The elimination of HBr from CH3Br by ground state of Cu+ and Au+ has been investigated by using DFT methods.A mechanism of one-step HBr elimination leading to CuCH2+ was identified.For the formation of AuCH2+,besides the one-step mechanism,another two-step HBr elimination pathway through a C-Br bond insertion intermediate was also found,and this pathway is energetically more favorable than the one-step mechanism.The calculated reaction energy barriers show that the formation of AuCH2+ is energetically much more favorable than the formation of CuCH2+.The experimental observations are well explained.