Stevenson s rule

Stevenson s Rule (Stevenson-Audier). A simple cleavage of a chemical bond in an odd electron ion may result in two pairs of ions and neutrals ... 

Stevenson s rule is applicable for the rearrangement processes as well. In this case a radical cation and a molecule are formed, that is, two molecules compete for the charge. The molecule with lower IE becomes the radical cation. 

Similarly to Stevenson s rule, Field s and Bowen s mles are applicable only to the reactions with thermodynamic control. Any reaction with kinetic control may lead to their violation. 

Unknown 3. What ions will form as a result of McLafferty rearrangement for the compounds listed below Estimate the intensities of olephine and enol peaks using Stevenson s rule. 

Stevenson s rule When a fragmentation takes place, the positive charge remains on the fragment with the lowest ionization energy. 

Fig. 6.4. Thermochemical description of Stevenson s rule Dab homol5dic bond dissociation energy of bond A-B, IE ionization energy.

Note The homolytic dissociation of a C-C bond always proceeds to yield both product pairs, their relative abundances being basically governed by Stevenson s rule. 

Note The validity of Stevenson s rule requires no reverse activation energy barrier to exist for the fragmentation pathway. This requirement is usually fulfilled for simple bond cleavages, but not in case of rearrangement fragmentations. 

When a ketone grows larger it does not necessarily imply that it has two identical alkyl groups at the carbonyl. In case of different alkyls at the carbonyl, Stevenson s rule may also be applied to decide which of them will dominantly be detected as part of the acylium ion and which should preferably give rise to a carbenium ion. Overall, a nonsymmetrical ketone will yield four primary fragment ions in its El mass spectrum... 

Example Propyl and pentyl loss from 2-ethyl-cyclohexylamine molecular ion, = 127 (odd m/z) are competitive (Fig. 6.12). Pentyl loss, m/z 56 (even m/z), is favored over propyl loss, m/z 84 (even m/z), in accordance with Stevenson s Rule. The peak at m/z 98 may be rationalized in terms of ethyl loss due to a minor contribution of 1,4-H shift, i.e., from position 3 instead of the predominant 1,5-H shift from position 2. The [M-CHs]" peak, m/z 112, is accompanied by a [M-NHs]" signal, m/z 110, which is typical of primary - and to a lower extend also secondary - amines. 

The McLafferty rearrangement itself proceeds via charge retention, i.e., as alkene loss from the molecular ion, but depending on the relative ionization energies of the respective enol and alkene products, the charge migration product, i.e., the corresponding alkene molecular ion is also observed. This is in accordance with Stevenson s rule (Chap. 6.2.2). 

The competition of homolytic cleavages is governed by Stevenson s rule (Chap. 6.2.2). Thermodynamic stability of the pairs of products formed is decisive in selecting the preferred fragmentation channel. 

This rule is illustrated by the two examples in Figure 7.6. However, Stevenson s rule must only be applied to the competitive formation of fragment ions. Further dissociation or additional formation by other pathways could respectively decrease or increase the abundance of the fragment ions and lead to erroneous interpretations. 

In principle, Stevenson s rule still applies. By this rule, the cleavage of the adjacent bond could involve radical migration and charge retention if the ionization energy of YR7 is less... 

By Stevenson s rule, the cleavage of the alpha bond could involve radical retention and charge migration if the ionization energy of CfI2 YR is more than of R. This counterpart reaction is classified as a special case of a a bond cleavage and can be written as follows ... 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

Stevenson s rule ° predicts comparable ion intensities near the thermodynamic onset for CH3+ and NO2+ produced by a C—N bond rupture of the nitromethane radical cation since CH3+ and NO2+ are close in ionization energies. However, the difference between the dissociation limit and the observed appearence energy amounts to 1.2 eV. The energy release with photon energy of 14.73 eV was 0.37 eV and 0.24 eV for NO2+ and CH3+, respectively. Thus, it was concluded that either CH3+ or NO2 was formed in an electronically excited state rather than in an electronic ground state". This suggests that the CH3+ may arise from a specific excited molecular ion. ... 

The spectrum of the heptyl ion is given in Table 18. The results for the source spectra are obtained from the spectrum of 1-iodo-n-heptane after correction for the fragments containing iodine. Bu" is by far the most abundant ion. The formation of propyl ion in the source might be astonishing, since it does not correspond to Stevenson s rule it could be formed by successive eliminations of two ethylenes, but we will show later... 

Retention of the charge is governed by Stevenson s rule, which states that a fragment that retains the charge is the one with the lowest ionization energy [22,23]. 

The origin of the peak at tn/z 43 in the El mass spectrum of acetone should be quite clear now and we may examine the formation of the CHs ion, m/z 15, next. In principle, the ionic charge may reside on either fragment, the acyclium or the alkyl. In case of acetone, the formation of the acylium ion, CHsCO, m/z 43, is preferred over the formation of the small carbenium ion, CHs, m/z 15. The question which of the two incipient fragments will preferably retain the charge can be answered by means of Stevenson s Rule. 

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