Molecular structure and internal dynamics of the antioxidant 2,6-di-tert-butylphenol

Antioxidants are a class of chemical compounds with particular chemico-physical properties that make them suitable for reducing oxidative stress. In this work we report the rotational spectroscopy analysis of the antioxidant 2,6-di-tert-butylphenol in a jet expansion. The rotational spectrum reveals both fine and hyperfine tunnelling components. The largest spectral doubling consists of two distinct groups of lines separated by ∼190 MHz, and is due to the torsional motion associated with the hydroxyl group. Each component of the doublet is further split into three fine components, with separations below 1 MHz. The spectrum was reproduced with a two-state torsion-rotation semirigid Hamiltonian for each pair of torsional states. Additional observation of all the singly-substituted 13C isotopologues allowed to determine the substitution structure by means of the Kraitchman equations. The comparison with the equilibrium structure obtained by computational calculations at B3LYP-D3BJ/def2-TZVP level validate the accurate determination of the carbon skeleton and tert‑butyl group positions. The investigation of intramolecular dynamics with a monodimensional flexible model demonstrates that the tunnelling phenomenon arises from the hydroxyl group's equivalent positions, with a double-minimum potential separated by a barrier of 1000(100) cm−1 allowing for this large amplitude motion. However, the three-fold fine structurte, while plausibly associated to internal motions within the tert‑butyl group, will require further exploration.