Endothermic reaction thresholds

Guided ion beam tandem mass spectrometry has also been employed to study the reaction of Ni+ with CS2 and COS.2410 The ground state ion Ni+ undergoes endothermic reaction to form NiS+ in both cases, as well as NiCS+ and NiCO+, respectively. Threshold values for the reactions and bond dissociation energies for the products have been determined. 

Consider an endothermic reaction A + B - C + D, or an excitation process if C and D are the same as A and B, with a threshold energy Eth > 0. Suppose that the interaction between C and D is of short range. Let the lowest possible angular momentum of the relative C-D motion be L. Then, the dominant term in the reaction or excitation cross section a (E) at energies E near and above the threshold Eth is... 

The determination of thermochemistry using threshold collision-induced dissociation (TCID) relies on a deceptively simple experiment determine the energy threshold for the endothermic reaction (Eq. 1), the dissociation of the molecular ion AB+ ... 

The D2O intermediate represents a potential well 2.6 eV deep with respect to reactants, excluding activation barriers. Since, in addition, the reaction is endothermic by 1.9 eV, the threshold for dissociation of the complex to products is correspondingly high. Moreover, for an endothermic reaction to proceed via stripping, the impact energy relative to one atom must be greater than the endothermicity. This condition is met... 

The endothermic reaction (95) has been found to have a threshold energy of 0.2 eV (CM). Complex formation at energies slightly above the threshold and stripping at higher energies have been recognized. At around 4 eV (CM), two peaks were observed in the velocity spectrum of the product, as in the reaction CD4 + CD4 described above. A characteristic... 

Although reactions (100) and (101) have been shown to proceed for Hj having less than threshold quantities of internal energy, reaction cross-sections for such reactants are extremely small compared to those for H2 with sufficient vibrational energy. For the latter reactants, the reactions are exothermic in a strict sense. Thus reactions (100) (for H2 in v = 0—2) and (101) (for in = 0—1) must be considered to be the examples of very inefficient endothermic reactions. The endothermic reactions... 

Using a guided ion beam instrument the translational energy dependent reaction cross sections of endothermic fragmentation processes can be determined [32]. Modelling these cross sections ultimately yields their energy thresholds and a great deal of valuable thermochemical information has been derived with this technique. Precision of 0.2 eV can be obtained for reaction thresholds. Bimolecular reactions can also be studied and reaction enthalpies derived from the analysis of the cross section data. 

Reactive scattering or a chemical reaction is characterized by a rearrangement of the component particles within the collision system, thereby resulting in a change of the physical and chemical identity of the original collision reactants A + B into different collision products C + D. Total mass is conserved. The reaction is exothermic when i j,gj(CD) > j.gj(AB) and is endothermic when j.gj(CD) < j.gj(AB). A threshold energy is required for the endothermic reaction. 

Figure 3.17 Energy-level diagram for endothermic and exothermic reactions. For endothermic reactions, the threshold energy is equal to [(m, + M2)/M2JQ. ...

The vibrational energy required for an endothermic reaction usually equals the activation barrier of the reaction and is much lower than the energy threshold of corresponding processes proceeding through electronic excitation. For example, dissociation of H2 through vibrational excitation requires 4.4 eV the same process through excitation of an electronically excited state S+ requires twice as much 8.8 eV... 

Monitoring the thresholds of endothermic reactions by the guided ion-beam technique is one of the methods used to obtain the most accurate bond energies (see also Chapter 2.12). Similar measurements can be performed employing the FTICR technique. General types of endothermic reactions studied to obtain BDEs are as follows (Mt is a bare or ligated metal ion). 

---