An Experimental and Theoretical Study of the Long-Lived Radical Cation of CH₃OCH₂CH₂OH

Abstract

The long-lived radical cation of 2-methoxyethanol, CH3OCH2CH2OH•+, is stable toward isomerization in the gas phase. This radical cation reacts with neutral reagents via three main channels in a Fourier-transform ion cyclotron resonance mass spectrometer. CH3OCH2CH2OH•+ abstracts an electron from reagents with ionization energies less than that of CH3OCH2CH2OH. Neutral reagents with higher ionization energies replace CH3OCH2 • or CH2O in the ion. Both replacement reactions are probably driven by the formation of a hydrogen bond between the entering nucleophile and the hydroxyl group in CH3OCH2CH2OH•+, which breaks the C−C bond in CH3OCH2CH2OH•+. For strong nucleophiles, this process is so exothermic that the resulting ion/molecule complex dissociates by loss of CH3OCH2 •. However, weak nucleophiles yield a longer-lived ion−molecule complex. Within this complex, CH3OCH2 • can react with the initially produced ion by replacement of CH2O (a weaker nucleophile) to yield the hydrogen-bridged complex of CH3OCH2 • and the nucleophile. This reaction provides a general approach for the gas-phase synthesis of different hydrogen-bridged radical cations. Ab initio molecular orbital calculations suggest that the most stable geometry of CH3OCH2CH2OH•+ is characterized by an unusually long C−C bond (1.775 Å at the UMP2/6-31G** level of theory). This finding is in agreement with the observed facile cleavage of the C−C bond in CH3OCH2CH2OH•+. The isomeric ions •CH2O+(CH3)CH2OH, (CH3)2O+CH2O•, •CH2OCH2CH2OH2 +, •CH2O(CH3)−H+···OCH2, and CH3OC(H)H···O(H)CH2 •+ were calculated to be less stable than CH3OCH2CH2OH•+ (at the UMP2/6-31G**//UHF/6-31G**+ZPE level of theory). Only the ion (CH3)2O-H···OCH•+ was found to lie lower in energy than CH3OCH2CH2OH•+ (by 7.2 kcal/mol at the UMP2/6-31G**+ZPVE level of theory). However, experimental evidence does not support the formation of this hydrogen-bridged ion upon ionization of 2-methoxyethanol.

Generation and characterization of distonic dehydrophenoxide radical anions under electrospray and atmospheric pressure chemical ionizations

“…In this way, distonic ions provide unique systems to study the chemistry of free radicals using mass spectrometry techniques [2][3][4][5]. With the pioneering work of Kenttämaa and coworkers as well as other researchers, numerous studies have been directed toward charge-inert radical cations [3,[6][7][8][9][10][11][12]. At the same time, the studies of distonic radical anions have also yielded interesting results [4,5,[13][14][15][16][17][18].…”