Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro‐methane (CF2Br2) and iodomethane (CH3I)/methane (CH4) as reagent gases is examined. The NICI mass spectrum of CF2Br2 contains Br−, [HBr2]− and [CF2Br3]− anions. Weak acids (i.e. those acids with approximately ΔH°(acid) values between 1674 and 1464 kJ mol−1) react with Br− to produce minor yields of the hydrogen−bonded bromide attachment [MH + Br]− anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°(acid) > 1464 kJ mol−1) produce primarily [MH + Br]− anions with a minor yield of proton transfer [M  H]− anion. The NICI spectrum of CH3I/CH4 is dominated by I−. Weak acids react with I− to yield minor amounts of [MH + 1]− or are unreactive. Strong acids produce only [MH + l]− anions. From a consideration of the gas‐phase basicity of the halide anion and the binding energy of the hydrogen‐bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.