Penning dissociation

Dichloro-bis (2,4-pentanedionato) zirconium(IV) is monomeric and a weak electrolyte in nitrobenzene solution n.m.r. chemical shifts,7 infrared and Raman spectra,8 and dipole-moment studies21 indicate that this compound exists in solution as the octahedral cis geometrical isomer. Chloro- and bromotris(2,4-pentanedionato) zirconium (IV) are seven-coordinate complexes which are monomeric in benzene and only slightly dissociated in nitrobenzene and 1,2-dichloroethane. Iodotris(2,4-pen-tanedionato) zirconium (IV), however, is appreciably dissociated both in nitrobenzene and in 1,2-dichloroethane.7... 

Figure 28 CO + NO reaction on Pd/MgO(l 00) model catalysts with various particle sizes (Pen = no = 5x10 8 Torr). (a) NO reaction probability as a function of sample temperature, (b) dissociation rate of NO as a function of temperature (from Ref. [90]).

Sheldon, Roger A., Igor V. Babich, and Yuri V. Plyuto, The rational design of Cr/AlP04-5 molecular sieve . Mend. Comm., Electronic Version, Issue 3, (1997), 94. DiBella, S., G. Lanza, and, I. Fragala, Ab Initio MO Study of the Molecular Structure, Vibrational Frequencies and Bond Dissociation Energy of Bis(2,4-pen-tanedionato-0,(9 )oxovanadium(IV) , J. Chem. Soc.. Faraday Trans. 1995, 91(17), 2709-14. 

Fraering PC, LaVoie MJ, Ye W, Ostaszewski BL, Kimberly WT, Selkoe DJ, Wolfe MS. Detergent-dependent dissociation of active y-secretase reveals an interaction between Pen-2 and PSl-NTF and offers a model for subunit organization within the complex. Biochemistry. In Press. 

It is a member of the Cys family of amino acids. It differs from D-Cys by the presence of two methyl groups on the /i-carbon. D-Cys inhibits carboxypeptidase A by displacing the zinc-bound water as might be expected since it contains a sulfhydryl binding group.D-PEN in marked contrast inhibits by increasing the zinc dissociation rate constant for CPD by 420-fold and then acting as the receptor of the freed zinc ion. ... 

As with histidine, cysteine is a major metal binding site in proteins. Penicillamine (j8, -di-methylcysteine), a degradation product of penicillin, finds use as a therapeutic chelating agent (see Volume 6, Chapter 62.2). Both Cys and Pen have three dissociable protons (pAT values approximately 2, 8 and 10) with Pen being the slightly more basic. Both amino acids undergo oxidation to the disulfide form (equation 6), a reaction which is catalyzed by metal ions, notably Cu and Fe . 

Most experimental results indicate that GTP is a classic anionic polymerization operating by reversible termination (dissociative mechanism) [55, 56], and/or degenerative transfer [57, 58]. For example, termination occurs once monomer is consumed by backbiting at the pen-penultimate carbonyl to generate cyclic /3-ketoester endgroups as in classic anionic polymerizations (Eq. 11) [59]. Acids with pA a< 18 also terminate group transfer polymerizations, whereas... 

We use the hydrolysis of A into P and Q as an illustration. Examples are the hydrolysis of benzylpenicillin (pen G) or the enantioselective hydrolysis of L-acetyl amino acids in a DL-mixture, which yields an enantiomerically pure L-amino acid as well as the unhydrolysed D-acetyl amino acid. In concentrated solutions these hydrolysis reactions are incomplete due to the reaction equilibrium. It is evident that for an accurate analysis of weak electrolyte systems, the association-dissociation reactions and the related phase behaviour of the reacting species must be accounted for precisely in the model [42,43]. We have simplified this example to neutral species A, P and Q. The distribution coefficients are Kq = 0.5 and Kp = K = 2. The equilibrium constant for the reaction K =XpXQ/Xj = 0.01, where X is a measure for concentration (mass or mole fractions) compatible with the partition coefficients. The mole fraction of A in the feed (z ) was 0.1, which corresponds to a very high aqueous feed concentration of approximately 5 M. We have simulated the hydrolysis conversion in the fractionating reactor with 50-100 equilibrium stages. A further increase in the number of stages did not improve the conversion or selectivity to a significant extent. Depending on the initial estimate, the calculation requires typically less than five iterations. 

Metastable atom bombardment (MAB) is another suitable ionization option for gas-phase organic compounds [11,12]. In this approach, ionization occurs through Penning processes of the type shown in reactions (2.21) and (2.22) by bombarding the sample with a beam of metastable species. The process depicted in reaction (2.21) involves an electrophilic reaction of the metastable atom (A ) to produce an odd-electron molecular ion of the analyte. The excitation energy of A must exceed the IE of the analyte molecule. With much higher excitation energy, dissociative ionization process also takes place [reaction (2.22)]. 

Phosphotungstic acid (1 12) is very soluble in water, in which it is completely dissociated. The pen-tahydrate has been shown to contain complex cations and should be formulated as 3(Hj05) PWjjO s. Decomposition to oxides occurs at 420°C. The crystalline 29 hydrate is built from PWjjO anions as in Figure 5.47a, and (H3 29H20) + units. This latter salt is an exceptionally good proton conductor and this is connected with the high freedom of movement of within the cavity structure of the hydrated cation units. 

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