Conformational studies of an N-acylated hindered amine light stabilizer

The low-energy conformations of a commercial, N-acylated, hindered amine li ght stabilizer, Tinuvin 440, are examined both theoretically and experiment ally. Candidate structures are determined from an empirical force-field sea rch algorithm and re-optimized using a variety of semiempirical and first-p rinciples quantum chemical methods. The global minimum is robustly predicte d to have a twist-boat configuration with the substituted N in its six-memb ered heterocycle not one of the "flagpole" substituents. First-principles H artree-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 disagr ees with earlier semiempirical results and contradicts their implication th at the oxidation and photostabilization behavior of Tinuvin 440 are strongl y influenced by a trans-annular, intramolecular hydrogen bond. An alternati ve boat conformation containing such a bond is predicted by first-principle s 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 negligi ble role at room temperature and are consistent with the theoretically pred icted ground state.