Lignite is an abundant natural resource that is a potential source of clean fuel and value-added chemicals. The mechanisms by which thermal and catalytic treatments deconstruct lignite remain elusive, which is where quantum mechanical calculations can offer fundamental insights. In order to investigate the cleavage of C—O bridge bond, which is the critical step in the thermal decomposition of lignite, the α-O-4 and β-O-4 types of structural units are selected as lignite model compounds to calculate the C—O bond dissociation enthalpies using several kinds of density functional theory methods (B3PW91, B3P86, PBE1PBE, BMK, M06-2X and M05-2X) at 6-31-G(d,p) level. By the comparison between the results and the theoretical benchmark values provided by CBS-QB3 method, M05-2X functional was applied for the calculations on C—O bond dissociation enthalpies. The present results indicate that the C—O average bond dissociation enthalpies are 51.0 kcal/mol and 66.1 kcal/mol for the α-O-4 and β-O-4 types of model compounds, respectively. Local substituents have a great effect on the C—O bond dissociation enthalpies, the C—O bond dissociation enthalpies will decrease when the adjacent arene rings are substituted by electron donating groups (OH, OCH₃ and CH₃), while the results are opposite for the electron withdrawing groups such as carboxyl group. Then the substituent effects are deeply analyzed on the basis of the ground-state effect and radical effect. An electron donating group can stabilize the phenoxy radicals (radical effect), however, an electron withdrawing group has the opposite effect. In most cases, the radical effect is more important than the ground-state effect. Furthermore, there is a negligible correlation between the C—O bond distances and strengths, and the C—O bond dissociation enthalpies cannot be predicted so easily. Interestingly, the C—O bond dissociation enthalpies can be significantly influenced by the intramolecular hydrogen bond, if the intramolecular hydrogen bond still exists after the cleavage of the C—O bond, the bond dissociation enthalpies will be lower.

Key words: density functional theory, bond dissociation enthalpy, lignite model compounds, substituent effect, intramolecular hydrogen bond