Physical Properties of 1,3,4-Oxadiazoles

In recent years molecular, electronic, and nuclear resonance spectra have been increasingly drawn upon in the elucidation of the structures of 1,3,4-oxadiazole derivatives. 

In the IR field, the oxadiazole ring has been characterized primarily by the bands at about 970 cm-1,37,151 1020-1030 cm-1,60 151 due to the C—O bond and at 1560-1640 cm-1,35 36 44a-55 85 181-152 due to the C=N valence vibration. 2,5-Dialkyl derivatives in this case fall in the longer-wavelength region35,36,44a 151 and oxadiazoline thiones in the shorter-wavelength region.65 The C=0 absorption in 1,3,4-oxadiazolin-5-ones lies at 1740-1785 cm-1 even in condensed systems.76,78,116 

4-oxadiazole system has an electronic spectrum equivalent to that of benzene and the maxima of the oxadiazole derivatives are only slightly shifted hypsochromically compared with benzene.103 Thus, for example, diphenyl and 2-phenyl-l,3,4-oxadiazole absorb almost identically at 248 ra.fi and p-terphenyl and 2,5-diphenyl-l,3,4-oxadiazole absorb at 276 and 280 mfi, respectively.103 The oxadiazole system is effective in the conjugative transmission of the effects of substituents.103 An extension of the system of conjugated rt bonds 

5-Diaryl-l,3,4-oxadiazoles exhibit strong fluorescence in solution on stimulation by UV or /3-irradiation, particularly 2-biphenylyl-5-phenyl- and 2-biphenyl-5-(l-naphthyl)-l,3,4-oxadiazole (cf. Section V). The quantum yield of the 1,3,4-oxadiazoles is lower than with p-terphenyl and its analogs.163-155 The band intensity with unsubstituted aryl-1,3,4-oxadiazoles and aromatic substituted oxadia-zoles is essentially the same.155 

In some cases for particular oxadiazole derivatives, such as poly-fluoroalkyloxadiazoles35 and derivatives of 5-phenyl-l,3,4-oxadia-zoline,115 nuclear magnetic resonance spectra have been utilized for characterization and clarification of the structure. 

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The following pages will be devoted mainly to the syntheses, the reactivity, the physical properties, and the uses of 1,3,4-oxadiazoles. 

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