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Sih; X2Π) with Dimethylacetylene (CH3CCCH3; X1a1g

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Sih; X2Π) with Dimethylacetylene (CH3CCCH3; X1a1g Article pubs.acs.org/JPCA Formation of the 2,3-Dimethyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reaction of the Silylidyne Radical (SiH; X2Π) 1 with Dimethylacetylene (CH3CCCH3;XA1g) Tao Yang, Aaron M. Thomas, Beni B. Dangi, and Ralf I. Kaiser* Department of Chemistry, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, United States Mei-Hung Wu, Bing-Jian Sun, and Agnes H. H. Chang* Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan *S Supporting Information ABSTRACT: We carried out crossed molecular beam experiments and electronic structure calculations to unravel the chemical dynamics of the 2Π reaction of the silylidyne(-d1) radical (SiH/SiD; X ) with dimethyla- 1 cetylene (CH3CCCH3;XA1g). The chemical dynamics were indirect and initiated by the barrierless addition of the silylidyne radical to both carbon atoms of dimethylacetylene forming a cyclic collision complex 2,3-dimethyl-1-silacyclopropenyl. This complex underwent unimolecular decomposition by atomic hydrogen loss from the silicon atom via a loose exit transition state to form the novel 2,3-dimethyl-1-silacycloprop-2- enylidene isomer in an overall exoergic reaction (experimentally: −29 ± 21 kJ mol−1; computationally: −10 ± 8 kJ mol−1). An evaluation of the scattering dynamics of silylidyne with alkynes indicates that in each system, the silylidyne radical adds barrierlessly to one or to both carbon atoms of the acetylene moiety, yielding an acyclic or a cyclic collision complex, which can also be accessed via cyclization of the acyclic structures. The cyclic intermediate portrays the central decomposing complex, which fragments via hydrogen loss almost perpendicularly to the rotational plane of the decomposing complex exclusively from the silylidyne moiety via a loose exit transition state in overall weakly exoergic reaction leading to ((di)methyl-substituted) 1-silacycloprop-2-enylidenes (−1to−13 kJ mol−1 computationally; −12 ± 11 to −29 ± 21 kJ mol−1 experimentally). Most strikingly, the reaction dynamics of the silylidyne radical with alkynes are very different from those of C1−C4 alkanes and C2−C4 alkenes, which do not react with the silylidyne radical at the collision energies under our crossed molecular beam apparatus, due to either excessive entrance barriers to reaction (alkanes) or overall highly endoergic reaction processes (alkenes). Nevertheless, molecules carrying carbon−carbon − double bonds could react, if the carbon carbon double bond is either consecutive like in allene (H2CCCH2) or in conjugation − with another carbon carbon double bond (conjugated dienes) as found, for instance, in 1,3-butadiene (H2CCHCHCH2). 1. INTRODUCTION In the case of ground state atomic carbon, the carbon atom During the past few years, the dimethylacetylene molecule (2- adds to either one or both carbon atoms, yielding triplet collision complexes, which eventually isomerize via ring butyne; CH3CCCH3) has received considerable attention by the reaction dynamics community to investigate sterical effects closure/ring opening to yield the acyclic triplet dimethylpro- of replacing one or both hydrogen atoms by bulky methyl pargylene intermediate (CH3CCCCH3) before ejecting atomic hydrogen from the methyl group forming the 1-methylbuta- group(s) (CH3) in the acetylene (C2H2) molecule, under 1,2 bimolecular reactions with the cyano radical (CN; X2Σ+), trienyl radical (CH3CCCCH2). On the singlet surface, dicarbon − boronyl radical (BO; X2Σ+),3 atomic carbon (C; 3P),4 6 singlet follows an indirect reaction mechanism governed by adding 1Σ + 3Π 7 and triplet dicarbon (C2;X g ,a u), and atomic boron (B; dicarbon to the central acetylene carbons of dimethylacetylene 2P) (Scheme 1).8,9 Considering the reactions with the diatomic followed by multiple isomerizations (ring closure/ring open- cyano and boronyl radicals, the reaction dynamics were dictated ing) to form eventually the 2,4-hexadiyne intermediate by adding doublet radicals with their respective radical center (CH3CCCCCH3), which then ejects atomic hydrogen leading located at the carbon and boron atom, to one of the carbons in the carbon−carbon triple bond followed by loss of atomic Received: July 13, 2016 hydrogen and/or the methyl group. The reaction dynamics Revised: August 26, 2016 with atomic carbon and dicarbon molecule are more complex. Published: August 29, 2016 © 2016 American Chemical Society 7262 DOI: 10.1021/acs.jpca.6b06995 J. Phys. Chem. A 2016, 120, 7262−7268 The Journal of Physical Chemistry A Article 2Π 1 Scheme 1. Reaction Products of Cyano (CN), Boronyl (SiH; X ) with dimethylacetylene (CH3CCCH3;XA1g) and (BO), Carbon (C), Dicarbon (C2), and Boron (B) with combine these investigations with electronic structure calcu- Dimethylacetylene (CH3CCCH3) Studied under Single lations. These studies will then be compared to the related Collision Conditions in a Crossed Molecular Beam Machine silylidyne−acetylene (SiH-HCCH)10 and silylidyne−methyla- − 11 cetylene (SiH CH3CCH) systems studied previously in our group. Finally, we contemplate the reaction dynamics of silylidyne with the corresponding C1−C4 alkanes and C2−C4 12 alkenes, as well as two special cases: allene (H2CCCH2) and 13 1,3-butadiene (H2CCHCHCH2). 