Ring-stretching band, pyridine coordinated

The basicity of the aromatic amine iV-oxide can be substantially varied by the introduction of substituents on the aromatic ring. IR spectra of free N-oxides display a prominent band between 1200 and 1300 cm-1, attributable to the nitrogen-oxygen stretching frequency v(NO). The more activating the substituent, the lower the energy of the absorption. Upon coordination, v(NO) is decreased by up to 60 cm-1. IR data for pyrazole and pyridine derivatives are given in Table 42.281-286... 

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