Lone pairs, delocalization

N-Nitroso compounds have been found to exist as syn and anti rotamers [30, 31] due to restricted rotation of the N-N bond resulting from nitrogen lone-pair delocalization (Fig. 3.2). This delocalization causes the hydrogens at the a-carbons to become acidic as evident by their base-catalyzed reactions, such as exchange with deuterium... 

A substantial divergence from the behaviour described up to now occurs with the 2,6-(V,(V-tetramethyl anilines (Table 7). The effect of the para substituent is no longer systematic, and no good correlation with substituent electronic properties is displayed. Evidently, the extent of nitrogen lone-pair delocalization in these compounds reflects a balance between electronic demand for, and steric inhibition of, delocalization that varies with the individual compounds. Nitrogen chemical shifts are a sensitive probe for these effects. 

Benzoannulation is thus seen to inhibit lone-pair delocalization in l//-azonine and its anion. The key operational question, of course, is, does this happen because of a reduction in the molecule s AH0 contribution to Eq. (1) or does it result from an increase in the a strain (AH0a term) possibly because of the development of skeletal restrictions not... 

Why are aniline (pWb = 9.4) and pyridine (pWb = 8.8) so much less basic than ammonia (pWb = 4.8) in aqueous solution For a long time, students of organic chemistry have been given reasons in terms of lone-pair delocalization and sp /sp hybridization of the nitrogen atom. From gas-phase studies, it is now clear that aniline and... 

MesE and (Me3Si)3E steadily increases down the group E = N, A = — 0.06 eV E = P, A = 0.39 eV E = As, A = 0.35 eV E = Sb, A = 0.91 eV. These results seem to argue for the importance of lone-pair delocalization from phosphorus and arsenic and the relative absence of such an interaction in the case of antimony. It is difficult to judge the possibility of alternative explanations, since direct structural information on the trimethylsilyl derivatives of As and Sb is not available. 

The ir-bond stretching of enamines is only slightly affected by interaction with the nitrogen lone pair. It might be argued that the relatively greater shift of the absorption for the less substituted isomer as compared with the more substituted isomer (relative to a tri- or tetra-substituted alkene at 1670 cm ) is an indication of greater lone pair delocalization in the former (Scheme 9). 

As a general conclusion, an interaction between the nitrogen lone pair and the adjacent 7T-system deshields the aniline-type nitrogen. It is therefore reasonable to expect that nitrogen chemical shifts can be used as probes for the influence of the ring substituents and the lone-pair delocalization. 

Guanidinium cation (H2N)3C Three N lone pairs delocalize plus... 

Acylium cation .. H3C-C=0 Lone-pair-delocalized vinyl cation... 

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