On the Fate of Protonated Aldehydes in the Gas Phase

Protonated aldehydes can be formed in the gas phase by proton-transfer reactions with atmospheric water. This work explored the unimolecular chemistry of protonated aldehydes in the gas phase using tandem mass spectrometry and theory, focusing on protonated propanal (1), butanal (2), isobutanal (3), crotonaldehyde (4), and methacrolein (5). An assessment of different density functional theory methods for calculating the minimum energy reaction pathway for water loss from 1 led to the use of the M06/6-311+G(d,p) level of theory. Results showed that the predominant reaction for 1-4 was the loss of H₂O to form a (substituted) allyl cation, while 5 showed CO loss as the major pathway. The primary mechanism for H₂O loss involved a combination of 1,2- and 1,4-H shift reactions forming water and a (methyl)allene ion. Minor neutral losses of CH₃ ^•, CH₄, and CO were observed from 1, CH₃ ^•, C₂H₄, and CH₂O from 3, CO and CH₂O from 4, and H₂O and CH₂O from 5.