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8.84 v. for Ivert(C6HsO). As would be expected, this is considerably higher than the ionization potential of the isoelectronic benzyl radical (7.76 v.).l 4 The appearance potential of the C6H50+fragment ion from anisole was found to be 11.92 f 0.1 v., in good agreement with the value obtained by Harrison, et ~ 1 . l Assuming this process to be a simple bond rupture in
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carbon atom as one of the vicinally coupled protons.' In vinyl compounds (11) it has been demonstrated that the cis-vicinal coupling constant (JH") is more sensitive than the trans-vicinal coupling constant (J"") t o the electronegativity of R," in qualitative ~agreement with the theoretical requirements.
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(14) J . B. Farmer, I . H. S. Henderson, C. A. McDowell, and F. P . Lossing, J . Chem. Phys., aa, 1948 (1954). (15) J . h l . S. T a i t , T . W . Shannon, and A . G. Harrison, J . A m . Chem. S O L ,84, 4 (1902). (16) H. A. Skinner, "Modern Aspects of Thermochemistry," Lectures, hlonographs and Reports S o . 3 , T h e Royal Institute of Chemistry. London,
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/-
\H
c=c
K
which the structural identity of the C6H50fragment is preserved, the relationship D(R-Y) .4(R+) - I(R) gives D(C&,O-CH3) L 71 kcal./mole. Evidence for the retention of the phenoxy structure in C6H50+ions from anisole a t the dissociation threshold is provided by the results of Harrison, et al., who found meta-para orientation to be preserved in R C s H 4 0 +ions produced in the dissociative ionization of RC6H40CH3 isomers. One might reasonably expect the 0-CH3 bond in anisole to be appreciably lower than this limit, however. The phenoxy radical is isoelectronic with benzyl radical, and should therefore exhibit considerable resonance stabilization. To a very rough approximation, then, D(CH30-CH3) - D(C6HsO-CH3) 2 D(CHx-CH3) D(C6H5CH2-CH3). Taking AHf(CH3) = 32.5 kcal./ mole16 and AHf(CsH5CHz) 43 kcal./mole,'' and the standard heats of formation of ethane and ethylbenzene,18 D(CH3-CH3) - D(C6HsCHz-CH3) 17 kcal. /mole. If D(CH30-CH3) = 77 k ~ a l . / m o l e , 'then ~ D(C6H50CH3) is: 60 kcal./mole. On this basis, the appearance potential of C6H50+ from anisole would include 11 kcal./mole of excitational energy. The validity of such a comparison is rather doubtful, however. Other phenoxy derivatives (phenol, phenyl ethyl ether, diphenyl ether) have such small intensities for C6H50+ ion that estimates of AHf(C6H50f)cannot be made from them. The value given above for A(C6H60+), together with AHr(aniso1e) = - 18.5 kcal./mole, leads to AHf(C6H50f) 224 kcal./mole and AH
