Fourier transform infrared (FTIR) spectroscopy is a powerful method to investigate the structures of key Tyr residues involved in various protein reactions. In this study, we have performed density functional theory (DFT) calculations for various hydrogen-bonded complexes of p-cresol, a simple model of a Tyr side chain, in different hydrogen-bond forms to develop explicit criteria for determining the hydrogen-bond structures of Tyr using FTIR spectroscopy. The CO stretching (nu CO) and COH bending (delta COH) vibrations were focused as markers and calculated results were compared with experimental data of p-cresol and Tyr. The calculated and experimental nu CO frequencies appeared at 1280-1260, 1260-1250, 1255-1235, and 1240-1220 cm(-1) in the hydrogen-bond donor, free, donor-acceptor, and acceptor forms, respectively. These frequencies, which showed little overlap between the individual hydrogen-bond forms, had a negative linear correlation with the CO lengths in optimized geometries. The delta COH frequencies were found at 1255-1210 cm(-1) in the donor form, while the free and acceptor forms showed relatively low delta COH frequencies at 1185-1165 and 1190-1160 cm(-1), respectively. In the donor-acceptor form, the vibrational mode with a considerable delta COH contribution was found at 1280-1255 cm(-1) with a weak IR intensity. This frequency and the nu CO frequency in the donor-acceptor form are similar to the nu CO and (delta COH frequencies, respectively, of the donor form, making it difficult to discriminate the two forms. These two forms can be clearly distinguished by detecting a strong nu CO(D) band in p-cresol-OD or Tyr-OD, in which the delta COD vibration largely downshifts to similar to 1000 cm(-1). The nu CO(D) frequency of the donor-acceptor form was found at 1260-1240 cm(-1), while that of the donor form was at 1270-1255 cm(-1). Practically, plotting the frequency of the lower-frequency strong IR band (nu CO of the donor-acceptor form or delta COH of the donor form) of undeuterated species against the nu CO(D) frequency is convenient for accurate discrimination. Because the donor form shows a positive linear correlation between delta COH and nu CO(D) frequencies, the two forms exhibited distinct areas in this plot. The effects of hydrogen-bond interactions on other potential IR and Raman markers are also discussed.