F. P. Lossing

Heats of formation of organic radicals from appearance energies

Abstract The heats of formation of eighteen organic radicals have been measured from appearance energies for fragmentations in which the daughter ion structure and heat of formation are known. The agreement with values obtained from kinetic measurements is generally good.

Stabilization energy and positional effects in halogen-substituted alkyl ions

Abstract The heats of formation, ΔH0f, of a series of halogen-substituted alkyl ions have been determined by appearance energy measurements with energy selected electrons. Halogen substitution at the formal charge site significantly reduces the heterolytic bond strength D[R+H−], the effect being not greatly different for Cl, Br and I. The β-substituted halo-ethyl ions clearly show stabilization resulting from the formation of bridged halonium ions but this effect is absent in the propyl and butyl analogues.

Concerning the heats of formation of CH2NH and CH2NH

Energy-resolved electron impact was used to measure the appearance energy of CH 2 NH + from azetidine and propargylamine and the adiabatic ionization energy of CH 2 NH formed in the pyrolysis of azetidine. The heats of formation of CH 2 NH and CH 2 NH + were determined to be 22 ± 3 and 250± 2 kcal mol −1 , respectively, although the presence of a competitive shift makes the results upper limits. A review of past thermochemical determinations leads to a selected value for Af H 298 0 (CH2NH) of 21 ± 4 kcal tool −1 .

Bond strengths in even-electron ions and the proton affinities of free radicals

Abstract Hydrogen atom affinities of even-electron ions have been shown to be dissimilar to homolytic bond dissociation energies in analogous neutral molecules. The proton affinities of free radicals are, in general, not the same as those of appropriate neutral molecules (each being protonated at the same functional group). These results are discussed with respect to their utility in estimating ionic heats of formation.

The effects of methyl substitution on the structure and thermochemistry of the cyanomethyl radical and cation

Energy-resolved electron impact appearance energy (AE) measurements were performed to determine the heat of formation of the cyanomethyl radical and cation as well as the ionization energy (IE) of the methyl substituted analogues, CH3ĊHCN and (CH3)2ĊCN. The results were ΔfH0298 (CH2CN) = 58 ± 3 kcal mol−1, ΔfH0298 (+CH2CN) = 287 ± 1 kcal mol−1, IE (CH3ĊHCN) = 9.25 ± 0.02 eV and IE ((CH3)2ĊCN) = 8.48 ± 0.02 eV, the latter two values yielding the corresponding cationic heats of formation, 263 ± 2 kcal mol−1 and 236 ± 2 kcal mol−1. The results were used to explore both π delocalization and the predictability of thermochemical properties in nitrile substituted cations and radicals.

The stability of [CCl4]+·, [Cl2CClCl]+·, their dications, and neutral counterparts

Electron impact ionization of tetrachloromethane has been shown to produce stable [CCl4]+· ions; their heat of formation is ⩽ 237 kcal mol−1. The ylid isomer, [Cl2CClCl]+·, generated upon dissociative ionization of Cl3CCOCl, is remarkably more stable; ΔH0f([Cl2CClCl]+·) < 224 kcal mol−1. The values for both isomers are significantly lower than calculated values. The corresponding neutrals, CCl4 and Cl2CClCl, are not stable on the time and energy scale of neutralization—re-ionization experiments. Although [Cl2CClCl]2+, was calculated to be stable, it was not present in the charge-stripping mass spectrum of the ylid ion.