24 -
I
j
1 6 - - 6 8 10 12 14Cp2^ Electronic energy/eV
F ig u re 6.4 Calculated cross-sections for the electron-transfer reaction between (CFz^^)* and NH3 as a function o f the
electronic excitation of the resulting Cp2'^ relative to its electronic ground state. The vertical line represents
the term energy o f the 4<3i electronic state o f that is assumed to dissociate to and 2F. The two curves correspond to two possible energies o f excited states of the reactant dication at 35 and 37.5 eV above the ground state o f CF2.
The fact that the signals for the ion are so weak is indicative of the relatively low abundance of the excited state of the reactant dication in the ion beam. A crude estimate may be made of the proportion of the dications in the excited state in the ion beam from the relative intensities of the product ions (Table 6.1). This estimate suggests that the ion beam contains ~1% o f dications in the excited electronic state. As stated above, the earlier work of Manning et al [5] also reported the presence of an excited state of in their dication beam. Although the earlier study made no mention of the abundance of the excited state, a consideration of the relative intensities indicates that the two beams contained similarly low abundances of the excited state.
The cross-section values listed in Table 6.10 are for the production of in the Aa\ electronic state, which, as stated above, has an energy of 11.7 eV with respect to the CFz^ ground state. This state lies higher in energy than both the asymptotes for the dissociation of CF]^ to C^4-2F and + CF which have energies of 10.5 eV and 11 eV with respect to the CF]^ ground state. Hence, the Aa\
electronic state of CF]^ can potentially dissociate to give either + 2F or F^ 4- CF. However, no
signal is observed in our experiments. So the appearance of a weak signal in our experiments perhaps indicates that the higher-lying excited electronic states of CF]^ preferentially dissociate to give C^-h2F.
6.4.2 Bond-forming reactivity
As stated in section 6.3, electron-transfer reactions dominate the product ion yield following collisions between the neutral NH3 and the reactant dication. However, the mass spectra indicate a small but significant signal due to the HNCF^ product ion at all the collision energies errçloyed in these experiments.
The observation o f HNCF^ is the first detection of a bond-forming reaction following collisions between molecular dications and a polyatomic neutral target. Hence, the available data relating to the bond-forming reaction for the CFi^'^/NH] collision system is non-existent. However, our data allows us to draw the first tentative conclusions regarding the mechanism of the bond-formation. The collision energy-dependence of the bond-forming reaction and electron-transfer cross-sections of this reaction system may be compared to other systems that have exhibited bond-forming reactivity and have been studied to a greater extent. Examination of Figure 6.2 clearly shows a very different collision energy- dependence of the bond-forming yield in this collision system than in the case of the corresponding reactions exhibited by the and D2/H2 collision systems. In the case of the CF2^"^ and D2/H2 collision systems, described in Chapter 5, the relative cross-section for bond formation rises steadily with decreasing collision energy [2]. This collision energy-dependence is indicative of a reactive pathway in which there is no energy barrier [19]. However, in the case of the bond-forming reaction between CF2^'^ and NH3, the yield reaches a maximum at centre-of-mass collision energies between 2 and 3 eV. Such a maximum can be considered to arise because of an energy barrier [19] which, in the absence of tunnelling, must be surmounted before the reaction can occur. Hence, at lower collision energies (< 2 eV) the yield of the HNCF^ ion is low as relatively few collision partners have sufficient energy to surmount the energy barrier. At centre-of-mass collision energies in excess of 3 eV, the ratio of cross-sections decreases because of a reduction in the reaction time, that is an effective reduction in the length of time in which the reactants are sufficiently close enough for chemical rearrangement.
The fundamental differences in the collision energy-dependencies of the CF2^^/H2/D2 and CF2^^/NH3 collision systems point to significant differences in the respective reaction mechanisms. This may also account for the much higher cross-sections for the bond-forming reaction in the
collision system. The bond-forming reaction for the CF2^ % 2/D2 collision system has been found to proceed via a direct, spectator-stripping mechanism [2,19-23]. Hence, the bond-forming reaction for the CF2^^/NH3 collision system would seem to proceed via a different reaction mechanism. Perhaps the HNCF^ ion is formed following the formation of a collision complex, as in Equation 6.10. The generation of a complex would certainly be assisted by the presence of the dipole moment associated with NH3, as such a dipole moment would invariably increase the interaction between the reactants leading to a ‘sticky’ collision and the formation of a complex.
C F f + N H 3 [H3NCF2] HNCF* + H* + H + F (6.10)
6.5 Conclusion
Relative yields of the electron-transfer and bond-forming product ions, following collisions between and NH3, have been recorded as a function o f the centre-of-mass collision energy. The relative cross-sections of the competing reaction processes are obtained from the recorded product ion intensities. The ion yields indicate that the two electron-transfer reactions leading to the formation of the CF^ and CF]^ product ions dominate the interaction of the dication with the neutral NH3 collision partner. However, a bond-forming reaction pathway that leads to the generation of the HNCF^ product ion is a significant reaction channel. The identification of a very weak ion signal is, in the absence of a charge-separating reaction channel, thought to arise through dissociative electron-transfer. This reaction cannot proceed with the reactant dicatiOn in its ground electronic state, and hence provides evidence for the presence of electronically excited dications in the reactant dication beam.
Our results indicate that the bond-forming reaction displays a significant collision energy- dependence. In this case, the ratio of the cross-sections of the bond-forming reaction to those of the electron-transfer reaction processes reaches a maximum value at a collision energy of approximately 2- 3 eV in the centre-of-mass frame. This behaviour points to the existence of a barrier to reaction. The CF2^^/NH3 collision system displays significant differences, both in terms of its collision energy- dependence and in the magnitude of its cross-section for bond formation, to the collision system, which has been found to undergo bond-forming reactions via a direct reactive pathway. The differences in the behaviour of the two collision systems point to a different reaction mechanism operating in the CF2^^/NH3 collision system. The presence of the dipole moment associated with the
NH3 reactant would tend to increase the interaction between the reactants. Such an increased interaction could lead to sticky collisions and would therefore favour complexation. The cross-sections for reactive pathways which involve complexation are usually much lower than those of reactions in which a direct mechanism operates. Hence, the operation of a complexation mechanism in the bond- forming reaction between ^ 2^"^ and NH3 would be consistent with the observed behaviour.
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