Direct observation of the forbidden hydrogen atom adduct to acetonitrile: A neutralization-reionization mass spectrometric and CCSD(T) ab initio/RRKM study

Collisional neutralization of protonated acetonitrile (6(+)) generates (E)- and (Z)-1-azapropen-2-yl radicals, CH3-C-.=N-H, (3 and 4, respectively) that represent the kinetically and thermodynamically forbidden adducts of hydrogen atoms to acetonitrile. Radicals 3 and 4 are formed with an excess of internal energy due to Franck-Condon effects and rapidly dissociate by loss of the imine hydrogen atom, a methyl hydrogen, and by CH3 --- C=N-H bond cleavage. The branching ratios for the loss of the imine hydrogen (H or D) and methyl group (CH3 or CD3) were determined for CH3-C-.=N-H, CD3-C-.=N-H, CH3-C-.=N-D, and CD3-C-.=N-D as 70/30, 66/34, 43/57, and 61/39, respectively. Ab initio calculations with coupled clusters with single, double, and perturbative triple excitations/aug-cc-pVTZ predict the loss of the imine hydrogen from 4 to have the lowest transition-state energy. The calculated branching ratios that were based on Rice-Ramsperger- Kassel-Marcus (RRKM) unimolecular rate constants agreed with the mass spectrometric data within experimental accuracy. However, the loss of the methyl hydrogen is predicted by RRKM to be incompetitive (< 0. 1 %) on the ground doublet electronic states of 3 and 4. The observed fraction (similar to 10%) is presumed to occur from an excited electronic state.