Unimolecular chemistry of protonated diols in the gas phase: Internal cyclization and hydride ion transfer

Unimolecular dehydration of protonated alpha,omega-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 alpha-carbon to the omega-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, respectively, and the competition between the two reactions being explained by the internal energy effect upon dissociation rate constants; (iii) for 1,4-butanediol and 1,5-pentanediol, the dehydration produces only the corresponding protonated cyclic ether with calculated critical energies equal to 110 and 107 kJ/mol, respectively.