2. EXPERIMENT The gas phase reactions of the silylidyne (SiH; X2Π) and 2Π silylidyne-d1 (SiD; X ) radical with dimethylacetylene 1 (CH3CCCH3;XA1g) were investigated respectively in a universal crossed molecular beam machine at the University − of Hawaii at Manoa.14 21 A pulsed beam of silylidyne radicals was prepared by photolysis of disilane (Si2H6; 99.998%; Voltaix) seeded in helium (He; 99.9999%; Gaspro) in a fraction of 0.5% as well as backing pressure of 2 atm.12 This gas mixture was fed into a piezoelectric valve operating at 120 Hz, −400 V amplitude and a pulse width of 80 μs, and the released pulsed beam was intercepted by a 1 mm × 4 mm laser spot from an ArF excimer laser (Compex 110, Coherent, Inc.) of 60 Hz and 30 mJ per pulse, which is located 1 mm downstream from the nozzle. The silylidyne-d1 radical beam was produced in situ by ablating a rotating silicon rod with the output of a Nd:YAG laser (Quanta-Ray Pro 270, Spectra-Physics) operat- ing at 30 Hz and 266 nm with pulse energies of 10−15 mJ and seeding the ablated species in deuterium gas (D2, 99.7% D; Icon Isotopes) released at 4 atm backing pressure by a piezoelectric valve operating at 60 Hz, −400 V amplitude, and pulse width of 80 μs.10 Prior to arriving at the center of interaction region, the silylidyne(-d1) radicals passed through a stainless steel skimmer and a four-slit chopper wheel operating at 120 Hz, and the latter was able to select the pulsed beam fi to the 1-methyl-1,4-pentadiynyl-5 radical (CH3CCCCCH2). with a well-de ned peak velocity (vp) and speed ratio (S)of On the triplet surface, dicarbon molecule adds barrierlessly to 1715 ± 13 m s−1 and 13.1 ± 1.9 for silylidyne, and of 1981 ± −1 ± both carbons concurrently or to one central carbon of 38 m s and 5.2 1.0 for silylidyne-d1, respectively. The dimethylacetylene leading to an intermediate CH3C(CC)- primary radical beam crossed perpendicularly the most intense CCH3, which further isomerizes prior to atomic hydrogen loss, part of a pulsed dimethylacetylene beam (C4H6, Sigma-Aldrich) − yielding the 3-methyl-1,4-pentadiynyl-3 radical (HCCC(CH3)- released by a second pulsed valve operating at 400 V CCH). Atomic boron also exhibits rather unique reaction amplitude, 120 Hz (for silylidyne plus dimethylacetylene)/60 dynamics. The reaction is initiated by the interaction of the Hz (for silylidyne-d1 plus dimethylacetylene), pulse width of 80 boron atom with the π electron density of the dimethylacety- μs at a backing pressure of 550 Torr. Peak velocity and speed lene molecule, which forms a 2,3-dimethyl-1-boracycloprope- ratio for the dimethylacetylene beam were determined to be ± −1 ± nylidenyl radical intermediate (c-BC2(CH3)2). However, rather 780 10 m s and 8.2 0.4, resulting in nominal collision ± −1 ± than undergoing ring opening, this intermediate isomerizes via energies (Ec)of33 1 kJ mol (SiH + C4H6) and 41 2kJ −1 a hydrogen shift from carbon to boron, followed by atomic mol (SiD + C4H6), as well as center-of-mass (CM) angles of hydrogen decomposition from the remaining methyl group. 40.0 ± 0.7° and 35.8 ± 1.1°, respectively. The rotational This process forms the cyclic 1,2-dimethylene-3-boracyclopro- temperature of the silylidyne radical beam (SiH; X2Π) was pane product. These considerations suggest that the reaction determined via laser-induced fluorescence (LIF) technique to dynamics involving dimethylacetylene with the doublet radicals be mainly 40 ± 10 K.12 of cyano and boronyl (addition−elimination) differ strongly In the triply differentially pumped detection system, the from those with carbon/dicarbon (addition−ring closure/ neutral products were first ionized by an 80 eV electron impact opening−atomic hydrogen loss) and in particular from atomic ionizer before filtering with a quadrupole mass spectrometer boron (addition−cyclization−hydrogen migration−atomic hy- (QMS; Extrel QC 150). Selected ions at a specific mass-to- drogen loss). charge ratio (m/z) traveled toward a doorknob shape stainless To date, these reactions with dimethylacetylene studied steel target of −22.5 kV, whose surface is coated with a thin under single collision conditions only involve atoms/radicals of layer of aluminum, thus enabling a subsequent electron cascade the elements in the second row of the periodic table: boron, upon the impact of the ions. Thereafter, the electrons flew carbon, nitrogen, and oxygen. To expand these investigations to toward an organic scintillator and generated a photon pulse that radicals carrying third row elements, we present data here on was further amplified by a photomultiplier tube (PMT, Burle, the crossed molecular beam reaction of the silylidyne radical Model 8850) of −1.35 kV.
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