C-0 bond stretching

Many studies have been made on the behavior of probe solutes in order to characterize the selective solvation in solvent mixtures, in particular depending on solvato-chromism. However, what is determined is the selective solvation of the probe, not of the binary solvent components with each other. It is conceivable that a spectroscopic investigation of a binary solvent mixture in the absence of a probe could be carried out if each component had a clear signal (absorption or fluorescence) at a characteristic exclusive wavelength. Such mixtures could be formamide + acetonitrile and formamide + dimethylsulfoxide, in which the C=0 bond stretching wave number of the hydrogen bond donor differs from the C=N and the S=0 bond stretching wave number of the acceptors (Alves and Santos 2007). 

Similarly, a C=0 bond stretches at a higher frequency (-1700 cm" ) than does a C—O bond (-1100 cm" ), and a C=N bond stretches at a higher frequency (-2200 cm" ) than does a C=N bond (-1600 cm" ), which in turn stretches at a higher frequency than does a C—N bond (-1100 cm" ) (Table 14.4). 

Hence, for a particular group, these two opposite effects determine the frequency of the vibration and it is therefore possible, in general, to give an approximate order for the C=0 bond stretching vibration frequency for different groups ... 

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