Estimation of Free Radical Ionization Energies by the Kinetic Method and the Relationship between the Kinetic Method and the Hammett Equation

Abstract

Substituted 1,2-diphenylethanes undergo competitive dissociations upon electron ionization (EI) to generate substituted benzyl cation and benzyl radical pairs. Application of the kinetic method to the previous reported EI mass spectra of these covalently bound precursor ions (data are taken from McLafferty et al. J. Am. Chem. Soc. 1970, 92, 6867)) is used to estimate the ionization energies of substituted benzyl free radicals. A correlation is observed between the Hammett σ constant of the substituents and the kinetic method parameter, ln(k(x)/k(H)), where k(x) is the rate of fragmentation to give the substituted product ion and k(H) is the rate to give the benzyl ion itself. Systems involving weakly bound cluster ions, including proton-bound dimers of meta- and para-substituted pyridines and meta- and para-substituted anilines, and electron-bound dimers of meta- and para-substituted nitrobenzenes, also show good correlations between the kinetic method parameter and the Hammett σ constant.

Dissociation of ionized benzophenones investigated by the kinetic method: effective temperature, steric effects and gas‐phase CO+• affinities of phenyl radicals

“…For example, Wenthold and Squires estimated Δ H acid of halogen‐substituted aromatic compounds by hydroxide‐induced cleavage and found the effective temperature of the siliconate ion to be of the order of 4400 K 45. In this laboratory, we found effective temperatures of 3073 ± 500 and 2527 ± 150 K for the EI dissociation of substituted 1,2‐diphenylethanes at 70 and 15 eV, respectively 41. In addition, Meurer et al 42 observed an effective temperature of 1705 K for the dissociation of ionized α‐diketones under mild conditions (CID at 5 eV collision energy).…”

“…Similar results were obtained by McLafferty et al ,1 as already mentioned in the Introduction. It is important to state that the Hammett equation is closely related to the kinetic method approach, as demonstrated previously 41…”

“…In cases where formal covalently bound precursor ions are involved, the same concepts can be applied in making thermochemical measurements if the conditions required for application of the method are fulfilled. For instance, the applicability of the kinetic method to allow estimates of the ionization energies ( IE ) of benzyl radicals by measuring the relative abundances of the two benzyl cations in the EI mass spectrum of substituted 1,2‐diphenylethanes41 has been demonstrated. More recently, Meurer et al 42 verified that the natural logarithm of the abundance ratio of both the acylium ion fragments [R 1 CO] + and [R 2 CO] + (summed to those of their respective secondary fragments) formed upon low‐energy (5 eV) collision‐induced dissociation of several ionized α‐diketones correlates linearly with the IE s of the corresponding R 1 CO • and R 2 CO • free radicals.…”

The Ionization Energy of CF3: When Does Entropy Matter in Gas-Phase Reactions?

“…The reference value of IE a (CF) provides an internal standard, which is lacking in the absolute appearance energy measurements and which is expected to compensate for most of the systematic error. Formally, this is similar to the mass spectrometric “kinetic method” of measuring the relative energetics of competing ion−molecule reactions. , In that context, it would be interesting to see the data of ref plotted and analyzed as a branching ratio. Another alternative analysis method would be in terms of competing kinetic processes, as described recently for the collision-induced dissociation of multiply ligated ions …”

Gaseous cation chemistry and chain‐length effects in electron ionization and collision‐induced dissociation mass spectraof symmetric 1,n‐bis(9‐anthracenyl)alkanes

“…Cooks' kinetic method typically uses observed abundance ratios of ions formed competitively from a common cluster ion to find their thermochemical properties when their structures are known 46, 47. There are reports of determination of relative ionization energies ( I ) by this method,48–51 and the following description of the method is derived from these. We shall take a different approach, however, using such observed abundance ratios (Table 3) with theoretical I to find the best fit among plausible isomeric structures and thus fragmentation mechanisms 52…”