3914
J. Am. Chem. Soc. 1996, 118, 3914-3921
An Experimental and Theoretical Study of the Long-Lived
Radical Cation of CH3OCH2CH2OH
Jaana M. H. Pakarinen,† Rebecca L. Smith,‡ Pirjo Vainiotalo,*,†
Tapani A. Pakkanen,*,† and Hilkka I. Kentta1maa*,‡
Contribution from the Departments of Chemistry, UniVersity of Joensuu, FIN-80101 Joensuu,
Finland, and Purdue UniVersity, West Lafayette, Indiana 47907-1393
ReceiVed June 29, 1995X
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 CH3OCH2-
CH2OH•+. 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+‚‚‚OdCH2, and CH3OC(H)H‚‚‚O(H)dCH2•+ were calculated to be less stable than CH3OCH2CH2-
OH•+ (at the UMP2/6-31G**//UHF/6-31G**+ZPE level of theory). Only the ion (CH3)2O-H‚‚‚OdCH•+ 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.
Introduction
Both types of ions have been established to represent stable
gas-phase species.1,4,6b,13
The removal of an electron from an organic molecule often
significantly affects its structure, relative thermodynamic stabil-
ity, and the barrier heights for isomerization and dissociation.1
Isomerization reactions are common for organic radical cations.
Indeed, the combined results obtained from mass spectrometry
experiments and high-level ab initio molecular orbital calcula-
tions have demonstrated that radical cations formed upon
removal of an electron from a stable neutral molecule can be
significantly less stable than their nonconventional isomers with
no stable neutral counterparts. Such nonconventional radical
cations include hydrogen-bridged radical cations and distonic
radical cations (i.e., ionized ylides, zwitterions, or biradicals).1-16
The 2-methoxyethanol radical cation CH3OCH2CH2OH•+ (1)
undergoes interesting unimolecular dissociation reactions,16-22
some of which have been proposed to involve hydrogen-bridged
(8) Postma, R.; van Helden, S. P.; van Lenthe, J. H.; Ruttink, P. J. A.;
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ReV. 1992, 92, 1649.
(14) Schaftenaar, G.; Postma, R.; Ruttink, P. J. A.; Burgers, P. C.;
McGibbon, G. A.; Terlouw, J. K. Int. J. Mass Spectrom. Ion Processes
1990, 100, 521.
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Org. Mass Spectrom. 1992, 27, 126.
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Spectrom. 1995, 6, 1030 and references therein.
† University of Joensuu.
‡ Purdue University.
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
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0002-7863/96/1518-3914$12.00/0 © 1996 American Chemical Society