Masoud Shekofteh - Department of Chemistry, University of Shahrood, Shahrood, masoud.
Zainab Moosavi-Tekyeh - Department of Chemistry, University of Shahrood, Shahrood,
Sayyed Faramarz Tayyari - Department of Chemistry, Ferdowsi University of Mashhad, Mashhad,

The structure and intramolecular hydrogen bond (IHB) of 1,7-bis(4- hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione; or Curcumin (CUR) have been investigated by means of density functional theory (DFT) at B3LYP level using 6-311++G**basic set. The results were compared with those of 5-hydroxy-1,7-di(phenyl)hepta-1,4,6-trien-3-one (CUR1), dibenzoylmethane (DBM) and acetylacetone (AA) as a parent molecule. To investigate the effect of styrene group on the hydrogen bond strength, the charge distributions and electron delocalization in CUR with the considered molecules are studied using the natural bond orbital (NBO) analysis. To characterize the nature of IHB, the computations were further complemented with an atoms-in-molecules (AIM) topological analysis.The estimated IHB energy (EHB) using AIM analysis (20.70, 20.08, 21.96 and18.19 kcal/mol, together with the values of 1HNMR for the enolated proton chemical shift (15.73, 14.49, 16.49 and 14.89 ppm, respectively), and the vibrational frequencies of the OH bond, (νOH: 2946 , 2974, 2922 and 3067 cm-1; γOH: 943, 940, 994 and 968 cm-1, respectively) for CUR, CUR1, DBM and AA, respectively, indicate that relatively stronger IHB in CUR1 than that in AA and weaker one than DBM.The molecule of CUR and CUR1 exhibit good nonlinear optical (NLO) activity and the predicted first order hyperpolarizability of these compounds are 62 and 33 times, respectively, greater than one of urea

Curcumin, Density functional theory (DFT), Theory of atoms in molecules (AIM), Natural bond orbital (NBO), Nonlinear optical properties (NLO)