Herman Halim

Mindo/3 and mndo calculations of closed‐ and open‐shell cations containing C, H, N, and O

Heats of formation of 119 closed‐ and open‐shell carbocations calculated by the semiempirical quantum chemical methods MINDO/3 and MNDO are reported and compared with experimental data. With proper consideration of failures in specific areas, both methods can be used for the thermodynamics of carbocations containing C, H, N, and O. MINDO/3 predicts unrealistic values for nitrogen containing cations with nitrogen multiple bonds and is not suited for closed‐shell cations containing oxygen. Saturated acyclic hydrocarbon radical cations often are computed with abnormally long CC bonds by MNDO. Otherwise, the standard deviation of the two methods is not very different, being in the range of ±13 kcal/mol. MINDO/3 tends to overestimate the cation stabilities, whereas MNDO calculates cations usually too high in energy. Some of the errors which were found in the calculations of the ions are related to the computed values for the parent neutral structures, but others are not.

On the formation of C2H5O2+ ions having the structure of hydroxy-protonated acetic acid

Experiments are reported which are best explained in terms of the formation of the long-sought hydroxy-protonated acetic acid, CH3C(O)OH2- This C2 H5O2+ species, generated upon dissociative ionization of 2,4-dihydroxy-2-methylpentane (consecutive losses of CH3. and C3H6), is characterized by a unique collisional activation mass spectrum and an extraordinarily small kinetic energy release for the unimolecular loss of H2O (T0.5<0.15 kJ mol−1). The latter observation is in good agreement with the results of semi-empirical molecular orbital calculations.

Remarkable rearrangement–dissociation processes of gaseous protonated methyl formate involving ion–dipole complexes

Collision induced decarbonylation of 13C, 18O-labelled protonated methyl formate, HC(OH)O*C*H3, involves the loss of both CO (40%) and CO*; the intermediacy of proton-bound dimers and ion–dipole complexes is invoked to explain the mechanisms.