The degree of proton transfer for XH⋯NH3 (X = F, Br, HS, and HCOO) heterodimers upon attachment of an excess electron

► Proton transfer for XH⋯NH3 upon an electron attachment was studied with DFT method. ► Attachment of an excess electron could increase the trend and degree of PT. ► The PT from HBr to NH3 is the barrier-free proton transfer (BFPT) in (HBr⋯NH3)− system. ► Waters could assist FH⋯NH3 and HCOOH⋯NH3 BFPT upon an excess electron attachment.