Dissociation threshold energy

Wenthold, P. G. Squires, R. R. Biradical thermochemistry from collision-induced dissociation threshold energy measurements. Absolute heats of formation of ortho-, meta-, and para-benzyne, /. Am. Chem Soc. 1994, 776, 6401-6412. 

Resonance IR MPE/D is a universal phenomenon for polyatomic molecules, although the average MP absorption cross section depends on the degree of complexity of the molecule. Figure 11.7 presents experimental data on the absorption cross section (Tabs and, accordingly, the MP dissociation threshold energy fluence < o for a nmnber of simple and complex molecules. On the left of Fig. 11.7, approximate values of the vibrational level density are presented, whose increase substantially lowers the threshold IR MP dissociation energy fluence. 

Fig. 11.7 The growth of the density of vibrational levels of a molecule in line with its increasing complexity leads to an increase in its average IR MP excitation cross section and a decrease in its IR MP dissociation threshold energy fluence.

Dissociation Threshold Energies of Protonated Glycine, Glycinamide, and Some Small Related Peptides and Peptide Amino Amides. 

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

Similar calculations with consideration of the GP effect have also been reported [12]. A total of 24, 24, and 50 levels of Aj, A2, and E symmetries have been found below tbe dissociation threshold of the lower surface, —1.0560 eV. These are therefore genuine bound states the cone states lying above sucb a dissociation threshold are pseudobound states. The lowest levels of A, A2, and E symmetries are found to lie at —1.3475, —1.3438, and —1.3989eV, respectively. The notable feature is that the energy levels have been shifted due to the... 

S.3.4.2. Collision-induced Dissociation/Translationally-driven Reactions Tandem-mass spectrometry can be used to determine the threshold energy for collision-induced dissociation (CID),... 

Butler and co-workers have taken a unique approach to study the unimolecular dissociation of the vibrationally and rotationally hot allyl radical.150-152 They have examined the secondary C-H dissociation of the allyl radicals that are produced with high internal energies above the allyl dissociation thresholds in the primary photodissociation of allyl chloride and allyl iodide at 193 nm. The production of allene versus propyne (both at mass 40) from the secondary dissociation of the hot allyl radicals are... 

Merged-beam measurements 23-26 have consistently shown that the measured recombination cross section depends on conditions in the ion source. The authors have ascribed the effect to differing vibrational distributions. In one of the later measurements,16 the Hj vibrational state was inferred from the threshold energy for electron-ion dissociative excitation,... 

Next, we discuss the J = 0 calculations of bound and pseudobound vibrational states reported elsewhere [12] for Li3 in its first-excited electronic doublet state. A total of 1944 (1675), 1787 (1732), and 2349 (2387) vibrational states of A, Ai, and E symmetries have been computed without (with) consideration of the GP effect up to the Li2(63 X)u) +Li dissociation threshold of 0.0422 eV. Figure 9 shows the energy levels that have been calculated without consideration of the GP effect up to the dissociation threshold of the lower surface, 1.0560eV, in a total of 41, 16, and 51 levels of A], A2, and E symmetries. Note that they are genuine bound states. On the other hand, the cone states above the dissociation energy of the lower surface are embedded in a continuum, and hence appear as resonances in scattering experiments or long-lived complexes in unimolecular decay experiments. They are therefore pseudobound states or resonance states if the full two-state nonadiabatic problem is considered. The lowest levels of A, A2, and E symmetries lie at —1.4282,... 

The Vibrational Energies of Ozone up to the Dissociation Threshold Dynamics Calculations on an Accurate Potential Energy Surface. 

Unfortunately, as with all oversimplified theories, there are limitations for the application of the latter equation to ions close to the dissociation threshold, hi these cases, the number of degrees of freedom has to be replaced by an effective number of oscillators which is obtained by use of an arbitrary correction factor. [7] However, as long as we are dealing with ions having internal energies considerably above the dissociation threshold, i.e., where E - Eo)/E = 1, the relationship is valid and can even be simplified to give the quasi-exponential expression... 

Mass spectrometry is one of the experimental methods for determining bond dissociation enthalpies. The mass spectrometer can provide a measure of the appearance potential for a given reaction, that is, the threshold energy necessary to produce a particular set of particles. The appearance potential for the following reaction of H2,... 

An ion formed with an internal energy higher than the dissociation threshold but with a sufficient lifetime that it can exit the source and enter the mass spectrometer where it dissociates... 

Resonances in open systems, that is, systems whose total energy is higher than the first dissociation threshold, are an old theme of scattering dynamics. For very elementary introductions the reader is referred to, for example, Messiah (Ref. 1, Chapter HI), Cohen-Tannoudji et al. (Ref. 2, Chapter Xffl), Satchler (Ref. 3, Chapter 4), and Schinke (Ref. 4, Chapter 7). TTiey... 

The coupling between the angle y and the dissociation coordinate R is always large if Jacobi coordinates are used. At low energies deep inside the well, this coupling is linear and normal coordinates are usually better suited for interpretation and assignment than are Jacobi coordinates. However, if the molecular dynamics above the dissociation threshold is studied, the normal-mode picture breaks down and scattering coordinates have to be employed. 

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