Protonation site

Electrostatic potential map for imidazole shows negatively-charged regions (in red) as likely protonation sites. 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 proto noted pyrrole) is lowest in energy Examine electrostatic potential maps to see if the lowest-energy form is also that in which the positive charged is best delocalized. Rationalize your result using resonance arguments. What should be the favored substitution product ... 

Optical detection of magnetic resonance (ODMR) was attempted for measurements of the pH effects on the triplet state of purine to investigate the protonation site of purine at low temperatures (78JA7131). The ODMR spectrum did not show the presence of more than one triplet state at liquid helium temperatures. Since the protonated tautomers 1H,9H (3a) and H,1H (3b) have similar bond structures, their triplets should have similar zero-field parameters and are thus not easy to distinguish by ODMR. 

In agreement with the previous data on the protonation site in azaindoles (76AHCS1, p. 529), an example of monoprotonation of 3-aryl substituted 6-azaindoles at the pyridine ring has been demonstrated by UV speetroseopy [88JCS(P2)1839]. 

Data on the protonation sites for a variety of azaindolizines have been summarized in the previous survey (76AHCS1, p. 536).Tlie measured magnetic circular dichroism spectra for 14 azaindolizines confirmed with some corrections the protonation sites of polyazaheterocycles previously reported and also established the conjugated acid structures of additional compounds (85JOC302).Tlie data are summarized in Scheme 1. 

Several polyazaindolizines (61-66) have been studied by and NMR spectroscopy, and the major protonation sites have been determined... 

At the DFT level the preferred protonation site was found to be O4 with a proton affinity of 208.8 kcal/mol at 298 K [98JCC989]. This value agrees nicely with the experimental value of 209.0 kcal/mol. 

Fig. 3-2. Molecular electrostatic potential with 6-31G //3-21G basis set in the molecular plane of (ii)-nitrous acid. Black dots refer to four different protonation sites in potential minima. For values of isopotential contours see Nguyen and Hegarty, 1984.

As an example of the MO treatment, let us consider the case of a diatomic molecule (AB) with two electrons in the presence of a bare proton. For our specific example, we will neglect the resonance interaction between the molecule and the ion using fiAC = jiBC = 0, where C designates an atomic orbital on the proton site. 

For determination of the number of protonic sites and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-i (due to pyridine chemisorbed on Lewis acid sites, L-Py) and 1490 cm i (due to both the L-Py and pyridine chemisorbed on protonic acid sites, B-Py) were used with the tangent background for all samples. The values obtained were normalized to the weight of the sample wafer. To obtain the apparent absorption coefficients of the bands, a known amount of pyridine was adsorbed on the sample, and the absorbance of each band was measured. Then, a small quantity of water which is sufficient to convert all Lewis acid sites into protonic acid sites was introduced into the IR-... 

The results are discussed in terms of strength and number of protonic sites and the presence of mesoporosity is shown to be important for the production of trimethylpentanes in the alkylation reaction. 

Sulfate-doped zirconia was found to have both Lewis and Brdnsted add sites on the surface, where the sulfate groups were the carrier of the protonic site. The ratio of Lewis to Br0nsted sites was found to vary depending on heat treatment, but the percentage of sulfate groups associated with a proton was found to be constant at 12-17%. 

Let us turn to the results obtained for nitrous oxide N2O. From a theoretical point of view, this is an interesting molecule as properties such as its dipole moment and protonation site have been found very difficult to calculate accurately [14,24], N2O is a linear species whose predicted bond lengths and dipole moment are presented in Table 2. 

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