Acetone dimers, protonated

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.

Munson and Haw (151) reported the first in situ NMR study of acetaldehyde in a zeolite. Figure 27 shows 13C spectra of this species reacting on HZSM-5 in the presence of water to form crotonaldehyde with high selectivity (an example of aldol condensation). We later reported a very detailed study of the aldol reactions of acetone and cyclopentanone on various zeolites (Scheme 4) (147). Dimerization of acetone followed by dehydration gives mesityl oxide (31), and the I3C isotropic shifts of this conjugated ketone are strongly dependent on state of protonation. Farcasiu and Ghen-ciu (152,153) have reported extensive measurements of the 13C shifts of 31... 

In their second paper Barnes et al.54) studied heteroassociations of alcohols in the gas phase at low alcohol pressures (about 2 Torr) and much higher pressures of the proton acceptors. In particular the IR spectra were measured for TFE with acetonitrile, acetone, methyl-isocyanide, diethylether and tetrahydrofuran and a number of amines. The dimers were obtained in all cases. Their paper contains a number of v and Av values for such systems. 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

This J] state is a diradical and can abstract a methine proton from the solvent, then a hydroxyl proton to give acetone and diphenyl hydroxy radical, which then dimerizes to give the product. 

FIGURE11.8 Plots of compensation voltage versus separation field for a homologous series of ketones from acetone (carbon number 3) to decanone (carbon number 10) for protonated monomer (top) and proton-bound dimers (bottom). These plots show field dependence of mobility for small molecules nonetheless, a mass relationship to the field-dependent behavior is clearly shown. (From Krylov et al.. Field dependence on mobilities for gas phase protonated monomers and proton bound dimers of ketones by planar field asymmetric waveform ion mobility spectrometer (PFAIMS). J. Phys. Chem. 2002. With permission.)... 

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). 

Preston and Rajadhyax who employed a hand-held ion mobility spectrometer, similar to those used in military applications, studied the correlation between the reduced mobility for ion processes at equilibrium and the arrival time of the single peak in the mobility spectrum [34]. Equilibrium constants were determined as a function of temperature for the formation of proton-bound dimers MZH as shown in Equation 13.13, where each of M and Z can be pyridine, 3-(3-methoxypropoxy) propanol (DPM), acetone, or water,... 

An enzyme-substrate complex has been detected in the oxidative dimerization of L-(-)-tyrosine by HRP(i) to give HRP(ii). The pH dependence of the reaction reveals an enzyme protonation at pATa 5.42 with a less reactive protonated form. The hydroperoxide of indole-3-acetic acid (lAA) is important in the autoxidation of lAA catalysed by HRP. Formation of HRP(i) by reaction with the hydroperoxide is easier for the neutral isoenzyme than for the acidic species at pH 4.4 and this determines the catalytic activity. HRP(ii) is detected as an intermediate in the reaction and it reacts with lAA to form radicals. The involvement of both HRP(i) and HRP(ii) is proposed in the autoxidation of 2-nitro-propane to acetone and HNO2, catalysed by HRP. 

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