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The Nitro Group

1991; Elsevier BV; Linguagem: Inglês

10.1016/b978-0-08-057116-4.50017-1

Daimay Lin-Vien, Norman B. Colthup, William G. Fateley, Jeanette G. Grasselli,

Spectroscopy and Laser Applications

This chapter discusses the NO2 stretching. The C—Ν stretching mode is used as a characteristic frequency of NO2 groupings only for small nitroalkanes. In the infrared (IR) spectra of unconjugated nitroalkanes, the intensity of the antisymmetric NO2 stretching band is strong while the symmetric stretching band is somewhat weaker. The chapter describes the influences of substituents on NO2 stretching frequencies. The substitution of halogen atoms (F, Cl, and Br), C=O, and NO2 groups on the α-carbon atom results in a high frequency shift of the antisymmetric NO2 stretching mode, while the symmetric NO2 stretching frequencies are lowered by 8–29 cm−1. Therefore, the NO2 antisymmetric and symmetric stretchings bands are farther apart than those of unsubstituted nitroalkanes. Conjugation with a C=C moiety lowers antisymmetric and symmetric NO2 stretchings through the resonance effect. As with the Raman spectra of nitroethylenes, the IR intensity of the symmetric NO2 stretching is enhanced by conjugation, and the antisymmetric and symmetric stretching bands in the IR spectra are found to be similar in peak intensity.