Conformational effect on gas‐phase protonation and NH 4 + clustering of 1,2‐diols

Ab initio molecular orbital calculations at the MP2/6–31G*//3–21G level are reported on conformers of neutral ethane‐1,2‐diol (ethylene glycol; 1), its protonated from 1H + and the ammonium adduct [1NH 4 ] + for an OCCO dihedral angle (θ) of 0–180°. The proton affinity ( PA ) of 1 was determined in ion cyclotron resonance bracketing experiments to be 820 ± 3 kJ mol −1 . An adduct stabilization energy of 134 kJ mol −1 was calculated for [1NH 4 ] + . Total optimization at the 3–21G level of cis ‐ and trans ‐cyclopentane‐1,2‐diol (2 and 3) and their protonated forms 2H + and 3H + showed distorted envelope structures with θ values of 47.2°, 73.4°, 33.0° and 58.0°, respectively. The calculation predicts PA (2) = 845 kJ mol −1 and PA (3) = 815 kJ mol −1 .