Bases proton affinity

There is the possibility of hydrogen bonding and multiple interactions of the adsorbed base with the zeolite. The case of adsorbed water is an interesting situation where the distinction between hydrogen bonded and protonated species is difficult to make. The initial formation of an hydrogen bonded complex may be followed by proton transfer from the acidic site to the adsorbate . As indicated earlier, the coupling of the acidic proton of the zeolite with its environment in the framework as well as the electronic structure of the adsorbate base ( proton affinity ) makes it rather difficult to establish a unique acidity scale based simply on (0(O-H) stretching frequencies. 

However, many substances, notably alcohols, have a greater proton affinity than the hydrogen fluoride molecule, and so behave as bases, for example ethanol ... 

Base Conjugale Acid Proton Affinity (experimental, kcal/mol) Proton Affinity (AM1, kcal/mol)... 

As early as 1889 Walker (320), using samples of thiazole, 2,4-dimethylthiazoie, pyridine, and 2,6-dimethylpyridine obtained from Hantzsch s laboratory, measured the electrical conductivity of their chlorhydrates and compared them with those of salts of other weak bases, especially quinoline and 2-methylquinoline. He observed the following order of decreasing proton affinity (basicity) quinaldine>2,6-dimethyl-pyridine>quinoline>pyridine>2,4-dimethylthiazole> thiazole, and concluded that the replacement of a nuclear H-atom by a methyl group enhanced the basicity of the aza-aromatic substrates. 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

The semi-empirical methods have better MAD s than th Hartree-Fock-based methods, indicating that their parametrization ha accounted for some of the effects of electron correlation. However, thei maximum errors are very large. Semi-empirical methods are especiall poor at predicting ionization potentials and proton affinities. 

Proton affinities of the bases calculated using the value AT/f (H ) = 367.2 kcal mol ... 

Gas-phase basity and proton affinity values for 3,4,6,7,8,9-hexahydro-2/f-pyrido[l,2-n]pyrimidine were determined and they were compared to other super bases, including its lower and higher piperidine ring homologs (94JP0725, 01JPO25). 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

As noted above, whole-cell MALDI-TOF MS was intended for rapid taxonomic identification of bacteria. Neither the analysis of specific targeted bacterial proteins, nor the discovery of new proteins, was envisioned as a routine application for which whole cells would be used. An unknown or target protein might not have the abundance or proton affinity to facilitate its detection from such a complex mixture containing literally thousands of other proteins. Thus, for many applications, the analysis of proteins from chromatographically separated fractions remains a more productive approach. From a historical perspective, whole-cell MALDI is a logical extension of MALDI analysis of isolated cellular proteins. After all, purified proteins can be obtained from bacteria after different levels of purification. Differences in method often reflect how much purification is done prior to analysis. With whole-cell MALDI the answer is literally none. Some methods attempt to combine the benefits of the rapid whole cell approach with a minimal level of sample preparation, often based on the analysis of crude fractions rather... 

Proton affinity in the gas phase and pXa values of strong neutral organic (super)bases in acetonitrile were calculated by DFT at the B3LYP level, among others for 34a (TBD, hppFI) and 34b (MTBD, hppM) <20010L1523>. 

These models refer to reactions with the simplest nucleophile, H, both under neutral conditions and in the protonated form. Chemical reactivity can be strongly altered by catalytic effects acid/base catalysis is of particular importance. We regard the studies on ga phase acidities and on proton affinities discussed in the above sections to bear special significance for quantitative modelling of acid/base catalysis in the future. 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

R = H, X = S, A = Et3N and Py). In solution the former is mainly in an ionic form the latter exists as a complex. The basicity of the amine is assumed to be important. Triethylamine is a stronger base than pyridine and the ionic form is stabilized. When the proton affinity is weak, the basicity in relation to the B(III) atom, a Lewis acid, plays an important role. This involves an equilibrium shift toward the complex. This assumption is confirmed by an easy displacement of pyridine by triethylamine. The reverse process demands more severe conditions. In the NMR spectra of the triethylamine complex the signal is shifted from 22 to 42 ppm as pyridine is added. The absence of signals of two separate forms is evidence in favor of their fast interconversion. The chemical shift of the signal in 3IP spectra is 22 ppm (EtOH), 26 ppm (Py, DMFA), and 42 ppm (EtOH, Py) for complexes with triethylamine and pyridine. 

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... 

In this equation, fH is the ionization potential for H (1312kJ mol-1), fB is the ionization potential for the base B, and EB+ is the energy of the B+-H bond. The term IB is subtracted from fH (the last term is of lesser importance), which leads to the conclusion that the smaller the value for IB, the greater the proton affinity. Because H+ reacts by removing electron density from B, the easier this process is to accomplish, the smaller the value of IB. For the molecules CH4, NH3, H20, and HF, the proton affinities are 527, 841, 686, and 469 kj mol-1, respectively. These values correlate well with the ionization potentials of the molecules, which are in the order NH3 < H20 < CH4 < HF. 

It is interesting to note that even a "saturated" molecule such as CH4 has a significant attraction for a proton. This demonstrates clearly that even a pair of electrons that is shared in a bond can be a binding site for H+. In general, the more acidic (or less basic) a compound is, the lower the value for its proton affinity. For example, the proton affinity for NH3 is 866kJ/mol, whereas that for PH3 is 774, in keeping with the fact that PH3 is the weaker base. 

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