Nitrogen acid-base changes

Table 7.1 Acid-base changes in nitrogen transformations in ricefields...

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

Substitution of the dimethylsilyl group by bis(tert-butyl)-stannyl does not change the structure in solution, e.g. 33 is found to be monomeric. A very interesting dimer is 26. In contrast to the centrosymmetrical dimer of 1 (C-Symmetry), 26 has a twofold axis (C2, see Fig. 9). This special structure may be due to intramolecular Lewis acid-base interactions between the boron and nitrogen atoms 39). Nevertheless,... 

Another field of investigation is the use of metal nitridooxophosphates such as A1PON (11) or ZrPON (12) as catalysts. The incorporation of nitrogen leads to a change in acid-base properties and to improved stability. 

AG a, where AG. c refers to the symmetrical proton transfer between two identical carbon bases and AG aa the symmetrical proton transfer for a catalysing acid-base pair HA/A. The inequality is justified because proton transfers between oxygen and nitrogen bases are much faster than those to and from carbon bases. In the simplified equation (20) changing the catalyst HA or A affects AG only through the thermodynamic term GA. Differentiation of... 

Reinvestigation of the excited state acid-base properties of 2-naphthylamine (Schulman and Capomacchia, 1972) showed that a reported change of hybridization from sp3 to sp2 on excitation had little effect on the entropy of protonation of nitrogen in the Sx state and that therefore the Forster cycle was still applicable. A pX (S1 )-value, calculated from the fluorescence maxima of the B and BH+ form, of —8-1 is in poor agreement with the value, —2, obtained from fluorescence titration measurements. From the acidity dependence of fluorescence intensity for 1- and 2-naphthylamine Liedke and Schulman (1973a) found that the decrease in emission of the B form occurred at lower acidities than the appearance of BH+ fluorescence. [A similar titration curve for the fluorescence of the neutral molecule was obtained by Seliskar and Brand (1971), who obtained a value of 0-64 for pkr(S1) from the decrease of the... 

Protonation by acid-base chemistry leads to an internal redox reaction (Fig. 11.19), without change of the number of electrons (Heeger, 2001 MacDiarmic, 2001). The semiconductor (emeraldine base, emeraldine salt, 100 S/cm). Complete protonation of the imine nitrogen atoms in emeraldine base by aqueous HC1 results in the formation of a delocalised polysemiquinone radical cation. This is accompanied by an increase in conductivity of more than 12 orders of magnitude. 

Table III includes the C/N, C/S, and N/S ratios for the various fractions to make the changes brought about by acid-base fractionation more apparent. The fact that the N/S ratio varies widely makes it clear that although sulfur is associated with nitrogen in some of the basic species, there must be other basic species that contain nitrogen without sulfur.

In the specific case of emissions of sulphur dioxide and oxides of nitrogen, the change in oxidizing nature of the atmosphere subsequent to chemical reactions in the atmosphere, and upon deposition, results in changes in the acid base and redox equilibria in the aqueous phases. 

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