The positions of the carbonyl stretching band in esters and ketones show that the pyridine 1- oxide ring can release electrons to the 4-position more easily than to the 3-position, but that the reverse is true of the pyridine ring. All the bands in a monosubstituted pyridine or oxide
[Ghodsinia, Sara S. E.; Akhlaghinia, Batool; Safaei, Elham; Eshghi, Hossein Journal of the Brazilian Chemical Society, 2013 , vol. 24, # 6 p. 895 - 903]
