The base peak in the mass spectrum of 2,2,4−trimethylpentane [(CH3)3CCH2CH(CH3)2] occurs at m/z = 57. Draw the ion responsible for this peak. draw structure ... Why is this ion is the most abundant fragment? 3° carbocations are the most stable of all carbocations. The two ions that are the most similar in mass will be the most stable. 2° carbocations have the most stable resonance forms. 1° carbocations are the most stable.