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 ph ase has been examined by both tandem mass spectrometry experiments, in cluding metastable ions decompositions, collisional activation, and ne utralization-reionization techniques, and molecular orbital calculatio ns up to the MP2/6-311G*//MP2/6-31G*+ZPE level. Two reaction mechanis ms were found to explain the experimental observations: one leading to a protonated cyclic ether via an internal nucleophilic substitution a nd one giving a protonated carbonyl species after hydride ion transfer from the alpha-carbon to the omega-position. Our major findings are t he following: (i) protonated 1,2 ethanediol exclusively leads to proto nated acetaldehyde via a concerted pinacol rearrangement with the calc ulated critical energy equaling 99 kJ/mol; (ii) protonated 1,3-propane diol gives protonated oxetane at low internal energy and protonated pr opanal at high internal energy with the calculated critical energies o f the reactions equaling 144 and 163 kJ/mol, respectively, and the com petition between the two reactions being explained by the internal ene rgy effect upon dissociation rate constants; (iii) for 1,4-butanediol and 1,5-pentanediol, the dehydration produces only the corresponding p rotonated cyclic ether with calculated critical energies equal to 110 and 107 kJ/mol, respectively.