Pyridine bond orders

Tire calculated Ca-N" bond lengths, Table III, of these pyridinium cations [as well as the bond orders, BO, and the reaction energies, zIT/r, for a hypothetical dissociation to give the corresponding carbenium ion 44 and pyridine (Scheme 13)] indicate that these bonds are significantly weakened... 

The separation of the same charged compounds were also accomplished on an ethyl-pyridine bonded silica surface and 30 0% methanol/C02 mobile phases without the need of added sulfonate modifier. Anionic compounds did not elute from the ethyl-pyridinium surface that lead the authors to hypothesize that the surface was positively charged. To further test this hypothesis, the separation of the same compounds on a strong anion exchange column, silica-based propyltri-methylammonium cationic surface, which exhibits are permanent positive charge was attempted. The same retention order was observed on the strong cation exchange surface. 

The cores of the W6 clusters are very similar to those of the Mo6 analogues, and they are composed of the regular octahedra of six tungsten atoms capped by eight sulfur atoms (41, 43, 44). The W-W distances are almost the same, and difference in terminal ligands has little effect on the geometry (Table I). Only in the case of tert-butylpyridine complexes has a very small compression of the octahedron been observed in the c-axis direction (44). The bond order for the W—N bonds in the pyridine complexes is much weaker than that for the triethyl-phosphine analogue (43). 

Recently, a series of models of 16 polynuclear pyridine-like heterocycles (Fig. 9 shows formulas of eleven of these) were treated using the HMO approximation7 (SN = 0.6, inductive effect not allowed for) and the following reactivity indices calculated 77-electron densities (q), bond orders (p), free valences (F, N x = Wheland s atom localization energies (A(,Ar,An), and superdelocalizabilities, both exact (Se,Sr,Sn) and approximate (S e,S r,S n). Atom-atom polarizabilities150 (773) had been calculated earlier.151 Some of the indices calculated are presented in Section VI, B. 

Protonation shifts of pyridine (Table 4.67) are much stronger but follow the trend of dilution shifts portrayed in Fig. 4.12. Shielding in the a position is attributed to a change of the N —C-a bond order. Deshieldings at C-3 and particularly at C-4 arise from an increased electron withdrawal of the positively charged nitrogen. Shift changes induced... 

Another interesting example of C-C bond formation can be found in reports from Li and co-workers who, in 2004, described the preparation of an MIP with peroxidase-like activity capable of dimerising the homovanillic acid (HVA) (73) [49]. In this case a polymer was prepared by using the HVA substrate, instead of a TSA, as a template and a haemin unit as catalytic centre (74). The polymerisation was carried out in the presence of acrylamide and vinyl-pyridine in order to add extra functionalities aiding substrate recognition. The imprinted polymer showed an enzyme-like activity, as confirmed by adherence to the Michaelis-Menten saturation model, and it was inhibited by ferulic acid (75), a structural analogue of the substrate, which is also capable of inhibiting the natural peroxidase. 

Table 1 X-ray bond lengths (A) and bond angles (°), bond orders, n-electron densities, and straight charges from INDO calculations of 177-pyrazolo[3,4-c]pyridine hydrochloride monohydrate <89ap<322)885>.

Phillips and Lee calculated the 15N isotope effect for the decarboxylation of 1-methyl orotate (lb) via 2-protonation (4b) and via 4-protonation (6b). They found that in both cases, the calculated isotope effect is normal 1.0043 for 2-protonation, and 1.0054 for 4-protonation. An examination of the optimized structures showed clearly that very little bond order change occurs at Nl, regardless of which oxygen is protonated. Phillips and Lee also benchmarked their calculations by computing the IEs for protonation of pyridine and for decarboxylation of picolinic acid (17) and A-methyl picolinic acid (18) the results of these calculations are in agreement with the experimental values mentioned above. Therefore, Philips and Lee asserted that... 

For these complexes, the isotropic and 15N chemical shifts and the 15N chemical shift tensor elements were measured as a function of the hydrogen bond geometry. Lineshape simulations of the static powder 15N NMR spectra revealed the dipolar 2H-15N couplings and hence the corresponding distances. The results revealed several correlations between hydrogen bond geometry and NMR parameters which were analysed in terms of the valence bond order model. It was shown that the isotropic 15N chemical shifts of collidine and other pyridines depend in a characteristic way on the N-H distance. A correlation of the and 15N... 

Adams and Slack compared the electron densities and mobile bond orders of the parent molecule (21) with those of thiazole (22) and pyridine (23). Overlap was neglected in these simplified calculations,... 

Addition of pyridine and acetylene results in generation and trapping of the complex shown in equation (87), which has a reduced bond order. In this complex, the acetylene bridges the two metals. This type of chemistry was initially developed by Schrock and coworkers for alkyne metathesis (see Alkyne Metathesis) reactions but has been subsequently developed by a number of other researchers. 

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