Kami K. Thoen

Synthesis of charged phenyl radicals and biradicals by laser photolysis in a Fourier-transform ion cyclotron resonance mass spectrometer

The feasibility of generating substituted phenyl radicals and biradicals (with a charged substituent) in the gas phase by laser photolysis was examined by using a Fourier-transform ion cyclotron resonance mass spectrometer. The precursors were generated by ipso-substitution of a halogen atom in the radical cation of a di- or trihalobenzene by various nucleophiles. Photolytic cleavage of the remaining carbon-halogen bond(s) with 266-nm radiation was found to produce many substituted phenyl radicals in greater yields than the earlier employed method, sustained off-resonance irradiated collision-activated dissociation (SORI-CAD). Furthermore, ion generation by photolysis leads to isomerization less often than collisional activation. Finally, not only phenyl-bromine and phenyl-iodine but also certain phenyl-chlorine bonds can be cleaved by photolysis, whereas the synthetic utility of SORI-CAD appears to be largely limited to the cleavage of phenyl-iodine bonds. Hence, laser photolysis greatly expands the variety of substituted phenyl radicals and biradicals that can be synthesized inside a mass spectrometer.

Hydrogen atom abstraction and addition reactions of charged phenyl radicals with aromatic substrates in the gas phase

Abstract In order to investigate competition between radical substitution and addition reactions, the gas-phase reactivity of phenyl radicals bearing a chemically inert, positively charged group and a neutral substituent (CH 3 , Cl, or Br), both at a meta position with respect to the radical site, was examined toward several aromatic substrates in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. The radicals undergo hydrogen atom abstraction from the substituent and/or addition to the phenyl ring of benzeneselenol, thiophenol, benzaldehyde, toluene, aniline, and phenol. The presence of an electron-withdrawing substituent Cl or Br on the phenyl ring of the radical slightly increases the rates for both hydrogen atom abstraction and addition due to favorable polarization of the reactions’ transition states. The observation of a stable ion-molecule addition product in most reactions was unexpected since in a low-pressure gas-phase environment, adducts are typically unable to release their excess energy before dissociation to products or back to reactants. However, the addition products discussed here are low in energy [addition is exothermic by 24–30 kcal/mol; B3LYP/6-31G d +ZPVE] and hence are long lived enough to become stabilized by infrared emission. The extent to which the charged radicals are able to abstract a hydrogen atom from the aromatic substrate and form stable products via addition to the aromatic ring was found to vary greatly. The outcome of this competition can be rationalized by reaction exothermicities only in extreme cases, i.e. for benzeneselenol and thiophenol, that predominantly react by hydrogen atom abstraction due to their especially weak heteroatom-hydrogen bonds and aniline that undergoes almost exclusive addition due to particularly stable resonance-stabilized addition products. For the other substrates, competition between the two reaction pathways is controlled by a complex interplay of polar effects that affects the energies of both transition states but to different extents.

Characterization of long-chain carboxylic esters with CH3OBOCH3+ in a small fourier-transform ion cyclotron resonance mass spectrometer

The usefulness of CH3OBOCH3+ as a chemical ionization reagent was examined by allowing the ion to react with carboxylic esters of various chain lengths in a small Fourier-transform ion cyclotron resonance mass spectrometer equipped with a permanent magnet. CH3OBOCH3+ is a strong electrophile and readily abstracts an oxygen-containing group from the carboxylic esters. Long-chain esters exclusively lose the alkoxide moiety to give the acylium ion. The same reaction was observed for saturated, unsaturated, branched and cyclic esters. In each case, the acylium ion reacts further with a neutral ester molecule by proton transfer to yield the protonated ester as a secondary product. This remarkably simple product distribution reveals the molecular weight of the ester, the chain length of its acid moiety, and the degree of unsaturation in the acid and alcohol moieties.

Distinguishing conventional and distonic radical cations by using dimethyl diselenide

Dimethyl diselenide is demonstrated to be among the most powerful reagents used to identify distonic radical cations. Most such ions readily abstract CH3Se from dimethyl diselenide. The reaction is faster and more exclusive than CH3S· abstraction from dimethyl disulfide, a reaction used successfully in the past to identify numerous distonic ions. Very acidic distonic ions, such as HC+(OH)OCH2·, do not undergo CH3Se· abstraction, but instead protonate dimethyl diselenide. In sharp contrast to the reactivity of distonic ions, most conventional radical cations were found either to react by exclusive electron transfer or to be unreactive toward dimethyl diselenide. Hence, this reagent allows distinction of distonic and conventional isomers, which was demonstrated directly by examining two such isomer pairs. To be able to predict whether electron transfer is exothermic (and hence likely to occur), the ionization energy of dimethyl diselenide was determined by bracketing experiments. The low value obtained (7.9±0.1 eV) indicates that dimethyl diselenide will react with many conventional carbon-, sulfur-, and oxygen-containing radical cations by electron transfer. Nitrogen-containing conventional radical cations were found either to react with dimethyl diselenide by electron transfer or to be unreactive.

Disulfide bond cleavage in neutral substrates by the dimethylene ketene radical cation inside a mass spectrometer

Abstract The reactivity of the dimethylene ketene radical cation (a distonic ion) toward organic disulfides was examined inside a Fourier-transform ion cyclotron resonance mass spectrometer. The radical cation efficiently cleaves the disulfide bond in all the disulfides studied. Hence, this radical cation provides a potentially useful tool for the mass spectrometric characterization and location of disulfide bonds in neutral substrates without the requirement of prior derivatization.

REACTIONS OF ALKYL HALIDES WITH DISTONIC ACYLIUM RADICAL CATIONS

Abstract Molecular orbital calculations (Becke3LYP/3-21G(d)) indicate that the distonic acylium ions • CH 2 CH 2 CH 2 CO + and • CH 2 CH 2 CO + are localized σ-radicals with a spin density of 0.95 e and 0.92 e at the terminal methylene carbon, respectively. Most of the positive charge (+0.88) is localized on the carbonyl carbon. The radical site carries a charge that is only slightly more positive than that in the neutral ethyl radical. Based on these values, the distonic acylium ions may be expected to behave like neutral alkyl radicals. However, an examination of the reactivity of gaseous • CH 2 CH 2 CO + and • CH 2 CH 2 CH 2 CO + toward a series of alkyl halides in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) revealed that their reactivity is greatly influenced by the charged group. The ions • CH 2 CH 2 CO + and • CH 2 CH 2 CH 2 CO + are unreactive toward alkyl chlorides but react with alkyl bromides and iodides by electron, halogen atom, alkyl radical and/or hydrogen atom abstraction. While hydrogen and halogen atom abstraction reactions are common for neutral alkyl radicals, electron and alkyl abstractions are not. A mechanism that involves catalysis by the charge site is proposed for the alkyl abstraction reactions.