Experimentally determined proton affinities of 4-methyl-3-penten-2-one, 2-propyl ethanoate, and 4-hydroxy-4-methyl-2-pentanone in the gas phase

Proton affinities have been determined for 4-methyl-3-penten-2-one, 2-propyl ethanoate, and 4-hydroxy-4-methyl-2-pentanone in the gas phase at 333 K. A quadrupole ion store (QUISTOR) was employed to study mass spectrometrically the equilibrium between a species of known proton affinity and one of the above compounds; equilibrium between protonated species was monitored over an ion storage duration of 100 ms. The values of the proton affinities were found to be 870.5 ± 0.8 kJ mol−1 for 4-methyl-3-penten-2-one (mesityl oxide); 842.7 ± 0.6 kJ mol−1 for 2-propyl ethanoate; and 831.6 ± 0.8 kJ mol−1 for 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol).