Gas-phase thermochemical information from triple quadrupole mass spectrometers: Relative proton affinities of amines

Proton-bound dimers of amines fragment to give the individual protonated bases in proportions which depend upon the proton affinities of the amines, the collision energy, and the target gas thickness in a triple quadrupole mass spectrometer. The behavior of these dimers observed with the triple quadrupole is similar to that observed with a reversed geometry mass spectrometer suggesting that the kinetic method of assigning amine proton affinities is applicable to the triple quadrupole. This is shown explicitly. Furthermore, the effect of precursor-ion internal energy on this measurement is delineated through MS/MS experiments covering excitation through collisions at various energies and target pressures. The effect of varying target thickness and collision energy are shown to be small when structurally similar amines comprise the dimer, but in the cases of dimers of phenylethylamine and pyridine with primary alkylamine partners, differences between literature values of proton affinities and the measured gas-phase basicities are observed. These entropic effects are interpreted on the basis of the relative steric hindrance to protonation of phenylethylamine (PEA), pyridine (PYR) and the primary alkylamines (ALK), that is, the order of steric hindrance appears to be PEA > ALK > PYR. For the primary alkylamine dimers, entropic effects appear to cancel within experimental error. Using data taken under various collision conditions, the proton affinity of 3-aminopentane is assigned as 221.7±0.3 kcal mol⁻¹.