Heteroatom effects, nitrogen resonances

Inhibiting effect of DM arises from the presence of aromatic rings in its structure. The aromatic compounds are known to influence the radiation-induced modification by effect of resonance energy dissipation [24-25]. In the structural formula of DM, heteroatoms, such as sulfur and nitrogen are also present. The sulfur moieties are known to inhibit the effect of ionizing radiation action on matter [26-27], due to the fact that sulfur groups act as sinks of the radiation energy [28]. 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

Resonance effects. Conjugation with a double bond increases the stability of a carbocation. Thus, allylic and benzylic cations are more stable than their saturated counterparts. (For example, see Problem 1.4.c.) Heteroatoms with unshared electron pairs, e.g., oxygen, nitrogen, or halogen, can also provide resonance stabilization for cationic centers, as in the following examples ... 

The situation is simplified if the substituent is at a para position to the reaction site since in this case there are no conditions for realizing the indirect resonance effect. The substituent and the reaction site cannot be simultaneously directly conjugated with the nitrogen heteroatom of the azine ring and, as a rule, the reactivity parameters correlate well with the Hammett ffp values. Examples include (1) the ionization constants of 5-substituted picolinic (57) and 6-substituted nicotinic acids (59) (59NKZ1293) (2) the rates of alkaline hydrolysis of 5-substituted methyl picolinates (58) (70JCS(B) 1063) ... 

The low values exhibited by imidoyls (1.20-1.85 G) could be accounted for through a spin polarization mechanism that induces negative spin density at the nitrogen, somewhat balancing the positive spin density resulting from resonance effect. In subsequent papers [5d,f], Roberts used isonitriles as the source of various a-heteroatom-substituted imidoyl radicals and calculated some rate constants for both the radical addition steps and the -fragmentation processes (Scheme 4, routes... 

The H-NMR spectra of quinazoline is complex due to the fused aromatic ring and six nonequivalent protons. HA and H2, which are deshielded by the nitrogen atoms in the diazine ring, are found downfield at 9.34 ppm and 9.28 ppm with HA being the most deshielded due to resonance effects. Correspondingly, H5-H% are less shielded, with //8 being the least shielded (7.99 ppm) due to the electron delocalization of the heteroatom. 

The question of which resonance structure is the principal contributor has been a point of considerable discussion. Since the nonpolar ylene resonance structures have 10 electrons at the phosphorus of sulfur atom, these structures imply participation of d orbitals on the heteroatoms. Structural studies indicate that the dipolar ylide structure is probably the main contributor. Molecular orbital calculations confirm the stabilizing effect that the second-row elements phosphorus and sulfur have in ylides, relative to the corresponding first-row elements nitrogen and oxygen. ... 

Another case in which the different characteristics of a group need to be considered is the effect of heteroatoms. When they are not directly attached to the carbon undergoing ionization, an electronegative atom such as O, N, or S destabilizes carbenium ions and slows SnI reactions due to inductive electron withdrawal. However, when they are directly attached, they accelerate SnI reactions due to resonance stabilization. Nitrogen is the best at stabilizing a carbenium ion and thereby accelerating SnI processes. Sulfur seems to have a variable effect in this resonance stabilization its orbitals are larger than those of C and hence the sizes are mismatched, but S is very polarizable. 

Because of the electronegativity of nitrogen, the dipole vector in both compounds points toward the heteroatom. The dipole moment of pyridine is larger than that of azacyclohexane (piperidine), because it is enhanced by the dipolar resonance forms in pyridine. In addition, the nitrogen is sp hybridized. (See Sections 11-3 and 13-2 for the effects of hybridization on electron-withdrawing power.)... 

The resonance effects that we adduced in amides and esters should also be visible in the lengths of the bond between the heteroatom and the carbon of the carbonyl, and this is indeed the case they are significantly shorter than normal C-X single bonds (Table 15.2). It is clear that there is no significant resonance donation from chlorine and that donation from the lone pair on nitrogen is greater than that from oxygen. 

---