Experimental determination of activation energies for gas-phase ethyl and n-propyl cation transfer reactions

Alkyl cation transfer reactions between ethanol and protonated ethanol and ethanol and protonated propionitrile, as well as between n-propanol and protonated n-propanol have been investigated experimentally by low-pressure Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The two ethyl cation transfer reactions were found to be the dominant reaction channels with association being only a minor reaction pathway. The n-propyl cation transfer reaction was found to compete with the association pathway resulting in an approximately 50:50 mixture of protonated di-n-propyl ether and the proton-bound dimer of n-propanol, depending on temperature and pressure. The enthalpies of activation were determined to be -16.1 +/- 0.8, -17.5 +/- 0.8, and -15.7 +/- 0.9 kJ mol(-1) for the ethanol/protonated ethanol, ethanol/protonated propionitrile, and n-propanol/protonated n-propanol alkyl cation transfer reactions, respectively. The entropies of activation were found to be essentially the same, -121 +/- 28 J K-1 mol(-1), for all three reactions. All experimentally determined thermochemical properties agreed very well with those calculated at the MP2/6-31+G(d)//B3LYP/6-31+G* level of theory. Furthermore, the enthalpies and entropies of activation for the methy, ethyl, and n-propyl cation transfer reactions between the neutral alcohols and the respective protonated alcohols were found to be the same within experimental uncertainty.