Appearance Potentials and Apparent Heats of Formation of the Methylene and Difluoromethylene Ions

Abstract

IN an earlier communication¹ we made use of the appearance potentials of the fluorobenzene ions from some trifluoromethyl benzenes to obtain an estimate of the heat of formation of the difluoromethylene radical eliminated in these electron impact processes. This work gave values of about − 20 kcal mol⁻¹ as the upper (most positive) limits for the heat of formation of the radical, compared with estimates of about −35 kcal mol⁻¹ given by other workers^2,3. If the latter estimates are the more correct, it would seem that the difluoromethylene radicals are eliminated in these impact processes with excess energy. Such behaviour is not unlikely in view of the demonstration that carbon monoxide is eliminated from benzalde-hyde by an electron impact process that involves excess energy (of about 3 eV)³. In an attempt to discover more about the thermochemistry of this radical, an investigation has been made of the appearance potentials of the difluoromethylene and ethylene ions. The measured appearance potentials (A.P.) and the derived apparent heats of formation of the ions, calculated using equations of the form: ΔH_f(CX₂⁺) = A(CX₂⁺) − ΔH_f(fragment) + ΔH_f(molecule) are given in Table 1. The apparent heats of formation derived from the methanes were calculated on the assumption that the other products of the impact processes were diatomic molecules. The other products of the impact processes involving ethylenes were assumed to be methylene radicals. If the impact processes involving the methanes had been assumed to produce atoms as the other fragments, then much smaller values of the apparent heats of formation would have resulted. In particular, the values for CF₂⁺ and CH₂⁺ would be about 160 kcal mol⁻¹ and 245 kcal mol⁻¹ from CF₂HCl and CH₃Cl, respectively. The assumption that these are the values associated with the minimum of excess energy would lead to greater thermochemical inconsistencies than do those we have chosen. The lowest values for the apparent heats of formation of the methylene ion are those derived from the methanes and are represented by the value 339 ± 2 kcal mol⁻¹. Although it is uncertain that this value includes no excess energy, we can be sure that the larger value found for the case of ethylene does include about 30 kcal or more excess energy. The value derived from CH₂=CF₂ will contain as little excess energy as the minimum derived from the methanes if the heat of formation of the difluoromethylene radical is –20 kcal mol⁻¹, but more if the heat of formation of this radicalis more negative, as is favoured by some workers. The spectroscopic value of the ionization potential of methylene is 10.35 eV (ref. 7). It follows from this and the heat of formation of the ion that the heat of formation of the methylene radical is equal to or less than 100 kcal mol⁻¹. This limit includes all the proposed values⁴.