Gas-Phase Synthesis of Boronylallene (H₂CCCH(BO)) under Single Collision Conditions: A Crossed Molecular Beams and Computational Study

Abstract

The gas phase reaction between the boron monoxide radical (BO; XΣ) and allene (HCCCH; XA) was investigated experimentally under single collision conditions using the crossed molecular beam technique and theoretically exploiting ab initio electronic structure and statistical (RRKM) calculations. The reaction was found to follow indirect (complex forming) scattering dynamics and proceeded via the formation of a van der Waals complex (BOCH). This complex isomerized via addition of the boron monoxide radical (BO; XΣ) with the radical center located at the boron atom to the terminal carbon atom of the allene molecule forming a HCCCHBO intermediate on the doublet surface. The chemically activated HCCCHBO intermediate underwent unimolecular decomposition via atomic hydrogen elimination from the terminal carbon atom holding the boronyl group through a tight exit transition state to synthesize the boronylallene product (HCCCHBO) in a slightly exoergic reaction (55 ± 11 kJ mol). Statistical (RRKM) calculations suggest that minor reaction channels lead to the products 3-propynyloxoborane (CH(BO)CCH) and 1-propynyloxoborane (CHCCBO) with fractions of 1.5% and 0.2%, respectively. The title reaction was also compared with the cyano (CN; XΣ)-allene and boronyl-methylacetylene reactions to probe similarities, but also differences of these isoelectronic systems. Our investigation presents a novel gas phase synthesis and characterization of a hitherto elusive organyloxoborane (RBO) monomer-boronylallene-which is inherently tricky to isolate in the condensed phase except in matrix studies; our work further demonstrates that the crossed molecular beams approach presents a useful tool in investigating the chemistry and synthesis of highly reactive organyloxoboranes.

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; X2Π) with Methylacetylene (CH3CCH; X1A1) and D4-Methylacetylene (CD3CCD; X1A1)

“…This holds in particular for bimolecular reactions between diatomic radicals and small hydrocarbons. Bimolecular reactions involving diatomic radicals such as boron monoxide (BO), [28][29][30][31][32][33][34][35] boron monosulfide (BS), [36][37] methylidyne (CH/CD), [38][39][40][41][42][43][44][45] cyano (CN), 32,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] dicarbon (C 2 ), 57,[61][62][63][64][65][66][67][68][69][70][71][72][73][74] hydroxyl (OH), [75]…”

A combined crossed molecular beam and quasiclassical trajectory study of the Titan-relevant N(2D) + D2O reaction

“…[10,11,17,26,28,29]) can lead to erroneous conclusions by neglecting important contributions coming from less exothermic channels leading to isomeric products, the distributions of which can be embedded in the global ones; and (2) in many reactive systems characterised by deep wells dynamical effects are important and statistical methods can fail, as already observed in previous work [10,30,31]. This is particularly true when more than one addition or insertion intermediate can be directly formed by the reactants interaction without entrance barriers [32][33][34][35][36]. In these cases only a dynamical treatment of the entrance channels can really describe the reactive system and lead to the correct determination of the product branching ratio.…”

Exploring the Gas Phase Synthesis of the Elusive Class of Boronyls and the Mechanism of Boronyl Radical Reactions under Single Collision Conditions

“…In this section, we report our experimental results on the reactions of the boronyl radical with multiple C2–C6 unsaturated hydrocarbons. To present the results, we have grouped these systems based on the functional groups of the involved hydrocarbons: (1) acetylenes carrying a carbon–carbon triple bond [acetylene (H–CC–H), methylacetylene (CH 3 –CC–H),and dimethylacetylene (CH 3 –CC–CH 3 )] to systematically explore the effect of the hydrogen atom substitution by one or two methyl groups (Table , Figure ); (2) olefins holding a carbon–carbon double bond [ethylene (H 2 CCH 2 ) and propene (CH 3 –CHCH 2 )] (Table , Figure ), and (3) highly unsaturated hydrocarbons with multiple double or triple bonds [allene (H 2 CCCH 2 ), diacetylene (H–CC–CC–H), 1,3 butadiene (H 2 CCH–CHCH 2 ), and benzene (C 6 H 6 )] (Table , Figure ). The reaction with allene when compared to the reaction with its isomer methylacetylene or the reaction with dimethylacetylene when compared to the reaction with its 1,3-butadiene isomer reveal effects associated with the stereospecificity of the boronyl radical attack.…”

“…In all cases, the CMB results have been interpreted in the light of electronic structure calculations performed ad hoc of the relevant stationary points along the minimum energy path of the underlying potential energy surface (PES). The calculations have been performed at various levels with the higher levels being CCSD­(T)/cc-pVTZ and CCSD­(T)/CBS by the groups of Chang, ,− , Mebel, , and Bartlett. , In addition, since these reactions are multichannel polyatomic reactions and normally more than one set of products is possible, statistical estimates of the product branching ratios by means of the Rice–Ramsperger–Kassel–Marcus (RRKM) method have been derived to be compared with the experimental determination. The main results, as well as some information on the experimental conditions, have been concisely summarized in the Tables –, while the contour maps for several systems are shown in Figures –.The structures of the novel product species identified for the first time as isolated molecules are reported in Figure .…”

“…In the case of the reaction with allene, once again the boron monoxide radical interacts with the π electron density without any entrance barrier to form the CH 2 CCH 2 (BO) intermediate via addition to one of the terminal carbon atoms or the more stable intermediate CH 2 C­(BO)­CH 2 through addition to the central carbon of the allene molecule. The formation of a shallow van der Waals complex precedes these additions.…”