Thermochemistry of demethylation of methoxy groups in aromatic compounds: Applicability of DFT, G4, and G4(MP2) methods

Abstract

This work analyses OC bond dissociation enthalpies (BDE) of aromatic compounds. BDE values were calculated for 23 meta- and para-substituted anisoles, together with polymethoxybenzenes and naturally occurring coumarins, using three theoretical methods: M06-2X, G4, and G4(MP2). Electron-donating groups decrease the OC BDE values, while electron-withdrawing groups increase them. The G4 and M06-2X methodologies show excellent agreement with experimental data, although M06-2X tends to overestimate BDEs. The applicability of other functionals was also assessed. G4 data for studied anisoles and phenols confirm that substituent effect on OC BDEs of anisoles and OH BDEs of phenols is identical. Found Hammett-type dependences can be employed for the gas-phase BDE estimations for anisoles and phenols possessing other substituents with known σ_m and σ_p⁺ constants. Obtained results also emphasize the accuracy and computational efficiency of the G4(MP2) method and its applicability for the investigation of demethylation thermochemistry of larger molecules.