Ionization and Dissociation by Electron Impact; Methylene, Methyl, and Methane

Abstract

The appearance potentials of C+, CH+, and CH2+ in the mass spectra of methylene, methyl, and methane are found to be mutually consistent and when combined with the spectroscopic value of the ionization potential of the carbon atom (11.26 ev) lead to 15.5±0.5 ev/molecule for the heat of atomization of methane. This value (15.5) when further combined with the heat of formation of methane (—0.78 ev/molecule) and the dissociation energy of H2 (4.48 ev/molecule) yields 5.76 ev/atmos or 133 kcal/mole for the heat of sublimation of graphite. The set of appearance potentials confirmed the spectroscopic value for the heat of dissociation of CH+ (3.6 ev/molecule) and yield the following additional energetic quantities: D(CH2–H) = 3.75±0.3 ev/molecule, D(CH–H) = 3.99±0.3 ev/molecule, D(CH3+–H) = 1.34±0.1 ev/molecule, D(CH2+–H) = 5.54±0.2 ev/molecule, and D(CH+–H) = 3.37±0.2 ev/molecule. Earlier determinations of Iz(CH4), Iz(CH3) and D(CH3–H) are confirmed (within the experimental error) and the ionization potential of the methylene radical, Iz(CH2), is found to be 11.75±0.1 ev/molecule. Combination of the appearance potentials of CH2+ in the methane and diazomethane mass spectra gives 46±6 kcal/mole for the heat of formation of diazomethane.

Photodissociation dynamics of the CNN free radical

“…Our heat of formation is just outside the error bars of the value derived from recent photoelectron and proton affinity measurements of Clifford et al However, the value obtained by Clifford et al involves a thermochemical cycle referenced to ⌬ r H 0 (H 2 CNN) for which the authors adopt a value of 2.85 eV based upon ab initio calculations, and this may be the cause of the discrepancy. Experimental 13,[38][39][40][41][42] and theoretical 15,[43][44][45][46][47] values for the heat of formation of diazomethane (H 2 CNN) vary from 2.2-3.3 eV.…”

The Reactions of Active Nitrogen With Organic Molecules

“…(38), so t h a t it would be expected to occur if less than about 20 ltcal. involved in reaction [21] resided as translational energy and vibrational excitation of the hydrogen cyanide. Since it certainly requires more than 7 kcal.…”

A Theoretical Investigation of the Structure and Bonding of Diazomethane, CH2N2

“…Although quite a few theoretical papers have appeared with the purpose of elucidating the electronic structure of the ground state of CH 2 N 2 , − there does not seem to exist a convergence of opinion upon the matter. In addition, concerning the CH 2 −N 2 bond dissociation energy, the existing experimental and/or theoretical data are in conflict. − …”

“…In addition, concerning the CH 2 -N 2 bond dissociation energy, the existing experimental and/or theoretical data are in conflict. [21][22][23][24][25][26][27] Most of the theoretical papers published employ small basis sets (minimal to DZ) in conjunction with the Hartree-Fock (HF) methodology and very limited CIs. Walsh and Goddard, 9 using a GVB(PP)-CI approach with a DZ basis and the experimental equilibrium geometry, 28 suggest that the CH 2 N 2 ground state is a singlet biradical (H 2 C ˙-N ¨dN ˙:), with the in situ CH 2 moiety in the ground 3 B 1 state and the N 2 in its B 3 Π g state.…”