Appearance energy decomposition

As noted after equation 4.17, the procedure to evaluate standard enthalpies of formation from appearance energies is somewhat controversial. When the threshold energies are determined from electron impact experiments, it has been argued that the correction terms (H%9S - Hq)a+ + (77298 - o)b - 6.197 in equation 4.17 should not be included in the calculation [66], Consider, for instance, reactions 4.21, and 4.22 where the ion CH2OH+ was produced from the decomposition of 1-propanol or methanol. 

Breakdown diagrams of CH4 and CD4 have been determined by a number of workers [131, 132, 797, 799, 877] and are explicable within the framework of QET. It was found [806] that there was no kinetic shift affecting the appearance energy of (CH3)+ from CH4. On the basis of translational energy releases measured with CH4 (and CD4), it has been suggested [798] that all rotational energy is available to assist the decomposition to (CH3)+ but that only 2 degrees of freedom contribute to formation of (CH2)t (from the molecular ion), cf. Sect. 2.3. 

Measured Appearance Energies and Partial Ionization Cross Sections at 70 eV for Various Fragment Ions Produced by Dissociative Electron Impact Ionization of TMS. The Fragment Ions are Arranged According to Their Decomposition Route, H loss by CH Decay, Removal of A Complete Methyl Group, and H Transfer from CH3 Groups, Respectively... 

The appearance energy of a cation A can be defined as the standard enthalpy of reaction (16), where a molecule AB in the gas phase at T= 0 K is ionized and excited to a state AB by means of an electron or a photon, followed by decomposition into the fragments A" and B, provided that A and B are in their ground states and all the species, including the electron, have zero translational energies. 

ABSTRACT. The use of mass spectrometry to collect data on the decomposition chemistry of nitramine compounds and the relevance of the data to the processes occurring in these materials when they are used in actu propellant and explosive applications is discussed. The simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) and time-of-flight (TOF) velocity-spectra techniques and then-application to the study of energetic materials are discussed. The means by which these techniques enhance the amount of information obtained from more conventional mass spectrometric experiments is illustrated with studies on the evaluation of the use of appearance energy measurements to study the thermal decomposition of HMX and on the identiHcation of the HMX pyrolysis products and the determination of their gas formation rates. 

The TOF velocity-spectra measurements can be used to evaluate the application of appearance energy measurements in mass spectrometry experiments to determine the initial products of HMX decomposition. In appearance energy measurements the energy of the ionizing electrons is lowered so that fragmentation of the ions is eliminated. For example, in experiments with HMX this would allow the 148 pyrolysis product to be observed from the Mowing reaction... 

---