Proton bound dimers and trimers

Proton-bound dimers and trimers are formed from the protonated alcohols and from other proton-bound compounds Uke ... 

In given concentrations of sample vapor neutrals, spectra for product ions can be altered by control of temperature, and this was seen at cryogenic tanperature, at which ion clusters not usually observed in mobility spectra were formed and appeared in mobility spectra. For example, proton-bound trimers of alcohols were observed when temperatures were decreased to -20°C and dissociated at temperatures from -20°C to +10°C. " Increases in temperatures will lead to dissociation of complex ions, such as proton-bound trimers and proton-bound dimers. As temperature is increased, the intensity of peaks for protonated monomer increase, and the peak abundance of proton-bound dimers decreases. This has been developed and explored for dimethyl methyl phosphonate (DMMP), " amines, and ketones. For example, proton-bound dimers of alkyl amines underwent dissociation above -30°C on a 2- to 20-ms time scale, which is within the range of drift times for these ions. Consequently, the dissociation pathway can be observed as a distortion in the peak shape and baseline of a mobility spectrum since an ion entering the drift region as a proton-bound dimer dissociates to a protonated monomer before arriving at the detector. These studies permitted the determination of kinetics of dissociation for thermalized ions and illustrated that the appearance of an ion in a mobility spectrum is governed by ion lifetimes in comparison to ion residence times in drift tubes, and ion lifetimes are controlled by temperature. 

Mobility spectra will exhibit protonated monomers for most polar or strongly polarizable molecules when the reactant ions are hydrated protons and vapor levels of analyte are more than 10 to 100 ppb, the detection limits for most such compounds. Mobility spectra may contain a proton-bound dimer when vapor levels are increased to 0.5 to 1 ppm, yet a proton-bound trimer or tetramer is never observed, even if vapor concentrations exceed those needed to form these higher cluster ions according to equilibrium calculations. In mobility spectrometers today, ions are formed and then drawn into purified air or gases excluding neutrals of sample. Thus, equilibrium does not exist in analytical mobility spectrometers, and ion passage through purified gas should be seen as a kinetic experiment. 

Ion lifetimes for protonated monomers are commonly much greater than 20 ms at temperatures up to lOO C and beyond. In contrast, some proton-bound dimers have lifetimes under a few milliseconds and thus are not observed unless temperatures are comparatively low (e.g., -20 C) molecules with strong dipoles have proton-bound dimers that are comparatively long lived and can drift for 20 ms or more. In contrast, the lifetimes for proton-bound trimers in a purified gas atmosphere at ambient pressure and temperatures are under 1 to 5 ms and undergo rapid decomposition these are never seen in analytical IMS drift tubes unless control of sample vapors is lost and ion-neutral reactions in the drift region occur. Higher clustered ions have even shorter lifetimes than proton-bound trimers and thus are not ever observed in mobility spectra. ... 

Figure 4. Variation of relative ionic abundances with reaction time, in a high-pressure source at 5-torr CH4, for negative ions derived from deprotonation of methanol. The exponential decay of rr /z 31 yields the bimolecular rate constant for formation of the proton-bound methoxide dimer, m/z 63. In addition, at this temperature (325 K) the subsequent reaction to generate the trimer anion (m/z 95) and attainment of equilibrium can be seen.

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