Carboxylic groups (COOH) of Asp and Glu side chains often function as key components in enzymatic reactions, and identifying their H-bond structures in the active sites is essential for understanding the reaction mechanisms. In this study, the correlation between the H-bond structures and the C=O stretching (vC=O) frequencies of COOH groups was studied using density functional theory calculations. The vC=O frequencies and their shifts upon OH deuteration were calculated. for model complexes of acetic acid and propionic acid H bonded at different sites with various compounds. Calculation results together with some experimental data showed that, upon direct H bonding at the C=O group, the vC=O frequencies downshift from the free value (1770-1780 cm(-1) in an Ar matrix) to 1745-1760 cm(-1), while H bonding at the OH hydrogen induce even larger downshifts to provide the frequencies at 1720-1745 cm(-1). In contrast, when the COH oxygen is H-bonded, the vC=O frequencies upshift to 1785-1800 cm(-1). In double and multiple H-bond forms, H-bonding effects at individual sites are basically additive, and complexes in which the C=O and the OH hydrogen are simultaneously H bonded exhibit significantly low vC=O frequencies at 1725-1700 cm(-1), while complexes H bonded at the oxygen of the COH in addition to either at the C=O or the OH hydrogen exhibit medium frequencies of 1740-1765 cm(-1). The vC=O frequencies linearly correlate with the C=O lengths, which are changed by H bonding at different sites. Upon OH deuteration, all the complexes showed vC=O downshifts mostly by similar to 10 cm(-1) and in some cases as large as similar to 20 cm(-1), and hence deuteration-induced downshifts can be a good indicator, irrespective of H-bond forms, for assignments of the vC=O bands of carboxylic groups. The results in this study provide the criteria for determining the H-bond structures of Asp and Glu side chains in proteins using their vC=O bands in Fourier transform infrared spectra.