Mass spectra bond dissociation energies

The mass spectrum of FClOs was measured (82, 138, 234). The vertical ionization potential ahd the F—ClOg bond dissociation energy were found to be 13.6 0.2 eV and 60 kcal mole , respectively. The average CIO bond dissociation energy and the heat of formation were estimated (82) to be 60 and —5.3 kcal mole", respectively. 

Stability considerations also play a role in molecular ion formation. Figure 15 illustrates that the intensity of MC1+ and MBr+ signals varies approximately exponentially with the bond dissociation energy. The mass spectrum of ice is dominated by H+(H20)n cluster ions, and M+(MX)n and X (MX)n clusters prevail in the spectra of MX alkali halides with n extending up to at least 100 [16]. 

Since O2F2 has not been observed in the O2F2 mass spectrum, the bond dissociation energy D(F-02F ) may be zero [1], see also [2]. 

From the infrared spectrum o of SnH4, the estimated bond dissociation energy for Sn-H is 73-7 kcal/mole, which compares well with the value of 70-3 kcal/mole obtained as a mean dissociation energy for Sn-H from mass spectrometry measurements . The thermochemical bond z energy of Sn-H has been obtained as 60-4 kcal/mole. 

The results of elctron-impact studies of phosphine by Halmann et al. are given in Table 3a. The authors used the appearance potentials, in conjunction with thermochemical data, to choose the probable reaction processes. In many simple cases the observed appearance potential A (Z) for an ion fragment Z from a molecule RZ is related to its ionisation potential 7(Z) and to the energy of dissociation 7)(R—Z) of the bond by the expression A (Z) = /(Z) + D (R—Z). This assumes that the dissociation products are formed with little, if any, excitation energy, and that /(Z) < /(R). The most abundant ion species in the usual mass spectrum of phosphine is PH, which is probably formed according to the following mechanism... 

A secondary hydrogen isotope effect has been observed in the loss of methyl radical from the -butylbenzene ion. In the El mass spectrum, the isotope effect IchJIct>3 was 1.1 and for metastable ions the isotope effect ranged from 1.5 to 1.9 [642], The isotope effect in the mass spectrum is equal to the ratio of the stretching frequencies of the dissociating bonds. The isotope effect on the metastable ions has been discussed in terms of an excited electronic state and specific radiationless transitions. The isotope effect is, however, explicable within QET. If the critical energy for CHj loss is 3.4 kJ mole-1 less than that for CDj loss, an isotope effect of about 2 is predicted by QET for metastable ions [853]. 

In summary, ionisation potentials, dissociation and cohesive energies for mercury clusters have been determined. The mass spectrum of negatively charged Hg clusters is reported. The influence of the transition from van der Waals (n < 13), to covalent (30 < n < 70) to metallic bonding (n > 100) is discussed. A cluster is defined to be metallic , if the ionisation potential behaves like that calculated for a metal sphere. The difference between the measured ionisation potential and that expected for a metallic cluster vanishes rather suddenly around n 100 Hg atoms per cluster. Two possible interpretations are discussed, a rapid decrease of the nearest-neighbour distance and/or the analogue of a Mott transition in a finite system. Electronic correlation effects are strong they make the experimentally observed transitions van der Waals/covalent and covalent/metallic more pronounced than calculated in an independent electron theory. 

The dissociation is facile because the bond energy for the dissociation is only 15 kcal/mol for acetic acid and 11 kcal/mol for ammonia. The net effect of the complete process [Eqs. 1.20 and 1.21] is equivalent to a proton transfer from the ionized basic site to the ionized acidic site. Thus the molecular weight of the protein was not changed, but the positive and negative groups were neutralized. Thus a clean mass spectrum of the protein is obtained without a mass change. 

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