Adduct ions dissociation

The nature of the solvent also determines the chemoselective outcome in the reaction products. Products arising from the incorporation of one solvent molecule are formed (besides dibromides) in alcohols, acetic acid and acetonitrile (Id-e), whereas dibromo derivatives are formed exclusively in chlorinated solvents, nitromethane and in ionic liquids. (9) Chemoselectivity depends on the relative nucleophilicity of the solvent and the counterion, although it is affected also by other phenomena (ion pairing, and ion dissociation) in methanol the addition process gives quasi-exclusively bromo-methoxy adducts, whereas in acetic acid dibromides are the main products, formed in addition to smaller amounts of the bromo-acetoxy derivatives. (70)... 

However, central to any truly accurate determination of the radiative rate are the integrated absorption (emission) intensities, A", which for gaseous ions are almost completely unknown as are, usually, the vibrational frequencies. Fortunately, however, ab initio and density functional methods have recently been shown to be quite accurate in their predictions of vibrational spectra for a wide variety of systems, and there is no reason to suspect that this accuracy would not carry over to comparable data for gaseous ions. The one caveat must be that the low-frequency modes that are common in cluster ions will be decidedly anharmonic, and prediction of both these frequencies and their intensities may be suspect. However, these modes are not generally expected to be dominant contributors to the overall radiative rate. In addition, standard RRKM procedures can be applied to the unimolecular dissociation of the same adduct ions and, in principle therefore, the overall kinetics of formation of stabilized association complexes can be accurately modeled. 

Figure 16. Metastable ion cyclotron resonance (MICR) spectra for the unimolecular dissociation of the chemically activated adduct ion derived from association of the methoxymethyl cation with pivaldehyde during a 2-s reaction delay at a pressure of pivaldehyde of 1.0 x 10 torr. The three spectra correspond to values of rf amplitude appropriate to eject transient intermediates with lifetimes longer than (a) 60 ps, (b) 80 ps, and (c) 1 70 ps. A partial pressure of CH4 of 1.0 x 10 torr was also present to thermalize ions. The peak at m/z 125 is a secondary reaction product of m/z 85.

All such reactions are characterized by a lowering of the cross section as the internal energy of the adduct is increased. The lifetime of the benzene dimer ion, as well as those of similar adduct ions, is very sensitive to its internal energy since it is very loosely bound (it has a binding energy of 8 kcal/mole). Increasing the product-ion internal energy by vibrational excitation of the reactant readily promotes the dissociation back into reactants. 

S. B. Levery, M. S. Toledo, R. L. Doong, A. H. Straus, and H. K. Takahashi, Comparative analysis of ceramide structural modification found in fungal cerebrosides by electrospray tandem mass spectrometry with low energy collision-induced dissociation of Li + adduct ions, Rapid Commun. Mass Spectrom., 14 (2000) 551-563. 

The binding of the CO ligand to SiF3+ is peculiar in that the adduct ion cannot undergo any simple elimination reaction other than the back dissociation of CO. At variance with... 

Polycyclic aromatic hydrocarbons such as octamethylbiphenylene and dodecamethyl-binaphthylene are also found to react with Me3Si+ in CI/MS, giving abundant adduct ions which on CID dissociate by loss of Me3Si, retaining the charge on the aromatic moiety127. 

The prediction of ab initio calculation on the ring expansion of TFIF, promoted by acylium ions, was confirmed by sequential product ion mass spectra collected after collision-induced dissociation (CID), which showed that tetrahydropyran failed to undergo six- to eight-membered ring expansion in reaction with [(Me)2N-C+=S] since its adduct (most likely the simple O-acylated ion) dissociated to regenerate exclusively the reactant ion [(Me)2N-C+=S] <2000CEJ897>. 

We have implied that the high syn selectivity in alkynes requires relatively tight association or bonding from beginning to end, whether the participants are covalent, ion pair, radicaloid, etc. By participants we mean the four reaction centres pictured in 27 to 30. Other ligands to copper, e.g. Br or (EtO)3P, may be labile and are effectively replaced by the triple bond as the addition occurs . At higher temperatures, when syn selectivity often decreases , one would have to allow the copper adduct to dissociate, isomerize and recombine. 

TSP spectra have much in common with the DLI spectra. Again the protonated or deprotonated aglycon is the major peak in the mass spectrum and thus fits the general characteristic of glucuronic acid conjugates (lfi). Also present were adduct ions, this time with NH4 + or acetate Ac from the 0.1M aqueous ammonium acetate in the mobile phase, which also contained 20% acetonitrile. These adducts were formed from the molecular ion, thermally derived BaPOH and a glucuronic acid derivative. The adduct ions have much in common with chemical ionization spectra (,17.18 ). This may be expected from the dissociation of ammonium acetate and extraction of the ions from the liquid. 

Product-ion scan The first quadrupole is set to transmit a selected precursor ion falling within a mass window of usually 1 Da. These ions enter the second quadrupole and are fragmented by collision induced dissociation. The fragment ions are mass resolved by the third quadrupole operated in the full-scan mode. The result is an electron impact-like mass spectrum obtained indirectly from a molecular-adduct ion or other characteristic ion formed in the ion source by a soft ionization method. Also known as a daughter ion scan... 

One of the most serious drawbacks that has been observed in the ionisation process with TSP, APCI, ESI interfaces, and also with FAB, is the soft ionisation of the analytes which mostly leads to molecular ions or molecular adduct ions. Though molecular mass information is provided, there is little or no structural information at all observable with PBI or electron impact (El) MS. This soft ionisation is clearly disadvantageous for any identification of environmental contaminants, since it generates either considerably less or no fragments at all, and hence is unable to confirm the presence of such compounds of environmental concern. With the commercial availability of tandem devices, tandem mass spectrometry (MS/MS) helped to overcome these identification obstacles via coUision-induced dissociation (CID) in MS/MS mode or via ion trap in MS mode. Today, even bench-top machines provide the possibility of MS . However, when TSP began to become the method of choice in environmental analysis and became commercially available, MS/MS technology was still quite expensive. Users of TSP ionisation with spectrometers not amenable for MS/MS had the possibility to record... 

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