Hydrogen-bridged Radical Cation Research Articles

The decarbonylation of ionized β-hydro×ypyruvic acid: the hydrogen-bridged radical cation [CH<sub>2</sub>=O <SUP>.</SUP> <SUP>.</SUP> <SUP>.</SUP>H <SUP>.</SUP> <SUP>.</SUP> <SUP>.</SUP>==C-OH]<SUP>.</SUP><SUP>+</SUP> studied by e×periment and theory

The decarbonylation of ionized β-hydro×ypyruvic acid: the hydrogen-bridged radical cation [CH<sub>2</sub>=O <SUP>.</SUP> <SUP>.</SUP> <SUP>.</SUP>H <SUP>.</SUP> <SUP>.</SUP> <SUP>.</SUP>==C-OH]<SUP>.</SUP><SUP>+</SUP> studied by e×periment and theory

The hydrogen-bridged radical cation [H 2O⋯H⋯OCOH ·+: A combined experimental and theoretical study of its stability and dissociation chemistry

Dissociative ionization of dihydroxyfumaric acid generated the hydrogen-bridged radical cation [H 2O⋯H⋯OCOH ·+, 1, as shown by a combination of tandem mass spectrometric techniques (metastable ion, collision-induced dissociation, and MS/MS/MS experiments) and computational chemistry (using ab initio MO and density functional theories). This hydrogen-bonded radical cation is predicted by theory to be more stable than the isomeric ions [H 2O⋯HO 2CH] ·+, 2/2a, [H 2O⋯HCO 2H] ·+, 3, and H 2OC(OH) ·+ 2, 4/4a, while HC(OH) ·+ 3 is not stable at all. The heat of formation of the most stable conformer of 1 was estimated as 73 kcal mol −1 (with an assigned uncertainty of ± 4 kcal mol −1) and its isomers 2, 2a, 3, 4 and 4a are calculated (Becke3LYP/6-31G ∗∗ + ZPE) to be higher in energy by 10.0, 9.4, 13.3, 18.1 and 19.1 kcal mol −1, respectively. Accordingly, the MP4SDTQ/6-31G ∗∗//MP2/6-31G ∗∗ + ZPE relative energies of 2a, 4 and 4a are 11.5, 19.2 and 21.3 kcal mol −1. Mass spectrometric experiments on isotopically labelled di[ 18O]hydroxyfumaric acid indicated that two 1,5-hydrogen transfers and sequential expulsions of CO lead, via an intermediate dihydroxyketene-water type of ion-neutral complex, to the hydrogen-bridged product ion 1. Unimolecular metastable dissociations of the latter lead to the proton-bound ion [H 2O⋯H⋯OCO] + and to the hydronium ion, H 3O +, as well as, upon collisional activation, to the dihydroxycarbene ion, HOCOH ·+.

Proton and electron transfers in O•H•O and C•H•O hydrogen-bridged ions: their role in the dissociation chemistry of ionized acetol, CH3C(=O)CH2OH•+

Low-energy acetol ions CH3C(=O)CH2OH•+, 1, dissociate to CH3C(H)OH+ and HC=O• by a double hydrogen transfer (DHT), a common reaction among oxygen-containing radical cations. Recent experimental work has shown that the isotopologue CH3C(=O)CH2OD•+ specifically loses HC=O• to produce CH3C(D)OH+. This finding refutes an earlier postulated attractive mechanism based on the behaviour of 1 in ion-molecule reactions. Using ab initio MO calculations (at the CEPA//RHF/DZP level of theory complemented with valence bond (VB) methods), a low-energy pathway was traced that may explain all of the available experimental observations. It is shown that the unimolecular chemistry of 1 can be understood in terms of two proton transfers, taking place in intermediate O•H•O and C•H•O bonded hydrogen-bridged radical cations. The two protons originate from the same moiety and a charge transfer complex is therefore implicated and shown to be involved. These concepts of proton and charge transfer may well be more generally applicable and they do correctly predict the unimolecular chemistry of ionized acetoin, CH3C(=O)CH(CH3)OH•+ and related α-ketols. Key words: ab initio calculations, hydrogen-bridged ions.

