Strong intramolecular hydrogen bonds. Part I. Vibrational frequencies of the OH group in some picolinic acid N-oxides predicted from DFT calculated potentials and located in the infrared spectra

Hydrogen bonding in picolinic acid N-oxide (I), its 4-nitro (III), 4-methox y (IV), 4-amino (V) derivatives and in quinaldic acid N-oxide (II) was char acterized by calculations (B3LYP/6-31G(d)) of metric parameters, H-bond ene rgies and one-dimensional proton potential functions with vibrational energ y levels. Solvent effects were estimated by the SCRF PCM method of Tomasi a nd coworkers (J. Tomasi, M. Persico, Chem. Rev. 94 (1994) 2027). The potent ial functions an strongly asymmetric with the energy minimum placed near th e carboxylic oxygen. The inflection near the NO oxygen develops into a seco nd, shallower minimum under the SCRF. Empirical assignments of the OH stretching and bending modes were made for (I)-(IV). The stretchings of (I, II) and (IV) in various solvents are obser ved in the region 1600-1300 cm(-1), but near 2600 cm(-1) for (III). The cal culated and observed frequencies are in fairly good agreement with theoreti cal predictions reflecting the electronic effects of the substituents upon the H-bond strength. The observed trends in the solvent effects upon variou s parameters characterizing the H-bonding also correspond to predictions. ( C) 2000 Elsevier Science B.V. All rights reserved.