Reza Fazaeli - Department of Chemistry, Faculty of Science, South Tehran Branch, Islamic Azad University, Tehran, Iran.
Abdullah Ghasemi - Department of Chemistry, Faculty of Science, Islamic Azad University of Arak, Arak, Iran

Abstract In studies performed on ۲- chloroacetalidehyde by abinitio beginning with the LC-WPBE, B۳LYP and m۰۶-۲x functionals and ۶-۳۱۱++G( d, p) basis set . Also by checking the total energy , HOMO-LUMO gaps and dipole moment, it was found that the keto form is more stable than the enol form .Theoretical calculations With LC-WPBE/۶-۳۱۱G(d,p) shows the more stability of I- conformer in compared to other conformers . NBO analysis was practical for illustrating the negative hyperconjugative effect on the conformers . In the basis of NBO analysis, the LP(۲)O and LP(۲)O Interactions were responsible of the negative hyperconjugation in the examined compounds. The deletion of all thw donor – acceptor electronic interaction from the fock matrices and off –diagonal elements , values of these interaction were reported.

۲- choroacetaldehyd, NBO ( natural bond orbital analysis), negative hyperconjugative effect, second order the stabilization energies ( E۲)