Cyano radicals, reactions

R.I.K is indebted to the Deutsche Forschungsgemeinschaft (DFG) for a Habilitation fellowship (IICl-Kal081/3-1) and D. Gerlich (University Chemnitz, Germany) for support N.B. thanks the Institute of Atomic and Molecular Sciences (lAMS), Taiwan, for a visiting fellowship (January-March-1999) and the Italian Space Agency (ASI) for partial support. The experimental work of the cyano radical reactions was further supported by Academia Sinica (November 1998 to July 1999). Both authors thank... 

NO formation occurs by a complex reaction network of over 100 free-radical reactions, and is highly dependent on the form of nitrogen in the waste. Nitro-compounds form NO2 first, and then NO, approaching equiHbrium from the oxidized side. Amines form cyano intermediates on their way to NO, approaching equiHbrium from the reduced side. Using air as the oxidant, NO also forms from N2 and O2. This last is known as thermal NO. ... 

In his first experiment (1987) Zewail studied the unimolecular disintegration of iodine cyanide into iodine atom and cyano radical ICN I + CN. They managed to observe a transition state corresponding to the I-C bond breaking. The whole reaction was over in 200 fs. 

The homolysis of AIBN occurs in a concerted process with simultaneous breakage of the two C— N bonds to yield nitrogen and 2-cyano-2-propyl radicals. Reaction of the radicals with each other can occur in two ways to yield tetramethylsuccinodinitrile and dimethyl-A-(2-cyano-2-isopropyl)ketenimine ... 

FIGURE 11.10 Products formed in the reactions of ethynyl and cyano radicals with unsaturated hydrocarbon molecules under single collision conditions. The reaction of ethynyl radicals with benzene is currently being studied in our laboratory. 

It should be stressed that in case of the ethynyl-acetylene reaction, a molecular hydrogen loss channel synthesizing the 1,3-butadienyl radical is open as well. Since the reactions of cyano and ethynyl radicals have no entrance barrier, are exoergic, and aU transition states involved are lower than the energy of the separated reactants, these reaction classes are extremely important to form nitriles and complex unsaturated hydrocarbons in low-temperature environments. On the other hand, the corresponding phenyl radical reactions are—due to the presence of an entrance barrier—closed in those environments. However, the elevated temperature in combustion systems helps to overcome these barriers, thus making phenyl radical reactions important pathways to form aromatic molecules in combustion flames. 

The reaction was initially believed to be a homolytic substitution reaction by ano-dically generated cyano radicals [214]. However, while significant oxidation of CN takes place at a potential as low as 0.5 V versus SCE, the cyanation process will occur only in the region around or above "1/2 of the substrate. This strongly implies a direct mechanism [215,219,220] analogous to that for acetoxylation. 

Since, further, the attack on a group of atoms may occur sometimes from the one side and sometimes from the other, a constituent part will at times have to be taken as belonging to the radical, while in other reactions it will appear to belong to the type. Even the very simplest compounds show such varying behavior, and in the most obvious manner. All cyano compounds, for instance, can in certain reactions be considered as compounds of the cyano radical N in other reactions (those in which nitrogen is offered the opportunity of forming ammonia) they appear as amide-like compounds, i.e. substances belonging to the ammonia type in which H is replaced by some residue, e.g. ... 

In the last decades there has been much interest in the reaction of the cyano radicals, CN(x2e+), with a variety of molecules, mainly because of their implication in combustion chemistry [1]. For this reason extensive studies of CN reactions with simple molecules of relevance in combustion, amongst which are simple unsaturated hydrocarbons, were performed [2 5]. More recently, a renewed interest in the reactions of CN radicals with simple unsaturated hydrocarbons has arisen because of their alleged role in some... 

After entering the collision chamber, the cyano radical beam collides with the molecules of the second pulsed beam with a collision angle of 90°. The unsaturated hydrocarbon beams were produced by expanding the pure gas or vapor at room temperature through a pulsed valve. For the experiments described here the typical collision energies are in the range 13.0—35.0kJ/mol. The reaction products are detected at different laboratory angles by the QMS which is preceded by an electron impact ionizer the velocity distributions... 

In the previous section, we have seen how the presence of a -CH3 group has a strong influence on the chemistry of CN addition to a triple acetylenic bond. In this section we further explore such an effect by moving to the fully CH3-substituted species, dimethylacetylene. Also, in this case, we expect a similar reactive approach with the cyano radical attacking the n-orbital of the dimethylacetylene molecule. The possible reaction routes are (Figure 14.10)... 

Balucani, N. Asvany, O. Chang, A.H.H. Lin, S.H. Lee, Y.T. Kaiser, R.L Bettinger, H.F. Schleyer, P.v.R. Schaefer III, H.F. Crossed beam reaction of cyano radicals with hydrocarbon molecules 1 chemical dynamics of cyano-benzene (CgHsCN X AJ and perdeutero cyanobenzene (CgDsCN X Aj formation from reaction of CN(X S ) with benzene, CgHe (X Aig) and dg-benzene, CgDg (X Aig). J. Chem. Phys. 1999, 111, 7457-7471. 

Azo compounds with functional groups that stabilize the radical are especially reactive. The stabilizing effect of the cyano substituent is responsible for the easy decomposition of azoisobutyronitrile (AIBN), which is frequently used as an initiator in radical reactions. 

Table 37 contains a summary of the gas phase data on Arrhenius parameters for the above class of reactions. Studies have recently been made of the H-abstraction reactions of cyano radicals [381] from nanosecond pulse radiolysis of (CN)2 at low pressures in an excess of... 

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