The role of isovalency in the reactions of the cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C₂H) radicals with unsaturated hydrocarbons acetylene (C₂H₂) and ethylene (C₂H₄)

Abstract

The classification of chemical reactions based on shared characteristics is at the heart of the chemical sciences, and is well exemplified by Langmuir's concept of isovalency, in which 'two molecular entities with the same number of valence electrons have similar chemistries'. Within this account we further investigate the ramifications of the isovalency of four radicals with the same X(2)Σ(+) electronic structure - cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C2H), and their reactions with simple prototype hydrocarbons acetylene (C2H2) and ethylene (C2H4). The fact that these four reactants own the same X(2)Σ(+) electronic ground state should dictate the outcome of their reactions with prototypical hydrocarbons holding a carbon-carbon triple and double bond. However, we find that other factors come into play, namely, atomic radii, bonding orbital overlaps, and preferential location of the radical site. These doublet radical reactions with simple hydrocarbons play significant roles in extreme environments such as the interstellar medium and planetary atmospheres (CN, SiN and C2H), and combustion flames (C2H, BO).

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…Computational studies on the reaction mechanism have indicated that there are two plausible channels from the direct CN + C 2 H 4 addition intermediate, the CH 2 CH 2 CN radical: 106,107 The photoionization spectrum of the m/z 53 product shows no evidence for the production of the isocyanoethene (CH 2 CH-N=C) species as the AIE of this isomer is calculated (CBS-QB3) to be 10.56 eV and no ion signal is detected until 10.9 eV. Calculations are in agreement with this finding, revealing that the H-elimination transition state from a CH 2 CH 2 NC is placed +12 kJ mol -1 above the entrance channel energy of the reactants.…”

Untangling the reaction dynamics of the silylidyne radical (SiH; X2Π) with acetylene (C2H2; X1Σg+)

“…This is well reflected in the difference in chemical bonding of the c-SiC 2 molecule, which exhibits a cyclic C 2v symmetric structure, while the tricarbon (C 3 ) molecule is (quasi)linear at its ground state molecular geometry. Similarly, the isovalent cyano (CN) and silicon nitride (SiN) radicals revealed significant discrepancies in reactivity with unsaturated hydrocarbons such as acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ), by bonding the carbon in the cyano (CN) and nitrogen in the silicon nitride (SiN) to the unsaturated hydrocarbons [12][13][14][15][16][17]. Considering the title reaction, the diverse chemical bonding of silicon analogous species is also evident, when comparing the silylidyne-acetylene (SiH-C 2 H 2 ) and methylidyne-acetylene (CH-C 2 H 2 ) potential energy surfaces (PESs) [18,19] as well as the cyclopropenylidene (c-CC 2 H 2 ) and silacyclopropenylidene (c-SiC 2 H 2 ) isomers in particular [20,21].…”

Reaction Dynamics in Astrochemistry: Low-Temperature Pathways to Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

“…Which experimental approach can best reveal the chemical dynamics involved in the formation of PAHs in the gas phase? Experiments conducted under single-collision conditions, in which particles A of one supersonic beam are made to collide only with particles BC of a second beam (Equation 1), are ideal to investigate the chemical dynamics of bimolecular gas phase reactions at the most fundamental, microscopic level (46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58). These are crossed molecular beam experiments.…”