The structural dynamics of 4-pyrimidone (4PMO) in the A-and B-band absorptions was studied by using the resonance Raman spectroscopy combined with quantum chemical calculations to better understand whether the excited state intramolecular proton-transfer (ESIPT) reaction occurs in Franck-Condon regions or not. The transition barrier for the ground state protontransfer tautomerization reaction between 3(H) (I) and hydroxy (II) was determined to be 165 kJÁmol À1 in vacuum on the basis of the B3LYP/6-311++G(d,2p) level of theory calculations. Two ultraviolet absorption bands of 4PMO were, respectively, assigned as p H !p* L and p H !p* L+1 transitions. The vibrational assignments were done on the basis of the Fourier transform (FT)-Raman and FT-infrared (IR) measurements, the density-functional theory computations and the normal mode analysis. The A-and B-band resonance Raman spectra of 4PMO were measured in water, methanol and acetonitrile. The structural dynamics of 4PMO was obtained through the analysis of the resonance Raman intensity pattern. We discuss the similarities in the structural dynamics of 4PMO and 2-thiopyrimidone (2TPM), and the results were used to correlate to the intramolecular hydrogen-atomtransfer process as observed by matrix-isolation IR experiments for 4PMO. A variety of NH/CH bend modes + C = O stretch mode mark the hydrogen-detachment-attachment or ESIPT reaction initiated in Franck-Condon region for 4PMO and 2TPM.