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Geometric and vibrational spectroscopic data (rotational constants, bond distances and angles, vibrational frequencies, IR intensities, and OH/OD isotope effects) of phenol, benzaldehyde, and salicylaldehyde as calculated at various levels of theory (HF/6-31G(d,p), HF/6-311++G(d,p), MP2/6-31G(d,p), B3P86/6-31G(d,p), BLYP/6-31G(d,p), B3LYP/6-31G(d,p), and B3LYP/6-311++G(d,p)) are reported. The theoretical results are discussed mainly in terms of comparisons with available experimental data. For geometric data (rotational constants and bond distances) the best agreement between theory and experiment is obtained at the MP2 and B3LYP levels. B3P86 calculated data are slightly worse, while HF and BLYP calculations yield distinctly too small and too large bond distances, respectively. B3LYP calculated vibrational spectroscopic data excellently agree with experimental IR spectra for phenol, phenol-OD, and benzaldehyde, and with minor restrictions, also for salicylaldehyde and salicylaldehyde-OD. Considering frequency sequences, IR intensities, and OH/OD isotope effects, reliable and consistent assignments are given. BLYP and B3P86 calculated vibrational spectroscopic data are slightly worse, whereas MP2 and HF calculations suffer from several shortcomings that are already known from calculations of smaller molecules, such as benzene.

IR gas-phase and quantum chemical data that characterize the intramolecular hydrogen bonds in a series of 2-hydroxybenzoyl compounds (Ph(OH)(COY):  Y = Cl, OH, SH, OCH3, SCH3, H, CH3, Ph, N(CH(CH3)CH2)2CH2, N(CH2CH2)2CH2, N(CH3)2, NHCH3, NH2, NHNH2) are reported and discussed. The sequences of ν(OH) and of ν(CO) frequencies calculated at HF/3-21G, HF/6-31G**, and B3LYP/6-31G** computational levels agree reasonably well with the corresponding experimental sequences. The consistency of the calculated data is demonstrated by exploiting several relations between calculated frequencies, force constants, and bond distances. It is shown that the sequence of hydrogen bond strengths, as defined either by ν(OH) frequency shifts or by R(O−H) bond distances, may qualitatively be interpreted in terms of two quantities, the R(H···O) hydrogen bond distances and the Q(O) oxygen partial charges, which characterize the intrinsic differences between the carbonyl groups.

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