Gernot Frenking

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.

Ab initio molecular orbital calculations on the interconversion of allene and propyne cation radicals and the mechanism for hydrogen loss from C3H4

Abstract Ab initio molecular orbital calculations (6-31G*/MNDO) of the C 3 H 4 +· potential energy surface (electronic ground state) reveal the following features. The molecular ions of allene ( 1 ) and propyne ( 2 ) are separated by substantial potential energy barriers which preclude easy interconversion. The minimal energy requirement path for the 1 ⇌ 2 isomerization proceeds via two successive 1,2-hydrogen migrations and involves the as yet unknown stable, linear C 3 H 4 +· ion 5. Isomerization via a direct 1,3-hydrogen migration is, if it is involved at all, energetically less favoured. The ion 5 also serves as the central intermediate for ring closure to the cation radical of cyclopropene ( 4 ), which itself is the actual precursor for loss of H · thus generating the cyclopropenylium ion ( 3 ). The complete geometric data of the various C 3 H 4 +· isomers and the transition states connecting them are reported.

CF22+ and CF2+, two unusually stable dications with carbon−fluorine double bonding

Abstract Ab initio calculations at MP2/6·31G * //MP2/6·31G * level of theory predict that CF 2 2+ and CF 2+ are very stable dications which in spite of strong Coulomb repulsion contain carbon-fluorine double bonding.

MNDO investigation of the 1,2-rearrangement of singlet carbenes and nitrenes

Abstract 1,2-Rearrangement of substituted methylcarbenes, carbonylcarbenes, methyinitrenes and carbonyltrenes have been calculated for singlet states by MNDO. The alteration in the electronic structure along the reaction course has been investigated by calculation and analysis of the transformation matrices between the MOs of the educts and the transition states. Migratory aptitudes are discussed as well as factors by which they may be determined. The effect of substituents at different positions has been investigated and conformational influences are discussed. As far as experimental results exist, the calculated data agree very well and offer a detailed understanding for the reaction mechanism and substituent effects.

Theoretical and experimental studies on the ground state potential energy surface of C2H4O dications

Abstract The C2H4O2+ ground state potential energy surface was examined by ab initio molecular orbital theory corrected for the effects of zero-point energies and for electron correlations by means of Moller-Plesset perturbation theory terminated to second order (MP2/6-31G) using 4-31G optimized geometries. The global minimum corresponds to H3C-C-OH2+ (1, C3v), which is 14.7 kcal mol−1 more stable than the dication of ring-opened ethylene oxide, i.e. H2C-O-CH22+ (2, D2d). Other stable C2H4O2+ isomers are the perpendicular forms of the anti- and syn-vinyl alcohol (3 and 4, 15.8 and 22.8 kcal mol−1 less stable than 1, an oxygen analogue of allene, 5, i.e. H2C-C-OH22+ (28.7 kcal mol−1 relative to 1), and an oxygen analogue of propyne, 7, i.e. HC-C-OH32+, which is substantially less stable than 1 (99.4 kcal mol−1). The end-on complex between water and acetylene, i.e HCCH-OH22+ (6), is also found to be a stable species on the 4-31G potential energy surface (38.0 kcal mol−1 relative to 1), in contrast to its side-on isomer. However, investigations at higher levels of theory indicate that 6 should not be regarded as a true minimum structure. The dications of acetaldehyde (8), planar vinyl alcohol (9) and ethylene oxide (10) are not found to exist in potential minima. The thermochemical and kinetic stabilities of 1, 2 and 3 are evaluated in detail. Although these dications are thermochemically highly unstable, dissociation is prevented by substantial barriers, thus making observation in the gas phase feasible. In fact, charge-stripping mass spectrometry provides evidence that 1, 2 and 3 can indeed be generated, and a comparison of the experimentally derived Qmin values with the calculated vertical ionization energies shows good agreement for the ions 2 and 3. It seems, however, quite impossible that C2H4O2+ could ever be generated as viable chemical intermediates in solution because the dications will strip an electron or an atom from an adjacent neutral (or negatively charged) species with avidity.

