Proton affinities of saturated aliphatic methyl esters

The kinetic method was used to determine the proton affinities of methyl ea ters of several saturated fatty acids. Decompositions of the proton-bound d imers of the methyl esters, AHB(+) were observed under different conditions with two instruments. The proton affinities (PAs) of the methyl esters inc rease continually with increasing carbon number in the acid. Equilibrium an d initial rate experiments were performed with a Fourier transform ion cycl otron resonance mass spectrometer on the methyl ester of the C-22 saturated acid (methyl behenate). These experiments give values for PA (methyl behen ate) that are perhaps slightly lower than those obtained with the kinetic m ethod. The Pas of the methyl esters of the fatty acids could be correlated with the equation: PA (ester) = (40.0 +/- 2.5)*log(n) + (784.7 +/- 3.9) kJ/ mol or PA (ester) = (864 +/- 2) - (479 +/- 41)/n, where it = number of atom s in the molecule. Proton affinities of smaller sets of 1-alkylamines and 1 -alkanols can be fit to similar equations. (C) 2000 American Society for Ma ss Spectrometry.