An experimental and theoretical study of ionized hydroxyacetone: a stable, hydrogen-bridged radical cation

Ion-molecule reactions and collision-activated dissociation experiments were used to examine the structure of the long-lived, low-energy molecular ion of hydroxyacetone [CH 3 C(O)CH 2 OH] .+ (1) in a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR). It is demonstrated that a significant proportion of the nonfragmenting ions have a noncovalent, hydrogen-bridged structure, most likely [CH 3 C=O...H...O=CH 2 ] .+ (2). This hydrogen-bridged ion undergoes reactions which are unique for a radical cation and which can best be described as solvent switching reactions: neutral reagents replace the acetyl radical in the ion

The gas phase chemistry of the methyl carbamate radical cation H 2NCOOCH +3[rad]: isomerization into distonic ions, hydrogen-bridged radical cations and ion—dipole complexes

The unimolecular chemistry of the methyl carbamate radical cation, H 2NCOOCH + 3, 1, has been investigated by a combination of mass spectrometry based experiments (metastable ion (MI), collisional activation (CA), colision-induced dissociative ionization (CIDI), neutralization—reionization (NR) spectrometry, 2H, 13C and 18O isotopic labeling, appearance energy (AE) measurements), and ab initio molecular orbital calculations, executed at the SDCI/G—31G**//4—31G level of theory and corrected for zero-point energies. These calculations indicate that besides ionized methyl carbamate there are at least seven other equilibrium structures inlcuding distonic ions and hydrogen-bridged radical cations. The most stable isomer is the hydrogen-bridged species [H 2NCHO ⋯ H ⋯ OCH] + which is best viewed as the carbenium ion H 2NCHOH + interacting with the formly dipole. The related species [H 2NCO ⋯ H ⋯ OCH 2] + in which the hydroxyaminocarbene ion H 2NCOH + interacts with the formaldehyde dipole is also a stable species. This hydrogen-bridged radical cation is the key intermediate in the spontaneous unimolecular dissociations of methyl carbamate ions. Experimentally, the metastable molecular ions form two sets of products, namely. H 2NCHOH + + HCO (the components of the most stable isomer) and [CH 2O ⋯ H ⋯ NH 2] + + CO. The minimum energy requirement paths have been located by ab initio calculations and the reactions follow multistep isomerizations. In the first step, H 2NCOOCH + 3 , 1, isomerizes via a 1,4-hydrogen shift to the distonic ion H 2NC(OH)OCH + 2 , 2, which then rearranges to the hydrogen-bridged radical ion [H 2NCO ⋯ H ⋯ OCH 2] + . The incipient formaldehyde molecule can then donate a hydrogen to the C atom of H 2NCOH, followed by loss of HCO or it can accept the hydroxyl hydrogen to form a CH 2OH radical; this radical then migrates within the electrostatic field of the H 2 N + CO ion towards the N atom to form the complex [H 2CO ⋯ H ⋯ NH 2CO + . This latter species, which can be viewed as a formaldehyde and a CO molecule interacting with NH + 3 lies in a shallow potential well only and sheds CO to produce [CH 2O ⋯ H ⋯ NH 2] , as observed experimentally. It is stressed that only with the aid of high level ab initio calculations could the above mechanisms be elucidated.

The role of hydrogen-bridged radical cations in the dissociation of ionized glycolaldehyde [HOCH 2CHO] +•

Ab initio molecular orbital theory calculations combined with mass spectrometric experiments show that the dissociation of metastable glycolaldehyde radical cations occurs at the thermochemical threshold and that this process proceeds via a CHO bridged ion [CH 2O(H)…H…CO] +•.