| Popis: |
This chapter discusses the NO 2 stretching. The C—Ν stretching mode is used as a characteristic frequency of NO 2 groupings only for small nitroalkanes. In the infrared (IR) spectra of unconjugated nitroalkanes, the intensity of the antisymmetric NO 2 stretching band is strong while the symmetric stretching band is somewhat weaker. The chapter describes the influences of substituents on NO 2 stretching frequencies. The substitution of halogen atoms (F, Cl, and Br), C=O, and NO 2 groups on the α-carbon atom results in a high frequency shift of the antisymmetric NO 2 stretching mode, while the symmetric NO 2 stretching frequencies are lowered by 8–29 cm −1 . Therefore, the NO 2 antisymmetric and symmetric stretchings bands are farther apart than those of unsubstituted nitroalkanes. Conjugation with a C=C moiety lowers antisymmetric and symmetric NO 2 stretchings through the resonance effect. As with the Raman spectra of nitroethylenes, the IR intensity of the symmetric NO 2 stretching is enhanced by conjugation, and the antisymmetric and symmetric stretching bands in the IR spectra are found to be similar in peak intensity. |
