Proton affinities protonation energetics

A mass spectrometer can be also used as a gas phase laboratory and a lot of reactions can be done inside it. Furthermore chemico-physical properties, such as proton affinity, the energetics of gas phase processes and many other thermochemical parameters can be... 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

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. 

It is important to stress that the energetics of reactions 4.25 and 4.27 are usually not amenable to direct experimental investigation. Indeed, proton affinities, gas-phase basicities, and gas-phase acidities reported in the literature were not... 

W1/W2 theory and their variants would appear to represent a valuable addition to the computational chemist s toolbox, both for applications that require high-accuracy energetics for small molecules and as a potential source of parameterization data for more approximate methods. The extra cost of W2 theory (compared to W1 theory) does appear to translate into better results for heats of formation and electron affinities, but does not appear to be justified for ionization potentials and proton affinities, for which the W1 approach yields basically converged results. Explicit calculation of anharmonic zero-point energies (as opposed to scaling of harmonic ones) does lead to a further improvement in the quality of W2 heats of formation at the W1 level, the improvement is not sufficiently noticeable to justify the extra expense and difficulty. 

A resolution into two or more components always occurs if the solvent has a high proton affinity, so that a solvent molecule can form a particularly stable association with a phenol molecule as a result of an energetically favourable mutual orientation. This is the case, for example, if benzene and toluene are used as the solvents. However, this effect is even more pronounced in the case of cyclohexene. Dielectric constant measurements for phenol in various solvents agree with this observation. In particular, the dipole moments in benzene and cyclohexene (1-45 and 1-79 D respectively), are considerably greater than the value of 1-32 in cyclohexane. Liittke and Mecke (1949) attributed this effect to the ability of this unsaturated solvent to act as a proton acceptor, i.e. to form 7r-complexes. 

In case of immonium ion fragmentations, the large difference of proton affinities, APA, between imine and alkene clearly favors the formation of immonium ion plus neutral alkene, whereas imine loss is restricted to highly energetic precursors. 

Second, there is an energetic effect. This effect is particularly important in a reaction in which charge separation occurs. Then, the ability of the solvent to accommodate the charges, the electron affinity, or the proton affinity can become the key factor to initiate the reaction. 

Figure 4-16. Gas phase proton affinities (PA in kcal mol-1) of B clusters versus /n (B = piperidine, ammonia, methanol, and water) (from Knochenmuss and Leutwyler 1989). The threshold proton affinity corresponds to the energetic limit for which excited state proton transfer occurs for 1-naphthol in small clusters.

For the stronger proton acceptors (ammonia, monoethylamine, and piperidine) a relation between the B proton affinities and the spectral shifts of the S3 <- S0 states of phenol(B) or naphthol(B) shows a linear dependence for proton affinities lower than a limit value situated around 10.4 eV (s240 kcal mol-1) for both phenol or naphthol molecules. Above this limit, the spectral shift is much larger and is different for phenol and 1-naphthol (see Figure 4-17). Nevetheless, this limit seems to correspond with the energetical limit of the proton transfer reaction. 

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