Protonated dimers

Examination of the mass spectrum of P2VPY taken during the maximum decomposition rate reveals the major decomposition products as methylpyridine (93 a.m.u.), protonated vinyl pyridine (106 a.m.u.), and protonated dimer (211 a.m.u.) with ion ratios 74 100 59 respectively. Trimeric and tetrameric protonated species (316 and 421 a.m.u.) are also observed but in relatively small amounts. Protonated ions, rather than the simple monomers and dimers observed for the decomposition of poly(styrene) by MS11, may be created by a mechanism similar to that reported for the decomposition of 2-(4-heptyl)pyridine12 in the mass spectrometer. 

It can be expressed as fhomo/. hetero = [Td-H] /[Td-To-H]+ / [To-H] + - -[T -H] /[To-T -H], where [To-H]" and [T -H]" correspond to the peak intensities of monomer ions produced by unimolecular decomposition of the relevant protonated dimer species. The experimental results (l homo/ hetero = 0-67 (diPT-D-tartrate - - diPT-D-tartrate-di4) 0.77 (diEt-D-tartrate - - diEt-D-tartrate-dio)) confirm the higher stability of the homochiral dimer relative to the heterochiral one by indicating that the latter has a higher tendency toward unimolecular decomposition. 

In order to assess the validity of such an approach, within the assumptions of the model outlined, we have recently undertaken an examination of deuterium isotope effects on both the radiative and unimolecular dissociation rates. One such case is that of the protonated dimer of acetone in which either the methyl groups, the protonated oxygen, or both, are deuterium substituted. Results for these four systems are shown in Figure 14 and the rate data derived are summarized in Table 1. 

Figure 15. Variation of the ratio of intensities of protonated acetone-c4 and pro-tonated acetone, [(CD3)2CO]H [(CH3)2CO]H arising from the unimolecular dissociation of the mass-selected mixed protonated dimer of acetone and acetone-ds (MIKES) as a function of the reciprocal ion source temperature.

Ions appearing at approximately twice the mass of the parent ion are known as multimers. These usually appear at (2M + H)" owing to the formation of a protonated dimer in the MS. 

The rate constant ratio of the competitive decomposition reaction is equal to the ratio of the two fragment ion intensities (/x) if the comparison is made for competitive dissociation of a weakly proton dimer having no other decomposition channel. In these conditions,... 

Dissociation of protonated dimers of acetaldehyde/ketone (homogenous or heterogenous clusters) leads to water loss [144,145], which is also the result of the corresponding ion molecule reactions [130]. Rearrangement of at least one of the reaction partners to the (protonated) enol seems likely. No comparable rates or barriers can be inferred from these data. 

Fig. 6a-c. Ion mobility spectra of bradykinin ions at m/z=1061 a the two features seen at 441 K are the singly protonated monomer (M+H)+ and the doubly protonated dimer (2M+2H)+ b at a cell temperature of 463 K the dimer dissociates into two monomer units accounting for the fill-in between the two peaks c at 510 K all of the dimer ions have disappeared upon exiting the cell... 

Mobile phase additives and impurities in the sample solution can add to the complexity of mass spectrum in LC/MS. Noncovalent complexes such as protonated dimeric ions [2M + H" ] are common. Other common adduct ions include complex cations ([M -i- [M -i- Na" ], [M -i- K" ], etc.), solvent adduct... 

The fast initiation step is followed by an equally fast propagation reaction. While the rate constant of the former has been measured by Kunitake and Takarabe the dimerisation kinetics have not been measured for this particular system. The following slow reactions consist of the proton transfer between the dimeric cation and the monomer to gjve alternatively the unsaturated dimer or the indanylic one. Finally, the protonated dimer can isomerise to a more stable configuration due to the direct interaction of the two phenyl groups through space polarisation effects Thus ... 

The simplest reactions of nonadsorbed species involve only mass and electron transfers. However, more complex reactions involve chemical reactions proceeding or following electron transfer, such as protonation, dimerization, or tautomerization. 

Figure 10.4 The measured angular distributions of scattered ion masses resulting from the electron impact ionization of the scattered neutral methanol clusters. The onset of the H loss fragment ion from the methanol monomer is shown at the limiting angle for the monomer (18.2°). Similarly, the dimer onset, producing the protonated monomer is shown at 9.2°, while the protonated dimer peak produced by reaction (1) is evident with a weak onset at 6.1°. Taken with permission from Buck et al. (1990).

Del Bene, Perera and Bartlett have also carried out the study of the spin-spin coupling constants in a number of positively charged systems with proton-shared hydrogen bond protonated dimers of H2O, H2CO, CO, HNO and HPO with 0H ---0 hydrogen bonds and protonated dimers... 

The protonated acetonitrile CHsCNH+ reacts with the neutral to form protonated dimer... 

Deuteration was also used to investigate the dimer of methionyl-tRNA synthetase [196], Mixture of the deuterated and protonated dimers DD and HH causes hydrid HD dimers to be formed through reversible dissociation. The equilibrium mixture of HH, DD and HD dimers is polydisperse in a //(O) matchpoint graph (cf. ferritin in Section 4.3.1) from which the amount of HD present is calculated. Comparison with the Rq values measured as a function of percentage H20 gave the of monomer and the Parallel Axes Theorem gave the distance between the monomers. 

Positive gas phase ion chemistry governs the reactions of nerve agents or simulants such as DMMP (dimethyl methyl phosphonate) in the IMS ionization source. The main reaction pathway between the reactant ion that is a protonated dimer of the dopant R2H+(H20) [where R could be acetone (CH3)2CO] by an organophosphorus molecule G involves displacement of an R molecule by G to form a heterogeneous proton-bound dimer GRH+(H20) i (Equation 13.1). 

The i-butane chemical ionization mass spectra of a number of saturated mono-hydroxylic alcohols have been determined to establish the general patterns of the spectra of this class of compounds. The spectrum of 2-hexanol is given as a typical example in Table X, and it may be seen that the following ion types comprise large fractions of the spectra of alcohols the alkyl ion formed from the hydrocarbon portion of the molecule R ) the ion formed by abstraction of the hydride from the molecule, (M l)" the protonated molecule, (M + 1) the association complex of the molecule with the mje = 39 ion of the i-butane plasma, (M -h 39) the association complex of the molecule with the mje = 57 ion of the /-butane plasma, (M -h 57) and the protonated dimer of the molecule, (2M -h 1). The intensity of this ion in the spectrum of 2-hexanol is so small that it is not included in Table X. 

In almost all of the compounds for which temperature studies have been made (esters of various types) ions with mje values higher than the molecular weight of the compound have been observed, and for many of these ions the relative intensities decreased sharply as the ionization chamber temperature increased. Thus, for example, in benzyl acetate an mje = 301 is observed, and this must comprise the (2M + 1) ion, i.e., the protonated dimer of benzyl acetate. At a source temperature of 37°C the relative intensity of this ion is 0.175, becoming negligibly small at 196°C. (2M + 1)" ions have also been observed with alcohols " (Section 3.7). 

From the value of the equilibrium constant and its temperature variation one calculates free energies, enthalpies, and entropies for the reactions. As an illustration of the results obtained we give in Table XVIII thermodynamic values for ionic equilibria in benzyl acetate and t-amyl acetate. The remarkable aspect of these results is the positive AS values obtained for the formation of the protonated dimers of benzyl acetate and r-amyl acetate and the BzAc C3H3 association complex. Reactions involving the association of two entities result in the loss of three translational degrees of freedom, which corresponds to an entropy change of — (30-40)... 

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