1 Chem Soc Rev 1 5 REVIEW ARTICLE 5 The role of isovalency in the reactions of the Q1 10 cyano (CN), boron monoxide (BO), silicon nitride Q2 10 Cite this: DOI: 10.1039/c3cs60328h (SiN), and ethynyl (C2H) radicals with unsaturated hydrocarbons acetylene (C2H2) and ethylene (C2H4) 15 D. S. N. Parker,a R. I. Kaiser*a and A. M. Mebel*b 15 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 20 investigate the ramifications of the isovalency of four radicals with the same X2S+ electronic structure – 20 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 X2S+ electronic ground state should dictate the outcome of their reactions with prototypical hydro- Received 16th September 2013 carbons holding a carbon–carbon triple and double bond. However, we find that other factors come into 25 DOI: 10.1039/c3cs60328h play, namely, atomic radii, bonding orbital overlaps, and preferential location of the radical site. These 25 doublet radical reactions with simple hydrocarbons play significant roles in extreme environments such as www.rsc.org/csr the interstellar medium and planetary atmospheres (CN, SiN and C2H), and combustion flames (C2H, BO). 30 30 a Department of Chemistry, University of Hawai’i at Manoa, Honolulu, HI 96822, 1. Introduction USA. E-mail: [email protected] b Department of Chemistry and Biochemistry, Florida International University, In 1919 Langmuir coined the concept of isovalency, in which Miami, FL 33199, USA. E-mail: [email protected] molecular entities with the same number of valence electrons 35 35 Dorian S. N. Parker received his Ralf I. Kaiser received his PhD in PhD in Chemistry from University Chemistry from the University of College London (United Mu¨nster (Germany) in 1994 and 40 Kingdom) in 2009. He conducted conducted postdoctoral work at 40 postdoctoral work at the UC Berkeley (Department of University of Hawaii, Chemistry Chemistry). During 1997–2000 Department using crossed he received a fellowship from the molecular beams to investigate German Research Council (DFG) 45 gas phase reaction dynamics in to perform his Habilitation at the 45 combustion and astrochemical Department of Physics (University environments. In 2013 he was of Chemnitz, Germany) and Insti- awarded a postdoctoral tute of Atomic and Molecular D. S. N. Parker fellowship with the NASA R. I. Kaiser Sciences (Academia Sinica, Tai- 50 Astrobiology Institute (NAI) to wan). He joined the Department 50 study the role of neutral–neutral reactions in forming prebiotic of Chemistry at the University of Hawaii at Manoa in 2002, where molecules in extraterrestrial environments. His interests lie in the he is currently Professor of Chemistry and Director of the W.M. chemistry of extreme environments such as in combustion and Keck Research Laboratory in Astrochemistry. He was elected Fellow astrochemistry, including the formation prebiotic molecules, as of the Royal Astronomical Society (UK) (2005), of the Royal Society 55 well as developing time-resolved imaging of reaction dynamics. of Chemistry (UK) (2011), of the American Physical Society (2012), 55 and of the Institute of Physics (UK) (2013). This journal is c The Royal Society of Chemistry 2013 Chem. Soc. Rev., 2013, 00,1À13 | 1 Review Article Chem Soc Rev 1 Table 1 Comparison table between four isovalent radicals CN, BO, SiN and C2H 1 Chemical name Cyano Boron monoxide Silicon nitride Ethynyl Formula CN BO SiN C2H 5 Lewis structure 5 Bond energy (kJ molÀ1) 74944 79944 438 728 I.E (eV) 13.645 13.3 Æ 0.545 10.345 11.61 Æ 0.0745 E.A (eV) 3.862 Æ 0.005 2.832 Æ 0.00845 2.949 Æ 0.00146 2.969 Æ 0.00145 Internuclear distance (Å) 1.172 1.205 1.574 1.203 Bond order Triple Double Double Triple 2 + 2 + 2 + 2 + 10 Electronic structure X S X S X S X S 10 and the same electronic structure have similar chemistries.1 Reactions of ethynyl radicals with unsaturated hydrocarbons The idea of isovalency allowed chemists to propel forward have also been proposed as a mass growth route to larger knowledge on reaction mechanisms involving organic and hydrocarbon molecules such as polycyclic aromatic hydrocar- 15 inorganic molecules based on their electronic structure. Small bons (PAHs) and polyacetylenes in the ISM and in planetary 15 di- and tri-atomic radicals – cyano (CN; X2S+), boron monoxide atmospheres.22,27–33 The facile formation of PAHs by barrier- 2 + 2 + 2 + (BO; X S ), silicon nitride (SiN; X S ), and ethynyl (C2H; X S ) less ethynyl additions is also important in combustion environ- have the same electronic structure and are isovalent. Gas phase ments where formation of PAHs and eventually soot are collisionally induced reactions between these radicals and unwanted competing reactions that negatively impact engine 20 unsaturated hydrocarbons play significant roles in extreme performance, health, and the environment.27,34,35 The silicon 20 