Frontorbitale und 1.2-Hydridwanderungen in Carbeniumionen/ Frontier Orbitals and 1,2-Hydrogen Shifts in Carbenium Ions

Abstract

<jats:title>Abstract</jats:title><jats:p>For various carbenium ions it is shown that the energy value and the degree of localization of the LUMO have a significant influence on the energy barriers of 1,2-hydrogen shifts. The structures of the transition states are in line with the Hammond-Postulate.</jats:p>

Isotopomer distributions of neutral products from a doubly labeled cation in the gas phase. Interconversion of 1-fluoro-1-propyl cation and 1-fluoroisopropyl cation on the C3H6F+ potential energy surface

“…Barrier-free rearrangements to 3 (via methyl shift) or 4 (via fluoride shift) prevail over hydrogen 1,2-shift to 1 (regardless of which conformation of 5 is chosen as the initial geometry).10 Previously reported semiempirical calculations for 4 -3 predict a 70 kJ mol"1 barrier (as well as surprisingly high barriers for exothermic hydrogen 1,2-shifts in other conformationally mobile, electron-deficient cations). 23 While that theoretical result is consistent with our ab initio calculations on ion 5, the computational results contradict a recent proposal that, in solution, species 19 (a formal equivalent of 5) rearranges exclusively to 1, as eq 13 portrays. 24 The EBFlow results for free ions are consistent with eq 13, but it turns out that there is more than one pathway to 1.…”

The calculated structures of the C4H8NO+ cations resulting from the unimolecular gas‐phase dissociation (CH3)2NCOCH2X·+ → C4H8NO+ + X· (X  CL, NO2)

“…One setback, the systematic underestimation of Hbridged structures, has been shown. 6 On the other hand, the relative stabilities of all the structures considered in our investigation is predicted nearly in the same way by both methods. Deviations occur mainly with regard to the relative stabilities of the different conformations (e.g., 8, 9, 17).…”

“…Another three-membered cyclic compound 6 may be formed from l(c) [reaction (6)]: (6) As the data in Table I1 show that 6 is rather high in energy and the process l(c) -6 is, in contrast to reaction (5), highly endothermic, 6 is not expected to be generated from 1, whereas 7 should be the only stable three-membered cyclic cation among the C~HSNO+ species discussed so far. A 1,2 H shift separates the substituted acylium ion 3 from structure 8:…”

A Quantum-Chemical Study of the C2H3F2+ and C2H3Cl2+ Isomers and Their Interconversion. CBS-QB3 Proton Affinities of Difluoroethenes and Dichloroethenes

“…CH 3 CX 2 + . Previous studies of the haloethyl cations indicated that the halogen substituents in the α-position stabilize the carbenium ion. − ,− , This suggests that the 1,1-dihaloethyl cations will be the most stable isomers of the difluoro- and dichloroethyl cations. Indeed, our calculations indicate that the CH 3 CX 2 + cations 1 and 2 (Figure ) are at the global minima on the C 2 H 3 X 2 + potential energy surfaces for both X = F and X = Cl.…”

“…To date the potential energy surfaces of the dihalogenated ethyl cations were not subjected to similar detailed studies, although some isomers were examined by quantum-chemical methods. − The heats of formation of the CH 3 CF 2 + , CH 2 FCHF + , and CHF 2 CH 2 + cations were calculated at the SCF level . Brum et al .…”