The N–O stretching vibrations in nitroalkanes occur near 1550 cm-1 (asymmetrical) and 1365 cm-1 (symmetrical), the band at 1550 cm-1 being the stronger of the two.
If the nitro group is attached to an aromatic ring, the N–O stretching bands shift to down to slightly lower wavenumbers: 1550-1475 cm-1 and 1360-1290 cm-1.
Summary:
Compare the spectra of nitromethane and m-nitrotoluene, below. In nitromethane, the N–O stretches are at 1573 and 1383, while in nitrotoluene, they are a little more to the right, at 1537 and 1358.
