The iodine-catalyzed nucleophilic addition reactions of pyrrole,furan,or thiophene with acetone were studied in gas and solvent by the density functional theory at the level of Lanl2DZ. It was seen that the halogen bond between iodine and carbonyl oxygen appeared to have an important catalytic effect on such reactions,and the first iodine molecule maximally diminished the barrier height by 41 kJ/mol,while the second iodine molecule could not improve such reactions largely. It was concluded that the C2-addition was generally more favorable than the C3-addition for the three heterocycles; however,iodine considerably more effectively catalyzed the C3-addition than the C2-addition for pyrrole. It was also revealed by PCM calculation that the iodine-catalyzed nucleophilic additions occurred more easily in solvent than in gas,which explained the experiment performed by Bandgar et al.. The iodine-catalyzed nucleophilic addition reactions of pyrrole, furan, or thiophene with acetone were studied in gas and solvent by the density functional theory at the level of Lanl2DZ. It was seen that the halogen bond between iodine and carbonyl oxygen had to an an important catalytic effect on such reactions, and the first iodine molecule maximally diminished the barrier height by 41 kJ / mol, while the second iodine molecule could not improve such reactions largely. It was concluded that the C2-addition was generally more favorable than the C3- addition for the three heterocycles; however, iodinearently more efficiently catalyzed the C3-addition than the C2-addition for pyrrole. It was also revealed by PCM calculation that the iodine-catalyzed nucleophilic additions occurred more easily in a solvent than in gas, which explained the experiment performed by Bandgar et al ..