Simulation of hydrogen bonds in low-rank coals with lignite-related complexes using dispersion corrected density functional theory

Phenol…phenol, phenol…benzene, phenol…oxydibenzene, phenol…quinoline, and benzoic acid…benzoic acid were selected as lignite-related complexes to investigate different hydrogen bonds formed by self-associated OH, OH-π, OH-ether O, OH-N, and COOH-COOH using density functional theory with dispersion correction, respectively. Moreover, the effects of substituents (CH₃-, CH₃O-, OH-, NH₂-, COOH-, and NO₂-) in donors on the hydrogen bonds were investigated. Geometry optimization, energy, Mulliken population, and frequency of all the complexes were calculated. It can be seen from optimized structures that there indeed are hydrogen bonds in the different complexes. Bond lengths of all O-H bonds in the different complexes become longer than that of free OH in phenol, which implies that intermolecular interactions exist in all the complexes. Among of them, bond lengths of O-H bonds in benzoic acid…benzoic acid are the longest. In addition, charge transfer can be observed via Mulliken population. Based on frequency analysis, all O-H stretching vibrations have obvious red shift, especially O-H bonds in benzoic acid…benzoic acid and phenol…quinoline, which gives evidence of using the infrared spectroscopy to analyze hydroxyl groups of coals. According to bond energies, the strength of the different hydrogen bonds decreases in the order: COOH-COOH > OH-N > self-associated OH ≈ OH-ether O > OH-π, which is consistent with the reported experimental results. Different substituents have distinct effects on the hydrogen bonds.