Structural and spectroscopic evidence of hydrogen bonding in 3-bromo-5-hydroxy-2,6-dimethylpyridine and 3-chloro-5-hydroxy-2,6-dimethylpyridine

Article abstract of DOI:10.1016/S0022-2860(96)09359-3

The crystal structures of 3-chloro-5-hydroxy-2,6-dimethylpyridine (CIHDMP) and 3-bromo-5 hydroxy-2,6-dimethyl-pyridine (BrHDMP) were determined at room temperature. Although CIHDMP is monoclinic, space group P2₁/c, and BrHDMP is orthorhombic, space group Pna2₁, the molecular conformation is essentially the same in both compounds. The structure consists of two parallel infinite chains of formula units linked through hydrogen bonds of the type O-H···N. There is no interconnection between the chains. Fourier transform IR and Raman spectra of CIHDMP and BrHDMP were measured in the 50-3500 cm^-1 frequency range at room temperature. The vibrational assignment of the observed bands is proposed. The appearance of the characteristic vibrational features in the spectra of these compounds is discussed in terms of strong asymmetric hydrogen bonding. The bands observed at approximately 2800-2850, 2500-255 and 1800-1850 cm^-1 are assigned to the O-H stretching modes and those at 1400-1410 and 900-950 cm^-1 to δ(O-H) and γ(O-H) modes, respectively. The v(O-H) vibrations of the hydrogen bonds appear at 130-140 cm^-1. The calculation γ(O-H) frequencies and other parameters describing the strong hydrogen bonding, such as the Fermi coupling constant |W|, the separation of the unperturbed vibrational levels (Δ(o)), etc., has been performed. The positions of γ(O-H) bands have been found to agree with those predicted from the perturbed v(O-H) patterns. Comparison of the structural and vibrational characteristics of hydrogen bonds in CIHDMP and BrHDMP has indicated that a stronger hydrogen bonding exists in the bromo-derivative.