Resonance technique, ion cyclotron

Using Fourier transform ion cyclotron resonance techniques, the proton affinities of the prototypical a, /3-unsaturated y- (42) and 5-lactones (43) have been determined as 836 and 862 kJ moU, respectively. This increase in basicity with the size of the ring also prevails for the saturated analogues (44) and (45). ... 

Gas-phase reactions of anions derived from CH3SCD2CN have been studied by Fourier transform ion cyclotron resonance techniques.87... 

The gas-phase ion chemistry of HCN, MeCN and various alkyl nitriles has previously been investigated111 using ion cyclotron resonance techniques (ICR)112-115. The developments in such techniques have made it possible to quantify base strengths relative to a variety of cationic reference acids in the gas phase116-130. 

Gas phase proton affinities of phosphabenzene and arsabenzene have been determined by ion-cyclotron resonance techniques 94>. These confirm the qualitative solution phase data (see Fig. 5). Phosphabenzene (PA = 194.5 kcal/mol) has a proton affinity nearly 30 kcal/mol less than trimethylphosphine and only slightly greater than that of phosphine. Arsabenzene (PA = 188.0 kcal/mol) has a proton affinity 23 kcal/mol less than trimethylarsine. In the case of arsabenzene, protonation occurred on carbon rather than arsenic so the As-basidty may be even lower. By contrast, the proton affinity of pyridine (PA = 218 kcal/mol) is only slightly less than that of trimethylamine (PA = 222 kcal/mol) but considerably larger than ammonia (PA = 202 kcal/mol). 

The reactions of orthoesters in solution played a key role in the formulation of ALPH, and Caserio et al. (1981) examined the reactivities of a diastereo-isomeric pair of orthoesters [34] with various proton donors in the gas phase, using ion cyclotron resonance techniques. With the isopropyl cation or methylthiomethyl cation as a proton donor, no difference in reactivity,... 

A second class of chemical compounds whose reactions are catalyzed by the fluoride ion, and by CsF are organosilanes (42). The diversity of chemistry exhibited by these compounds is considerable, and with the potential for expanded valence in these compounds to five and six coordinate intermediate anions, the application of the salt/molecule technique was suggested. The gas phase ion/molecule reactions of several silane systems have been investigated through ion cyclotron resonance techniques (53,54), providing thermochemical information about the product ions in these systems. Perhaps the simplest reaction is that of F with SiFi, the intermediate anion SiFs has been stabilized under carefully controlled circumstances (55), while the 2 1 adduct SiFe is well known ( ). Less is known about the effects of alkylation on the stability of the product ions. 

Hehre et 3 . who have used ion-cyclotron resonance techniques to obtain criteria of aliphatic and aromatic carbocation stabilities. 

Kricmler and Buttrill [1611 studied the formation of positive and negative ion-molecules fron ethyl nitrate by ion cyclotron resonance technique. They found three kinds of reactions of pt> itivc ions... 

Whether the C-H bond of DME or MeOH is sufficiently nucleophilic to undergo substitution by CHi" " is debatable. The gas phase reaction of CH3+ with MeUH was studied by Smith and Futrell [18] using ion cyclotron resonance techniques. Hydride abstraction forming CH4 and CH20H+ were seen to be the predominant reaction (85-90%). 

What are the principal features and advantages of these two techniques In the flow reactor (6), the solvent of a solution is replaced by a bath of inert gas (typically helium) (Figure 1). The bath gas cannot of course solvate the reactants what the bath gas does is to undergo many millions of collisions with the reactants thus, the reactants are at the bath temperature. This bath temperature can be varied (typically from 100 to 500 K) to measure rate constants as a function of temperature. Because the flow reactor succeeds in defining the temperature unambiguously, it offers advantages over ion cyclotron resonance techniques. 

The careful establishment of the merging-beams technique is an excellent example of what is needed. McDaniel s recent formulation of necessary criteria to be met for a reliable drift-tube measurement is another example. In contrast, one notes the lack of the necessary control experiments during the past development of the mass-spectrometer ion-source technique (Section 3.4.1) and during the recent development of the ion cyclotron resonance technique (Section 3.4.7). The necessary techniques may not always be available, but the effort must be directed to that end and not to the premature acquisition of data of unproven reliability. 

Pathway identifications are handled elegantly in the ion cyclotron resonance technique. However, the velocity distribution is clearly suprathermal the distribution of states is unknown the deduction of a rate constant is involved and indirect. It is too early in the development of this technique to foresee its future role. 

A search for PHg in interstellar and circumstellar sources is described and the results are discussed with respect to the depletion of P in dense and diffuse interstellar clouds [62]. Laboratory experiments on the gas-phase ion-molecule chemistry of phosphorus using the ion cyclotron resonance technique predicted that the PHg molecule is formed only with difficulty in dense interstellar clouds [63]. 

Relative proton affinities are determined by measuring the enthalpy of proton transfer from one base B to a second base B (equation 6), using high-pressure mass spectrometry, the SIFT/flow afterglow or ion cyclotron resonance techniques. This overlap method achieves an ordering of proton affinities, but to obtain absolute values these must be anchored on some independent measurement. 

The use of tandem mass spectrometers as an experimental tool and the type of information derived from such studies are closely related to the crossed-beam studies of these reactions discussed by Herman and Wolfgang in Chapter 12, to the charge-exchange studies Lindholm discussed in Chapter 10, and to the ion cyclotron resonance technique described by Henis in Chapter 9. The relationship of these techniques is illustrated by Fig. 1, which shows a highly sophisticated, idealized apparatus suitable for studying all these problems. All four approaches have the characteristic that, with appropriate care, one can isolate a particular elementary reaction and study it without interference from the many complex, interacting parameters present in a system which does not involve some method of species selection. 

There is a wealth of information on gas phase ion thermodynamics because of the power of mass spectrometry and ion cyclotron resonance techniques. Before we discuss carboca-tion, and subsequently carbanion, stabilities, keep in mind that ionic structures are much more sensitive to environmental influences than radicals. The polarity, nucleophilicity, and hydrogen bonding ability of the solvent are important influences, as are the nature of the counterion. As such, thermodynamic information is a less reliable predictor of reactivity for carbocations and carbanions than it is for radicals. Nevertheless, gas phase thermodynamics is an excellent starting point, defining the intrinsic stabilities of ions. Any deviation in trends between gas phase and solution studies is likely a consequence of solvation effects, a theme we will visit many times throughout this book. 

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