[HB]

tetrahydrocarbazole was reacted with chloroacetyl chloride to give 9-(chloroacetyl)-tetrahydrocarbazole.
although the C=O is amidic, it appeared in IR at 1714 cm-1.
and upon nucleophilic substitution of chlorine, the C=O band returned to 1680 cm-1.

could that be explained in terms of -I effect of chlorine which increase the double bond character of C=O, and if that is true what about the +M effect of nitrogen (I mean the reseonance effect which decrease the double bond character and decrease the frequency)

[Corribus]

It's possible that it's just a mass effect. It's often hard to decouple electronic effects from mass effects when comparing IR stretching frequencies.