Theoretical and experimental investigation of the electron affinities of allene and propyne

In qualitative agreement with semi-empirical (MNDO) and ab initio calculations, the electron affinities measured by electron transmission spectroscopy are found to be strongly negative for allene (− 1.9 ± 0.1 eV) and propyne (−2.8 ± 0.1 eV).

Mechanismus der protonkatalysierten gasphasendehydratisierung von furfurylalkohol

Zusammenfassung Die protonkatalysierte H 2 O-Abspaltung aus Tetrahydrofufurylalkohol ( 1 ) liefert in der Gasphase ausschliesslich das ringerweiterte Oxonium-Ion 12 . Fur eine [1.2]-Hydridwanderung und Erzeugung von 11 gibt es keinerlei experimentelle Hinweise. Aus der Analyse spezifisch 13 C-markierter Substrate kann in Kombination mit Stossaktivierungsspektren klar gefolgert werden, dass die Ringerweiterung uber den Bruch der Sauerstoff/Kohlenstoffbindung (via 14 ) verlauft. Ein Vergleich der experimentellen Befunde mit ab initio Rechnungen (4-31G) belegt, dass das freie Carbenium-Ion 7 wahrend der Gasphasesolvolyse nicht durchlaufen wird. Die Eliminierung von H 2 O ist vielmehr ein durch den Ethersauerstoff anchimer assistierter Prozess (via 15 ).

Structures and stabilities of ion/dipole complexes

Abstract The stabilities towards dissociation, geometries and electronic structures of ion/dipole complexes can be related to the donor/acceptor properties of the ion and dipole, respectively. This is demonstrated for the three radical cations CH2XH+. (X = F, OH, NH2) by means of ab initio investigations. MNDO and UMNDO calculations show qualitative the same results but differ considerably in detail. The relative stability of the ion/dipole complexes compared to the ‘normal’ structures CH3X+. does not reflect the strength of the bonding between ion and dipole.

On the origin of the different activation energies for hydrogen additions at the C and O centres of RCO+ ions (R = H, CH3): A theoretical interpretation

Abstract The different activation energies for hydrogen addition at the carbon and oxygen positions of HCO + and CH 3 CO + are explained using frontier orbital arguments.The reactions are considered as “nucleophilic” attack of the hydrogen atom. Addition at the carbon centres yields H 2 CO + · and CH 3 CHO + ·, respectively, and is favoured in comparison to addition all the oxygen (which would result in the formation of RCOH + ·) because of the larger LUMO coefficient at the carbon centre. This explanation is in agreement with the calculated and experimentally derived activation energies.That the SOMO(H)-LUMG(RCO + ) interaction is indeed the dominant one is demonstrated by the transformation matrix between the MOs of the educt and transition state.

MNDO-rechnungen zum mechanismus der H2O-abspaltung aus protoniertem cyclohexanon in der gasphase

Abstract The potential energy surfaces for some reactive C 6 H 11 O + ions generated under chemical ionisation conditions in a mass spectrometer are explored by semiempirical quantum mechanical calculations (MNDO). It is shown that unimolecular H 2 O loss from protonated cyclohexanone (4) proceeds via three competitive pathways, e.g. direct [1.3] and [1,4]-elimination from cyclohexanone-type intermediates and a process involving ring-contracted cations. The computational results do not only account for the experimentally observed fact that all methylene groups of cyclohexanone contribute to H 2 O loss: moreover, they explain why only a minor fraction of protonated cyclohexanone undergoes unimolecular H 2 O loss and also account for the fact that the non-decomposing [MHH 2 O] + ions generated from protonated cyclohexanediols rearrange to protonated cyclopentane carbaldehyd9e (3). The experimental result that non-decomposing 3 and 4 do not interconvert is also reflected by the MNDO calculations.