berkeley, Chemistry Head-Gordon, Martin; University of California, Berkeley, Chemistry ACS Paragon Plus Environment Page 1 of 32 The Journal of Physical Chemistry 1 2 3 4 A Combined Experimental and Theoretical Study on the 5 6 Formation of the 2-Methyl-1-silacycloprop-2-enylidene 7 8 Molecule via the Crossed Beam Reactions of the Silylidyne 9 2 1 10 Radical (SiH; X ΠΠΠ) with Methylacetylene (CH3CCH; X A1) 11 1 12 and D4-Methylacetylene (CD3CCD; X A1) 13 14 15 16 17 Tao Yang, Beni B. Dangi, Ralf I. Kaiser* 18 19 Department of Chemistry, University of Hawai'i at Manoa, Honolulu HI 96822 20 21 22 Luke W. Bertels, Martin Head-Gordon* 23 24 Department of Chemistry, University of California, Berkeley, Berkeley CA 94720 25 26 27 28 29 30 Corresponding Authors: 31 32 Professor Dr. Ralf I. Kaiser; Email: [email protected] ; Phone: +1-808-956-5731 33 34 35 Professor Martin Head-Gordon; Email: [email protected] ; Phone: +1-510-642-5957 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 32 1 2 3 Abstract 4 5 6 The bimolecular gas phase reactions of the ground state silylidyne radical (SiH; X 2Π) 7 1 1 8 with methylacetylene (CH3CCH; X A1) and D4-methylacetylene (CD3CCD; X A1) were 9 -1 10 explored at collision energies of 30 kJ mol under single collision conditions exploiting the 11 12 crossed molecular beam technique and complemented by electronic structure calculations. These 13 studies reveal that the reactions follow indirect scattering dynamics, have no entrance barriers, 14 15 and are initiated by the addition of the silylidyne radical to the carbon-carbon triple bond of the 16 17 methylacetylene molecule either to one carbon atom (C1; [i1]/[i2]) or to both carbon atoms 18 19 concurrently (C1-C2; [i3]). The collision complexes [i1]/[i2] eventually isomerize via ring- 20 closure to the c-SiC H doublet radical intermediate [i3], which is identified as the decomposing 21 3 5 22 reaction intermediate. The hydrogen atom is emitted almost perpendicularly to the rotational 23 24 plane of the fragmenting complex resulting in a sideways scattering dynamics with the reaction 25 -1 -1 26 being overall exoergic by -12 ± 11 kJ mol (experimental) and -1 ± 3 kJ mol (computational) to 27 form the cyclic 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC H ; p1). In line with 28 3 4 29 1 computational data, experiments of silylidyne with D4-methylacetylene (CD 3CCD; X A1) depict 30 31 that the hydrogen is emitted solely from the silylidyne moiety, but not from methylacetylene. 32 2 33 The dynamics are compared to those of the related D1-silylidyne (SiD; X Π) – acetylene (HCCH; 34 1 + 35 X ∑g ) reaction studied previously in our group, and from there we discovered that methyl group 36 acts primarily as a spectator in the title reaction. The formation of 2-methyl-1-silacycloprop-2- 37 38 enylidene under single collision conditions via a bimolecular gas phase reaction augments our 39 40 knowledge of hitherto poorly understood silylidyne (SiH; X 2Π) radical reactions with small 41 42 hydrocarbon molecules leading to the synthesis of organosilicon molecules in cold molecular 43 44 clouds and in carbon-rich circumstellar envelopes. 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 ACS Paragon Plus Environment Page 3 of 32 The Journal of Physical Chemistry 1 2 3 1. Introduction 4 5 Over the past few decades, crossed molecular beam experiments have led to an 6 7 unprecedented advancement in our understanding of fundamental principles of chemical 8 9 reactivity and reaction dynamics. Detailed experimental studies of three-atom systems such as 10 1-2 3 4 5-6 11 bimolecular collisions of chlorine (Cl), fluorine (F), deuterium (D), carbon (C), nitrogen 12 7-10 11-12 13-14 (N), oxygen (O), and sulfur (S) with molecular hydrogen (H 2) established 13 14 experimental benchmarks. The crossed beam approach has been successfully extended to four- 15 15-17 16, 18-19 20 21 16 atom [OH/CO, OH/H 2, CN/ H2 ], five-atom [C/C 2H2 ], and even six-atom systems 17 22-23 24-26 18 [Cl/CH 4, F/CD 4 ] bridging our theoretical understanding of reactive