Relative protonicity

In many cases, a protonated molecular ion (M - - H)+ is the only ion observed in a thermospray spectrum but if ammonium acetate buffer is used, depending upon the relative proton affinities of the species present, an ammonium adduct (M - - NH4)+ may be the predominant ion. In addition, clusters may be formed with components of the mobile phase. Although the thermospray ionization process involves less energy than conventional Cl, and very little intense fragmentation is usually observed, the presence of ions due to the elimination of small molecules, e.g. water, methanol and ketene, is not unknown. These latter ions are usually of relatively low intensity when compared to the protonated or... 

This is either an (M + 1)+ or an (M - -18)+ ion, depending upon the relative proton affinities of the analyte and ammonia. The molecular weight is therefore either 221 or 204 Da. Since the molecule contains an even number of nitrogen atoms, it must have an even molecular weight. The molecular species must therefore be (M - -18)+ and so the molecular weight is 204 Da. 

The relative binding enthalpies, 5 >[(CpNi-L)+] show a good linear correlation with the relative proton affinities of SZ>[(L-H)+] of the ligands, except that ligands having ir- bonding ability (NO, MeCN, MeNC) are more strongly bound to nickel than would be expected from their proton affinity alone. 

Not all ionization methods rely on such strictly unimolecular conditions as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

Stmcture Elucidator is not used for every unknown impurity or degradant problem in the author s laboratory, the program package does serve as a very useful tool when particularly challenging unknown stmctures have to be identified, especially if those stmctures are relatively proton deficient in nature. 

This observation has been rationalized on the basis of the relative proton affinities (PA) of the substrates and that of the nucleophile. In fact, the relative order of proton affinities reveals that /M(C2H5C1) > PA(H20) > P/f(CH3CI). Thus, for the first case, rapid proton transfer dominates over nucleophilic displacement, since proton transfer involving species which do not require any electronic or bond reorganization will in general be much faster than a displacement. 

Another important question related to substituent effects on gas-phase basicities of thiocarbonyl compounds is whether these substituent effects arise from interactions within the neutral or within the protonated species, or both. It is useful to define the relative proton affinities along the monosubstituted series of compounds by means of the process shown in equation 54. 

By fixing the scale of mass number to proton excess at 8 1, the latter quantity is mapped on a 44 x 44 square lattice, gauged on atomic number. The isotopes of any element map on to straight lines perpendicular to OZ, with Ru on the diagonal. The relative proton excess, 1 — tN/Z, varies... 

For proton affinity determination, the kinetic method involves the formation of the proton bound heterodimer between the two bases whose affinities are to be compared. By tandem mass spectrometry, the appropriate cluster ion [BiHB2]+ is selected and its spontaneous or collisional dissociation is observed. As shown in Figure 4.16, the competitive dissociation leading to the two protonated monomers is analysed and the relative abundances of the monomers [BiH]+ and [B2H]+ are measured. From these abundances, the relative proton affinities of the two bases Bi and B2 can be calculated and the proton affinity of one of the two bases can be determined, if the proton affinity of the other is known. 

The reaction affords two products, an oxolane Pi and an oxetane P2, which exhibit a mirror-image relationship of their CIDNP patterns. The three most strongly polarized signals, of Hi, H7, and H7, with intensity ratios of about —2 to + 3 to +3.5, have been shown in the figure all the other protons are also polarized, but more weakly. The observed pattern is found to be in excellent agreement with the relative proton hyperfine coupling constants of the neutral benzosemiquinone radical and of the tert-butoxybicyclo[2.2.1]heptenyl radical, which were tested as model compounds for the two radical moieties.The biradical BRi is thus the source of the polarizations. It is formed in a triplet state, its singlet exit channel produces the oxolane Pi, and its triplet exit channel the oxetane P2. 

It is possible to quantify these relationships between the degree of proton transfer from acid to base and the relative proton affinities of the two partners. This treatment illustrates the unlikelihood that HF will form an ion pair with any acceptor, no matter how basic. HCl is more susceptible to the nature of the acceptor its proton can be pulled away from the Cl as the basicity of the acceptor is raised. HBr is more sensitive still, as the nature of its complexes with the amines take on more and more ion pair character for the more basic amines. 

NPAD Normalized Proton Affinity Difference. A measure of the relative proton... 

Strong oxazole infrared (IR) absorbance occurs in the range of 1555-1590cm . The value can shift up to 1600 or down to 1500 depending on the substituents on the heterocycle. IR spectroscopy has been used to established the relative proton affinities of the nitrogens of 2,5-diphenyloxazole 33, 5-phenyl-2-(2-thienyl)oxazole 34, and 2-(2-furyl)-5-phenyloxazole 35 toward phenol. The trend was phenyl < thienyl < furyl <1999RJC1810>. 

Aqueous solutions do not contain free protons and free electrons, but it is nevertheless possible to define relative proton and electron activities. The pH,... 

Before processes of the type shown in Eq. (22) become general for the synthesis of hydro complexes of the platinum metals, it will be necessary to study this reaction more fully in order to be able to predict the direction of the possible equilibrium. This depends on the relative proton affinities of the conjugate bases, the strength of the metal-ligand bond, and possibly on the relative volatilities or solubilities of the acids. A further complication arises from the type of reaction shown in Eq. (23) which must also be considered when predicting the final result. 

A number of proton-transfer equilibrium constants for reactions similar to those shown in Eq. (3) have been measured by ion cyclotron resonance, high-pressure mass spectroscopy, flowing afterglow, MIKES, and MIKES/CID techniques. These studies allowed the relative proton affinities of a variety of bases to be determined with an accuracy of better than +0.2 kcal mol" and compared with related thermodynamic data measured in solution. 

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