Salicylaldehyde Benzoyl Hydrazone: Isomerization Due to Water. A Structural Analysis Using a Combination of NMR,IR, and Theoretical Investigations

Solutions of salicylaldehyde benzoyl hydrazone (SBH) in mixed DMSO/H₂O solvents have been studied by IR and NMR spectroscopy. Adding water to dilute solutions in DMSO changes the major conformation from the fully extended E isomer denoted B2E to another E isomer, B1E, where the carbonyl is associated with water. A minor Z form, B2Z, is also detected both in DMSO and in the mixed solvent. Near saturation in the mixed solvent, the major species has an imidinol function and the ratio B2Z/B1E increases. Raising the pH also favors the Z configuration. According to quantum mechanical calculations, the B2Z imidinol or monodeprotonated forms are stabilized by several kcal-mol^–1 with respect to their B1E and B2E counterparts, whereas the reverse is true for free or hydrated ketones. Free B2E ketone is calculated to be slightly more stable than its B1E counterpart, but the specific association with water reverses the situation. The balance between B1E, B2E, and B2Z forms appears to depend mainly on steric strain release and on hydrogen bond formation involving either both nitrogens, N₁ and N₂, of the hydrazone or at least one of them, N₁ for the E configuration or N₂ for the Z one. Calculation indicates that the first deprotonation occurs at the NH site for the B1E and B2E forms, but at the OH phenol site for B2Z.