The reaction dynamics to form the 1-cyano-1-methylallene isomer CNCH3CCCH2 in its 1A8 ground state via the radical–closed shell reaction of the cyano radical CN(X 2S1) with dimethylacetylene CH3CCCH3 (X 1A18) are unraveled in a crossed molecular beam experiment at a collision energy of 20.8 kJ mol21 together with state-of-the-art electronic structure and Rice–Ramsperger–Kassel– Marcus ~RRKM! calculations. Forward convolution fitting of the laboratory angular distribution together with the time-of-flight spectra verify that the reaction is indirect and proceeds by addition of the CN radical to the p orbital to form a cis/trans CH3CNCvCCH3 radical intermediate. This decomposes via a rather lose exit transition state located only 6–7 kJ mol21 above the products to CNCH3CCCH2 and atomic hydrogen. The best fit of the center-of-mass angular distribution is forward–backward symmetric and peaks at p/2 documenting that the fragmenting intermediate holds a lifetime longer than its rotational period. Further, the hydrogen atom leaves almost perpendicular to the C5H5N plane resulting in sideways scattering. This finding, together with low frequency bending and wagging modes, strongly support our electronic structure calculations showing a H–C–C angle of about 106.5° in the exit transition state. The experimentally determined reaction exothermicity of 90620 kJ mol21 is consistent with the theoretical value, 80.4 kJ mol21. Unfavorable kinematics prevent us from observing the CN versus CH3 exchange channel, even though our RRKM calculations suggest that this pathway is more important. Since the title reaction is barrierless and exothermic, and the exit transition state is well below the energy of the reactants, this process might be involved in the formation of unsaturated nitriles even in the coldest interstellar environments such as dark, molecular clouds and the saturnian satellite Titan.
Crossed beam reaction of cyano radicals with hydrocarbon molecules II: chemical dynamics of 1,1-cyanomethylallene formation from reaction of CN(X 2 Sigma + ) with dimethylacetylene, CH 3 CCCH 3 (X 1 A 1g )
BALUCANI, Nadia;
1999
Abstract
The reaction dynamics to form the 1-cyano-1-methylallene isomer CNCH3CCCH2 in its 1A8 ground state via the radical–closed shell reaction of the cyano radical CN(X 2S1) with dimethylacetylene CH3CCCH3 (X 1A18) are unraveled in a crossed molecular beam experiment at a collision energy of 20.8 kJ mol21 together with state-of-the-art electronic structure and Rice–Ramsperger–Kassel– Marcus ~RRKM! calculations. Forward convolution fitting of the laboratory angular distribution together with the time-of-flight spectra verify that the reaction is indirect and proceeds by addition of the CN radical to the p orbital to form a cis/trans CH3CNCvCCH3 radical intermediate. This decomposes via a rather lose exit transition state located only 6–7 kJ mol21 above the products to CNCH3CCCH2 and atomic hydrogen. The best fit of the center-of-mass angular distribution is forward–backward symmetric and peaks at p/2 documenting that the fragmenting intermediate holds a lifetime longer than its rotational period. Further, the hydrogen atom leaves almost perpendicular to the C5H5N plane resulting in sideways scattering. This finding, together with low frequency bending and wagging modes, strongly support our electronic structure calculations showing a H–C–C angle of about 106.5° in the exit transition state. The experimentally determined reaction exothermicity of 90620 kJ mol21 is consistent with the theoretical value, 80.4 kJ mol21. Unfavorable kinematics prevent us from observing the CN versus CH3 exchange channel, even though our RRKM calculations suggest that this pathway is more important. Since the title reaction is barrierless and exothermic, and the exit transition state is well below the energy of the reactants, this process might be involved in the formation of unsaturated nitriles even in the coldest interstellar environments such as dark, molecular clouds and the saturnian satellite Titan.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
