A theoretical study of the rotational isomerization of glyoxal (OHC-CHO) and its fluorinated (OHC-CFO and OFC-CFO) and chlorinated (OHC-CClO) derivatives is presented. It is found that halogen substitution lowers the torsional energy barriers due to changes in local specific interactions, These interactions are treated through partition of the torsional potential and torsional forces into two contributions. through bond and through space. As a result we obtained that in most cases the torsional potential barrier is mainly determined by the through bond interactions. (C) 2002 Elsevier Science B.V. All rights reserved.