scattering dynamics 19 on chemically accurate potential energy surfaces with experimental observations.27 With the 20 21 development of powerful theoretical models, attention has turned during the last years to more 22 23 complex systems of fundamental applications in catalysis, combustion processes, and interstellar 24 25 chemistry along with planetary atmospheres. This holds in particular for bimolecular reactions 26 between diatomic radicals and small hydrocarbons. Bimolecular reactions involving diatomic 27 28 radicals such as boron monoxide (BO), 28-35 boron monosulfide (BS), 36-37 methylidyne 29 38-45 32, 46-60 57, 61-74 75-86 32 30 (CH/CD), cyano (CN), dicarbon (C 2), hydroxyl (OH), silicon nitride (SiN) 31 87 32 and silylidyne (SiH/SiD), synthesized important transient species in extreme environments 33 34 ranging from low temperature molecular clouds to high temperature combustion settings, 35 interstellar, and chemical vapor deposition environments (Table 1). 36 37 38 39 Among these diatomic radicals, reactions of the silylidyne radical (SiH; X2Π) with hydro- 40 41 carbons have received particular attention as these bimolecular reactions are expected to result in 42 43 the formation of small organosilicon molecules (SiCxHy, x≤6, y≤6). These silicon-carbon- 44 bearing molecules are of essential interest to the astrochemistry community since those species 45 46 are suggested to comprise nearly 10% of all molecules by mass that have been identified in 47 88 48 interstellar and circumstellar environments. However, the underlying reaction pathways, how 49 50 organosilicon molecules are formed, are unknown to date. Proposed to be a potential key source 51 of refractory material injected into the interstellar medium (ISM), 89 carbon-rich Asymptotic 52 53 Giant Branch (AGB) stars such as the infrared carbon star IRC+10216 are ideal natural 54 55 laboratories to test astrochemical reaction networks synthesizing organosilicon molecules in 56 90,91 57 extreme environments (Figure 1). Nevertheless, until now, bimolecular ion-molecule 58 59 60 3 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 4 of 32 1 2 3 reactions, photochemical processing of circumstellar grains, and reactions on grain surfaces 4 5 cannot account for the observed fractional abundances of key silicon species such as silicon 6 92 7 carbide (SiC) and silicon dicarbide (c-SiC 2). These discrepancies are the effect of insufficient 8 9 laboratory data such as rate constants and reaction products in particular from reactions 10 90, 92-94 11 involving two neutral species. Therefore, it is crucial to systematically explore the 12 chemical dynamics of neutral-neutral reactions involving silicon-bearing reaction partners such 13 14 as the silylidyne radical (SiH; X2Π) with hydrocarbon molecules leading to the formation of 15 95-98 16 simple organosilicon molecules (SiC xHy, x≤6, y≤6) under single collision conditions. 17 18 19 These organosilicon molecules have also attracted great interest from the physical organic 20 chemistry community due to the distinct chemical bonding of silicon versus its isovalent carbon 21 22 counterpart.99-103 Studies utilizing the crossed molecular beam approach involving isovalent 23 24 cyano (CN) and silicon nitride (SiN) radicals with acetylene (C 2H2) and ethylene (C 2H4) under 25 26 single collision conditions revealed the formation of molecules that are distinct in their molecular 27 structures: nitriles (HCCCN, cyanoacetylene; C H CN, vinyl cyanide) 32 and silaisocyano 28 2 3 29 32 products (HCCNSi, silaisocyanoacetylene; C2H3NSi, silaisocyanoethylene) (Figure 2). Further, 30 31 crossed beam reactions of methylidyne (CH) and D1-silylidyne (SiD) with acetylene lead to 32 33 HCCCH (propargylene)/H 2CCC (vinylidene carbene) and to a minor amount to c-C3H2 (cyclo- 34 38 35 propenylidene) formation [methylidyne reaction], whereas in the D1-silylidyne – acetylene 36 system, the cyclic isomer c-SiC 2H2 (silacyclopropenylidene) was formed exclusively (Figure 37 38 2).
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