Conformational studies of an N-acylated hindered amine light stabilizer

The low-energy conformations of a commercial, N-acylated, hindered amine light stabilizer, Tinuvin 440, are examined both theoretically and experimentally. Candidate structures are determined from an empirical force-field search algorithm and re-optimized using a variety of semiempirical and first-principles quantum chemical methods. The global minimum is robustly predicted to have a twist-boat configuration with the substituted N in its six-membered heterocycle not one of the "flagpole" substituents. First-principles Hartree-Fock, post-Hartree-Fock, and density functional theory calculations agree well on the relative stabilities of different conformers, while force-field and semiempirical methods prove unreliable. The present study disagrees with earlier semiempirical results and contradicts their implication that the oxidation and photostabilization behavior of Tinuvin 440 are strongly influenced by a trans-annular, intramolecular hydrogen bond. An alternative boat conformation containing such a bond is predicted by first-principles methods to lie at least 3.4 kcal/mol above the global minimum. NMR and IR spectral data confirm that intramolecular hydrogen bonding plays a negligible role at room temperature and are consistent with the theoretically predicted ground state.