Proton affinity decomposition

Two other molecules considered in the same context, but with proton affinities larger than ammonia, were pyridine (C5H5N) and trimethylamine (TMA) (CH3)3N whose proton affinities are 220.8 cal mol-1 and 225.1 cal mol-1, respectively.190 Metastable decomposition studies of NH3(C5H5N)mH+ (m = 1-5) yield the following results ... 

The decomposition of NH4HSO4 and the appropriate thermochemical cycle for determining the proton affinity of the HS04-(g) ion can be shown as follows ... 

In Chapter 7, it was shown how the enthalpy of decomposition of an ammonium salt can be used to calculate the proton affinity of the anion. The proton affinity is a gas-phase property (as is electron affinity) that gives the intrinsic basidty of a species. The reaction of H+ with a base B can be shown as... 

While a great deal of research has been done on condensations of oxometalates in aqueous solutions, there has been little such work in the field of thiometalates. The condensation reactions subsequent to protonation of thiometalate ions take place at lower pH than those of the oxoanions, since the proton affinity of S is appreciably lower than that of O. Investigation of condensation behavior of thiometalate ions is more difficult, since various complex decomposition processes are also involved. For example, no definite condensation product has yet been isolated from an aqueous solution containing MoS2-. 

The above equation can be simplified to give an equation that shows the relation of the proton affinity (PA) with the rate constant ratio of the decomposition reaction ... 

Formation of a-carbon radicals either from cation decomposition or through abstraction of H by OH- is considered here because it can be construed formally as loss of an electron followed by proton transfer. If the cation deprotonates from the peptide chain by transfer to components of high proton affinity then the a-carbon radical is formed directly. Abstraction by OH in fluid systems gives the same result. 

The overall order of alkane acidities deduced from the decomposition of silicon anions is the same as that obtained from the decomposition of alkoxides " " . The ethyl anion has the highest proton affinity, i.e., ethane is the weakest acid. The acidities of propane (secondary hydrogen) and of cyclobutane are both lower than that of methane, but all other alkanes are more acidic. The acidity of methane is particularly enhanced by a cyclopropyl substituent and by a t-butyl group. 

APCI is a relatively soft ionization technique. In fact, only few fragment ions are normally recorded. Nevertheless, analyte decomposition may occur due to heating. It can be used in the analysis of polar and low polar analytes with molecular weights up to 1500 Da [29] as long as the proton affinity of the analytes is higher than that of the solvents. On the other hand, ionization techniques such as ESI and matrix-assisted laser desorption/ionization (MALDI) are mainly used in the analyses of polar, less volatile, and thermally labile analytes, for example, large biomolecules. These two ionization techniques are discussed in the following (Sections 2.4 and 2.6). 

Even-electron ions. As discussed in Section 4.4, the favored decomposition of an EE" ion yields another EE" ion and a molecule as the neutral product. Product molecules of low proton affinity are favored, such as those of Equations 4.26-4.28 (PA = 7.4,6.2, and 7.2 eV, respectively Field s Rule, Section 4.4). Equations 4.26 and 4.27 are two-step pathways for forming the same products produced by Equations 4.22 and 4.25, respectively. However, these are energetically less favorable by 0.2-0.8 eV (20-80 kj/mol) than their one-step counterparts. 

Charge competition in EE decompositions Field s Rule, Although Stevenson s Rule is also applicable to EE" ions, of additional predictive value is a proposal by Field (1972), amplified by Bowen et al. (1978). This points out the energetic favorahility of forming neutrals with low proton affinities (PA) in dissociations of different EE ions yielding the same products. (As a corollary of Field s Rule, an intermediate proton-bound complex should dissociate preferentially to form the lower PA neutral.)... 

Vmin, -Pvmin and 7s,min are closely related to the electrostatic, polarization and charge-transfer components, respectively, of the Morokuma decomposition of proton affinity. It has been shown [197] that the spectroscopic scale Ziv(OH) (related to methanol), the phenol affinity, the diiodine affinity and the proton affinity of 42 nitrogen, oxygen and sulfur bases can be correlated by the triple-scale Equation 1.133 ... 

For anion-radicals, air (i.e., oxygen, carbon dioxide, and water [moisture]), on the whole, is an active component of the medium and so it should be removed before conducting reactions. Understandably, air inhibits anion-radical reactions The anion-radicals primarily formed are consumed at the expense of oxidation, carboxylation, and protonation. Certainly, oxidation can take place only if the acceptor organic molecule possesses a lower affinity for an electron than oxygen does or if one-electron oxidation of the anion-radical by oxygen proceeds more rapidly than the anion-radical decomposition into a radical and an anion (RX R + X ). 

The term nucleophilicity refers to the relative rate of reaction of an electron donor with a given electrophile, as distinct from basicity, which refers to its relative affinity for a proton in an acid-base equilibrium. A quantitative relationship between rate and equilibrium constants was discovered by Brpnsted and Pedersen (1) in 1924. These authors found that the rate constants for the catalytic decomposition of nitramide by a family of bases, such as carboxylate ions (GCH2C02 ), could be linearly correlated with the acidities of their conjugate acids, pKHB. This observation led to the discovery of general base catalysis and the first linear free-energy relationship, which later became known as the Brpnsted equation ... 

---