Nitroalkanes shifts 247

This section considers aUcylamines, nitroalkanes, isonitriles, pyridines, and diazines and offers comparisons between NMR shifts and calculated atomic charges. Most of... 

Here we are concerned mainly with alkylamines. Detailed SCF charge analyses [43] indicate that any gain in total charge translates into a downfield shift, which is the trend exhibited by alkylamines, but also by nitroalkanes and isonitriles. Examples are offered in Table 6.5 (see also Fig. 6.6), along with pertinent ionization potentials, indicated in kcal/mol ... 

Similar conclusions hold for nitroalkanes as well they were calculated by means of Eq. (12.12), with scn= 53.00kcal/mol. The NMR shifts reported in Ref. 138 were used for the carbons those of nitrogen are from [156]. The conclusions are in no way affected by the simplifying assumption RE = 0 for NO2, which may well be revised in the future. 

Straightforward applications of the theory are presented in the atom-by-atom approach, as exemplified in Table 15.4, using charges deduced from NMR shifts, Eq. (6.8) for the carbon atoms, and Eqs. (6.12)-(6.14) for the nitrogen atoms. (CN bond dissociation energies and comparisons with the corresponding intrinsic bond energies are described in Chapter 12 for both alkylamines and selected nitroalkanes.)... 

The N-15 resonance shifts of alkylamines and nitroalkanes are expressed in ppm from HNO3, in methanol. Separate correlation lines are observed for mono-, di- and trialkylamines, namely... 

Table 4.48. 13C Chemical Shifts of Representative Nitroalkanes [363] and Nitroalkenes (<5C in ppm).

Electron attracting substituents in the nearest vicinity to the nitro group tend to increase the screening constant for the latter. Nitrogen chemical shifts differentiate clearly nitroalkanes> nitroaromatics, Af-nitro and 0-nitro groups. They are also helpful in distinguishing between the true nitro structure and that of an aci-nitro compound ... 

Additivity rules were found for the nitrogen chemical shifts in nitroalkanes [491. 

The nitrogen chemical shifts of the N03 group in nitroalkanes and conjugated nitro compounds were extensively examined before 1972 (ref. Id, pp. 233-244). There is a clear /2-effect on the chemical shift in nitroalkanes (Id),... 

In the review period, additional data became available (118) for nitroalkene systems (Table XXVI, note h). The structures with a double bond system separated from the N02 group by at least one saturated carbon atom give N02 shifts within the range characteristic of nitroalkanes. However, the presence of a double bond in a position conjugated to the N02 system results in an increased screening (Table XXVI) by 10-20 ppm [55], [56]. 

A number of papers deal with the N-NMR spectra of anions derived from mono- and polynitroalkanes. The change of structure from that of a true anion of aci-nitroalkane to that of a true nitrocarbanion was suggested in the series of anions of mono-, di-, and trinitroalkanes on the basis of the N spectra [SO]. Some data on the N, N, and resonance shifts were reported for nitroalkane anions [51—53] and O for nitroalkanes [54]. 

TABLE 3. chemical shifts of some aliphatic amines and nitroalkanes ... 

In nitroalkanes, a hydrogens, those hydrogen atoms that are attached to the same carbon atom to which the nitro group is attached, have a characteristically large chemical shift. 

Nalidixic acid, 139-41 Narrow component, 87 Nephelauxetic effect, 330,338,455 Neutron diffraction, 271 Niobium-93 NMR, 446,454 Niobium-93—oxygen-17 couplings see Oxygen-17—niobium-93 couplings Nitriles, N chemical shifts, 217 Nitroalkanes, 44-46... 

It is generally accepted that the condensation of nitroalkanes 1 with aldehydes proceeds with the nitronates 2 as the intermediates. The role of the base is to shift the tautomeric equilibrium towards the formation of nitronic acids, or ac/-nitroalkanes 3. Since 3 are much stronger acids (with a pAa range of 2 6) than 1 (with a pAa range of 9 10), they are more readily deprotonated with the base. After the deprotonation, the formed nucleophilic... 

Figure IX-H-19. Comparison of the absorption spectra of (CH3)3CNO (McMillan, 1966), CF3NO (Mason, 1963), CH3NO2, and CF3NO2. The region of absorption of the RNO compounds results in a rare characteristic they are blue-colored gases. The visible band of the nitrosoalkanes is missing in the nitroalkanes as absorption is shifted to the ultraviolet region.

---