Exothermicity, potential energy surfaces

The potential energy surface [47] for this reaction (Fig. 5) shows many potentially competitive pathways, labeled A-F, leading to the two most exothermic product channels. Many of these pathways can be isotopically separated by reaction of 02 with HCCO in normal abundance, as diagramed in Fig. 5. Zou and Osbom used time-resolved Fourier transform emission spectroscopy to detect the CO and CO2 products of this reaction [47]. Rotationally resolved infrared (IR) spectroscopy can easily identify all the possible isotopologs. For example. Fig. 6 shows a single... 

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

Studies of kinetic energy release distributions have implications for the reverse reactions. Notice that on a Type II surface, the association reaction of ground state MB+ and C to form MA+ cannot occur. In contrast, on a Type I potential energy surface the reverse reaction can occur to give the adduct MA+. Unless another exothermic pathway is available to this species, the reaction will be nonproductive. However, it is possible in certain cases to determine that adduct formation did occur by observation of isotopic exchange processes or collisional stabilization at high pressures. 

FeO O) (g), is more exothermic by 13 eV than for the +2 ion. Relationship to Potential Energy Surface Diagrams... 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

Semi-empirical PM3 calculations" reveal that ylide 23 is a minimum on the potential energy surface and that both steps are exothermic. The enthalpy of the reaction of ylide formation in CH3CN was estimated to be —43 kcal/ mol and the enthalpy of reaction of the second step, 1,2-hydrogen shift, was calculated to be —12.5kcal/mol. 

Fig. 1. The potential energy surface for the nuclear motion in the cases of electron localization on the core of the donor, Jl/ (q), and on the core of the acceptor, 7/r(<7). q is the nuclear coordinate, -tt,(q ) is the activation energy of the electron transfer in the case of the classical nuclear motion, J is the reaction exothermicity, and Er is the reorganization energy.

Fig. 5. The potential energy surfaces of the initial and final states at different ratios between the reaction exothermicity, J, and the reorganization energy, Er. As a rule, for exothermic reactions in condensed media, J is assumed to be equal to the change of the Gibbs free energy, - AG°.

Further support for structure 7a is obtained by AMI molecular orbital calculations performed on the reaction of C o and silylenes (Ph2Si ). These calculations show that Ph2Si adds across the junction of two six-membered rings in Cgo to give the 1,2-addition silirane analogous to 7a with an exothermicity of 61.3 kcalmol-1. The analog of 7b was not located on the potential energy surface. Also, 7a was 19.4 and 10.7 kcalmol-1 more stable than the 1,6-adducts 11a and lib, respectively (Scheme 4). 

General Features of Late Potential Energy Surfaces for Exothermic Reactions... 

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