Unimolecular Chemistry of Protonated Diols in the Gas Phase: Internal Cyclization and Hydride Ion Transfer

Unimolecular dehydration of protonated α,ω-diols in the gas phase has been examined by both tandem mass spectrometry experiments, including metastable ions decompositions, collisional activation, and neutralization−reionization techniques, and molecular orbital calculations up to the MP2/6-311G**//MP2/6-31G*+ZPE level. Two reaction mechanisms were found to explain the experimental observations:  one leading to a protonated cyclic ether via an internal nucleophilic substitution and one giving a protonated carbonyl species after hydride ion transfer from the α-carbon to the ω-position. Our major findings are the following:  (i) protonated 1,2-ethanediol exclusively leads to protonated acetaldehyde via a concerted pinacol rearrangement with the calculated critical energy equaling 99 kJ/mol; (ii) protonated 1,3-propanediol gives protonated oxetane at low internal energy and protonated propanal at high internal energy with the calculated critical energies of the reactions equaling 144 and 163 kJ/mol, respectiv...