X-ray, spectroscopic and computational studies of the tautomeric structure of a new hydrazone of 5-nitrosalicylaldehyde with indole-3-acetic hydrazide

A new hydrazone of 5-nitrosalicylaldehyde with indole-3-acetic hydrazide (NSIAH) has been obtained and its structure has been studied by X-ray, FT-IR and CPMAS in the solid as well as H-1, C-13 and N-15 NMR in the CH3CN-d(3) and DMSO-d(6) solutions. The hydrazone crystallises in the Pbca space group from the orthorhombic system with a = 9.5435(7) angstrom, b = 11.0932(8) angstrom and c = 29.672(2) angstrom. According to Hirsfeld surface analysis, the whole structure is dominated by H center dot center dot center dot O interactions (ca. 32.2%) and by H center dot center dot center dot H interactions (ca. 30%). The other important contributions come from: H center dot center dot center dot C interactions 19.3%, C center dot center dot center dot C interactions (pi center dot center dot center dot pi electrons: ca. 6.6%) and H center dot center dot center dot N interactions (also ca. 5.9%). The crystal structure has provided clear evidence that the hydrazone exists in the imine (hydroxy) form in which one intramolecular O-H center dot center dot center dot N hydrogen bond exists. The existence of this tautomeric form is well reflected by the presence of characteristic bands in the FT-IR spectrum of NSIAH. In CH3CN-d(3) solution the imine (hydroxy) form with the intramolecular O-H center dot center dot center dot N hydrogen bond is preserved, whereas in DMSO-d(6) this bond is partially broken and stabilized by the intermolecular O-H center dot center dot center dot O ones with the solvent molecule. As follows from the PM5 calculations of the NSIAH structures solvated by three DMSO-d(6) molecules, structure A including the intramolecular O-H center dot center dot center dot N hydrogen bond and structure B including only intermolecular hydrogen bonds, are energetically equivalent. (C) 2010 Elsevier B.V. All rights reserved.