Hypervalent Ammonium Radicals. Effects of Alkyl Groups and Aromatic Substituents

Abstract

Neutralization by collisional electron transfer of gaseous benzylalkylammonium ions produces transient hypervalent radicals whose dissociations depend on the substituents in the aromatic ring and at the amine nitrogen atom. Benzylammonium radical, C6H5CH2NH3 •, dissociates mainly by N−H bond cleavage to give benzylamine. Dissociation of the CH2−N bond to benzyl radical and ammonia is less abundant. Benzylmethylammonium, C6H5CH2NH2CH3 •, dissociates by CH2−N, N−CH3, and N−H bond cleavages to give methylamine, benzyl radical, benzylamine, and N-methylbenzylamine. Benzyldimethylammonium, C6H5CH2NH(CH3)2 •, undergoes loss of dimethylamine and hydrogen, while the loss of methyl is less important. (2,3,4,5,6-Pentafluorobenzyl)dimethylammonium radical, C6F5CH2NH(CH3)2 •, dissociates mainly by fission of the pentafluorophenyl ring to give C n F m fragments with CF• as the dominating product, while bond dissociations at the hypervalent nitrogen atom are less important. The relative stabilities of pentafluorobenzyl and tropyl cations and radicals are assessed by ab initio calculations. (3,5-Dinitrobenzyl)dimethylammonium radical, (NO2)2C6H3CH2NH(CH3)2 •, undergoes competitive losses of hydrogen and NO and intramolecular proton transfer onto the dinitrophenyl ring. Mechanisms for these reactions are suggested involving dissociative electron attachment at the aromatic ring and formation of hypervalent ammonium radicals and zwitterionic intermediates.

The effect of radical trap moieties on electron capture dissociation spectra of substance P

“…Other small molecule losses from both singly-tagged peptides included carbon monoxide and ammonia losses, both of which have been previously reported and mechanisms for their losses discussed [24,26,27]. Finally, the singly-tagged peptides contained losses of the tag as well as a tag fragment.…”

Gas-phase Protonation of Pyridine. A Variable-time Neutralization-Reionization andAb InitioStudy of Pyridinium Radicals

“…29,30 In vertical neutralization, electron capture in a vacant orbital, followed by internal conversion of the excited state to the ground state, may provide a mechanism 1A 1 for excitation of pyridine molecules and cause their unimolecular dissociations, as discussed recently for aromatic amines. 31 NeutralizationÈreionization of protonated pyridine, here denoted as 1H`regardless of its structure, also gave substantial survivor ions attesting to the stability of the intermediate pyridinium radicals (Fig. 2).…”

Cycloreversion and other gas‐phase reactions of neutral and cationic pyrrolidine‐fused chlorins and isobacteriochlorins under ion bombardment and electrospray

“…In the molecular ion zone the [M-H] þ ion (m/z 1117) can result from a hydrogen radical loss followed by one-electron reduction (or deprotonation), during the desorption/ionization process, of one of the central NH groups (Scheme 4), but the formation of the singly charged ion at m/z 1118, when 3-NBA is used, is more difficult to explain, as it must involve one-electron reduction of the pyrrolidinium M 2þ ion. Hypervalent radical pyrrolidinium cations are transient species, 25 but Turecek and collaborators 26,27 have reported that phenyl groups bearing electronegative substituents, such as fluorine, can stabilize the neutralized species by capturing the incoming electron to form a metastable zwitterion. The presence of four C 6 F 5 groups in compound IIB increases the probability of electron capture and the macrocyclic structure allows for delocalization of the radical, thus increasing the stability of the zwitterion (Scheme 5) long enough to give a measurable signal.…”