Calculated stabilities and structures for carbocations and singlet carbenes bearing electron‐withdrawing groups

Abstract

Carbocations and carbenes, as electron-deficient species, require electron donation from the remainder of the molecule to the carbon center by whatever means available. Classical interactions include resonance, polar and polarizability effects, but neighboring group participation of several sorts can also serve as stabilizing factors. Simple carbocations with directly attached electron-withdrawing groups (EWGs), that is, EWG-CH 2 + ions, comprise one group by which these interactions may be probed. This article provides computational evidence at the MP2/6-311 + G** level for variable but significant stabilizing interactions between the carbocation center and common EWGs, many via bridging (partial or symmetrical), neighboring group participation, homoconjugation or π interactions. Bridging from atoms possessing nonbonding electron pairs is a common motif. Removal of bridging by application of geometric constraints nullifies bridging stabilization, but does not eliminate the possibility of other stabilizing interactions, for example polar, polarizability and π donation from the EWG. The potential for π donation from strong EWGs has not been widely appreciated in the past. A carbocation stabilization enthalpy, CSE+, is defined as the enthalpy of the isodesmic reaction CH 3 + + R-H → CH 4 + R + . Comparisons are made with singlet carbenes bearing EWGs for which many structural features are similar.

Photoinduced Formation of H-Bonded Ion Pair in HCFC-133a

“…Consequently, the energy of the ionic channel decreases, leading to smaller binding energy for the bonded ion pair. The F migration of CF 3 C + H 2 cation has been previously characterized at the MP2 level . Additional calculations at other correlated levels, along with previous results at the HF level, indicate that such spontaneous migration can only be captured at post-Hartree–Fock levels.…”

“…The F migration of CF 3 C + H 2 cation has been previously characterized at the MP2 level. 32 Additional calculations at other correlated levels, along with previous results at the HF level, indicate that such spontaneous migration can only be captured at post-Hartree−Fock levels. The computed value of 10.78 eV for the vertical ionization potential (at the MP2/augcc-pVTZ level) is in excellent agreement with the experimental value of 10.6 eV given by Fisher et al 52 The C + −H•••Cl − moiety is a relatively uncommon structure.…”

“…This choice is due to the F migration, from C1 to C2, observed at correlated levels. 32 These orbitals are shown in from the charge-center (the C1 atom, as the final optimized structure is [CF2CFH2] + due to F migration) and collinear with both C atoms. This supermolecule has then been used to obtain the dissociation limit of the ion-pair channel (with the valence space).…”

“…The cation fragment [CF 3 CH 2 ] + , necessary to obtain the dissociation limit of the ion-pair channel, has been treated using an active space (at both CASSCF and MR-CISD levels) consisting of five orbitals, that is, the three lone pair orbitals of the F atom in the symmetry plane, along with the 2p σ orbitals of both C atoms. This choice is due to the F migration, from C1 to C2, observed at correlated levels . These orbitals are shown in Table S5 of the Supporting Information.…”

“…This choice is due to the F migration, from C1 to C2, observed at correlated levels. 32 These orbitals are shown in Table S5 of the Supporting Information. Three (2A′ and 1A″) singlet states with equal weights have been included in the state-averaged CASSCF calculations.…”