Electron-Rich Radicals by Neutralization—Reionization Mass Spectrometry. Generation, Dissociations and Energetics of the Hydrogen Atom Adduct to Acetamide

Abstract

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.

Peptide Radicals and Cation Radicals in the Gas Phase

“…135 The hot hydrogen atom hypothesis was subsequently scrutinized by experiment and quantum chemistry calculations. Addition of a H atom to amide carbonyl oxygen atoms, albeit exothermic, 115,219,220 requires overcoming potential energy barriers of 38−68 kJ mol −1 that would make a thermal reaction very slow, with rate constants calculated at k ≈ 10 −18 molecules cm −3 s −1 . 169 A comparable energy barrier was found for H-atom addition to the amide carbonyl carbon atom forming a transient oxygen-based radical.…”

Electron Super-Rich Radicals in the Gas Phase. A Neutralization−Reionization Mass Spectrometric and ab Initio/RRKM Study of Diaminohydroxymethyl and Triaminomethyl Radicals

“…have been studied experimentally and computationally with the aim of relating the stability and electronic properties of these radicals to the nature of the substituents. − Of much interest have also been combined (captodative) effects of π-electron donors and π-electron attractors in multifunctional carbon-centered radicals . Recently, carbon-centered radicals carrying amino groups have become of interest in connection with electron-transfer, electron−ion, and ion−ion recombination reactions in the gas phase in which α-amino alkyl radicals are thought to play an important role. − …”

“…6 Recently, carbon-centered radicals carrying amino groups have become of interest in connection with electron-transfer, 9 electron-ion, 10 and ion-ion recombination reactions 11 in the gas phase in which R-amino alkyl radicals are thought to play an important role. [12][13][14][15][16] Experimental studies of functionalized carbon-centered radicals have mostly relied on generating these transient species in the gas phase by femtosecond electron transfer to the corresponding cations or by collisional electron detachment from gasphase anions. We and others have used this approach to generate several radicals and study their stability and unimolecular dissociations in an isolated state in the rarefied gas phase, as reviewed recently.…”

Where Does the Electron Go? Electron Distribution and Reactivity of Peptide Cation Radicals Formed by Electron Transfer in the Gas phase

“…The structures of c + and z +• ions are not known with certainty, although the former are usually represented by enolimine structures, while z +• ions are thought to be C α radicals. It is noteworthy that enolimines are substantially less stable than their amide tautomers and the possibility of exothermic prototropic rearrangement to amides during the dissociation cannot be excluded.…”