Appearance energies

As explained by the Franck-Condon diagram, almost no molecular ions will be generated in their vibrational ground state. Instead, the majority of the ions created by El is vibrationally excited and many of them are well above the dissociation energy level, the source of this energy being the 70 eV electrons. Dissociation of 

or fragmentation as it is usually referred to in mass spectrometry, leads to the formation of a fragment ion, mi, and a neutral, a process generally formulated as 

Reaction 2.10 describes the loss of a radical, whereas reaction 2.11 corresponds to the loss of a molecule, thereby conserving the radical cation property of the molecular ion in the fragment ion. Bond breaking is a endothermal process and thus the potential energy of the fragment ion is usually located at a higher energy level (Fig. 2.4). 

Definition The amount of energy needed to be transferred to the neutral M to allow for the detection of the fragment ion mi is called appearance energy (AE) of that fragment ion. The old term appearance potential (AP) is still found in the literature. 

Consider a di- or a polyatomic molecule AB in the gas phase, at T = 0. By means of an electron or a photon, this molecule can be ionized and excited to a state AB+, which subsequently decomposes into the fragments A+ and B  

If A+ and B are formed in their ground states and if these species and the electron have zero translational energies, then the standard enthalpy of reaction 4.10 at T = 0 is equal to the appearance energy of A+ at T = 0, Ao(A+/AB). It becomes obvious from this definition that when reporting a value for an appearance energy, it is essential to state the parent molecule (indicated by /AB). Otherwise, we cannot identify the remaining species in the net reaction 4.10. 

The appearance energy (formerly known as appearance potential) is a widely used concept in threshold mass spectrometry experiments, which involve measuring the minimum energy required to cause a certain process. However, there are a number of theoretical and practical problems associated with the determination of reliable values of H o(A+/AB). In the following paragraphs we summarize the discussion of this subject made by the groups of Traeger for photoionization [64,65] and Holmes for electron impact [66]. 

It can be shown that ifH ,exp(A+/AB) is obtained by linear extrapolation of the ionization efficiency curve [64], the products have only the translational energy required to conserve momentum, and the relationship between d/icxp(A+/AB) (or ArH ) andH b(A+/AB) is 

Now figure 4.3 shows that another relation between AEexP(A+/AB) and AE0(A+/AB) is 

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Appearance energy. The minimum energy that must be imparted to an atom, molecule, or molecular moiety in order to produce a specified ion. The use of the alternative term appearance potential is not recommended. 

Potzinger and coworkers determined ionisation and appearance energies for the molecular and major fragment ions of several dialkylsulfoxides, R SOR (R =Me R = Me, Et, i-Pr, and i-pentyland R = R = Et or i-Pr). In addition to the evaluation of dissociation energies in the ions and their enthalpies of formation, a value of 280 + 30kJmol" for the C—S dissociation energy in neutral dialkyl sulfoxides was derived. 

Appearance Energy (AE) The cation appearance energy is the 298 K enthalpy required to form a cationic fragment from a neutral precursor. 

The appearance energy for any fragment critically depends on the precursor from which it is formed. 

The experimental approaches described above are examples of relative methods, wherein a thermochemical property is measured with respect to that of a standard, or an anchor. The quality of these measurements ultimately depends on the quality of the anchor. Alternatively, there are methods of determining thermochemical properties, in which the energy for a chemical process is measured on an absolute basis. Among the more common of these are the appearance energy measurements, in which the threshold energy for formation of an ionic fragment from an activated precursor is measured. There are mauy differeut methods of activation that can be used. Some of these are discussed here. 

A large nnmber of the measured ionization energies for stable neutral molecules come from electron impact appearance energy studies, but it has also been adapted for the direct study of reactive neutral molecules. If the reactant molecule is RH, then the appearance energy for R+ from RH, designated AE(R+, RH),... 

An alternate positive ion approach, similar to that in Eq. 5.4a is to obtain a carbon-halogen BDE, R X, from which it is possible to obtain the enthalpy of formation of the radical from which the hydrocarbon BDE can be derived. The advantage of this approach is that it is easier to measure the R appearance energy from RX than it is from RH because of the weaker RX bond. However, a limitation of the approach is that the enthalpies of formation of organic halides, required to determine the enthalpies of formation of the cations, are generally not known as accurately as those for hydrocarbons. 

Appearance energies for CH2 have been measnred from a large nnmber of precursors, including methane, ketene, and even methyl radical. 

Unfortunately, appearance energy measnrements become more complicated with larger substrates, where the cations are more prone to rearrangement during ionization. Eor example, numerous attempts have been made to measnre the energy for formation of o-benzyne cation, CgH4+ by using benzonitrile as a precursor (Eq. 5.6). 

Does not include values obtained using Eq. 5.4a because of questions regarding the ion structure in the appearance energy measurement see text. 

Fig. 4. Electron-impact efficiency curves at m/e = 32 and rri/e = 16 for a supersonic beam of O2. Arrows indicate the literature values72 of the ionization energy of O2 and of the appearance energy of 0+.

The Lozier tube, illustrated in Figure 3, has been used by several groups for a variety of different studies, including the determination of appearance energies and kinetic energies of ion fragments produced in electron impact-induced dissociation... 

Figure 4.3 Thermochemical cycle, showing how the experimental appearance energy of A+(A exp) is related to the appearance energies at 0 K and temperature T. AH(1), AH(2), and T are defined in the text. Adapted from [64].

A second likely error source in the experimental determination of the appearance energy has also a kinetic origin. As shown in figure 4.4, recombination of the products A+ and B may involve an activation barrier (Etec). Therefore, even if Akin = 0, when Eiec is not negligible the measured appearance energy will be an upper limit of the true (thermodynamic) value. 

In summary, both the kinetic shift and the recombination barrier lead to thermodynamic values of the appearance energy that are too large and to upper limits of A 7/°(A+, g). We now illustrate the procedures and conventions just described... 

The appearance energy of C2H+ has been measured by many groups using a variety of precursors and experimental techniques [67], One of the values obtained for XE expiC H /C2H5Br), 1067.1 1.0 kJ mol 1(11.06 O.OleV), was reported by Traeger and McLoughlin [64] and refers to reaction 4.18 ... 

As noted after equation 4.17, the procedure to evaluate standard enthalpies of formation from appearance energies is somewhat controversial. When the threshold energies are determined from electron impact experiments, it has been argued that the correction terms (H%9S - Hq)a+ + (77298 - o)b - 6.197 in equation 4.17 should not be included in the calculation [66], Consider, for instance, reactions 4.21, and 4.22 where the ion CH2OH+ was produced from the decomposition of 1-propanol or methanol. 

The experimental appearance energies of CH2OH+, 1080.6 kJ mol-1 (11.20 eV) and 1127.9 kJ mol-1 (11.69 eV), respectively, were measured in both cases by using a monoenergetic electron beam [68,69]. Because they have been directly identified with the enthalpies of reactions 4.21 and 4.22 at 298.15 K, we can write... 

In addition to the concepts reviewed in the last two sections (appearance energy, ionization energy, and electron affinity), three others are relevant in gas-phase molecular energetics, namely, proton affinity, gas-phase basicity, and gas-phase acidity. 

J. L. Holmes, F. P. Lossing, A. Maccoll. Heats of Formation of Alkyl Radicals from Appearance Energies. J. Am. Chem. Soc. 1988,110, 7339-7342. 

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