Electron-Impact Measurements

The quantity K, the momentum transferred to the molecule, is, from the preceding paragraphs, a fundamental quantity in the theory of electron scattering. It may be obtained from purely kinematic considerations. Consider Fig. 2a (i.e., Fig. 1 a of Hamnett et al.23). If k, and ky are the initial and scattered momenta of the incident electron, we have, from conservation of momentum, 

if we desire to make K 2 as small as possible, we desire E to be very small compared to E0 and 9 also to be very small. Under these circumstances, (11.40) may be written to second order in 9 and the dimensionless parameter x = (E/2E0) 

If the half angle of acceptance of our detector is 90 and we are concerned with 90 very small, that is, with forward scattering, integration with respect to 9 may be carried out analytically. We find23 

Lassettre et al.33 were able to show that even if the Born approximation does not hold, we still find 

Unfortunately, in practice the lowest value of K that may be conveniently measured at impact energies less than 500 eV is still too large for the limit described by (11.47) to be obtained accurately. However, the approach has been proven sufficiently useful for a number of absolute measurements to be made, and comparison with optical data has been very fruitful.10 

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While these disadvantages are severe, electron impact determinations play a useful role in suggesting the pattern of variation in bond enthalpy contributions in molecules which have not been studied by conventional thermochemical techniques. A few examples of this are shown in Table 11. Electron impact measurements also indicate that D (Ru-Cp) in ruthenocene is ca. 100 kJ mol-1 greater than D (Fe-Cp) in ferrocene82). 

The standard enthalpies of formation of the gaseous compounds and the enthalpy of disruption derived therefrom are given in Table 13. An interesting problem arises as to how these results are to be evaluated. If the value of AHf [M(CO)s, g] derived15,1 ) from electron impact measurements on M2(CO)io (M = Mn, Re) is used, then as outlined earlier this will be expected to give an upper limit to the value of D(M-M). It has been shown16) that for all values of Z)(M-M) below specified upper limits the following relation holds... 

Simple alkylcarbonium ions that have been discussed in the chemical literature were considered until very recently only as transient entities. Their existence had been inferred from the study of the course of certain reactions. No reliable physical measurements other than electron impact measurements were reported until recently for any of the simple alkylcarbonium ions. 

The two ring-Mg bond energies in CpiMg and CpMg have been determined from electron impact measurements for reactions 25-27. 

Early Unsuccessful Attempts. Until the early 1960s, simple alkyl cations were considered only as transient species.15 Their existence has been inferred from the kinetic and stereochemical studies of reactions. No reliable physical measurements, other than electron impact measurements in the gas phase (mass spectrometry), were known. The formation of gaseous organic cations under electron bombardment of alkenes, haloalkanes, and other precursors has been widely investigated in mass spectrometric studies.81 No direct observation of carbocations in solutions was achieved prior to the early 1960s. 

Table II. SFC-FTMS, Electron Impact Measured Masses for Caffeine...

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

Further examples of the use of electron impact measurements to the determination of dissociation energies in polyatomic molecules are given in the discussion of particular values of dissociation energies in Chap. 9. It may be said here that in most cases the results are consistent with other information, suggesting that kinetic energy effects are often unimportant. 

Electron impact measurements on the dissociation of CO have been most detailed, and an account of the work is given in Section 5.2. They lead to Z)(CO) =9 6 0 2 eV, whence Lq == 136 4 kcal, and as far as can be seen do not admit of any other interpretation. Gaydon s criticism of them is also discussed in Section 5.2. 

Electron impact measurements and the study of photosensitized reactions both provide values for D CH) in ethylene. Indeed, electron impact work, perhaps overgenerously, provides two values, which differ by 30 kcal ... 

Z)(Et -Et) in -butane has been deduced from electron impact measurements to be 77 6 kcal470. This depends on the same data as those leading to Z)(Et -H) —96 4 5 kcal, and is therefore subject to at least that error. Z)(Et Et) may of course be deduced thermochemically from Z)(Et --H). 

Electron impact measurements of the appearance potentials of various ions in the mass spectrum of methylamine and other amines by Collin 2 lead to upper limits to Z)(GN) in these compounds which appear to be very much larger than the most probable values. These probable values are not directly determined, but can be obtained from the heats of formation of the amines and the radicals concerned. Collin, for example, gives 140 kcal as an upper limit to D(CH -NH2), whereas the most probable value is about 80 kcal. 

The molecular formula of a molecule can also be defined by high resolution mass spectrometry (hrms). The observed mass for the molecular ion or pseudo molecular ion must normally be within 5ppm of the calculated mass for El (electron impact) measurements, or within lOppm for Cl (chemical ionization) measurements. It is important to note that high resolution mass spectrometry confirms that some molecules of a particular molecular formula are present in the sample, but does not give any indication of purity. Some other evidence of compound purity will therefore be required. 

Some weak bands should, of course, exist such as those corresponding to the forbidden components of the (2t2,4p) transition (Aj, E, Ti) or to valence-shell states of the same sjmimetry. Whether or not there is a band at 64100 cm due to a separate electronic transition is a problem of some importance. Further experimental work, in particular electron impact measurements, would be needed as well as advanced quantum chemical calculations to determine the extent of interaction between the two T2 states arising from the (2t2,4s) and (2t2,4p) transitions, and to locate the forbidden bands. The main point at this stage is, however, that the 64100 cm band cannot belong to the (2t2,4s) transition. 

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