Very strong intramolecular hydrogen bonding of 1,2-dithenoylcyclopentadiene; DFT and spectroscopic studies

Structure and intramolecular hydrogen bond (IHB) of 1,2-dithenoylcyclopentadiene (DTCP) have been investigated with quantum mechanical calculations using density functional theory at B3LYP level using some Pople-style basis sets augmented with polarized and diffuse functions. The full isomeric and conformational analyses have been done, and the relative stabilities for different chelated and non-chelated enol and different keto forms have been reported. The estimated IHB energy and the barrier height for proton transfer (19.34 and 0.31 kcal/mol, respectively), together with the value of 0.082 Å for the Q Gilli’s symmetry coordinate and the σ-skeleton of the molecule (O⋯O distance of 1.463 Å), confirm a very strong, nearly symmetric, linear and short distance resonance assisted IHB (RAHB) in DTCP. The values of 18.29 ppm for the enolated proton chemical shift and 1128 cm−1 for out-of-plain bending of the OH bond, besides investigations on other IR and Raman bands associated with the IHB strength and electron delocalization in different segments of the molecule, confirm the results of the theoretical studies and a very strong RAHB in DTCP.