Resonant activation

In the resonant mode, the acceleration of the ions is done by applying a radio-frequency of type Vcoscat with o) = 2jtf between the endcap electrodes. The value of f corresponds to that of the secular frequency of the precursor ion whose effect is to make it enter into resonance.  

Under the effect of the resonance phenomenon, the ion is accelerated and the multiple collisions with helium become activating. The daughter ions issued from the dissociation of this ion have their own secular frequencies that are different from that of 

FIGURE 5.21 Principle of resonant activation in ion trap mass spectrometer. 

It is to be noted that activation by resonance resembles axial modulation (see Chapter 4) because both require the application of a radiofrequency of type Vcostot between the endcap electrodes. There are nevertheless two important differences between the two concepts. First, the value of V is around 3 to 4 volts in axial modulation and 0.2 to 1.5 V in MS/MS where the point is not to eject the precursor ions. The second difference is the value of the radiofrequency trapping amplitude applied to the ring electrode during the application of the radiofrequency of resonance. This value is high in axial modulation and low in MS/MS where the point is to trap the daughter ions formed by collision. 

The resonant mode is generally used for structural analysis because it allows accurate control of the activation energy of the ions. This mode is also used in dosage methods. In this case, the more selective the sample preparation, the more efQdent the resonant mode. In the opposite case, the interfering compounds from the matrix present in the ion trap can lead to small differences in the secnlar freqnencies of the precursor ions. This phenomenon is exhibited by a decrease in efficiency of the resonance phenomenon. 

---

In comparing resonance activation by nitrogen in the same ring with that by nitrogen in the adjoining ring of a bicyclic azine, other effects must be considered besides the volume over which the charge is distributed (Section IV, A). 

The A-oxidation of 3-chloropyridazines increases their reactivity toward methoxide and sulfanilamide anions.The reactivity of 4-chloro- or 4-nitro-quinoline and of chloropyridines toward methoxide ion and piperidine is less than that of the corresponding A-oxides (see Tables II and XI, pp. 270 and 338). The activating effect of the A-oxide moiety in 3-halopyridine A-oxides is greater than that of a nitro group, and in fluoroquinoline A-oxides the activation is transmitted to resonance-activated positions in the adjoining rings. 

In 2-chloro-8-nitroquinoline (196), where resonance activation by the nitro group in the adjoining ring is possible, the chloro group is rapidly (10 min) displaced in boiling aqueous acid (cf. Table XI, p. 338). 

The activation energy of substitution of an unactivated aromatic halide (e.g., fiuorobenzene and 2-chloronaphthalene ) is over 30 kcal while that of activated compounds is 5-20 kcal. For the tabulated reactions (Tables II-VIII) with alkoxide and with primary, secondary, or tertiary amines, resonance activation (cf. 278 and 279) by ortho or para nitrogens is found to be greater than inductive activation (cf. 251). This relation is qualitatively demonstrated in... 

When a hydrogen atom is peri to an azine-nitrogen, there is no steric inhibition of resonance activation as there is in 1-nitronaph-thalene (4-methoxy-dechlorination of its 4-chloro derivative seems to be thereby decelerated only 2-fold in rate). Steric hindrance of nucleophihc substitution by the co-planar peri hydrogen is sometimes... 

Resonance activation in the 8-substituted-isoquinolines (344) or -2-nitronaphthalenes is predicted to be greater than that in 5-substituted-quinolines (345) or -1 -nitronaphthalenes due to the lower energy charge-... 

The kinetic data for the structures in Schemes II and III lead to the following relationships of the rates of piperidino-dehalogenation (type of activation given in parentheses ind. = inductive activation, res. = resonance activation) ... 

The reduction in rate of nucleophilic substitution when the resonance-activating center is transferred to the adjoining ring is 10 -10 -fold for... 

The irregularity in the bond lengths of naphthalene in the ground state (misleadingly called bond fixation) has been used to explain the poor transmission of resonance activation in 2,3-orienta-... 

This numbering system is especially useful since all the reactivity characteristics summarized above can be recognized just from the name of the compound. Resonance activation of the leaving group (Le) for alkoxylation or alkylamination has the following observed characteristics (Section IV, A, 2) ... 

The relations in Scheme V take into account intranuclear and intemuclear resonance activation and the special cases of 2-Le-3-aza (poor) and 4-Le-8-aza (extremely poor) activation as well as the two groups of inductive activation the possibility of an accelerative peri effect in substitutions with protic nucleophiles is indicated by the sign for 4-Le-5-aza compounds. 

The activation of all the positions in quinoline in relation to each other and to those in naphthalene can be judged from the kinetic study of piperidino-debromination (Tables IX and X) summarized in the following tabulation (res. = resonance activation, ind. = inductive activation). 

As summarized in the following tabulation, the relative rates of piperidino-debromination of the halo-l-nitronaphtholenes (data from Table XII, numbered to show relation to quinolines) provide a good confirmation of the relation of induction (ind.) to resonance activation (res.) and of the extent of transmission of activation to an adjoining ring. Here again, as in the quinoline series, the 8-isomer (346) is more reactive than its resonance-activated 5-bromo isomer (345) and its inductively activated 3- and 6-bromo isomers (351 and... 

---