Although mechanistic generalizations based on physical-organic chemistry have been relatively successful in providing rationalizations for the decomposition reactions in a large proportion of observed mass spectra, many of these generalizations show little apparent relationship to the quasi-equilibrium theory (QET), whose validity for small molecules appears to be well established. In the case of substituent effects on mass spectra, the absence of a quantitative correlation found for a number of systems has cast doubt on the basic assumptions of these mechanistic interpretations. This paper attempts to show that the effect of substituents on normal and metastable ion abundances, as well as ionization and appearance potentials, in 1,2-diphenylethanes is consistent with the predictions of the QET, as shown by rate constants calculated utilizing the RRKM theory with exact enumeration of states. A substantial number of factors have been identified which are correlated by U + or up+ substituent constants; many of these factors correspond closely to previously accepted spectra-structure relationships, such as the driving forces provided by product ion stability and bond lability. Other factors identified in this study were less well recognized previously; these include the possible magnitude of the kinetic shift and the effect of the activated complex geometry in simple cleavage reactions. The distribution of internal energy values of the molecular ionis a major determinant of ion abundances, as predicted; the photoelectron spectrum convoluted with the thermal energy appears to be a useful approximation of this distribution, providing a rationalization for a dramatic effect of temperature on metastable ion abundances. The overall consistency of the experimental results with the rate constants calculated using the QET approach suggests that these combined techniques may be valuable for study of the mass spectra of other larger molecules. elationships between the mass spectral behavior R of molecules and their structure have, in general, been developed using two basic approaches which have been found to be valuable for relating the chemical behavior of neutral molecules to their structure. The first approach utilized physical-theoretical principles, for which the quasi-equilibrium theory (QET)3*4 and thermochemical energy relationships5 form the bases for most investigations. The second approach uses the generalizations of mechanistic organic c h e m i~t r y ,~?~ such as those employing inductive, resonance, and similar effects. Applications of the QET to simple molecules such as propane have been quite successful, but extension of these calculations in a rigorous manner to much larger molecules is difficult with the methods of approximation and information on necessary molecular parameters which are presently a~a i l a b l e .~ Al-(1) Paper XIV: F. W. McLafferty and L.
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