Proton structures

H3O" is strictly the oxonium ion actually, in aqueous solutions of acid this and Other solvated-proton structures exist, but they are conveniently represented as... 

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

Dlugosz M, Antosiewicz JM, Robertson AD (2004) Constant-pH molecular dynamics study of protonation-structure relationship in a heptapeptide derived from ovomucoid third domain. Phys RevE 69 021915. 

FIGURE 25 Two proposed structures structure A with nine exchangeabie protons, structure B with eight exchangeabie protons. 

The nucleus is a bound system of strongly interacting particles. Unfortunately, modern QCD does not provide us with the tools to calculate the bound state properties of the proton (or other nuclei) from first principles, since the QCD perturbation expansion does not work at large (from the QCD point of view) distances which are characteristic for the proton structure, and the nonper-turbative methods are not mature enough to produce good results. 

Fortunately, the characteristic scales of the strong and electromagnetic interactions are vastly different, and at the large distances which are relevant for the atomic problem the influence of the proton (or nuclear) structure may be taken into account with the help of a few experimentally measurable proton properties. The largest and by far the most important correction to the atomic energy levels connected with the proton structure is induced by its finite size. 

Radiative corrections to the nuclear polarizability a(Za) m to S -levels are described by the diagrams in Fig. 7.16 and in Fig. 7.17 (compare with the diagrams in Fig. 6.4). As usual for muonic hydrogen the dominant polarization operator contribution is connected with the electron loops, while heavier loops are additionally suppressed. The contribution of the diagrams in Fig. 7.16 was calculated in [52] on the basis of the experimental data on the proton structure functions... 

Up to now we considered only the contributions of order Za)Ep to h q)erfine splitting in hydrogen generated by the elastic intermediate nuclear states. As was first realized by Iddings [13] inelastic contributions in Fig. 11.5 admit a nice representation in terms of spin-dependent proton structure functions Gi and G2 [13, 11]... 

The theoretical situation for the hyperfine splitting in hydrogen always remained less satisfactory due to the uncertainties connected with the proton structure. 

First, peak heights are measured at five points in the NMR spectra (Figure 2). All NMR spectra of fulvic acids described in this study were determined as the sodium salt in D20 at pH 8 (21). Peak heights were used rather than peak areas to minimize overlapping spectral contributions from various proton structures. From structural-model considerations, peak 1 appears to be a combination of methylene and methine protons in aliphatic alicyclic rings and branched methyl groups located beta to carbonyl groups of a carboxylic acid, ester, or ketone. The structural model rules out meth-... 

Electrophilic Aromatic Substitution. The 7t-excessive character of the pyrrole ring makes the indole ring susceptible to electrophilic attack. The reactivity is greater at the 3-position than at the 2-position. This reactivity pattern is suggested both by electron density distributions calculated by molecular orbital methods and by the relative eneigies of the intermediates for electrophilic substitution, as represented by the protonated structures (7a) and (7b). Structure (7b) is more favorable than (7a) because it retains the benzenoid character of the carbocydic ring (12). 

The cyanodiazonium ion NCN2+ proposed as a transient intermediate in solution,503 has been observed in the gas phase by Cacace et al.504 It was generated by ionization of NF3 and cyanamide and characterized by collisionally activated dissociation. The linear structure of C, , v symmetry (216) has been found442 to be the global minimum by high-level ab initio calculations [MP2(FU)/6-31G ]. Of the protonated structures the dication protonated on the cyano nitrogen is 55.0 kcal mol 1 more stable. 

Generation of the hydroxydiazonium ion HON2+ as long-lived species under superacidic conditions was not successful.507 According to an early theoretical study, the ion has a structure similar to that of methoxydiazonium ion 218. Recent high-level calculations442 [MP2(FU)/6-31G level] have found the O-protonated structure of Cs symmetry to be the global minimum but it is only 2.5 kcal mol-1 more stable than A-protonated nitrous oxide. Of the dications formed by a second protonation, the 0,A-diprotonated nitrous oxide is more stable than the O, (9-diprotonated nitrous oxide by 7.5 kcal mol... 

On the basis of spectral data, structure 5B was excluded. Structure 5A was established on the basis of IR spectrum, which showed an absorption band at 1665 (5a), 1659 (5b), 1790 cm 1 (5c) characteristic to (CO). The H NMR spectrum which revealed a broad single at 3 7.47 ppm (5a) and at 7.33 ppm (5c) characterised for NH proton. Structures 3-6 were established by spectral data and analogy with our previous work (Scheme 1). 

Abstract. The usefulness of study of hyperfine splitting in the hydrogen atom is limited on a level of 10 ppm by our knowledge of the proton structure. One way to go beyond 10 ppm is to study a specific difference of the hyperfine structure intervals 8Au2 — Avi. Nuclear effects for axe not important this difference and it is of use to study higher-order QED corrections. 

The hyperfine splitting of the ground state of the hydrogen atom has been for a while one of the most precisely known physical quantities, however, its use for tests of QED theory is limited by a lack of our knowledge of the proton structure. The theoretical uncertainty due to that is on a level of 10 ppm. To go farther with theory we need to eliminate the influence of the nucleus. A few ways have been used (see e. g. [1]) ... 

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