The mass spectra of the title compounds show peaks corresponding to the primary fragmentation of H, CH, and (H, + C,Hz). Kinetic study suggests that the molecular ion of 1phenylcyclobutene and 2-phenyl-l,3-butadiene loses H and CH8 through formation of a common intermediate. This is confirmed by appearance potential and ionisation potential measurements, fragmentation of deuterium labelled compounds and the corrected Z values.
IN AN earlier study1 of the mass spectrometric fragmentation of isomeric hydrocarbons, we studied the variation of [F]+/[M]+ratios (2 values) as a function of electron energy (E), where [F]+ and [MI+. are the concentrations of a primary fragment and of the molecular ion, respectively. We observed that the 2 functions could be described by straight lines when E varied from threshold to 10 to 15 eV above threshold. We suggested that comparison of the ratio of the slopes of Z functions corresponding to two competitive fragmentations (2, and Z,, Scheme 1) could serve in detecting whether two isomeric molecular ions [Ml]+and [M,]+. rearrange to a common intermediate [c]+. before further fragmentations. In that case, the ratio slopes of the 2, and 2, are equal for both compounds M, and M,.
MI [MI]+'
r r i +.
\ SCHEME 1
We also noted that in such cases the straight lines, 2, or Z , do not necessarily overlap but may be parallel. We interpreted this observation by suggesting that the decay processes were represented by the same k(E) and P(E) functions (those corresponding to the overall energy distribution in the common intermediate [c]+*), and that the parallelism of the 2 functions was a consequence of a constant difference in the internal energy of [el+. when generated from either M, or M,. We wish to report here a further study of the mass spectrometric fragmentation of the two hydrocarbons I and I1.t t For 70 eV spectra of compounds I and I1 see Ref. la. a $ 91, 352 (1969).
2 These values are consistent with ions with a I-methylindene-like structure5 (calc AHi(C,,H,) = 225 kcal/mol). These values are consistent with ions with an indene-like structure6 (Calc AHi(C,H,) = 250 kcal/mol).
0 B ' These values were calculated by assuming a primary loss of an hydrogen molecule from the molecular ion. A value of 279 kcal/mol is reported for the AHi of the phenylacetylene molecular ion: J.