Effects of electronegative group s F,Cl,Br on Hydro gen Bond intraction in Propanoic A cid Dime r using Q M.

In this project we study hydrogen bondin g in mono mers and di mers Propanoic acid and it’s derivatives .The substituent effect can be analyzed by compa ring the bin ding energies of propa noic acid and fluoro,chloro and Bromopropanoi c acid mon omers and dimers۱-۳.We have been obtained binding energy and sy mmetrical dimer of propanoic ac id calculated at B۳LYP/۶-۳۱۱G ( d,p) level was found as -۱۶.۱۴ kcal/mol and our calculatio ns show tha t with replacing of one hydrogen with an electronegative substituent ( ۱….۱ F,Cl atom) hydrogen bondi ng strength decreases .B ut in the substituents ( ۱…۱ Br , ۲ …۲ Cl,Br) h ydrogen bonding strength increases.The calcula ted vibrational spectra of dimers give evidence of the form ation of stro ng hydrogen bonds involving O-H …O=C For the two dimers, elongat ion of O-H bonds results in downshift of the stretching vibrational fre quencies that Br`substituent exhibits a greater shift than F,Cl ۴-۵.NBO analysis has been performed to calculate ch arges on atoms , Due t o complexation, hydrogen atoms in hydrogen b onding ga in more positive charges, the oxygen atoms acting as hydrogen acceptor gain more negative charges, and the charges o n the oxygen atoms acting as hydrogen donor diminish as compared with monomers.

Keywords:

Hydrogen bonding , Propan oic acid , DF T , electron egative groups F , Cl , Br , NBO analysis.