Resonance Effects. Part 1: Formaldehyde, H₂C=O.
The CO stretching vibration is strong and highly characteristic for carbonyl compounds. Substitution of one of the H-atoms by another atom or group X, affects the frequency of the CO stretching vibration in HCOX. If X is a pi-donating group, then resonance will weaken the CO bond and cause the frequency of the CO stretching vibration to be lowered. The molecules H₂C=O, HCOOH, HCOCl and HCONH₂ nicely exemplify the effect of resonance. Look for their CO stretching frequencies in the output files behind the links "Freqs".