environments such as extraterrestrial,2,3 combustion,4 and nitride radical has also been identified in the interstellar atmospheric environments5 as well as in industrial settings medium such as in circumstellar envelopes of dying carbon like chemical vapor deposition (CVD).6 stars9 suggesting, based on the isovalency between cyano and The majority of small reactive di- and tri-atomic species in silicon nitride, that the formation of organo-silicon molecules 25 the interstellar medium (ISM) are radicals, and their chemistry such as silaisocyanoacetylene and silaisocyanoethylene could 25 plays an important part in its chemical evolution.5,7,8 The cyano follow similar mechanisms in extraterrestrial environments.36–38 (CN), silicon nitride (SiN), and ethynyl (C2H) radicals have been In CVD processes, silicon nitride reactions with unsaturated identified in the ISM.7,9–12 Cyanoacetylene (HCCCN) and vinyl hydrocarbons are competing radical reactions that need to be 6,39 cyanide (C2H3CN) were among the first nitriles molecules correctly characterized in order to reduce their disruption. 30 identified13,14 and are thought to be formed through collision- Finally, in rocket propulsion systems the combustion of boron 30 ally induced bimolecular reactions of the cyano radical with offers three times the energy release than carbon, however boron abundant acetylene and ethylene molecules, respectively.15–20 A readily forms oxides like boron monoxide that disrupt the range of cyano radical reactions with unsaturated hydrocarbons combustion process by undergoing competing reactions with can also explain the variety of cyano substituted molecules fuel components such as unsaturated hydrocarbons.40–43 The 35 found in the hydrocarbon rich planetary atmospheres.21–26 multitude of simultaneous reactions in extreme environments 35 makes for a difficult area of study. Only by systematically investigating the chemical dynamics of each contributing ele- mentary reaction under single collision conditions will we be Alexander M. Mebel received his able to effectively model these complex environments. 40 PhD in physical chemistry from Table 1 summarizes the properties of the isovalent radicals 40 Kurnakov’s Institute of General CN, BO, SiN, and C2H. The locations of the electron density and Inorganic Chemistry of corresponding to the radical can be understood in terms of Russian Academy of Science in each radicals Lewis structure. In cyano radical and boron Moscow, Russia. His first faculty monoxide the electron density of the radical preferentially 45 appointment was at the Institute resides on the most electropositive atom – carbon (2.55) and 45 of Atomic and Molecular Sciences boron (2.04), compared with nitrogen (3.04) and oxygen (3.44), (Academia Sinica, Taiwan) and in respectively. In cyano, carbon is left without its valency of 4 2003 he joined the Department of filled after forming a triple bond with nitrogen and nitrogen Chemistry and Biochemistry of forms a lone pair, in boron monoxide, boron forms a double 50 Florida International University bond with oxygen leaving oxygen to form two lone pairs. Silicon 50 A. M. Mebel in Miami, Florida, USA, where nitride and the ethynyl radicals however, host their electron he is currently Professor of density on the more electronegative atoms: nitrogen (3.04) Chemistry. His current research interests involve theoretical rather than the silicon atom (1.90), and carbon (2.55) rather quantum chemical studies of mechanisms, kinetics, and dynamics than carbon bound to hydrogen (2.48). Here, silicon nitride 55 of elementary chemical reactions related to combustion, only forms a double bond with nitrogen, presumably due to its 55 atmospheric, and interstellar chemistry. large atomic radius, resulting in a lone pair residing on the 2 | Chem. Soc. Rev., 2013, 00,1À13 This journal is c The Royal Society of Chemistry 2013 Chem Soc Rev Review Article 1 silicon atom and one lone pair on the nitrogen atom, and leaving differing atomic compositions and radical locations cause 1 the radical electron density to reside on the nitrogen atom. significant divergences from their predicted chemical charac- Similarly, carbons triple bond to C–H leaves one of its valences ter. Within this article we shall discuss the chemical reactivity unfilled resulting in the electron density residing there. Despite of small isovalent radicals that possess the electronic structure 2 + 5 the same number of valence electrons on each atom in silicon X S – CN, BO, SiN and C2H – with simple unsaturated 5 nitride and the cyano radical, the large silicon radius (1.10 Å) and hydrocarbon reactants acetylene (C2H2) and ethylene (C2H4), diffuse p-type orbitals result in a significantly longer internuclear and as prototype representatives of organic molecules carrying distance of 1.574 Å.