J. Phys. Chem. 1992, 96, 5212-5216
5272
measurable peak energy of the CTTS absorption band. This choice controls the value of E, and while hv,,, should correspond to absorption at the most probable solvent configurationaround the anion, it is still somewhat arbitrary in view of the breadth of CTTS spectra. However, it is observed that this band breadth is fairly constant for many anions.35aThus, if hu, is used consistently in eq 6 for all the anions, and with the same value of E (eq 7), the effects on R of any uncertainty in assigning the CTTS level are minimized. (36) If AS can be neglected, Ex(sol) is actually the absolute reduction potential. This is indicated by the expression used earlier to calculate Ex(sol) of organic molecules from their El/2: Ex(so1) = 412 + 4.4.16 (37) A similar conclusion can be reached from an earlier estimate that (in our notation) AHs(H+) + B - Rs 12 eV.'j But the value of Atfs(H+), the enthalpy of solvation of H+,is 11.7 eV (Friedman, H. L.; Krishnan, C. V. In Water, Comprehensive Treatise; Franks, F.,Ed.; Plenum Press: 1973; Vol. 3,pl),i.e.,E-R-0.3eV. (38) Since in some cases the difference may yet be appreciable, Ex(sol) should not simply be replaced by hvcm as is done in ref 26. (39) Herzberg, G. Injrared and Raman Spectra; Van Nostrand: Princeton, NJ, 1945. (40) Comprehensive Inorganic Chemistry; Bailar, J. C., Emeleus, H. J., Nyholm, R., Trotman-Dickenson,Eds.; Pergamon: New York, 1972; Vol. 2, pp 1368, 1416. (41) Shizuka, H.; Nakamura, M.; Morita, T. J . Phys. Chem. 1980, 84, 989. (42) Atkins, P. W.; Symons, M. C. R. The Srructure of Inorganic Radicals; Elsevier: Amsterdam, 1967; Chapter 8. (43) Zachariasen, W. H.; Buckley, H. E. Phys. Reu. 1931, 37, 1295.
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(44) Delahay, P.; Dziedzic, A. J. Chem. fhys. 1984, 80, 5793. (45) Stanbury, D. M.; Lednicky, L.A. J . Am. Chem. Soc. 1984,106,2847 and references therein. (46) Stanbury, D. M.; de Maine, M. M.; Goodloe, G.J . Am. Chem. Soc. 1989, 111, 5496. (47) Ram, M. S.; Stanbury, D. M. J . Am. Chem. Soe. 1984, 106, 8136. (48) In this connection, see: Fairbank, M. G.; McAuley, A. Inorg. Chem. 1987, 26, 2844 and references therein. (49) Ram, M.S.; Stanbury, D. M.J . fhys. Chem. 1986,90, 3691. ( 5 0 ) For AQS/NO,- and NQ/N02-, ORMx was determined from the intercepts of the @,/OR vs [NO2-] plots (see text). (51) As a typical group I anion, SCN- reduces the triplet at concentrations which are much higher than that required for q~enching.~.'~ (52) This value is somewhat higher than that reviously reported." (53) The difficulty in directly determining kqNc-for the NQ system (see Experimental Section) may reflect interference of some overlapping absorp tion. This may perhaps originate from an exciplex, a possibility which should be further investigated. The lifetime of triplet NO2- in aqueous solution is unknown. Phosphorescencelifetimes of frozen solutions (alcoholic mixtures at 77 K) of various heav metal nitrites have values varying from 0.25 s (Cd(N02)2)to -5 X IO-Y s ((Pb(N02), and TINO,) (Maria, H. J.; Armstrong, A. T.; McGlynn, s. P. J. Chem. fhys. 1968,48,4694). The natual lifetime of triplet NO2- in concentrated N a N 0 , solution, as estimated from the area of the SI T I absorption, is 0.1 s (see: Maria, H. J.; Wahlborg, A.; McGlynn, S. P. J. Chem. fhys. 1968,49, 4925). (54) See, e.&: Levine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics; Oxford University Press: London, 1974; pp 100-103, 217-221.
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Blmolecular Reactions of the @-Dlstonic Isomer of the Ethanol Radical Cation: 'CH2CH20H2+ Krista G. Stirk and Hikka I. Kenttiimaa* Department of Chemistry, Purdue University, West hfayette, Indiana 47907 (Received: January 21, 1992; In Final Form: March 17, 1992)
The first experimental study on the properties of the long-lived,low-energy radical cation 'CH2CH20H2+is reported. Bimolecular reactions of this ion and its conventional isomer, the radical cation of ethanol, have been investigated in a dual-cell Fourier transform ion cyclotron reSonance mass spectrometer. Strikingly different reactivity is observed for these two radical cations. The distonic ion undergoes thermoneutral exchange of a water molecule when reacted with 2H-or 1801abekedwater. Acetonitrile readily replaces water in this ion, as well. The ethanol radical cation predominantly reacts by proton transfer with all of these reagents. The same is true for protonated ethanol. Thus, the chemical properties of the distonic ion do not reflect the ground-state structure, a covalently bound, protonated radical, which is expected to react like protonated ethanol with bases and nucleophiles. The observed reactivity is rationalized on the basis of an electrostatically bound intermediate, in accordance with the description of the distonic ion as a loosely bound ion-dipole complex of ionized ethylene and water.
Introduction The 8-distonic' isomer of the radical cation of ethanol, 'CH2CH20H2+,has been the subject of intensive research for over 10 years. This ion was first introduced2 in 1976 by Golding and Radom, who proposed, on the basis of theoretical calculations, that certain protonated radicals may occur as intermediates in reactions catalyzed by adenosylcobalamin (a derivative of vitamin B,z). The ion 'CH2CH20H2+was calculated to be about 10 kcal/mol lower in energy than the radical cation of ethanol and to have a relatively large binding energy with respect to C2H4" and H 2 0 (20 k ~ a l / m o l ) . ~ JThese initial theoretical studies2" sparked a wide interest in this unusual isomer of ionized ethanol. Terlouw et ale4reported in 1981 the first experimental study on the ion 'CH2CH20H2+. They discovered that the fragment ion C2H60*+of 1,3-propanediol has a structure for which no stable neutral counterpart exists: the dissociation reactions of this ion are distinct from those of the two conventional isomers, CH3CH20H'+and CH30CH3'+. It was proposed that this new C2H60'+ ion could be represented as 'CH2CH20H2+,or as a a-bonded complex:
The heat of formation of the ion (AHf = 175 f 2 kcal/mol) was experimentally determined in 1982 by Holmes et alasaby a p pearance energy measurements. A few years later, McLafferty et al. used neutralization-reionization mass spectrometry to show that ionized ethanol and its &distonic isomer are stable toward isomerization:
