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 (