Atmospheric Chemistry of Methyl Isocyanide-An Experimental and Theoretical Study

The reaction of CH₃NC with OH radicals was studied in smog chamber experiments employing PTR-ToF-MS and long-path FTIR detection. The rate coefficient was determined to be k_CH = (7.9 ± 0.6) × 10^-11 cm³ molecule^-1 s^-1 at 298 ± 3 K and 1013 ± 10 hPa; methyl isocyanate was the sole observed product of the reaction. The experimental results are supported by CCSD(T*)-F12a/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemistry calculations showing the reaction to proceed primarily via electrophilic addition to the isocyanide carbon atom. On the basis of the quantum chemical data, the kinetics of the OH reaction was simulated using a master equation model revealing the rate coefficient to be nearly independent of pressure at tropospheric conditions and having a negative temperature dependence with k_OH = 4.2 × 10^-11 cm³ molecule^-1 s^-1 at 298 K. Additional quantum chemistry calculations on the CH₃NC reactions with O₃ and NO₃ show that these reactions are of little importance under atmospheric conditions. The atmospheric fate of methyl isocyanide is discussed.