Negative proton abstraction

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

There are different ways to ionize a molecule (M, Scheme 2.1) extraction of an electron from gas phase molecules (Mg), yielding radical cations [Equation (2.1)], as occurs in electron ionization, or addition of one [Equation (2.2) Cl, MALDI, etc.)] or more protons [Equation (2.3) ESI]. Similarly, molecules can be ionized by the formation of negative ions due to single [Equation (2.4)] or multiple proton abstraction [Equation (2.5)]. 

In negative ion mode, (M-H) is typically formed by the proton abstraction by OH. ... 

When careful measurements are made in a flame, very little HCO+ or e is found. This is because these species further react very quickly. The first loses H+ (proton abstraction) to any of a variety of species on collision. The most stable product is and its polymerized form (H2O) H+. It is such polymerization about a charged species which makes raindrops. Such watery charge is what is collected. The free electron can be captured by radicals, most importantly, OH, but also by H2O and O2. Thus the negative ionic species weighs about the same as the final positive ion. 

Often, the basic group that is responsible for the proton abstraction is also the nucleophilic group in the Michael addition. Thus, most of the suicide inhibitors made so far have been aimed at enzymes that catalyze the formation of carban-ions or carbanion-like intermediates. Suicide inhibitors are typically based on acetylenic compounds (as in equation 9.8), /3, y-unsaturated compounds (as in equation 9.9), or /3-halo compounds (as in equation 9.10). (The a protons in such compounds are acidic because the negative charge in the carbanion is delocalized by the conjugation with X.)... 

Curved Airrhenius plots and negative standard isotopic entropies have been observed for proton abstraction from a hydrocarbon acid (Bell et al., 1956) and for proton transfer to hydrocarbon anions (Caldin and Kasparian, 1965). A negative A8° value has also been associated with ku/kd for proton transfer to allylmercuric iodide (Kreevoy et al., 1966b). 

In aprotic medium, on the other hand, pyrimidine gives a reversible diffusion-controlled le wave at a very negative potential, with formation of a radical anion which is deactivated via two pathways rapid formation of the anionic, probably 4,4 -, dimer, with a rate constant of 8 x 105 L mol-1 sec-1, and proton abstraction from residual water in the medium at a much lower rate constant, 7 L mol-1 sec-1 98). This is rapidly followed by a further le reduction to produce, ultimately, 3,4-dihydropyrimidine 98). In the presence of acid there is also a le reduction wave corresponding to formation of a free radical which, as in aqueous medium, dimerizes, most likely to 4,4 -Z w-(3,4-dihydropyrimidine). Examination of the mechanism of reduction in acetonitrile in the presence of acids supported the conclusion that reduction of pyrimidine in aqueous medium is preceded by its protonation98). 

For the acid-catalysed reaction, substituent effects have been examined principally in water and acetic acid-water mixtures. The main feature is that the effects are usually small. In Table 3 are also listed the Hammett p-values obtained for acetophenones and arylmethyl phenyl ketones. The slightly negative p-values account for the cationic character of the transition state and for opposite effects on the pre-equilibrium constant and on the elementary rate constant for proton abstraction. 

In an APCI interface the column effluent enters a heated nebulizer where the pneumatically assisted desolvation process is almost completed. While still in the spray chamber, ionization of analytes is initiated by corona discharge. The ionization mechanisms in APCI are almost identical to those in conventional medium pressure chemical ionization (7). Positive ion formation can be achieved by proton transfer, adduct formation or charge exchange reactions, while in the negative mode ions are formed due to proton abstraction, anion attachment and electron capture reactions. The APCI interface is compatible with flow rates exceeding 1 ml/min and will... 

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