Protonated, characterization

A nucleus is composed of protons and neutrons, each of which has unit atomic mass. The number of protons characterizes each element. In going from one element to the next, the total number of protons increases by one. Thus the simplest element, hydrogen, has atoms having only one proton in the nucleus, and the next simplest, helium, has two protons in the nucleus. 

However, in the (A.. H.. B) system (more particularly in those represented by (A—H—A) and (B—H—B)+) containing strong hydrogen bonds (Table 1), other types of potential curves may exist. This is the case of the hesitating proton characterized by a small or zero potential barrier. We can distinguish, 1. the asymmetric double minimum, 2. the symmetric double minimum, and 3. the symmetric single minimum by spectroscopic means (see Section III). 

A combination of physicochemical, topological, and geometric information is used to encode the environment of a proton, The geometric information is based on (local) proton radial distribution function (RDF) descriptors and characterizes the 3D environment of the proton. Counterpropagation neural networks established the relationship between protons and their h NMR chemical shifts (for details of neural networks, see Section 9,5). Four different types of protons were... 

Figure 10.2.6. Special distance measures for the characterization of proton environments a) distance r and angle a, to double bonds b) distance and angle Oc, to single bonds c) dihedral angle a, to the third bond from the hydrogen atom.

The 4-Hydroxy-thiazoles are characterized by infrared absorption near 1610 cm (KBr) (3) or 1620 to 16.S0cm (CCI4) (8), indicating a strongly polarized carbonyl group. H-5 resonates near 5.6 ppm in the NMR spectrum like similar protons in other mesoionic compounds (3). Two fragmentations of the molecular ion are observed in the mass spectra. The first involves rupture of the 1,2 and 3,4 bonds with loss of C2R 0S . In the second, the 1,5 and 3,4 bonds are cleaved with elimination of C2R 0. ... 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 13 10 A doublet of doublets characterizes the signals for the protons of the type shown (where W X Y and Z are not H or atoms that split H themselves)... 

Methyl ketones such as 2 butanone m Figure 17 18 are characterized by sharp singlets near 8 2 for the protons of CH3C=0 Similarly the deshieldmg effect of the carbonyl causes the protons of CH2C=0 to appear at lower field (8 2 4) fhan m a CH2 group of an alkane... 

Section 20 21 Acyl chlorides anhydrides esters and amides all show a strong band for C=0 stretching m the infrared The range extends from about 1820 cm (acyl chlorides) to 1690 cm (amides) Their NMR spectra are characterized by a peak near 8 180 for the carbonyl carbon H NMR spectroscopy is useful for distinguishing between the groups R and R m esters (RCO2R ) The protons on the carbon bonded to O m R appear at lower field (less shielded) than those on the carbon bonded to C=0... 

Elimination unimolecular (El) mechanism (Section 5 17) Mechanism for elimination characterized by the slow for mation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene Enamine (Section 17 11) Product of the reaction of a second ary amine and an aldehyde or a ketone Enamines are char actenzed by the general structure... 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

Weak bases only partially accept protons from the solvent and are characterized by a base dissociation constant, kj,. For example, the base dissociation reaction and base dissociation constant for the acetate ion are... 

Equivalent Weights Acid-base titrations can be used to characterize the chemical and physical properties of matter. One simple example is the determination of the equivalent weighf of acids and bases. In this method, an accurately weighed sample of a pure acid or base is titrated to a well-defined equivalence point using a mono-protic strong acid or strong base. If we assume that the titration involves the transfer of n protons, then the moles of titrant needed to reach the equivalence point is given as... 

A neutron is characterized by having no electrical charge but has one unit of atomic mass, the same as that of a proton (Figure 46.2). Neutrons, like protons, reside in the atomic nucleus and contribute to the mass of the atom. The chemistry of an atom, like its size, is determined by the electrons in the atom. The mass of the atom is characterized mainly by the total number of neutrons and protons in the nucleus (atomic binding energies are ignored in this discussion). For mass spectrometric purposes of measurement, it is the mass that is important in establishing m/z values. 

While the number of protons in an atomic nucleus characterizes each element, the mass of the nucleus comprises the total number of protons and neutrons. 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

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