Linear IR spectra show "free" and intramolecular hydrogen-bonded (IHB) -OH groups in vicinal diols as two separate O-H stretching absorption bands. Here we present a case study of four linear vicinal diols with different alkyl groups: 1,2-ethylene glycol, 1,2-propanediol, 2,3-butanediol, and 1,2-butanediol. By carefully removing contributions from anharmonic vibrational coupling and local structural effect, "pure" IHB-resulted frequency separation and peak enhancement are obtained and found to exhibit a linear relationship between them. The results suggest that the IHB formation energy in the diols is structurally dependent, and 1,2-propanediol has the smallest vibrational contribution to the IHB energy among the four diols examined.