Carbon-nitrogen bonds absorptions

Nitro-compounds fRNOj) are isomeric with nitrites, but their electronic structure, excited states and photochemistry are very different. There is no very low-lying (n.jt ) state, and nitroalkanes show n — 3i absorption with a maximum around 275 nm ( —201 mol - cm In cyclohexane solution, nitromethane (CH1NOi) is photoreduced to nitrosomethane(CH,NO, but nitroethane under the same conditions gives rise to a nitroso-dimer derived from the solvent CS.47). The latter process is probably initiated by cleavage of the carbon-nitrogen bond in the nitroalkane. In basic solution (when the nitroalkane is converted to a nitronate anion) irradiation can lead to efficient formation of a hydroxamic acid (S.48), and this reaction most likely proceeds through formation of an intermediate three-mem bered cyclic species. 

The most readily recognized carbon-nitrogen bond is the triple bond of a nitrile (Figure 12-14). The stretching frequency of the nitrile C=N bond is close to that of an acetylenic C=C triple bond, about 2200 cm however, nitriles generally absorb above 2200 cm (2200 to 2300 cm ), while alkynes absorb below 2200 cm . Also, nitrile triple bonds are more polar than C=C triple bonds, so nitriles usually produce stronger absorptions than alkynes. 

Bohlmann et al. (118-121) observed that an infrared absorption band between 2700-2800 cm is characteristic of a piperidine derivative possessing at least two axial carbon-hydrogen bonds in antiperiplanar position to the free-electron pair on the nitrogen atom. The possibility of forming an enamine by dehydrogenation can be determined by this test. Compounds which do not fulfill this condition cannot usually be dehydrogenated (50, 122,123). Thus, for example, yohimbine can be dehydrogenated by mercuric acetate,whereas reserpine or pseudoyohimbine do not react (124). The quinolizidine (125) enamines (Scheme 4), l-azabicyclo(4,3,0)-nonane, l-azabicyclo(5,3,0)decane, l-azabicyclo(5,4,0)undecane, and l-azabicyclo(5,5,0)dodecane have been prepared in this manner (112,126). 

N-Substituted carbene complexes show y(CN) absorption in the 1470-1620 cm 1 range of the IR spectrum. These data are consistent with the crystallographic evidence for substantial carbon-nitrogen multiple bonding in these compounds. 

Absorption bands for carbon-nitrogen double bonds occur at somewhat lower wavenumbers than those for carbonyl groups. However, compounds containing this functional group are relatively uncommon, so their IR spectra are not discussed in detail in this book. 

Carbon-nitrogen double bonds absorb in the same region as C=C double bonds, around 1660 cm-1 however, the C=N bond gives rise to stronger absorptions because of its greater dipole moment. The C=N stretch often resembles a carbonyl absorption in intensity. 

The carbon-nitrogen double bond behaves in the IR region as would be expected, exhibiting stretching absorptions in the range 1690 to 1640 cm . No IR spectra for imine anions appear to have been reported. 

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